A NEW PROPOSAL FOR POWER QUALITY AND CUSTOM

POWER IMPROVEMENT OPEN UPQC

ABSTRACT

Power quality (PQ) is very important to certain customers.

For this reason, many utilities could sell electrical energy at

different prices to their customers, depending on the quality of

the delivered electric power. Since most end users are connected

to secondary. Currently, the quality of supplied power is

important to several customers. Power quality (PQ) is a service

and many customers are ready to pay for it. In the future,

distribution system operators could decide, or could be obliged

by authorities, to supply their customers with different PQ

levels and at different prices. A new device that can fulfill

this role is the OPEN unified power quality conditioner (UPQC),

composed of a power-electronic series main unit installed in the

medium-voltage/low-voltage (LV) substation, along with several

power-electronic shunt units connected close to the end users.

The series and parallel units do not have a common dc link,

so their control strategies are different than traditional UPQC

control techniques. This device can achieve general improvement

in PQ, reducing the most common disturbances for all customers

that are supplied by the mains (PQ) by using only the series

unit. Additional increments in PQ (i.e., mains power

interruptions), can be provided to the customers who need it

(custom power) by the shunt units. Therefore, this new solution

combines an improvement in PQ for all end users, with a cost

reduction for those that need high quality power. The proposed

solution has been analyzed and described, and a model of a 400-

kVA LV grid is considered a test network to evaluate the steady-

state performance and functioning limits. The results obtained

under steady-state conditions justify the configuration chosen

and good device performance.

The OPEN UPQC apparatus is a good compensation system if

wide installation of shunt units is needed. An increase in the

percentage of the protected load enhances the voltage

stabilization interval over which the OPEN UPQC can significantly

improve the power quality, especially if the load power factor

takes a high value.

If the power factor of load is less than one, the power

factor in section increases, to avoid nonnative power absorption

from the mains. For low values of the parameter, the OPEN UPQC

becomes expensive if there are few shunt units. In this case, it

is better to install other compensation device typologies (as

UPS, UPQC, etc.) near the sensitive loads, and a non active

compensator system near the nonsensitive loads if necessary. It

is possible to conclude that installation of the series unit is a

cost-effective way for distributors to improve the power quality

level in the distribution networks in order to achieve the

standards imposed by the authorities. Compensation improvement

for the sensitive end users can be achieved by installing a shunt

unit near them, instead of the more expensive UPS device. The

OPEN UPQC working conditions are reported.

I. INTRODUCTION

Power quality (PQ) is very important to certain customers.

For this reason, many utilities could sell electrical energy at

different prices to their customers, depending on the quality of

the delivered electric power. Since most end users are connected

to secondary distribution networks, at low voltage (LV), it could

be important to monitor and compensate the main disturbances on

the LV grid. Specifically, it has been reported in a survey that,

in the Southeastern region of the U.S., most monitored industrial

customers and main end users did not suffer long outages. Rather,

they experienced numerous short duration voltage sags and

momentary interruptions. Therefore, local utility companies had

to re-configure their systems to keep their most important

customers on-line.

Fig. Chronology of voltage sags occurring in the Southeastern

states of the

U.S. on December 4 and 5, 2002.

Fig. shows the chronology of voltage sags occurring in the

Southeastern states of the U.S. on December 4 and 5, 2002. As can

be seen, most sags take place around the 10%–20% level. Various

solutions are available to compensate for these disturbances. One

solution involves increasing the short circuit level of the

distribution network, i.e., revamping all the LV distribution

cables or raising the power of the MV/LV substation transformer,

thus increasing the power quality for all end users. In this way,

an incoming disturbance from a load (i.e., harmonics) or from a

fault in a line is reduced at the point of common coupling (PCC).

Therefore, this solution effectively reduces the depth of the

voltage variations, but does not protect the loads against

transients and short interruptions.

A second solution that can compensate any kind of

disturbance, including interruptions, is installation of on-line,

off-line, line interactive and hybrid UPS systems. In all of

these cases, only the end users that decide to install them are

protected, while all of the other costumers do not receive any

improvement in PQ. Often, these solutions cannot be adopted by

the local utility companies or by the end users, because they are

too expensive relative to the increase in power quality that they

produce. However, many cheaper solutions are available. In

particular, several electronic devices have been developed,

studied and proposed to the international scientific community

with the goal of improving supplied power quality. Different

connection topologies (series or shunt types) are used to realize

these devices.

The series devices are connected upstream of the protected

lines, while the shunt devices are connected in parallel to the

sensitive loads. In general, both types of conditioning devices

increase the power quality level at the loads. Other studies have

been carried out to consider combinations of the previous single

apparatus solutions (as UPS, UPLC, UPQC, etc.). The unified power

quality conditioner (UPQC) compensator seems to be a particularly

promising power conditioner device. This apparatus is constituted

of a series and a shunt unit, with a common dc section through

which power can be exchanged. Its function is to improve the

quality levels of the current absorbed at the mains and the load

supply voltage.

However, these devices do not allow local distributors to

guarantee different quality demand levels to the final customers,

because they improve power quality for all the supplied end

users. The installation investments are also quite high relative

to the power quality level obtained. A solution that has similar

performances and advantages, but also makes cost reduction

possible, is the proposed OPEN UPQC. This new solution, analyzed

in, starts from the UPQC configuration, removes the common dc

connection and splits the shunt unit into several shunted

devices.

Therefore, the control strategy is different than the

traditional combined series and shunt converters, but the

improvements to load voltage and network current quality are

quite similar. Above all, the OPEN UPQC can stabilize load

voltage, increase the network power factor, leading to keep load

voltage and network current sinusoidal and balanced as well.

The series main unit is installed in the MV/LV substation.

In a grid connected configuration, it can stabilize load voltage

at the LV bus bar (PCC) as the series devices analyzed. The shunt

units do not affect the dynamic behavior of the series unit,

because their dynamic responses are very slow under these

operating conditions. The transient behavior of a single dynamic

voltage restorer) device was analyzed and simulated, and its

working limits were determined. In particular, the device

behavior in the presence of voltage sags (i.e. 10%–20%) is

described.

The several shunt units are connected near the end users

that need high power quality. If a storage system is present,

they can exchange active power and nonnative power with the

electrical system. Especially in a grid-connected configuration,

no active power can be exchanged with the mains in order to

enhance the series unit performance and extend its working

limits. Otherwise, the users can disconnect themselves when the

PCC voltage is out of the operating limits, and the load will be

supplied in back-up mode.

II. POWER SYSEM AND POWER QUALITY

POWER QUALITY

The contemporary container crane industry, like many other

industry segments, is often enamored by the bells and whistles,

colorful diagnostic displays, high speed performance, and levels

of automation that can be achieved. Although these features and

their indirectly related computer based enhancements are key

issues to an efficient terminal operation, we must not forget the

foundation upon which we are building. Power quality is the

mortar which bonds the foundation blocks. Power quality also

affects terminal operating economics, crane reliability, our

environment, and initial investment in power distribution systems

to support new crane installations.

To quote the utility company newsletter which accompanied

the last monthly issue of my home utility billing: ‘Using

electricity wisely is a good environmental and business practice

which saves you money, reduces emissions from generating plants,

and conserves our natural resources.’ As we are all aware,

container crane performance requirements continue to increase at

an astounding rate. Next generation container cranes, already in

the bidding process, will require average power demands of 1500

to 2000 kW – almost double the total average demand three years

ago. The rapid increase in power demand levels, an increase in

container crane population, SCR converter crane drive retrofits

and the large AC and DC drives needed to power and control these

cranes will increase awareness of the power quality issue in the

very near future.

POWER QUALITY PROBLEMS

For the purpose of this article, we shall define power

quality problems as: ‘Any power problem that results in failure

or misoperation of customer equipment, manifests itself as an

economic burden to the user, or produces negative impacts on the

environment.’

When applied to the container crane industry, the power

issues which degrade power quality include:

• Power Factor

• Harmonic Distortion

• Voltage Transients

• Voltage Sags or Dips

• Voltage Swells

The AC and DC variable speed drives utilized on board

container cranes are significant contributors to total harmonic

current and voltage distortion. Whereas SCR phase control creates

the desirable average power factor, DC SCR drives operate at less

than this. In addition, line notching occurs when SCR’s

commutate, creating transient peak recovery voltages that can be

3 to 4 times the nominal line voltage depending upon the system

impedance and the size of the drives. The frequency and severity

of these power system disturbances varies with the speed of the

drive. Harmonic current injection by AC and DC drives will be

highest when the drives are operating at slow speeds. Power

factor will be lowest when DC drives are operating at slow speeds

or during initial acceleration and deceleration periods,

increasing to its maximum value when the SCR’s are phased on to

produce rated or base speed.

Above base speed, the power factor essentially remains

constant. Unfortunately, container cranes can spend considerable

time at low speeds as the operator attempts to spot and land

containers. Poor power factor places a greater kVA demand burden

on the utility or engine-alternator power source. Low power

factor loads can also affect the voltage stability which can

ultimately result in detrimental effects on the

life of sensitive electronic equipment or even intermittent

malfunction. Voltage transients created by DC drive SCR line

notching, AC drive voltage chopping, and high frequency harmonic

voltages and currents are all significant sources of noise and

disturbance to sensitive electronic equipment

It has been our experience that end users often do not

associate power quality problems with Container cranes, either

because they are totally unaware of such issues or there was no

economic Consequence if power quality was not addressed. Before

the advent of solid-state power supplies, Power factor was

reasonable, and harmonic current injection was minimal.

Not until the crane Population multiplied, power demands

per crane increased, and static power conversion became the way

of life, did power quality issues begin to emerge. Even as

harmonic distortion and power Factor issues surfaced, no one was

really prepared. Even today, crane builders and electrical drive

System vendors avoid the issue during competitive bidding for new

cranes. Rather than focus on Awareness and understanding of the

potential issues, the power quality issue is intentionally or

unintentionally ignored. Power quality problem solutions are

available. Although the solutions are not free, in most cases,

they do represent a good return on investment. However, if power

quality is not specified, it most likely will not be delivered.

Power quality can be improved through:

• Power factor correction,

• Harmonic filtering,

• Special line notch filtering,

• Transient voltage surge suppression,

• Proper earthing systems.

In most cases, the person specifying and/or buying a

container crane may not be fully aware of the potential power

quality issues. If this article accomplishes nothing else, we

would hope to provide that awareness.

In many cases, those involved with specification and

procurement of container cranes may not be cognizant of such

issues, do not pay the utility billings, or consider it someone

else’s concern. As a result, container crane specifications may

not include definitive power quality criteria such as power

factor correction and/or harmonic filtering. Also, many of those

specifications which do require power quality equipment do not

properly define the criteria. Early in the process of preparing

the crane specification:

• Consult with the utility company to determine regulatory

or contract requirements that must be satisfied, if any.

• Consult with the electrical drive suppliers and determine

the power quality profiles that can be expected based on the

drive sizes and technologies proposed for the specific project.

• Evaluate the economics of power quality correction not

only on the present situation, but consider the impact of future

utility deregulation and the future development plans for the

terminal.

THE BENEFITS OF POWER QUALITY

Power quality in the container terminal environment impacts

the economics of the terminal operation, affects reliability of

the terminal equipment, and affects other consumers served by the

same utility service. Each of these concerns is explored in the

following paragraphs.

1. ECONOMIC IMPACT

The economic impact of power quality is the foremost

incentive to container terminal operators. Economic impact can be

significant and manifest itself in several ways:

A. POWER FACTOR PENALTIES

Many utility companies invoke penalties for low power factor

on monthly billings. There is no industry standard followed by

utility companies. Methods of metering and calculating power

factor penalties vary from one utility company to the next. Some

utility companies actually meter kVAR usage and establish a fixed

rate times the number of kVAR-hours consumed. Other utility

companies monitor kVAR demands and calculate power factor. If the

power factor falls below a fixed limit value over a demand

period, a penalty is billed in the form of an adjustment to the

peak demand charges. A number of utility companies servicing

container terminal equipment do not yet invoke power factor

penalties.

However, their service contract with the Port may still

require that a minimum power factor over a defined demand period

be met. The utility company may not continuously monitor power

factor or kVAR usage and reflect them in the monthly utility

billings; however, they do reserve the right to monitor the Port

service at any time. If the power factor criteria set forth in

the service contract are not met, the user may be penalized, or

required to take corrective actions at the user’s expense.

One utility company, which supplies power service to several

east coast container terminals in the USA, does not reflect power

factor penalties in their monthly billings, however, their

service contract with the terminal reads as follows:

‘The average power factor under operating conditions of

customer’s load at the point where service is metered shall be

not less than 85%. If below 85%, the customer may be required to

furnish, install and maintain at its expense corrective apparatus

which will increase the Power factor of the entire installation

to not less than 85%. The customer shall ensure that no excessive

harmonics or transients are introduced on to the [utility]

system. This may require special power conditioning equipment or

filters.

The Port or terminal operations personnel, who are

responsible for maintaining container cranes, or specifying new

container crane equipment, should be aware of these requirements.

Utility deregulation will most likely force utilities to enforce

requirements such as the example above.

Terminal operators who do not deal with penalty issues today

may be faced with some rather severe penalties in the future. A

sound, future terminal growth plan should include contingencies

for addressing the possible economic impact of utility

deregulation.

B. SYSTEM LOSSES

Harmonic currents and low power factor created by nonlinear

loads, not only result in possible power factor penalties, but

also increase the power losses in the distribution system. These

losses are not visible as a separate item on your monthly utility

billing, but you pay for them each month.

Container cranes are significant contributors to harmonic

currents and low power factor. Based on the typical demands of

today’s high speed container cranes, correction of power factor

alone on a typical state of the art quay crane can result in a

reduction of system losses that converts to a 6 to 10% reduction

in the monthly utility billing. For most of the larger terminals,

this is a significant annual saving in the cost of operation.

C. POWER SERVICE INITIAL CAPITAL INVESTMENTS

The power distribution system design and installation for

new terminals, as well as modification of systems for terminal

capacity upgrades, involves high cost, specialized, high and

medium voltage equipment. Transformers, switchgear, feeder

cables, cable reel trailing cables, collector bars, etc. must be

sized based on the kVA demand. Thus cost of the equipment is

directly related to the total kVA demand.

As the relationship above indicates, kVA demand is inversely

proportional to the overall power factor, i.e. a lower power

factor demands higher kVA for the same kW load. Container cranes

are one of the most significant users of power in the terminal.

Since container cranes with DC, 6 pulse, SCR drives operate at

relatively low power factor, the total kVA demand is

significantly larger than would be the case if power factor

correction equipment were supplied on board each crane or at some

common bus location in the terminal. In the absence of power

quality corrective equipment, transformers are larger, switchgear

current ratings must be higher, feeder cable copper sizes are

larger, collector system and cable reel cables must be larger,

etc. Consequently, the cost of the initial power distribution

system equipment for a system which does not address power

quality will most likely be higher than the same system which

includes power quality equipment.

2. EQUIPMENT RELIABILITY

Poor power quality can affect machine or equipment

reliability and reduce the life of components. Harmonics, voltage

transients, and voltage system sags and swells are all power

quality problems and are all interdependent. Harmonics affect

power factor, voltage transients can induce harmonics, the same

phenomena which create harmonic current injection in DC SCR

variable speed drives are responsible for poor power factor, and

dynamically varying power factor of the same drives can create

voltage sags and swells. The effects of harmonic distortion,

harmonic currents, and line notch ringing can be mitigated using

specially designed filters.

3. POWER SYSTEM ADEQUACY

When considering the installation of additional cranes to an

existing power distribution system, a power system analysis

should be completed to determine the adequacy of the system to

support additional crane loads.

Power quality corrective actions may be dictated due to

inadequacy of existing power distribution systems to which new or

relocated cranes are to be connected. In other words, addition of

power quality equipment may render a workable scenario on an

existing power distribution system, which would otherwise be

inadequate to support additional cranes without high risk of

problems.

4. ENVIRONMENT

No issue might be as important as the effect of power

quality on our environment. Reduction in system losses and lower

demands equate to a reduction in the consumption of our natural

nm resources and reduction in power plant emissions. It is our

responsibility as occupants of this planet to encourage

conservation of our natural resources and support measures which

improve our air quality.

FACTS

Flexible AC Transmission Systems, called FACTS, got in the

recent years a well known term for higher controllability in

power systems by means of power electronic devices. Several

FACTS-devices have been introduced for various applications

worldwide. A number of new types of devices are in the stage of

being introduced in practice.

In most of the applications the controllability is used to

avoid cost intensive or landscape requiring extensions of power

systems, for instance like upgrades or additions of substations

and power lines. FACTS-devices provide a better adaptation to

varying operational conditions and improve the usage of existing

installations. The basic applications of FACTS-devices are:

• Power flow control,

• Increase of transmission capability,

• Voltage control,

• Reactive power compensation,

• Stability improvement,

• Power quality improvement,

• Power conditioning,

• Flicker mitigation,

• Interconnection of renewable and distributed generation

and storages.

Figure shows the basic idea of FACTS for transmission

systems. The usage of lines for active power transmission should

be ideally up to the thermal limits. Voltage and stability limits

shall be shifted with the means of the several different FACTS

devices. It can be seen that with growing line length, the

opportunity for FACTS devices gets more and more important.

The influence of FACTS-devices is achieved through switched

or controlled shunt compensation, series compensation or phase

shift control. The devices work electrically as fast current,

voltage or impedance controllers. The power electronic allows

very short reaction times down to far below one second.

The development of FACTS-devices has started with the

growing capabilities of power electronic components. Devices for

high power levels have been made available in converters for high

and even highest voltage levels. The overall starting points are

network elements influencing the reactive power or the impedance

of a part of the power system. Figure 1.2 shows a number of basic

devices separated into the conventional ones and the FACTS-

devices.

For the FACTS side the taxonomy in terms of 'dynamic' and

'static' needs some explanation. The term 'dynamic' is used to

express the fast controllability of FACTS-devices provided by the

power electronics. This is one of the main differentiation

factors from the conventional devices. The term 'static' means

that the devices have no moving parts like mechanical switches to

perform the dynamic controllability. Therefore most of the FACTS-

devices can equally be static and dynamic.

The left column in Figure 1.2 contains the conventional

devices build out of fixed or mechanically switch able components

like resistance, inductance or capacitance together with

transformers. The FACTS-devices contain these elements as well

but use additional power electronic valves or converters to

switch the elements in smaller steps or with switching patterns

within a cycle of the alternating current. The left column of

FACTS-devices uses Thyristor valves or converters. These valves

or converters are well known since several years. They have low

losses because of their low switching frequency of once a cycle

in the converters or the usage of the Thyristors to simply bridge

impedances in the valves.

The right column of FACTS-devices contains more advanced

technology of voltage source converters based today mainly on

Insulated Gate Bipolar Transistors (IGBT) or Insulated Gate

Commutated Thyristors (IGCT). Voltage Source Converters provide a

free controllable voltage in magnitude and phase due to a pulse

width modulation of the IGBTs or IGCTs. High modulation

frequencies allow to get low harmonics in the output signal and

even to compensate disturbances coming from the network.

The disadvantage is that with an increasing switching

frequency, the losses are increasing as well. Therefore special

designs of the converters are required to compensate this.

CONFIGURATIONS OF FACTS-DEVICES:

SHUNT DEVICES:

The most used FACTS-device is the SVC or the version with

Voltage Source Converter called STATCOM. These shunt devices are

operating as reactive power compensators. The main applications

in transmission, distribution and industrial networks are:

• Reduction of unwanted reactive power flows and therefore

reduced network losses.

• Keeping of contractual power exchanges with balanced reactive

power.

• Compensation of consumers and improvement of power quality

especially with huge demand fluctuations like industrial

machines, metal melting plants, railway or underground train

systems.

• Compensation of Thyristor converters e.g. in conventional HVDC

lines.

• Improvement of static or transient stability.

Almost half of the SVC and more than half of the STATCOMs

are used for industrial applications. Industry as well as

commercial and domestic groups of users require power quality.

Flickering lamps are no longer accepted, nor are interruptions of

industrial processes due to insufficient power quality. Railway

or underground systems with huge load variations require SVCs or

STATCOMs.

SVC:

Electrical loads both generate and absorb reactive power.

Since the transmitted load varies considerably from one hour to

another, the reactive power balance in a grid varies as well. The

result can be unacceptable voltage amplitude variations or even a

voltage depression, at the extreme a voltage collapse.

A rapidly operating Static Var Compensator (SVC) can

continuously provide the reactive power required to control

dynamic voltage oscillations under various system conditions and

thereby improve the power system transmission and distribution

stability.

Applications of the SVC systems in transmission systems:

a. To increase active power transfer capacity and transient

stability margin

b. To damp power oscillations

c. To achieve effective voltage control

In addition, SVCs are also used

1. in transmission systems

a. To reduce temporary over voltages

b. To damp sub synchronous resonances

c. To damp power oscillations in interconnected power

systems

2. in traction systems

a. To balance loads

b. To improve power factor

c. To improve voltage regulation

3. In HVDC systems

a. To provide reactive power to ac–dc converters

4. In arc furnaces

a. To reduce voltage variations and associated light flicker

Installing an SVC at one or more suitable points in the

network can increase transfer capability and reduce losses while

maintaining a smooth voltage profile under different network

conditions. In addition an SVC can mitigate active power

oscillations through voltage amplitude modulation.

SVC installations consist of a number of building blocks.

The most important is the Thyristor valve, i.e. stack assemblies

of series connected anti-parallel Thyristors to provide

controllability. Air core reactors and high voltage AC capacitors

are the reactive power elements used together with the Thyristor

valves. The step up connection of this equipment to the

transmission voltage is achieved through a power transformer.

SVC building blocks and voltage / current characteristic

In principle the SVC consists of Thyristor Switched

Capacitors (TSC) and Thyristor Switched or Controlled Reactors

(TSR / TCR). The coordinated control of a combination of these

branches varies the reactive power as shown in Figure. The first

commercial SVC was installed in 1972 for an electric arc furnace.

On transmission level the first SVC was used in 1979. Since then

it is widely used and the most accepted FACTS-device.

SVC

SVC USING A TCR AND AN FC:

In this arrangement, two or more FC (fixed capacitor) banks

are connected to a TCR (thyristor controlled reactor) through a

step-down transformer. The rating of the reactor is chosen larger

than the rating of the capacitor by an amount to provide the

maximum lagging vars that have to be absorbed from the system.

By changing the firing angle of the thyristor controlling

the reactor from 90° to 180°, the reactive power can be varied

over the entire range from maximum lagging vars to leading vars

that can be absorbed from the system by this compensator.

SVC of the FC/TCR type:

The main disadvantage of this configuration is the

significant harmonics that will be generated because of the

partial conduction of the large reactor under normal sinusoidal

steady-state operating condition when the SVC is absorbing zero

MVAr. These harmonics are filtered in the following manner.

Triplex harmonics are canceled by arranging the TCR and the

secondary windings of the step-down transformer in delta

connection. The capacitor banks with the help of series reactors

are tuned to filter fifth, seventh, and other higher-order

harmonics as a high-pass filter. Further losses are high due to

the circulating current between the reactor and capacitor banks.

Comparison of the loss characteristics of TSC–TCR, TCR–FC

compensators and synchronous condenser

These SVCs do not have a short-time overload capability

because the reactors are usually of the air-core type. In

applications requiring overload capability, TCR must be designed

for short-time overloading, or separate thyristor-switched

overload reactors must be employed.

SVC USING A TCR AND TSC:

This compensator overcomes two major shortcomings of the

earlier compensators by reducing losses under operating

conditions and better performance under large system

disturbances. In view of the smaller rating of each capacitor

bank, the rating of the reactor bank will be 1/n times the

maximum output of the SVC, thus reducing the harmonics generated

by the reactor. In those situations where harmonics have to be

reduced further, a small amount of FCs tuned as filters may be

connected in parallel with the TCR.

SVC of combined TSC and TCR type

When large disturbances occur in a power system due to load

rejection, there is a possibility for large voltage transients

because of oscillatory interaction between system and the SVC

capacitor bank or the parallel. The LC circuit of the SVC in the

FC compensator. In the TSC–TCR scheme, due to the flexibility of

rapid switching of capacitor banks without appreciable

disturbance to the power system, oscillations can be avoided, and

hence the transients in the system can also be avoided. The

capital cost of this SVC is higher than that of the earlier one

due to the increased number of capacitor switches and increased

control complexity.

STATCOM:

In 1999 the first SVC with Voltage Source Converter called

STATCOM (STATic COMpensator) went into operation. The STATCOM has

a characteristic similar to the synchronous condenser, but as an

electronic device it has no inertia and is superior to the

synchronous condenser in several ways, such as better dynamics, a

lower investment cost and lower operating and maintenance costs.

A STATCOM is build with Thyristors with turn-off capability

like GTO or today IGCT or with more and more IGBTs. The static

line between the current limitations has a certain steepness

determining the control characteristic for the voltage.

The advantage of a STATCOM is that the reactive power

provision is independent from the actual voltage on the

connection point. This can be seen in the diagram for the maximum

currents being independent of the voltage in comparison to the

SVC. This means, that even during most severe contingencies, the

STATCOM keeps its full capability.

In the distributed energy sector the usage of Voltage Source

Converters for grid interconnection is common practice today. The

next step in STATCOM development is the combination with energy

storages on the DC-side. The performance for power quality and

balanced network operation can be improved much more with the

combination of active and reactive power.

STATCOM structure and voltage / current characteristic

STATCOMs are based on Voltage Sourced Converter (VSC)

topology and utilize either Gate-Turn-off Thyristors (GTO) or

Isolated Gate Bipolar Transistors (IGBT) devices. The STATCOM is

a very fast acting, electronic equivalent of a synchronous

condenser.

If the STATCOM voltage, Vs, (which is proportional to the dc

bus voltage Vc) is larger than bus voltage, Es, then leading or

capacitive VARS are produced. If Vs is smaller then Es then

lagging or inductive VARS are produced.

6 Pulses STATCOM

The three phases STATCOM makes use of the fact that on a

three phase, fundamental frequency, steady state basis, and the

instantaneous power entering a purely reactive device must be

zero. The reactive power in each phase is supplied by circulating

the instantaneous real power between the phases. This is achieved

by firing the GTO/diode switches in a manner that maintains the

phase difference between the ac bus voltage ES and the STATCOM

generated voltage VS. Ideally it is possible to construct a

device based on circulating instantaneous power which has no

energy storage device (ie no dc capacitor).

A practical STATCOM requires some amount of energy storage

to accommodate harmonic power and ac system unbalances, when the

instantaneous real power is non-zero. The maximum energy storage

required for the STATCOM is much less than for a TCR/TSC type of

SVC compensator of comparable rating.

STATCOM Equivalent Circuit

Several different control techniques can be used for the

firing control of the STATCOM. Fundamental switching of the

GTO/diode once per cycle can be used. This approach will minimize

switching losses, but will generally utilize more complex

transformer topologies. As an alternative, Pulse Width Modulated

(PWM) techniques, which turn on and off the GTO or IGBT switch

more than once per cycle, can be used. This approach allows for

simpler transformer topologies at the expense of higher switching

losses.

The 6 Pulse STATCOM using fundamental switching will of

course produce the 6 N1 harmonics. There are a variety of

methods to decrease the harmonics. These methods include the

basic 12 pulse configuration with parallel star / delta

transformer connections, a complete elimination of 5th and 7th

harmonic current using series connection of star/star and

star/delta transformers and a quasi 12 pulse method with a single

star-star transformer, and two secondary windings, using control

of firing angle to produce a 30phase shift between the two 6

pulse bridges.

This method can be extended to produce a 24 pulse and a 48

pulse STATCOM, thus eliminating harmonics even further. Another

possible approach for harmonic cancellation is a multi-level

configuration which allows for more than one switching element

per level and therefore more than one switching in each bridge

arm. The ac voltage derived has a staircase effect, dependent on

the number of levels. This staircase voltage can be controlled to

eliminate harmonics.

SERIES DEVICES:

Series devices have been further developed from fixed or

mechanically switched compensations to the Thyristor Controlled

Series Compensation (TCSC) or even Voltage Source Converter based

devices.

The main applications are:

• Reduction of series voltage decline in magnitude and angle

over a power line,

• Reduction of voltage fluctuations within defined limits

during changing power transmissions,

• Improvement of system damping resp. damping of

oscillations,

• Limitation of short circuit currents in networks or

substations,

• Avoidance of loop flows resp. power flow adjustments.

TCSC:

Thyristor Controlled Series Capacitors (TCSC) address

specific dynamical problems in transmission systems. Firstly it

increases damping when large electrical systems are

interconnected. Secondly it can overcome the problem of Sub

Synchronous Resonance (SSR), a phenomenon that involves an

interaction between large thermal generating units and series

compensated transmission systems.

The TCSC's high speed switching capability provides a

mechanism for controlling line power flow, which permits

increased loading of existing transmission lines, and allows for

rapid readjustment of line power flow in response to various

contingencies. The TCSC also can regulate steady-state power flow

within its rating limits.

From a principal technology point of view, the TCSC

resembles the conventional series capacitor. All the power

equipment is located on an isolated steel platform, including the

Thyristor valve that is used to control the behavior of the main

capacitor bank. Likewise the control and protection is located on

ground potential together with other auxiliary systems. Figure

shows the principle setup of a TCSC and its operational diagram.

The firing angle and the thermal limits of the Thyristors

determine the boundaries of the operational diagram.

ADVANTAGES

Continuous control of desired compensation level

Direct smooth control of power flow within the network

Improved capacitor bank protection

Local mitigation of sub synchronous resonance (SSR). This

permits higher levels of compensation in networks where

interactions with turbine-generator torsional vibrations or

with other control or measuring systems are of concern.

Damping of electromechanical (0.5-2 Hz) power oscillations

which often arise between areas in a large interconnected

power network. These oscillations are due to the dynamics of

inter area power transfer and often exhibit poor damping

when the aggregate power tranfer over a corridor is high

relative to the transmission strength.

SHUNT AND SERIES DEVICES

DYNAMIC POWER FLOW CONTROLLER

A new device in the area of power flow control is the

Dynamic Power Flow Controller (DFC). The DFC is a hybrid device

between a Phase Shifting Transformer (PST) and switched series

compensation.

A functional single line diagram of the Dynamic Flow

Controller is shown in Figure 1.19. The Dynamic Flow Controller

consists of the following components:

• a standard phase shifting transformer with tap-changer

(PST)

• series-connected Thyristor Switched Capacitors and

Reactors (TSC / TSR)

• A mechanically switched shunt capacitor (MSC). (This is

optional depending on the system reactive power

requirements)

Based on the system requirements, a DFC might consist of a

number of series TSC or TSR. The mechanically switched shunt

capacitor (MSC) will provide voltage support in case of overload

and other conditions.

Normally the reactance of reactors and the capacitors are

selected based on a binary basis to result in a desired stepped

reactance variation. If a higher power flow resolution is needed,

a reactance equivalent to the half of the smallest one can be

added.

The switching of series reactors occurs at zero current to

avoid any harmonics. However, in general, the principle of phase-

angle control used in TCSC can be applied for a continuous

control as well. The operation of a DFC is based on the following

rules:

• TSC / TSR are switched when a fast response is required.

• The relieve of overload and work in stressed situations is

handled by the TSC / TSR.

• The switching of the PST tap-changer should be minimized

particularly for the currents higher than normal loading.

• The total reactive power consumption of the device can be

optimized by the operation of the MSC, tap changer and the

switched capacities and reactors.

In order to visualize the steady state operating range of

the DFC, we assume an inductance in parallel representing

parallel transmission paths. The overall control objective in

steady state would be to control the distribution of power flow

between the branch with the DFC and the parallel path. This

control is accomplished by control of the injected series

voltage.

The PST (assuming a quadrature booster) will inject a

voltage in quadrature with the node voltage. The controllable

reactance will inject a voltage in quadrature with the throughput

current. Assuming that the power flow has a load factor close to

one, the two parts of the series voltage will be close to

collinear. However, in terms of speed of control, influence on

reactive power balance and effectiveness at high/low loading the

two parts of the series voltage has quite different

characteristics. The steady state control range for loadings up

to rated current is illustrated in Figure 1.20, where the x-axis

corresponds to the throughput current and the y-axis corresponds

to the injected series voltage.

Fig. Operational diagram of a DFC

Operation in the first and third quadrants corresponds to

reduction of power through the DFC, whereas operation in the

second and fourth quadrants corresponds to increasing the power

flow through the DFC. The slope of the line passing through the

origin (at which the tap is at zero and TSC / TSR are bypassed)

depends on the short circuit reactance of the PST.

Starting at rated current (2 kA) the short circuit reactance

by itself provides an injected voltage (approximately 20 kV in

this case). If more inductance is switched in and/or the tap is

increased, the series voltage increases and the current through

the DFC decreases (and the flow on parallel branches increases).

The operating point moves along lines parallel to the arrows in

the figure. The slope of these arrows depends on the size of the

parallel reactance. The maximum series voltage in the first

quadrant is obtained when all inductive steps are switched in and

the tap is at its maximum.

Now, assuming maximum tap and inductance, if the throughput

current decreases (due e.g. to changing loading of the system)

the series voltage will decrease. At zero current, it will not

matter whether the TSC / TSR steps are in or out, they will not

contribute to the series voltage.

Consequently, the series voltage at zero current corresponds

to rated PST series voltage. Next, moving into the second

quadrant, the operating range will be limited by the line

corresponding to maximum tap and the capacitive step being

switched in (and the inductive steps by-passed). In this case,

the capacitive step is approximately as large as the short

circuit reactance of the PST, giving an almost constant maximum

voltage in the second quadrant.

UNIFIED POWER FLOW CONTROLLER:

The UPFC is a combination of a static compensator and static

series compensation. It acts as a shunt compensating and a phase

shifting device simultaneously.

Fig. Principle configuration of an UPFC

The UPFC consists of a shunt and a series transformer, which

are connected via two voltage source converters with a common DC-

capacitor. The DC-circuit allows the active power exchange

between shunt and series transformer to control the phase shift

of the series voltage. This setup, as shown in Figure 1.21,

provides the full controllability for voltage and power flow. The

series converter needs to be protected with a Thyristor bridge.

Due to the high efforts for the Voltage Source Converters and the

protection, an UPFC is getting quite expensive, which limits the

practical applications where the voltage and power flow control

is required simultaneously.

OPERATING PRINCIPLE OF UPFC

The basic components of the UPFC are two voltage source

inverters (VSIs) sharing a common dc storage capacitor, and

connected to the power system through coupling transformers. One

VSI is connected to in shunt to the transmission system via a

shunt transformer, while the other one is connected in series

through a series transformer.

A basic UPFC functional scheme is shown in fig.1

The series inverter is controlled to inject a symmetrical

three phase voltage system (Vse), of controllable magnitude and

phase angle in series with the line to control active and

reactive power flows on the transmission line. So, this inverter

will exchange active and reactive power with the line. The

reactive power is electronically provided by the series inverter,

and the active power is transmitted to the dc terminals. The

shunt inverter is operated in such a way as to demand this dc

terminal power (positive or negative) from the line keeping the

voltage across the storage capacitor Vdc constant. So, the net

real power absorbed from the line by the UPFC is equal only to

the losses of the inverters and their transformers.

The remaining capacity of the shunt inverter can be used to

exchange reactive power with the line so to provide a voltage

regulation at the connection point.

The two VSI’s can work independently of each other by

separating the dc side. So in that case, the shunt inverter is

operating as a STATCOM that generates or absorbs reactive power

to regulate the voltage magnitude at the connection point.

Instead, the series inverter is operating as SSSC that generates

or absorbs reactive power to regulate the current flow, and hence

the power low on the transmission line.

The UPFC has many possible operating modes. In particular,

the shunt inverter is operating in such a way to inject a

controllable current, ish into the transmission line. The shunt

inverter can be controlled in two different modes:

VAR Control Mode: The reference input is an inductive or

capacitive VAR request. The shunt inverter control translates the

var reference into a corresponding shunt current request and

adjusts gating of the inverter to establish the desired current.

For this mode of control a feedback signal representing the dc

bus voltage, Vdc, is also required.

Automatic Voltage Control Mode: The shunt inverter reactive

current is automatically regulated to maintain the transmission

line voltage at the point of connection to a reference value. For

this mode of control, voltage feedback signals are obtained from

the sending end bus feeding the shunt coupling transformer.

The series inverter controls the magnitude and angle of the

voltage injected in series with the line to influence the power

flow on the line. The actual value of the injected voltage can be

obtained in several ways.

Direct Voltage Injection Mode: The reference inputs are

directly the magnitude and phase angle of the series voltage.

Phase Angle Shifter Emulation mode: The reference input is

phase displacement between the sending end voltage and the

receiving end voltage. Line Impedance Emulation mode: The

reference input is an impedance value to insert in series with

the line impedance

Automatic Power Flow Control Mode: The reference inputs are

values of P and Q to maintain on the transmission line despite

system changes.

ACTIVE POWER CONDITIONER

A power conditioner (also known as a line conditioner or

power line conditioner) is a device intended to improve the

quality of the power that is delivered to electrical load

equipment. While there is no official definition of a power

conditioner, the term most often refers to a device that acts in

one or more ways to deliver a voltage of the proper level and

characteristics to enable load equipment to function properly. In

some usages, power conditioner refers to a voltage regulator with

at least one other function to improve power quality (e.g. noise

suppression, transient impulse protection, etc.).

The terms "power conditioning" and "power conditioner" can

be misleading, as the word "power" refers to the electricity

generally rather than the more technical electric power.

Conditioners specifically work to smooth the voltage of the

electricity they supply.

Power conditioners can vary greatly in specific

functionality and size, with both parameters generally determined

by the application. Some power conditioners provide only minimal

voltage regulation while others provide protection from half a

dozen or more power quality problems, such as harmonics, reactive

power, power line flicker and resonance. Units may be small

enough to mount on a printed circuit board or large enough to

protect an entire factory. Small power conditioners are rated in

volt-amperes (V·A) while larger units are rated in kilovolt-

amperes (kV·A).

While no single power conditioner can correct all power

quality problems, many can correct a variety of them.

It is common to find audio power conditioners that only

include an electronic filter and a surge protector with no

voltage regulating capability

PULSE WIDTH MODULATION

Pulse Width Modulation (PWM) is the most effective means to

achieve constant voltage battery charging by switching the solar

system controller’s power devices. When in PWM regulation, the

current from the solar array tapers according to the battery’s

condition and recharging needs consider a waveform such as this:

it is a voltage switching between 0v and 12v. It is fairly

obvious that, since the voltage is at 12v for exactly as long as

it is at 0v, then a 'suitable device' connected to its output

will see the average voltage and think it is being fed 6v -

exactly half of 12v. So by varying the width of the positive

pulse - we can vary the 'average' voltage.

Similarly, if the switches keep the voltage at 12 for 3

times as long as at 0v, the average will be 3/4 of 12v - or 9v,

as shown below

and if the output pulse of 12v lasts only 25% of the overall

time, then the average is

By varying - or 'modulating' - the time that the output is

at 12v (i.e. the width of the positive pulse) we can alter the

average voltage. So we are doing 'pulse width modulation'. I said

earlier that the output had to feed 'a suitable device'. A radio

would not work from this: the radio would see 12v then 0v, and

would probably not work properly. However a device such as a

motor will respond to the average, so PWM is a natural for motor

control.

PULSE WIDTH MODULATOR

So, how do we generate a PWM waveform? It's actually very

easy, there are circuits available in the TEC site. First you

generate a triangle waveform as shown in the diagram below. You

compare this with a d.c voltage, which you adjust to control the

ratio of on to off time that you require. When the triangle is

above the 'demand' voltage, the output goes high. When the

triangle is below the demand voltage, the

When the demand speed it in the middle (A) you get a 50:50

output, as in black. Half the time the output is high and half

the time it is low. Fortunately, there is an IC (Integrated

circuit) called a comparator: these come usually 4 sections in a

single package. One can be used as the oscillator to produce the

triangular waveform and another to do the comparing, so a

complete oscillator and modulator can be done with half an IC and

maybe 7 other bits.

The triangle waveform, which has approximately equal rise

and fall slopes, is one of the commonest used, but you can use a

saw tooth (where the voltage falls quickly and rinses slowly).

You could use other waveforms and the exact linearity (how good

the rise and fall are) is not too important.

Traditional solenoid driver electronics rely on linear

control, which is the application of a constant voltage across a

resistance to produce an output current that is directly

proportional to the voltage. Feedback can be used to achieve an

output that matches exactly the control signal. However, this

scheme dissipates a lot of power as heat, and it is therefore

very inefficient.

A more efficient technique employs pulse width modulation

(PWM) to produce the constant current through the coil. A PWM

signal is not constant. Rather, the signal is on for part of its

period, and off for the rest. The duty cycle, D, refers to the

percentage of the period for which the signal is on. The duty

cycle can be anywhere

from 0, the signal is always off, to 1, where the signal is

constantly on. A 50% D results in a perfect square wave. (Figure

1)

A solenoid is a length of wire wound in a coil. Because of

this configuration, the solenoid has, in addition to its

resistance, R, a certain inductance, L. When a voltage, V, is

applied across an inductive element, the current, I, produced in

that element does not jump up to its constant value, but

gradually rises to its maximum over a period of time called the

rise time (Figure 2). Conversely, I does not disappear

instantaneously, even if V is removed abruptly, but decreases

back to zero in the same amount of time as the rise time.

Therefore, when a low frequency PWM voltage is applied

across a solenoid, the current through it will be increasing and

decreasing as V turns on and off. If D is shorter than the rise

time, I will never achieve its maximum value, and will be

discontinuous since it will go back to zero during V’s off period

(Figure 3).* In contrast, if D is larger than the rise time, I

will never fall back to zero, so it will be continuous, and have

a DC average value. The current will not be constant, however,

but will have a ripple.

At high frequencies, V turns on and off very quickly,

regardless of D, such that the current does not have time to

decrease very far before the voltage is turned back on. The

resulting current through the solenoid is therefore considered to

be constant. By adjusting the D, the amount of output current can

be controlled. With a small D, the current will not have much

time to rise before the high frequency PWM voltage takes effect

and the current stays constant. With a large D, the current will

be able to rise higher before it becomes constant.

DITHER

Static friction, stiction, and hysteresis can cause the

control of a hydraulic valve to be erratic and unpredictable.

Stiction can prevent the valve spool from moving with small input

changes, and hysteresis can cause the shift to be different for

the same input signal. In order to counteract the effects of

stiction and hysteresis, small vibrations about the desired

position are created in the spool. This constantly breaks the

static friction ensuring that it will move even with small input

changes, and the effects of hysteresis are average out.

Dither is a small ripple in the solenoid current that

causes the desired vibration and there by increases the linearity

of the valve. The amplitude and frequency of the dither must be

carefully chosen. The amplitude must be large enough and the

frequency slow enough that the spool will respond, yet they must

also be small and fast enough not to result in a pulsating

output.

The optimum dither must be chosen such that the problems of

stiction and hysteresis are overcome without new problems being

created. Dither in the output current is a byproduct of low

frequency PWM, as seen above. However, the frequency and

amplitude of the dither will be a function of the duty cycle,

which is also used to set the output current level. This means

that low frequency dither is not independent of current

magnitude. The advantage of using high frequency PWM is that

dither can be generated separately, and then superimposed on top

of the output current.

This allows the user to independently set the current

magnitude (by adjusting the D), as well as the dither frequency

and amplitude. The optimum dither, as set by the user, will

therefore be constant at all current levels.

WHY THE PWM FREQUENCY IS IMPORTANT:

The PWM is a large amplitude digital signal that swings from

one voltage extreme to the other. And, this wide voltage swing

takes a lot of filtering to smooth out. When the PWM frequency is

close to the frequency of the waveform that you are generating,

then any PWM filter will also smooth out your generated waveform

and drastically reduce its amplitude. So, a good rule of thumb is

to keep the PWM frequency much higher than the frequency of any

waveform you generate.

Finally, filtering pulses is not just about the pulse

frequency but about the duty cycle and how much energy is in the

pulse. The same filter will do better on a low or high duty cycle

pulse compared to a 50% duty cycle pulse. Because the wider pulse

has more time to integrate to a stable filter voltage and the

smaller pulse has less time to disturb it the inspiration was a

request to control the speed of a large positive displacement

fuel pump. The pump was sized to allow full power of a boosted

engine in excess of 600 Hp.

At idle or highway cruise, this same engine needs far less

fuel yet the pump still normally supplies the same amount of

fuel. As a result the fuel gets recycled back to the fuel tank,

unnecessarily heating the fuel. This PWM controller circuit is

intended to run the pump at a low speed setting during low power

and allow full pump speed when needed at high engine power

levels.

MOTOR SPEED CONTROL (POWER CONTROL)

Typically when most of us think about controlling the speed

of a DC motor we think of varying the voltage to the motor. This

is normally done with a variable resistor and provides a limited

useful range of operation. The operational range is limited for

most applications primarily because torque drops off faster than

the voltage drops.

Most DC motors cannot effectively operate with a very low

voltage. This method also causes overheating of the coils and

eventual failure of the motor if operated too slowly. Of course,

DC motors have had speed controllers based on varying voltage for

years, but the range of low speed operation had to stay above the

failure zone described above.

Additionally, the controlling resistors are large and

dissipate a large percentage of energy in the form of heat. With

the advent of solid state electronics in the 1950’s and 1960’s

and this technology becoming very affordable in the 1970’s & 80’s

the use of pulse width modulation (PWM) became much more

practical. The basic concept is to keep the voltage at the full

value and simply vary the amount of time the voltage is applied

to the motor windings. Most PWM circuits use large transistors to

simply allow power On & Off, like a very fast switch.

This sends a steady frequency of pulses into the motor

windings. When full power is needed one pulse ends just as the

next pulse begins, 100% modulation. At lower power settings the

pulses are of shorter duration. When the pulse is On as long as

it is Off, the motor is operating at 50% modulation. Several

advantages of PWM are efficiency, wider operational range and

longer lived motors. All of these advantages result from keeping

the voltage at full scale resulting in current being limited to a

safe limit for the windings.

PWM allows a very linear response in motor torque even down

to low PWM% without causing damage to the motor. Most motor

manufacturers recommend PWM control rather than the older voltage

control method. PWM controllers can be operated at a wide range

of frequencies. In theory very high frequencies (greater than 20

kHz) will be less efficient than lower frequencies (as low as 100

Hz) because of switching losses.

The large transistors used for this On/Off activity have

resistance when flowing current, a loss that exists at any

frequency. These transistors also have a loss every time they

“turn on” and every time they “turn off”. So at very high

frequencies, the “turn on/off” losses become much more

significant. For our purposes the circuit as designed is running

at 526 Hz. Somewhat of an arbitrary frequency, it works fine.

Depending on the motor used, there can be a hum from the

motor at lower PWM%. If objectionable the frequency can be

changed to a much higher frequency above our normal hearing level

(>20,000Hz).

PWM CONTROLLER FEATURES:

This controller offers a basic “Hi Speed” and “Low Speed”

setting and has the option to use a “Progressive” increase

between Low and Hi speed. Low Speed is set with a trim pot inside

the controller box. Normally when installing the controller, this

speed will be set depending on the minimum speed/load needed for

the motor. Normally the controller keeps the motor at this Lo

Speed except when Progressive is used and when Hi Speed is

commanded (see below). Low Speed can vary anywhere from 0% PWM to

100%.

Progressive control is commanded by a 0-5 volt input signal.

This starts to increase PWM% from the low speed setting as the 0-

5 volt signal climbs. This signal can be generated from a

throttle position sensor, a Mass Air Flow sensor, a Manifold

Absolute Pressure sensor or any other way the user wants to

create a 0-5 volt signal. This function could be set to increase

fuel pump power as turbo boost starts to climb (MAP sensor). Or,

if controlling a water injection pump, Low Speed could be set at

zero PWM% and as the TPS signal climbs it could increase PWM%,

effectively increasing water flow to the engine as engine load

increases. This controller could even be used as a secondary

injector driver (several injectors could be driven in a batch

mode, hi impedance only), with Progressive control (0-100%) you

could control their output for fuel or water with the 0-5 volt

signal.

Progressive control adds enormous flexibility to the use of

this controller. Hi Speed is that same as hard wiring the motor

to a steady 12 volt DC source. The controller is providing 100%

PWM, steady 12 volt DC power. Hi Speed is selected three

different ways on this controller: 1) Hi Speed is automatically

selected for about one second when power goes on. This gives the

motor full torque at the start. If needed this time can be

increased ( the value of C1 would need to be increased). 2) High

Speed can also be selected by applying 12 volts to the High Speed

signal wire. This gives Hi Speed regardless of the Progressive

signal.

When the Progressive signal gets to approximately 4.5 volts,

the circuit achieves 100% PWM – Hi Speed.

HOW DOES THIS TECHNOLOGY HELP?

The benefits noted above are technology driven. The more

important question is how the PWM technology Jumping from a

1970’s technology into the new millennium offers:

• LONGER BATTERY LIFE:

– reducing the costs of the solar system

– reducing battery disposal problems

• MORE BATTERY RESERVE CAPACITY:

– increasing the reliability of the solar system

– reducing load disconnects

– Opportunity to reduce battery size to lower the system cost

• GREATER USER SATISFACTION:

– get more power when you need it for less money

ELECTRICAL SUBSTATION

An electrical substation is a subsidiary station of an

electricity generation, transmission and distribution system

where voltage is transformed from high to low or the reverse

using transformers. Electric power may flow through several

substations between generating plant and consumer, and may be

changed in voltage in several steps.

A substation that has a step-up transformer increases the

voltage while decreasing the current, while a step-down

transformer decreases the voltage while increasing the current

for domestic and commercial distribution. The word substation

comes from the days before the distribution system became a grid.

The first substations were connected to only one power station

where the generator was housed, and were subsidiaries of that

power station.

ELEMENTS OF A SUBSTATION

Substations generally have switching, protection and control

equipment and one or more transformers. In a large substation,

circuit breakers are used to interrupt any short-circuits or

overload currents that may occur on the network. Smaller

distribution stations may use recloser circuit breakers or fuses

for protection of distribution circuits. Substations do not

usually have generators, although a power plant may have a

substation nearby. Other devices such as capacitors and voltage

regulators may also be located at a substation.

Substations may be on the surface in fenced enclosures,

underground, or located in special-purpose buildings. High-rise

buildings may have several indoor substations. Indoor substations

are usually found in urban areas to reduce the noise from the

transformers, for reasons of appearance, or to protect switchgear

from extreme climate or pollution conditions.

Where a substation has a metallic fence, it must be properly

grounded (UK: earthed) to protect people from high voltages that

may occur during a fault in the network. Earth faults at a

substation can cause a ground potential rise. Currents flowing in

the Earth's surface during a fault can cause metal objects to

have a significantly different voltage than the ground under a

person's feet; this touch potential presents a hazard of

electrocution.

TRANSMISSION SUBSTATION

A transmission substation connects two or more transmission

lines. The simplest case is where all transmission lines have the

same voltage. In such cases, the substation contains high-voltage

switches that allow lines to be connected or isolated for fault

clearance or maintenance. A transmission station may have

transformers to convert between two transmission voltages,

voltage control/power factor correction devices such as

capacitors, reactors or static VAr compensators and equipment

such as phase shifting transformers to control power flow between

two adjacent power systems.

Transmission substations can range from simple to complex. A

small "switching station" may be little more than a bus plus some

circuit breakers. The largest transmission substations can cover

a large area (several acres/hectares) with multiple voltage

levels, many circuit breakers and a large amount of protection

and control equipment (voltage and current transformers, relays

and SCADA systems). Modern substations may be implemented using

International Standards such as IEC61850.

DISTRIBUTION SUBSTATION

A distribution substation in Scarborough, Ontario, Canada

disguised as a house, complete with a driveway, front walk and a

mown lawn and shrubs in the front yard. A warning notice can be

clearly seen on the "front door".

A distribution substation transfers power from the

transmission system to the distribution system of an area. It is

uneconomical to directly connect electricity consumers to the

high-voltage main transmission network, unless they use large

amounts of power, so the distribution station reduces voltage to

a value suitable for local distribution.

The input for a distribution substation is typically at

least two transmission or sub transmission lines. Input voltage

may be, for example, 115 kV, or whatever is common in the area.

The output is a number of feeders. Distribution voltages are

typically medium voltage, between 2.4 and 33 kV depending on the

size of the area served and the practices of the local utility.

The feeders will then run overhead, along streets (or under

streets, in a city) and eventually power the distribution

transformers at or near the customer premises.

Besides changing the voltage, the job of the distribution

substation is to isolate faults in either the transmission or

distribution systems. Distribution substations may also be the

points of voltage regulation, although on long distribution

circuits (several km/miles), voltage regulation equipment may

also be installed along the line.

Complicated distribution substations can be found in the

downtown areas of large cities, with high-voltage switching, and

switching and backup systems on the low-voltage side. More

typical distribution substations have a switch, one transformer,

and minimal facilities on the low-voltage side.

LAYOUT

Tottenham Substation, set in wild parkland in North London,

United Kingdom

The first step in planning a substation layout is the

preparation of a one-line diagram which shows in simplified form

the switching and protection arrangement required, as well as the

incoming supply lines and outgoing feeders or transmission lines.

It is a usual practice by many electrical utilities to prepare

one-line diagrams with principal elements (lines, switches,

circuit breakers, and transformers) arranged on the page

similarly to the way the apparatus would be laid out in the

actual station.

Incoming lines will almost always have a disconnect switch

and a circuit breaker. In some cases, the lines will not have

both; with either a switch or a circuit breaker being all that is

considered necessary.

A disconnect switch is used to provide isolation, since it

cannot interrupt load current. A circuit breaker is used as a

protection device to interrupt fault currents automatically, and

may be used to switch loads on and off. When a large fault

current flows through the circuit breaker, this may be detected

through the use of current transformers. The magnitude of the

current transformer outputs may be used to 'trip' the circuit

breaker resulting in a disconnection of the load supplied by the

circuit break from the feeding point. This seeks to isolate the

fault point from the rest of the system, and allow the rest of

the system to continue operating with minimal impact. Both

switches and circuit breakers may be operated locally (within the

substation) or remotely from a supervisory control center.

Once past the switching components, the lines of a given

voltage connect to one or more buses. These are sets of bus bars,

usually in multiples of three, since three-phase electrical power

distribution is largely universal around the world.

The arrangement of switches, circuit breakers and buses used

affects the cost and reliability of the substation. For important

substations a ring bus, double bus, or so-called "breaker and a

half" setup can be used, so that the failure of any one circuit

breaker does not interrupt power to branch circuits for more than

a brief time, and so that parts of the substation may be de-

energized for maintenance and repairs. Substations feeding only a

single industrial load may have minimal switching provisions,

especially for small installations.

Once having established buses for the various voltage

levels, transformers may be connected between the voltage levels.

These will again have a circuit breaker, much like transmission

lines, in case a transformer has a fault (commonly called a

'short circuit').

Along with this, a substation always has control circuitry

needed to command the various breakers to open in case of the

failure of some component.

SWITCHING FUNCTION

An important function performed by a substation is

switching, which is the connecting and disconnecting of

transmission lines or other components to and from the system.

Switching events may be "planned" or "unplanned".

A transmission line or other component may need to be de

energized for maintenance or for new construction; for example,

adding or removing a transmission line or a transformer.

To maintain reliability of supply, no company ever brings

down its whole system for maintenance. All work to be performed,

from routine testing to adding entirely new substations, must be

done while keeping the whole system running.

Perhaps more importantly, a fault may develop in a

transmission line or any other component. Some examples of this:

a line is hit by lightning and develops an arc, or a tower is

blown down by a high wind. The function of the substation is to

isolate the faulted portion of the system in the shortest

possible time.

There are two main reasons: a fault tends to cause equipment

damage; and it tends to destabilize the whole system. For

example, a transmission line left in a faulted condition will

eventually burn down, and similarly, a transformer left in a

faulted condition will eventually blow up. While these are

happening, the power drain makes the system more unstable.

Disconnecting the faulted component, quickly, tends to minimize

both of these problems.

III. UNIFIED POWER QUALITY CONDITIONER

The provision of both DSTATCOM and DVR can control the power

quality of the source current and the load bus voltage. In

addition, if the DVR and STATCOM are connected on the DC side,

the DC bus voltage can be regulated by the shunt connected

DSTATCOM while the DVR supplies the required energy to the load

in case of the transient disturbances in source voltage. The

configuration of such a device (termed as Unified Power Quality

Conditioner (UPQC)) is shown in Fig. 14.15. This is a versatile

device similar to a UPFC. However, the control objectives of a

UPQC are quite different from that of a UPFC.

CONTROL OBJECTIVES OF UPQC

The shunt connected converter has the following control

objectives

1. To balance the source currents by injecting negative and zero

sequence components required by the load

2. The compensate for the harmonics in the load current by

injecting the required harmonic currents

3. To control the power factor by injecting the required reactive

current (at fundamental frequency)

4. To regulate the DC bus voltage.

The series connected converter has the following control

objectives

1. To balance the voltages at the load bus by injecting negative

and zero sequence voltages to compensate for those present in the

source.

2. To isolate the load bus from harmonics present in the source

voltages, by injecting the harmonic voltages

3. To regulate the magnitude of the load bus voltage by injecting

the required active and reactive components (at fundamental

frequency) depending on the power factor on the source side

4. To control the power factor at the input port of the UPQC

(where the source is connected. Note that the power factor at the

output port of the UPQC (connected to the load) is controlled by

the shunt converter.

OPERATION OF UPQC

The operation of a UPQC can be explained from the analysis

of the idealized equivalent circuit shown in Fig. 14.16. Here,

the series converter is represented by a voltage source VC and

the shunt converter is represented by a current source IC. Note

that all the currents and voltages are 3 dimensional vectors with

phase coordinates. Unlike in the case of a UPFC (discussed in

chapter 8), the voltages and currents may contain negative and

zero sequence components in addition to harmonics. Neglecting

losses in the converters, we get the relation

where X,Ydenote the inner product of two vectors, defined by

Let the load current IL and the source voltage VS be decomposed

into two

Components given by

Where I1p L contains only positive sequence, fundamental

frequency components. Similar comments apply to V 1pS . IrL and V

rS contain rest of the load current and the source voltage

including harmonics. I1pL is not unique and depends on the power

factor at the load bus. However, the following relation applies

for I1p L .

This implies that hIrL ; VLi = 0. Thus, the fundamental

frequency, positive sequence component in IrL does not contribute

to the active power in the load. To meet the control objectives,

the desired load voltages and source currents must contain only

positive sequence, fundamental frequency components and

where V ¤ L and I¤S are the reference quantities for the

load bus voltage and the source current respectively. Ál is the

power factor angle at the load bus while Ás is the power factor

angle at the source bus (input port of UPQC). Note that V ¤ L(t)

and I¤S (t) are sinusoidal and balanced. If the reference current

(I¤C ) of the shunt converter and the reference voltage (V ¤ C)

of the series converter are chosen as

Note that the constraint (14.30) implies that V 1p C is the

reactive voltage in quadrature with the desired source current,

I¤S . It is easy to derive that The above

equation shows that for the operating conditions assumed, a UPQC

can be viewed as a inaction of a DVR and a STATCOM with no active

power °ow through the DC link. However, if the magnitude of V ¤ L

is to be controlled, it may not be feasible to achieve this by

injecting only

reactive voltage. The situation gets complicated if V 1p S is not

constant, but changes due to system disturbances or fault. To

ensure the regulation of the load bus voltage it may be necessary

to inject variable active voltage (in phase with the source

current). If we express

This implies that both ¢VC and ¢IC are perturbations involving

positive sequence, fundamental frequency quantities (say,

resulting from symmetric voltage sags). the power balance on the

DC side of the shunt and series converter. The perturbation in VC

is initiated to ensure that

Thus, the objective of the voltage regulation at the load bus may

require exchange of power between the shunt and series

converters.

REMARKS

1. The unbalance and harmonics in the source voltage can arise

due to uncompensated nonlinear and unbalanced loads in the

upstream of the UPQC.

2. The injection of capacitive reactive voltage by the series

converter has the advantage of raising the source voltage

magnitude.

IV. MODELLING OF CASE STUDY

THE OPEN UPQC

Most end user disturbances are characterized by short

duration and small amplitude, though they can still cause

interruptions in production processes. Most voltage sags have

small depth and short durations. More than 95% of voltage sags

can be compensated by injecting a voltage of up to 60% of the

nominal voltage, with a maximum duration of 30 cycles. This

information is primarily used to evaluate a suitable size for the

OPEN UPQC.

The series unit of the OPEN UPQC, sized to supply 60% of the

LV network power and equipped with a small storage system, can

compensate for most of the voltage disturbances.

Fig.. Example of distribution of voltage disturbances reported in the

EPRI event coordination chart

Fig. 3. Multiwire power diagram of the new proposed solution.

Each shunt unit is sized in relation to the supplied load

power, and can protect its sensitive load against interruptions.

The shunt unit’s function is similar to that of the UPS output

stage, but is less expensive because it only has one conversion

stage and involves less power loss. The series unit consists of a

coupling transformer (TR), with the primary circuit connected in

series with the mains line and a secondary one supplying the

reversible ac/dc power converter. The output stage of the pulse

width modulation (PWM) voltage controlled converter contains

passive RC shunt filters, to compensate for the harmonic currents

at switching and multiple frequencies. Neglecting the active

power to compensate the converter losses, the series unit is

controlled to act as a purely reactive inductor when the supply

voltage is within its operation limits. This fact is of

fundamental importance, because in this range the loads must be

supplied by the mains 95% of the time, as established by the IEEE

Std 1159 “IEEE Recommended Practice for Monitoring Electric Power

Quality” and European EN50160; therefore, the storage system must

not discharge itself. Outside of this range, active power can be

used to compensate the disturbances, in the same way as the usual

series compensation devices, when a storage system is present.

The shunt units consist of an ac/dc power converter, similar

to the one used in the series unit, connected to an energy

storage system and a set of static switches (SS). The shunt unit,

depending on the state of the network voltage, can supply either

the entire load, or a part of the load. There are two different

modes of OPEN UPQC operation:

COMPENSATOR: when the PCC voltage is within its operation

limits, the SS are closed, the series unit works as a three

phase voltage generator and the shunt units work as current

generators.

BACK-UP: when the PCC voltage is outside of its operation

limits, the SS are open, decoupling the network and the

load-compensator system. Each sensitive load is supplied by

its shunt unit, which acts as a sinusoidal voltage

generator, using the energy stored in the storage system as

an energy source.

OPEN UPQC PERFORMANCEThis section is focused on understanding the OPEN UPQC

compensation limits. The analysis will be carried out under

steady state conditions, to evaluate the compensation capacity of

the device in normal operation mode . It is

important to remember that the power absorbed by the loads and

the shunt units influences the performance of the series unit,

and therefore of the whole OPENS UPQC.

Therefore, when considering a particular set of load

conditions, it is possible to find operating conditions for the

shunt units that increase the compensating limits of the series

unit. Depending on whether or not storage systems are present,

the series and shunt units can exchange only non active power or

both non active power and active power with the mains. In the

latter case, as will be shown in the following, the OPEN UPQC can

better compensate for short duration disturbances.

In the following cases, all of the solutions will be

analyzed under the assumption that the voltages are sinusoidal

and are constituted of only the positive sequence component in

the different network buses. It is important to emphasize that

suitably coordinating the various units of the OPEN UPQC allows

for a wide compensation range, comparable with the UPS, but more

economical.

This coordination requires a communication system (i.e.,

based on the carrier waves) between the series unit and the shunt

units, but this system cannot be very fast. Moreover, in

transient analysis, the communication between the series unit and

the shunt units cannot be included (the communication could be

slow, could be out of order, etc.). Therefore, each unit

necessarily works alone.

Fig. Voltage compensation, exchanging only nonnative power. Case

(a1): it is possible to obtain a power factor equal to 1 in s

section in low-voltage situations. Case (a2): the power factor is

always less than 1.

A. NONNATIVE AND POWER EXCHANGE

The conditions under which all of the converters exchange

only non active power must be confirmed in situations when the

system voltage Vs is near the contractual limits (normal

operation mode). In normal operation mode, the maximum voltage

drop in the LV lines of the network must be less than 5% to

maintain low power loss. Therefore, if all the converter units

are operating to stabilize the voltage in the PCC at its nominal

value (100%), the load voltage value will be at least 95% of the

nominal voltage.

This result allows an improvement of one of the aspects of

the supply quality, the stability of the real value of the supply

voltage, for all customers. Therefore, the OPEN UPQC works to

stabilize the nominal voltage at the PCC. The phasor diagram of

the OPEN UPQC is shown in Fig. In order to avoid active power

injections, the series voltage has to be in quadrature with the

mains current.

The current Is is primarily composed of the current of

unprotected loads Us (whose phase difference with respect to Vpcc

cannot be varied) and the current of protected loads (whose

phase difference with respect to Vpcc can be changed by the shunt

units) as reported in (2), where Pu12 and Qu12 are the active and

reactive power of the equivalent load respectively, Plosses m and

Qlosses are the active and reactive power lines losses,

respectively, and Qb is the reactive power injected by all the

shunt units

Therefore, the angle can oscillate between the upper

and lower limits , obtained

when and respectively, in the area highlighted in Fig. 4. The

angle can be calculated by the equation shown at the bottom of

the page.

The current phasor Ia can move along the black dotted line,

varying the reactive power of the shunt units. In case (a1) in

particular, it is possible to obtain a power factor equal to 1 in

the section in low voltage situations, because the line

intercepts the black dotted line. In case (a2), the power factor

is always less than 1. The quantities can be

obtained with (4) and (5), as shown at the bottom of the page.

Assuming that , the range

the range amplitude can be obtained with

It can be seen that the compensating range amplitude

depends on the value that the series unit can

inject, and on the non active power. The non active power is

susceptible to exchanges by the shunt units (length of the black

dotted line, proportional to the loads apparent power) and to

the power factors of the equivalent loads and .

In normal operation mode, the compensation strategy can be

implemented in various ways. For example, power factor

maximization in the s section (corresponding to minimization of

the current Is) is a compensation strategy that can be

implemented by coordinating the series unit and the shunt ones.

Therefore, communication between all the units is required. The

simplest solution is to employ a slow communication system that

allows the OPEN UPQC to stabilize the voltage at the PCC,

maximizing the power factor in normal operation conditions and

increasing its compensation limits outside of normal operation.

Obviously, in the case of large disturbances in that the series

unit cannot compensate, each shunt unit can supply the load in

back-up mode.

B. NONNATIVE Qb AND Qx AND ACTIVE Px POWER EXCHANGE

In this case, the series converter produces only non active

power, but the shunt units can exchange active and non active

power with the mains. This condition could be represented as an

active network into which dispersed generations are inserted.

Fig. 5 depicts the new phasor diagram of the OPEN UPQC under the

above operating conditions.

In order to avoid active power injections by the series

unit, the voltage Vx and the mains current Is have to be in

quadrature with each other. In Fig. 5, the light grey areas

indicate the field in which Vx can lay without active power

exchanges by the shunt units, and the dark gray areas indicate

the possible values of with active power exchanges by shunt

units. In this case, the compensating range amplitude

is greater than without active power exchanges, but it is

important to note that the difference is small. The phasor

current can move inside of the gray dotted circle, varying the

active and non active power of the shunt units (movement on the

black dotted line regards only non active power exchange).

C. NONCCTIVE Qb AND Qx AND ACTIVE Px POWER EXCHANGE

In order to exchange active power with the mains, a storage

system connected to the dc section of the series unit is needed.

The storage system size does not need to be very large, because

little energy is required to compensate most of the disturbances

For example, to compensate most of the voltage variations

reported in Fig. 2 (voltage sag 60% deep for 30 cycle) for a 400-

kW load, an energy equal to 120 kJ is needed, corresponding to a

battery capacity of about 0.4 Ah at 96 V or a capacitor or super

capacitor bank of about 1.5 F at 400 V. Given a storage system

with twice the abovementioned capacity, in order to allow

bidirectional energy exchange with the mains, it is possible to

compensate voltage disturbances in that are outside of the

contractual limits.

In the case of mains interruptions lasting longer than 30

cycles, the SS of the shunt units switch off, and the loads are

supplied in back-up mode Considering compensation of transient

disturbances, such as voltage sags, swells, etc., various

compensation strategies are available for the OPEN UPQC,

including minimizing the energy required by the storage system of

the series unit. The new phasor diagram of the OPEN UPQC

operation is shown in Fig. 6. In the light gray areas, the series

voltage and the mains current have to be in quadrature with each

other, because active power exchanges by the series unit are not

allowed.

In these areas, the behavior of the OPEN UPQC is the same

as that of the cases described previously. In the case of

transient disturbances, the series unit can compensate the

voltage over a very large range (the compensating range amplitude

is ) compared with all the cases previously

analyzed. Indeed, the series unit can exchange active power with

the mains in the dark gray areas, but this is only possible for

transient disturbances due to the small size of the series unit

storage system.

CONTROL STRATEGYThe following describes a control strategy that can be

employed in normal operation mode , under

steady state conditions, and elucidates the device performance.

The dynamic response during transient events has not been

considered in this work, because it is described in detail in.

For example, considering the dynamic behavior of the series unit,

it can be seen that the series unit cannot be affected by the

shunt units during a transient event. This is due to the fact

that the communication system between them is slow, and does not

allow a fast coordinated control strategy.

In order to compensate for the voltages in normal

operation, the strategy that maximizes the power factor

(corresponding to the current minimization) can be chosen. With

this choice, it is possible to minimize the apparent power

required by the mains. The mains current Is is reported in (2),

and the compensated voltage Vpcc is

Neglecting power losses and considering that the voltage has

to be in quadrature with the current , it is possible to write

the following relation:

Where is the equivalent reactance of the series unit, giving

a voltage proportional to . Solving (7) and (8) is not

mathematically easy due to the nonlinearity of the problem, and

implementing them into a controller is not useful. It is more

convenient to implement two PI controllers: one to evaluate the

voltage of the series unit, and another to evaluate a signal

related to the non active power that all of the shunt units have

to inject.

The conditions under which the series unit can exchange only

nonactive power can be obtained by applying the Park transform to

the three phase currents , and , and calculating

the two components and in a rotating reference frame, as reported

in (9)

For the constant Kvx to be independent of the load

conditions, the previous expressions must be normalized with

respect to the load current module

Fig. Voltage control loop of the series unit and nonactive power

control loop of the shunt units in the OPEN UPQC system

The constant is obtained by a PI controller that keeps

the voltage at the output of the series unit equal to the

rated value , as reported in the block diagram of Fig. 7.

The second control loop acts to minimize the angle between the

voltage and the current downstream of the MV/LV transformer, in

order to maximize the power factor absorption in the section. In

this case, the PI controller produces a signal , which varies

from 0 to 1, and is equal to the ratio between the desired non

active power inject able by the shunt units and the maximum

inject able non active power. This signal is sent to all shunt

units by the communication system. Thus, the injected non active

power of the th shunt unit is equal to

Where Ak is the unit’s rated power. The total non active power

injected by all the shunt units is

Obviously, this compensation strategy, which is useful for

its fast series unit response, requires non active power

injection by the shunt units to be capable of achieving a wide

compensation range. To enhance the entire system’s performance,

the power losses and the voltage drops in the LV lines generally

must increase. However, if the power factor at the is PCC kept

high ( 0.8), these increments are negligible. Moreover, this

increment can be reduced by sending a different signal Kqbk to

each shunt unit. This allows the closest shunt units to be used

to inject more non active power, avoiding useless non active

power flows.

TEST NETWORK AND EVALUATION OF OPERATION LIMITS

Fig. shows a simplified 400-kVA LV grid, used to validate the

OPEN UPQC

System compensation structure

The protected loads are grouped into the equivalent load ,

so all of the shunt units are represented by means of an

equivalent unit. In the same way, all of the unprotected loads

are grouped in the equivalent load U2.All of the parameters of

the three-phase MV/LV transformer used for the simulations are

reported in Table I. The LV cables used for the following

analysis, with different power factors and loads u1 and u2 , are

reported in Table II. In each analysis, the correct 200-m cable

is chosen as a function of the current needed to supply the

equivalent load with a voltage drop of less than 3%, without

considering the OPEN UPQC. In this way, it is possible to neglect

power loss and the voltage drop on the LV grid.

In this study, all of the converters are represented as

ideal controlled voltage or current sources. Moreover, the series

unit is not equipped with a storage system. For these reasons,

the OPEN UPQC limits are evaluated mainly in the normal operation

mode in the following. Therefore, the series unit cannot exchange

active power with the mains. The following figures and tables

report the power factor and the mains current in the

section as functions of the network voltage. Each diagram is

represented for a fixed load power factor and, and is parametric

in . This parameter indicates the ratio between the apparent

powers of the total loads of shunt units Au1 and the total

apparent power of loads Aref

The reference current is expressed in per unit (p.u.), as

the ratio between the power reference and the voltage reference.

Since the network cables are correctly designed and their

parameters are constant, the voltage drop variation when the OPEN

UPQC is present can be neglected under maximum load conditions

when the load power factor is equal to 0.9 and it is connected at

the end of the line. The operation limits reported in Figs. 9 and

10, which allow the voltage to be fixed at the nominal value,

were obtained by assuming the above hypothesis and that the

maximum inject able voltage by the series unit is equal to 0.6

p.u.. The following results for the proposed solution were

obtained by converting the vector diagrams of Fig. 4 into

geometrical equations.

In the following, the maximum nonactive power injected by

all the shunt units can reach the apparent power . In

this case, if the control strategy can keep the voltage

equal to the nominal value, then these relations have to be true

The figures reveal that:

1) the OPEN UPQC is well-adapted when the power factor of

the load is low. Fig. 9 shows the interval that can be

compensated by exchanging only nonactive power when the power

factor of the load is equal to 0.8. In this case, the OPEN UPQC

produces excellent voltage stabilization, especially when the

parameter is greater than 0.4.

2) The OPEN UPQC is not well-adapted when the power factor

of the load is high. Fig. 10 shows the interval that can be

compensated by exchanging only nonactive power when the power

factor of the load is equal to 1. In this case, the OPEN UPQC

does not produce good voltage stabilization, because it is too

limited. It is possible to obtain voltage stabilization in normal

operation mode range (between 0.9 p.u. and 1.1 p.u.) only with a

high value.

TABLE III MAXIMUM AND MINIMUM MAINS VOLTAGE RANGE VARIATION THAT THE SYSTEM CAN COMPENSATE, AS A FUNCTION OF THE

MAXIMUM NONACTIVE POWER THAT THE SHUNT UNIT CAN INJECT, WITHOUT CONSIDERING THE MAINS POWER FACTOR

It is possible to estimate the power of the series unit,

given the maximum current value. This value is equal to the

product between the maximum injectable voltage (equal to 0.6

p.u.) and the maximum line current (equal to 1.1 p.u. when and as

shown in Fig. 9). Therefore, with slight over-sizing of the

series unit, good stabilization of the mains voltage is possible.

The usual working conditions present an interesting case, when

the power factor of load is between 0.9 and 1, and the mains

voltage is inside of the contractual limits (normal operation). The

distribution power losses should be estimated, in order to

understand the energy cost associated with this solution.

Under these conditions, it is always possible to compensate

the voltage , without considering the power factor in section ,

if is greater than or equal to 0.5, as reported in Table III.In

the case of smaller spread among the shunt units , it is always

possible to compensate for the voltage by decreasing the power

factor in the section. However, the power factor will always be

greater than 0.8. When the power factor of the load is equal to

one, the power factor in the section is always close to one, and

the compensation limits previously mentioned can be maintained.

The mains voltage limits reported in Table III change to

those reported in Table IV when it is important to keep the mains

power factor between 0.9 and 1.

COST EVALUATIONTo evaluate the costs of power quality improvement and the

economic convenience of the proposed solution, an analysis of the

400-kVA LV distribution network has been carried out. It was

supposed that the line L1 represents an equivalent line in which

all of the sensitive loads U1 that make up the OPEN UPQC are

connected, while the line L2 represents an equivalent line that

supplies only the nonsensitive loads U2 . Therefore, each load

that belongs to the set needs to be protected against

disturbances and network interruptions, while the ones that

belong to set only require general improvement of the power

quality.

TABLE IV

MAXIMUM AND MINIMUM MAINS VOLTAGE RANGE VARIATION THAT THE SYSTEM CAN COMPENSATE, AS A FUNCTION OF THE MAXIMUM

NONACTIVE POWER THAT THE SHUNT UNIT CAN INJECT TO KEEP THE MAINS POWER FACTOR BETWEEN 0.9 AND 1 (LIMITATIONS OF THE

VOLTAGE DROP IN THE LINE

Several solutions are available for compensating each load.

In the following description, only two possible solutions are

considered. The first solution consists of the installation of an

UPS for each end user, while the second one is the installation

of a shunt unit for each load. Instead, in order to obtain

general power quality improvement for all loads, it is possible

to rebuild the LV distribution system to increase the short

circuit level in the load connection point or to install a series

unit in the MV/LV substation. Therefore, three different methods

for improving the power quality have been considered:

• installation of a UPS for each end user. In this case, it

is not possible to improve the power quality of the distribution

network. However, it is possible to compensate for all voltage

disturbances for the end users;

• revamping of all of the LV distribution cables. In this

case, it is not possible to compensate for all voltage

disturbances;

• installation of an OPEN UPQC. In this case, it is possible

to compensate for most of the voltage disturbances. The last

solution consists of the installation of a series unit sized for

66% of the total power loads supplied (264 kVA), while each shunt

unit has an assumed size of 5 kVA. Moreover, each UPS is assumed

to have the same power, and the input stage of each UPS is

composed of PFC rectifiers. It is important to clarify that the

storage systems cost for UPS and OPEN UPQC solutions in this

analysis is not considered, because it primarily depends on the

technologies and autonomies required. The revamping cost

considered here can be found in the economical analysis carried

out.

The total cost for each solution is reported in Fig. 11.

They include only the devices and various materials; the

installation cost is assumed to be 50% of the device cost. Fig.

11 shows that to compensate for most of the disturbances in the

whole network, installing the series unit only is a better

solution than revamping all of the LV distribution system. To

compensate for the loads , it is necessary to install a UPS or a

shunt unit close to them, which increases the total cost as a

function of their power. Referring to Fig. 11, it can be seen

that the UPSs are a good solution if only a few sensitive loads

are present.

However, if it is necessary to improve the power quality of

the whole network, they become too expensive to use. They can

only be more convenient than the proposed OPEN UPQC if the total

power of the sensitive loads is lower than about 80 kVA (20% of

the total load) when, and only when, it is not necessary to

increase the PQ of the network

TABLE VOPEN UPQC UNITS ACTIONS

V. MATLAB DESIGN OF CASE STUDY & RESULTSCASE: I

Fig. Simulink model of OUPQC

Fig. voltage vectors

Fig. Power factors of the system and maximum line

currents in case 1, for different values.

Fig. load voltage

Fig. UDC

Fig. load current

Fig. power factor

CASE: II

Fig. Simulink model of OUPQC

Fig. voltage vectors

Fig. Power factors of the system and maximum line

currents in case 2, for different values.

Fig. load voltage

Fig. UDC

Fig. load current

Fig. power factor

VI. CONCLUSION

The OPEN UPQC apparatus is a good compensation system if

wide installation of shunt units is needed. An increase in the

percentage of the protected load enhances the voltage

stabilization interval over which the OPEN UPQC can significantly

improve the power quality, especially if the load power factor

takes a high value. If the power factor of load is less than one,

the power factor in section increases, to avoid nonnative power

absorption from the mains. For low values of the parameter, the

OPEN UPQC becomes expensive if there are few shunt units. In this

case, it is better to install other compensation device

typologies (as UPS, UPQC, etc.) near the sensitive loads, and a

nonnative compensator system near the nonsensitive loads if

necessary.

It is possible to conclude that installation of the series

unit is a cost-effective way for distributors to improve the

power quality level in the distribution networks in order to

achieve the standards imposed by the authorities. Compensation

improvement for the sensitive end users can be achieved by

installing a shunt unit near them, instead of the more expensive

UPS device. The OPEN UPQC working conditions are reported in

Table V. At this moment, the OPEN UPQC study is still under

investigation. The dynamic behavior, considering changing

operating modes, of a 5 kW prototype shunt unit is developed and

experimental results are presented in [18]. As shown in Fig. 11,

large investments are needed to analyze the completed solution,

and availability of electrical distribution operators for an

infield test will be required.

VII. REFERENCES

[1] D. M. Divan, W. E. Brumsickle, G. A. Luckjiff, J. W.

Freeborg, and R. L. Hayes, “Realizing a nation-wide power quality

and reliability monitoring system,” 2003. [Online]. Available:

http://www.softswitch. com/docs/Realizing%20a%20Nationwide

%20System%202003.pdf.

[2] J. M. Guerrero, L. Garcia de Vicuna, and J. Uceda,

“Uninterruptible power supply systems provide protection,” IEEE

Ind. Electron. Mag., vol. 1, no. 1, pp. 28–38, Spring, 2007.

[3] B.Wang, G. Venkataramanan, and M. Illindala, “Operation and

control of a dynamic voltage restorer using transformer coupled

H-bridge converters,” IEEE Trans. Power. Electron., vol. 21, no.

4, pp. 1053–1061, Jul. 2006.

[4] D. M. Vilathgamuwa, H. M. Wijekoon, and S. S. Choi, “A novel

technique to compensate voltage sags in multiline distribution

system—The interline dynamic voltage restorer,” IEEE Trans. Ind.

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[5] C. N. M. Ho and H. S. H. Chung, “Fast transient control of

singlephase Dynamic Voltage Restorer (DVR) without external DC

source,” in Proc. 37th IEEE Power Electronics Specialists Conf.,

Jeju, Korea, Jun. 18–22, 2006, pp. 2105–2111.

[6] M. R. Banaei, S. H. Hosseini, and M. D. Khajee, “Mitigation

of voltage sag using adaptive neural network with dynamic voltage

restorer,” in Proc. 5th Int. CES/IEEE Power Electronics and

Motion Control Conf., Shanghai, China, Aug. 14–16, 2006, vol. 2,

pp. 1–5.

[7] A. Ghosh and G. Ledwich, “Compensation of distribution system

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1030–1036, Oct. 2002.

[8] M. K. Mishra, K. Karthikeyan, and P. K. Linash, “A

development and implementation of DSP based DSTATCOM to

compensate unbalanced nonlinear loads,” presented at the IEEE

Power India Conf., New Delhi, India, Apr. 10–12, 2006.

[9] B. Singh, A. Adya, A. P. Mittal, and J. R. P. Gupta,

“Analysis, simulation and control of DSTATCOM in three-phase,

four-wire isolated distribution systems,” presented at the IEEE

Power India Conf., New Delhi, India, Apr. 10–12, 2006., Jun. 18–

22, 2006.