OREGON TECH. CLASS REE511, PROJECT 1: STATE-OF-THE-ART PATENT SEARCHING.

Abstract— The rising of the world demand for energy together

with the resistance to the usage of fossil fuels due to

environmental impacts have pushed wind energy to grow

remarkably in terms of total installed capacity. Wind Power

Plants are more numerous and have higher capacity. The trend

has pushed companies, researchers around the world to work

hard on R&D to reduce cost, increase efficiency, and increase

reliability. Their work resulted in an impressive number of

patents in design and control of Wind Turbine Systems (WTS).

This paper review recent patents and patent applications in

design and control of FSC-based WTS. This emerging technology

has superior advantages over older technologies.

Index Terms—Wind energy, power conversion, full scale

power converter, wind turbine systems, recent patents.

I. NOMENCLATURE

WTS Wind Turbines System

WPP Wind Power Plant

FSC Full Scale Converter DFIG Doubly Fed Induction Generator

GC Grid Code

FOC Field Oriented Control PWM Pulse Width Modulation

FRT Fault Ride-Through

PMSG Permanent Magnet Synchronous Generator

II. INTRODUCTION

The fifth Scientific Assessment on global climate by IPCC

(the Intergovernmental Panel on Climate Change) concluded:

The earth doesn’t stop warming, and human is the main cause

for that. The assessment goes thousands of pages, comprises

prudently checked reports by hundreds of scientists around the

world [2]. Scientists agree that with the current burning rate of

fossil fuels, the climate will soon come to a critical point

where we cannot turn back [2]. On the other hand, latest

catastrophes happening around the world, like typhoon Haiyan

or hurricane Katrina, with the intensity at a record level in

history further confirm the looming danger that scientists have

anticipated [2]. Not only causing detrimental effect to our

climate, fossil fuels also have limited resources and with the

quickly increasing of the world’s energy demand, they will

sooner or later run out.

Renewable sources of energy, like wind energy, on the

contrary, have many advantages. The wind is abundant, and it

doesn’t seem to stop blowing at any time soon. Harnessing

wind energy is a clean process and almost harmless to the

environment. In some cases, electricity generated from the

wind has even at lower cost per kWh compared coal.

However, a drawback of wind energy is its variability. The

intermittent nature of wind shrank the capacity factor of wind

farms to really low, at about 22%. Today’s new technology in

converter design together with the large-scale implementation

of WPP increase the capacity factor of wind farm

substantially. Some new wind farms have capacity factor

higher than 50%. In near future, new grid-scale energy storage

technologies potentially can bring the capacity factor to even

higher. For example: Ocean Renewable Energy Storage from

MIT [20], Wind Hydrogen storage systems are very

promising. In addition, today’s advanced technologies have

enable Wind Power Plant (WPP) to behave just as good as

conventional fossil fuel or nuclear power plans. Gigantic WPP

nowadays can actively participate in grid voltage/frequency

regulation on both primary and secondary control, eliminate

all the anxiety about high wind energy penetration might make

the grid become unstable. Renewable sources of energy will

be an important part of the solution to the climate and energy

problems. With all the advantages, wind energy will be

involved to be a main source of energy for the world.

In recent years, technologists and companies have been

trying to lower the cost of wind energy by increasing the

output power by making Wind Turbine bigger and bigger (Fig.

1). Recently, the utilization of full scale power electronics

converter in WTS has allowed a complete control of power

conversion and optimization process, increases wind turbine

efficiency as well as reliability and allows WTS to meet the

Recent Patents in Design and Control of

FSC-based Wind Turbine Systems

Tai Huynh, Student Member, IEEE.

Fig. 1. The evolution of WTS. Source: IPCC (2011), “Special report on

renewable energy”

OREGON TECH. CLASS REE511, PROJECT 1: STATE-OF-THE-ART PATENT SEARCHING.

most stringent Grid Code (GC) requirements.

Today’s high levels of renewable energy penetration has

driven the need for “renewable generators” having the

capability to provide active power control (APC) services,

which are inherent in “traditional generators”. These ancillary

services features in new FSC for WTS are crucial for grid

reliability and lay the foundation for renewable energy to

completely replace traditional sources of energy. The use of a

Full Scale Converter in WTS seems to be the most successful

configuration for the near future, winning the DFIG actual

market share [4], [1].

This review studies recent advancements of the design and

control of Full Scale Converter for WTS. This emerging

technology has many advantages over other Wind Turbines’

technologies including: higher efficiency, higher reliability,

and especially future GC conformity.

III. SEARCH METHODOLOGY AND INCLUSION CRITERIA

A search was conducted to locate issued patents and patent

applications published by the United States Patent &

Trademark Office (USPTO) in the last five years (since 2008).

The search criteria required a patents to disclose the design

and control of Full Scale Converter for WTS. The search

excluded patents that are related to maintenance and safety

issues of WTS. Also excluded are patents related to DFIG

converter technology. Although, both DFIG and FSC based

WTS can offer compliance with current Grid Codes (GCs),

there is a strong trend to migrate towards FSC as future GCs

can challenge DFIG conformity, especially in the case of

asymmetrical grid faults [5]. In addition, the search criteria

excluded patents related to Current Source Converter (CSC) or

Z-Source Converter (ZSC). Although CSC and ZSC are very

promising technologies for High Power, Medium Voltage

WTS, a lot of work need to put into R&D to make these types

of converters practicable for commercial applications. Voltage

Source Converter is still the dominant technology in FSC for

WTS right now.

The search results were found under several USPTO

classifications. The most common subclass is 290/44, which

directs toward Prime-Mover Dynamo Plants including control

of, by or through an electric circuit. The next most relevant

subclass was 363/37, which is for electric power conversion

systems by semiconductor rectifier and inverter.

The strategy for patents searching was composed from the

most relevant keywords with time information. For example a

query: ("full scale" OR "full-scale") AND power AND

(converter OR inverter) AND wind AND "wind turbine"

reveals 172 publications. When limit searching results to last 5

years, revealed results showed 122 publications. Limiting

searching results to 3 years reveals 72 publications. So

approximately, 70% of the patents in this field were invented

in the last 5 years and 42% were invented in the last 3 years.

This paper reviews 14 latest and novel patents and patent

applications disclosing the design and control of Full Scale

Converter for WTS.

IV. REVIEW OF RECENT PATENTS

Patents reviewed in this paper can be classified into three

categories. The first category is related to topology of circuit

design in FSC. The second category is related to control

techniques for FSC and the third category is related to Fault

Ride-Through capability of WTS. Table I summarizes all the

patents being reviewed.

A. Design Topology

Typically, in a LV 690V/2MW Wind Turbine Systems,

there is a phase current of 1700A power transmitted from a

nacelle (which is a chamber at the top of a wind tower, can be

as high as 100 ft. in the air) to the ground. This requires a

parallel connection of multiple cables per phase and a

considerable voltage drop and conductor loss in the

transmission line. The drawback can be mitigated by

employing a transformer in the nacelle to increase the voltage

before transmission. However, the structure to backing the

nacelle weight introduces extremely higher costs [6]. Issued

US Patent 8129853 discloses a transformerless converter

topology in harmony with generator to produce a high voltage

output, which then can be transmitted to the tower bottom

efficiently Fig. 2.

The general concept of the invention involves cascading of

power stages to create high voltage output from multiple

isolated low voltage power supplies. To be able to do

cascading of power stages, the invention requires multiple

isolated power supplies from Wind Turbines to feed directly

into each power stage. Each power stage may receive at least

one of the isolated power supplies. In an embodiment of the

invention, each power stage is a single phase full-bridge

TABLE I

SUMMARY AND CLASSIFICATION OF PATENTS REVIEWED

Design Topology Control of WT Fault Ride-through

US Pat. No. 8129853 US20130181532

US Pat. No. 8395360 US20120056602

US20120313593

US Pat. No. 8089171 US20120255215

US20130200620

US20130200621

US Pat. No. 8471534 US20130207394

US20130234522

US Pat. No. 8183706 US Pat. No. 8487462

Fig. 2. Block diagram of a multi-level wind energy conversion system in

accordance with one embodiment of the invention [6]

OREGON TECH. CLASS REE511, PROJECT 1: STATE-OF-THE-ART PATENT SEARCHING.

converter. By serial cascading of power stages we can built a

high voltage converter efficiently from low voltage

semiconductor devices.

In another embodiment of the invention, a power stage may

be built from a multilevel converter which can also produce a

relatively high voltage output from low voltage switching

devices. An eminent drawback of multilevel converter is

capacitor voltage balancing problem. However, with multiple

isolated power supplies input directly from generator, there is

no problem with capacitor voltage balancing anymore.

Another aspect of the invention is a generator that can

provide multiple isolated power outputs directly to multiple

power stages of a power converter [6]. The idea is illustrated

by a design example in the patent publication (Fig. 2a). The

invention goes further by suggesting a double layer windings

pattern to reduce overall machine size and weight.

Said methods to build a high voltage, transformerless

converter has many advantages: lower harmonic distortion,

lower switching loss, lower voltage change rate (dv/dt),

smaller filter elements, lower cost, etc. However the

disadvantages are complicated wiring and control. The idea of

cascading voltage is not new, but Teco-Westinghouse Motor

Company was among the first who saw the need of high-

voltage converter for WTS and successfully applied the idea to

remove the weighty transformer in their design of

Transformerless High-voltage converter. The patent was

granted and published on March 06, 2012. However, their

priority dated back from June 30, 2009.

Patent application publication US 20130181532 also

discloses a different approach for removing the traditional

step-up transformer. The idea was also serially connecting low

voltage switching units so that together they can work under

high voltage. However, connecting switching devices in series

is known to be a pretty difficult task due to voltage balancing

problems arising from different switching times of the

switching devices. If the voltage across serial connected

switching devices are not properly balanced, overvoltage

levels typically ends up on a particular switching devices [7]

and create damage to the whole system. So the patent further

discloses circuit to balance voltage between serially connected

switching units Fig. 15.

In one aspect of the invention (Fig. 3.), a high-voltage

power converter comprising a number of switching blocks,

each of which comprising a pluralities of series connected

switching units, said high-voltage power converter further

comprising: a resonance circuit comprising a link inductor and

a link capacitor, and a plurality of serial connected clamping

circuits, each clamping circuit comprising a clamping

capacitor and a clamping switching unit [7]. An advantage of

this topology is simple wiring and control but disadvantages

are high voltage change rate (dv/dt), higher switching loss, and

harmonic distortion. Once again, the old idea was first applied

by Vestas Wind Systems A/S to assuage strong needs and

wishes to remove step-up transformer from WTG.

B. Control of Wind Turbine.

Field Oriented Control (FOC) has been the most popular

control techniques in the industry, for motor control as well as

generator and grid converter control. FOC is based on a

synchronous rotating reference frame (dq frame) attached to

the rotor of generator, with q axis (quadrature axis)

perpendicular to the rotor magnetic field. The idea behind

FOC is to control the quadrature id and direct iq components of

the stator current phasor on the synchronous dq reference

frame. Theoretically, this allows decouple control of flux and

torque component created by stator current id and iq. Thus, a

non-linear control problem become two linear ones, which can

be solved easily with a simple PI controller. However, in real

control systems, there are always errors in sensors reading,

calculations and the response of PI control loop is not

instantaneous, creating some shortcomings, for example, (a) a

difficulty of maintaining correct decoupling between the flux

and torque producing components of the stator currents, (b)

controlling the currents using at high speeds and high

modulation index [8]. US Patent 8395360 discloses a method

to directly control the flux vector generated by stator. Unlike

previous implementation of FOC which indirectly controls

stator flux by controlling the stator current id and iq, the new

method directly gets feedback and controls the stator flux.

This leads to a more robust and simpler system [8].

Fig. 2a. A cross-section view of an exemplary three-phase, six-pole 72 slot

double layer windings permanent magnet generator [6]

Fig. 3. A high-voltage inverter including series-connected chopper/dump

load circuits in the DC-link [7]

OREGON TECH. CLASS REE511, PROJECT 1: STATE-OF-THE-ART PATENT SEARCHING.

According to an embodiment of the invention, stator’s flux

vector of the generator is estimated and feedback to the

control system. Then the PWM for switching devices are

calculated and updated based on the different between said

feedback flux vector and a set point Fig. 6. The invention also

discloses a method to quickly derive the optimum PWM for

switching devices based on the flux vector difference. Overall,

the improved FOC method allows not only better dynamic

response, but also has a simpler system design and results in

reduction of CPU processing power [8].

In another effort of trying to improve the traditional FOC

techniques, U.S. Patent Application publication 20120056602

discloses a complicated amendment for the FOC combining

PID, fuzzy logic and adaptive control. The patent publication

includes a very detailed mathematical analysis of traditional

FOC algorithms. It is shown that some implementations of

FOC neglect the difference between the d- and q- axis

synchronous inductances of stator, Ld, Lq, and the effect of

rotor flux linkage ψf . As a consequence, the control dynamic

output is not good and WTS don’t really operate at the

Maximum Power Point. According to the application, the d-

and q- axis component of stator current depend on both d- and

q- axis component of voltage [9]:

So controlling q-axis of voltage output have effect on the d-

axis current and vice versa. The publication then discloses a

correct control block diagram which take into account the

effect of Ld, Lq, and ψf .

The application goes further by showing in a simulation that

even with the precise mathematical model for the traditional

FOC control system, the output is not good when generator

rotates at high speed. This is because the required Vdq output

exceeds the linear modulation limit of the converter. Based on

that, the application suggest a non-linear control method

combining PID, fuzzy and adaptive control techniques. With

the proposed non-linear control technique, the publication

shows in a simulation that there is a lots of improvement in

tracking the maximum power point under variable and gust

wind. Besides the output power is also more stable compared

to the traditional FOC method.

Renewable energy sources are said to “lack natural inertia.”

Wind and solar are not something that we can control (e.g.,

putting more “fossil fuel”). Controlling of wind generator to

behave as good as conventional generators do without

compromising for less energy harvested or higher cost with

added battery is one of the topics keeping researchers from

universities around the world busy. There are some patents

and patent applications aim to do just that. These advanced

control techniques help to make WTS capable of supporting

grid stability just as good as conventional generator do. Not

surprisingly, most of the features are mandated by new GC.

U.S. Patent Application 20120313593 disclose and advanced

control method to stabilize the grid when there is a frequency

disturbance due to outage, generation loss or sudden increase

in consumed power, etc.

Previously, in November 2008, issued U.S. Patent 7345373

already disclosed a method for stabilizing the swing in grid

frequency by implementing an integrator that emulates a

virtual frequency inertia with a custom magnitude m. The

ideas of electrical frequency inertia is exactly what we see in

mechanical world: inertia of a body measured by its mass m.

By implementing an ability to sluggishly response to change

we can stabilize the oscillation. The bigger the inertia

magnitude m, the higher the ability it can absorb oscillations.

Choosing inertia magnitude is a trade-off between control

system’s dynamic response and the ability to absorb

oscillation.

A simple Inertia Response (IR) is a good way to stabilize

the grid frequency most of the time. However it is not enough

to do well in some circumstances. U.S. Patent Application

20120313593 discloses many necessary improvements to the

old method in issued U.S. Patent 7345373. In an embodiment

of the invention, the virtual Inertia Response is implemented

by controlling the amount of active power P injected into the

grid in such a way that the frequency oscillation is reduced. In

power generation systems, grid frequency reflects the balance

of generated and consumed active power. So if frequency is

lower than the nominal frequency, we need to reduce the

injected power to bring the frequency back to normal and vice

versa.

In another embodiment of the invention, the IR may be

combined with a frequency recovery function, which makes

sure that activation of the IR function does not subsequently

lead to over frequency events. If the total frequency at the

point of interconnection above a certain threshold, the soft

recovery function is activated and power output is adjusted

according to a specified droop curve [10].

In another embodiment of the invention, the IR function is

only activated when the error in grid frequency is above a

specified threshold. So most of the time when grid frequency

is within a specified range of the nominal frequency, (which is

Fig. 4. A schematic representation of a fault ride-through switch shown in

accordance with an embodiment of the invention [8]

OREGON TECH. CLASS REE511, PROJECT 1: STATE-OF-THE-ART PATENT SEARCHING.

50Hz in US) the IR function is disable. The thresholds for

activating and deactivating the IR function are different. The

hysteresis in the operation of IR function is needed to keep the

overall system more stable.

In another embodiment of the invention, the normal

operation of Wind Turbine is controlled to avoid a “double

dip”, which may occur when system return to normal

operation after an Inertia Response [10]. After an IR, the

Turbine’s Rotor rotation speed might be higher or lower than

before the IR. If we reactivate the normal operation

immediately after the IR, a “double dip” may occur. To avoid

this undesirable effect, the real power reference is temporarily

controlled to allow turbine’s rotor rotation speed return to that

of the present or determined optimal operating condition [10].

And then the normal operation is allowed to be reactivated.

The Inertia Response might create oscillation in the

mechanical systems of wind turbine which might create

system failure in the long run. So the invention goes further by

suggesting that if the number of IR is over a certain limits,

depending on the current rotor rotational speed, WTS may

decide not to do an IR to stabilize grid’s frequency disturbance

in order to protect the Wind Turbine itself.

Fig. 6 showed a graph of an actual grid frequency according

to the conventional method compare to an embodiment of a

method in this patent application.

In wind turbines nowadays, generators play the most

important role in generating power. They are like the heart of

turbine. The principle of generator is the winds captured by

the blades will, in result, twist the shaft of the wind turbines.

This shaft can be connected directly to the rotor shaft or

through a gear box. The rotor will rotate proportionally to the

speed of the shaft, and therefore the rotor flux which is created

by permanent magnet or windings will cross the stator flux.

By Faraday’s Law, when this incidence takes place in

generator, it will generate a power at the output, or in other

word, wind turbine’s generators convert mechanical power to

electrical power. The electromagnetic power of generator can

be determined by the magnitude of the stator flux and its

position respect to the rotor flux. Therefore, it is possible to

apply a voltage to the stator accordingly to the position of the

rotor which can position the stator flux to get a desired output

voltage, at a rotational speed. A method to determine the

position of rotor is disclosed in U.S. Patent 8089171 issued to

Li, Bing, et al, on Jan 3, 2012 [13]

The method comprising three steps as illustrated in Fig. 6.

The first step is to determine the voltage of electrical

generator; this duty is assigned for block 401. After the

voltage of generator is known, block 403 will determine rotor

position angle based on this information (second step). Block

405 is called feedback block which means it will combine the

rotor position angle estimate (block 404) with the voltage of

generator, and feed the combined information to block 403 to

determine the subsequent rotor position (third step). With this

achieved rotor position, an appropriate voltage will be applied

to the stator to get a desired a desired output voltage [13].

In full scale wind turbines, faults may occur in a power

electronic converter. To avoid the faults of converter causing

the output of WTS disturb, a plurality of converters are

connected in parallel to share of the power generated by wind

turbine. When one converter is not operational or failure, the

WTS can continue to operate with the power reduced to the

lower combined converter capacity. With the plurality of

converters used in WTS, the switching inside converters will

generate the frequency other than the frequency of the grid.

This frequency is called the switching harmonic. With the

strict of grid codes in some countries, this harmonic has to be

resolved so that the frequency generated by WTS will be

matched with the grid frequency. In order to do that, it

requires significant filter requirements to filter out the

harmonic. There is a system so-called “interleaving” wherein

the switching harmonic is spread over with higher frequencies

and lower amplitude by displacing the switching waveforms

of each converter over a switching period. It has proven to

reduce the cost also the requirements of harmonic filtering.

However, since the interleaving system operates based on a

synchronization clock which may add to the complexity and

the cost of the design; more importantly, all the converters

have to restart and be synchronized to the clock all over again

if one converter is strip; therefore, according to general terms

of the embodiments, the present invention U.S. Patent

Application 20120155125 published on Jun 21, 2012 proposes

a method that might not require the synchronization clock and

Fig. 5. Actual grid frequency according to the conventional method (401)

compare to an embodiment of a method in this patent application (402) [10]

Fig. 6. Block diagram for determining rotor position in accordance with an

embodiment of the invented method [13]

OREGON TECH. CLASS REE511, PROJECT 1: STATE-OF-THE-ART PATENT SEARCHING.

restarting of converters when the failure occurs in another

converter. The method comprises monitoring an indicator of

the harmonics at the switching frequency and minimizing that

indicator by adjusting the delay of one or more interleaved

converters. Fig. 7 illustrates the flow diagram for this method

[14].

In the diagram, an individual converter (202) is chosen as a

reference for all the converters in wind turbine generator

(WTG). This reference converter might usually be the grid

side converter because it has the closet frequency with the grid

frequency. A second converter (204) is the enabled delay

adjustment converter. The magnitude of the harmonic signal at

the switching frequency will be measured (206) at a certain

time t. The delay of the switching wave from will be

incremented at time = t + 1 (208). Then, the magnitude of

harmonic at t + 1 will be determined (210), if it is smaller than

the prior measured magnitude at time t, the delay will be

further incremented (214), if no, the delay is decremented

(216). The process (210-216) will be continuously repeated to

match the harmonic signal of the second converter (204) with

the harmonic signal of the reference converter; and after a

period of time, the delay adjustment will stop for the second

converter and be enabled for the third individual converter.

The steps (204-216) will be similarly applied for the third

converter. After the set of period, the process will be disabled

for the third converter and repeated for the next converter and

so on. Until one all power converters have gone through, the

process will come back to the second converter (204), and

therefore, the cycle of harmonic adjustment will be repeated

[14].

Traditionally, wind turbine converters can be classified into

two forms based on the valve type and the circuit connection,

or it is also called the level of converter. The first form of

converter is symmetric structure wherein both grid-side

converter and generator-side converter have two levels; on the

other hand, the second form is similar to the first one but both

gird-side and generator-side converters have three levels. As

discussed above, because of the switching, converters can

generate harmonic signal which can follow the output power

and enter into the electric grid. In some cases, this harmonic

signal or harmonic current can be large, and when entering, it

might cause many undesired effects to the grid. Therefore,

suppressing harmonic is one of the first priorities when

designing power converter for wind turbines. Some prior

inventions have been proposed to resolve the harmonic

problem for both forms of converters. In two-level converters,

LCL (inductor-capacitor-inductor) filter is used to filter out

the harmonic; however, it might lead to more complexity in

design and even cause more harmonic oscillation condition.

The problems seem to be solved with three-level symmetric

structure converters, but with the over-size of power

components used in the main circuit which leads to the higher

cost of the system, those problems still remain unsolved.

Consequently, there are a lot of ideas attempting to design a

converter structure which can greatly reduce the harmonic

current, the cost of power components and the loss of wind

turbine. The structure proposed in US20130124522 patent

application published on Jun 19, 2012 is one of those ideas

[15].

Fig. 8 above shows the configuration to control the

generator-side converter and grid-side converter of a three-

level converter structure. According to the invention, the

generator-side converter includes a control circuit comprising

a power regulator and a current regulator. Under normal

operation conditions, an instruction signal reflecting the

current power including the real power, the rotational speed,

and the torque from generator will be sent from the a power

regulator TR to a current regulator CR. Current regulator will

then combine this instruction signal with three-phase current

signals received from generator to output a PWM

(Pulse width modulation) control signal to generator-side

control circuit to turn on and off the switching valve of the

generator-side converter. In an embodiment, a power

comparator is arranged in the between the power generator

and the power regulator. The purpose of this comparator is to

output a difference value - the instruction signal - to the power

Fig. 7. Flow diagram for minimizing the harmonic signal at the switching

frequency according to an embodiment of the invention [14]

Fig. 8. Configuration to control three-level converter structure in accordance

with an embodiment of the invention [15]

OREGON TECH. CLASS REE511, PROJECT 1: STATE-OF-THE-ART PATENT SEARCHING.

regulator based on the difference between the received

parameters reflecting the current power of the generator and

the preset parameter. A current comparator is also arranged

between the power regulator and the current regulator. Its

purpose is to receive the instruction signal and compare it with

the three-phase current to output a difference value to the

current regulator [15].

On the gird-side converter, the converter structure includes

grid-side control circuit which contains voltage regulator and a

current regulator. A voltage comparator will combine the DC

voltage and the reference voltage to output a

difference value or instruction signal. The voltage regulator

will receive this instruction signal and sent it to a current

comparator which is settled between the voltage regulator and

the current regulator. The current comparator will combine the

instruction signal and the three phase signals and

to give a difference value to the current regulator CR. Based

on this instruction signal or difference value, the current

regulator will send a PMW control signal to the grid-side

control circuit to turn on and off the power switching valves of

the grid-side converter.

Patent application 20130200621 published on Aug 8, 2013

proposes a general method to motoring harmonic signals, and

modifying the output power based on the detected oscillation

to a pre-defined reference value. Fig. 9 illustrates

schematically the disclosed method according to one

embodiment of the invention [19].

According to one embodiment of the invented method, a

sensor (119) will measure the output quantity including the

voltage, real power and reactive power at point close to the

output terminal (123). The signal containing all the

information of the output quantity (121) will be sent to the

arrangement (103). The arrangement will detect whether the

output has any oscillation according to a set of reference

value. Depend on the amplitude of the harmonic, the

arrangement will derive an instruction signal supplied to the

wind turbine controller to adjust the output quantity. The

arrangement might also further send a signal to a switch (139),

which in the case of faults will disconnect the wind turbine

from the power grid [19].

The advantage of this disclosed method is that it means to

control the output quantity of the wind turbine to be at a pre-

defined reference output quantity; however, with the

characteristic of disconnecting the wind turbine from the grid

in the event of faults, it does not adhere to the new grid codes

today any more.

C. Fault Ride-through.

As mandatory by new GCs, high power WTS are required

to have fault ride-through (FRT) capability. FRT is the ability

of WTS to stay connected to the grid during temporary grid

fault without tripping offline, which could further destabilize

the grid. Many kinds of faults may occur such as voltage

surges (over voltage), voltage sag (voltage dip), short-circuit.

When there is a grid fault, WTS cannot export power into

the grid suddenly, resulting in an impulsive change in

generator torque. The step reduction in torque can cause

detrimental mechanical oscillation in the drive train of the

WTS [11] Fig. 10. This effect can be mitigated by introducing

a dynamic braking resistor (DBR) to absorb at least part of the

power that cannot be exported into the grid during fault. If the

DBR is fully rated, then all of the power can be absorbed until

power can start to be exported into the grid again Fig. 11.

However the DBR which can absorb 3MW in a typical FSC

based wind turbine generator is very costly, switching devices

to support dynamic braking are also very expensive and they

are normally required liquid cooling. U.S. Patent Application

20130207394 discloses a cost saving method to control the

generator torque by control the DBR during grid fault, so that

there is no vibration and only require a much smaller DBR.

Fig. 9. Method of detecting harmonic and controlling the output voltage

according to one embodiment of invention. [19]

Fig. 10. The inability to push power into the grid during grid fault, results in nearly step reduction in generator torque and harmful vibration to WTS. [11]

OREGON TECH. CLASS REE511, PROJECT 1: STATE-OF-THE-ART PATENT SEARCHING.

In an embodiment of the invention, a WTS comprising a

generator and a rotating mechanical drive train having a

natural period Tn. When there is a fault condition, the

generator torque is controlled to decrease in smaller step

changes describe by the formula below [11]:

Where t is time, n is an integer and C is a constant.

In contrast to absorbing all the energy during grid fault, the

new method only needs to absorb part of the energy and still

can eliminate almost all the mechanical oscillations Fig. 12.

And as a consequence, the DBR can have much smaller rating

compared to old method where the DBR is fully rated. The

total system cost can be greatly reduced by having smaller

DBR, cooling system, power electronics, etc.

Issued US Patent 8471534 also discloses an inexpensive

though effective fault ride through switch for power

generation systems, including WTS. During grid fault, the

switch can automatically absorb electrical power to reduce the

quick acceleration and harmful vibration in power generation

systems. The switch design doesn’t employ any expensive

power electronics device Fig. 13.

In an embodiment of the invention, the fault ride through

switch is coupled in series between power generator and grid

electricity. The switch has two branches. The first branch has

a LC resonance circuit at each phase and almost no resistance,

which is configured to transfer electric current during normal

operation conditions. The second branch is coupled in parallel

with the first branch and has a resistor and an inductor coupled

in series at each phase [12] Fig. 13.

During normal operation, the first branch acts as a short

circuit to transfer all electricity from WTS to utility grid Fig.

14. In an event of fault, the power is diverted from the first

branch and flow through the second branch which has a

resistor to absorb at least part of the power during fault

conditions reducing quick acceleration and harmful vibrations

in WTS. Fig. 15.

Fig. 13. A schematic representation of a fault ride-through switch shown in

accordance with an embodiment of the invention [12]

Fig. 11. All power is absorbed by a fully rated DBR during grid fault. [11]

Fig. 12. By controlling the generator torque to decrease in smaller step sizes, all mechanical oscillation can be reduced substantially. [11]

OREGON TECH. CLASS REE511, PROJECT 1: STATE-OF-THE-ART PATENT SEARCHING.

Wind generators convert wind kinetic energy to electrical

energy for power grid. In variable-speed wind turbine, the

power generated from the generators will be transferred to the

grid through the power converter. Under normal conditions,

the generated power will be equal to the power supplied to the

grid, or it can be called as power balance in technical terms. In

the events of wind gust or grid fault, the power generated will

be larger than the power fed to the grid causing the power

imbalance. Such power imbalance might lead to undesired

tower oscillations, drive train damage or turbine stripping.

Normally, a wind turbine will manage wind gust by pitching

the blades to reduce the rotational speed of rotor leading to a

lower excess power generated. However, in some cases, the

pitching does not respond fast enough to sudden wind gust; so

pitching wind turbine blades is not an effective way to handle

this kind of situation. Using power dissipating unit such as

crowbar or load dump seems to be very promising, and has

been proposed to use widely in wind turbines recent years.

The US20130200620 patent application published on May

22, 2012 discloses a method using load dump to dissipate the

excess power over during converter fault or gird fault.

According to Fir. 16, in full-scale wind turbine, the generator

is usually connected to the power grid (210) though a

transmission path. This transmission path contains a generator

converter side (204) that is connected to the output of the

generator, and a grid converter side (206) connected to the

power grid. The generator converter side is used to convert

AC-(alternative current) to-DC (direct current), on the other

hand, the grid converter side converts DC-to-AC, and this

converter might be connected to the power grid through a

transformer (210). There exists a DC link (209) comprising a

DC capacitor to couple AC-to-DC converter and DC-to-AC

converter. A controlling unit which is called power converter

controller is arranged in wind turbine to detect the failures of

either the grid or the converter. One or two load dump is used

to dissipate the over power in case of faults occur. The first

load dump is directly connected to the output of the generator,

thus this load dump is usually called AC load dump (212), and

it will be turned on by power converter controller when the

converter failure occurs. The second load dump is call DC

load dump (216) because it is connect across the DC link. This

load dump comprises a bank of resistor to dissipate power in

case of grid fault. Due to the weight of converter and the size

of nacelle, both load dumps can share one bank of resistors

and be located in one container. [16].

As in Fig. 17, when generator operates normally, the switch

SW1 and SW2 are close so the full output power from

generator will go through two converters and merge into the

gird. Switch SW3, SW5 and switches SW5, SW6 remain

open. When power converter controller detects a fault

occurring in any of the converters, or in case of any of the

converters is not operational, the power controller will open

the switch SW1 to cut off the power path to the AC-to-DC

converter, and it will close switch SW3 and some of the fifth

switches SW5 so that the output power from generator will be

dissipated in one or more than one resistors in resistor bank.

The switch SW4 and switches SW6, in this case, still remain

open. In case of the grid failure, the controlling unit will send

the signal to close the switch SW4 and some of the switches

SW5, SW6. This will create a path for the output power to

Fig. 14. A diagrammatic illustration of the fault ride-through switch of Fig.

4 operating in pre-fault and post-fault conditions [12]

Fig. 15. A diagrammatic illustration of the fault ride-through switch of Fig.

4 operating in a fault condition [12]

Fig. 16. Load dump arrangement in accordance of with an embodiment of

the invention [16]

Fig. 17. Resistor bank and switches of load dump in accordance with an

embodiment of the invention [16]

OREGON TECH. CLASS REE511, PROJECT 1: STATE-OF-THE-ART PATENT SEARCHING.

dissipate in one or some resistors, and depend on the condition

of the gird at that time, partial of the real output power or none

of it will be sent to the grid.

Load dump offers the way to dissipate the excess power in

the events of imbalance power. However, it is still not the

most effective way, since the resistor in the dump load might

experience the enormous stress over a long time of power

dissipation. Hence, the US8183706 issued on May, 22, 2012

discloses an improved way to handle the excess power in case

of imbalance power. Fig. 17 illustrates the arrangement of load

dump in wind turbine power converter.

In Fig. 18, the load dump unit used in converter is a chopper

circuit containing a switch SW1 and a resistor (114). It is

arranged across the DC link 113, and in the between

generator-side converter (110) and grid-side converter (111).

A converter controller (120) comprising a generator-side

converter controller (121) and grid-side converter controller

(122) will be under control of wind turbine controller (123),

which is in charge of controlling the operation of the wind

turbine overall. To assure not to leave the load dump under

stress, the switch SW1 is open when the power is in balance

and close in a period of time when the imbalance power

occurs. The control of the operation of the switch SW1 can be

understood by describing Fig. 19 below

One of the factors used to close the switch SW1 and

activate the chopper circuit is the voltage at DC link; this is

because the DC link voltage reflects the excess power which is

not fed to the grid when there is imbalance power. Generally,

in Fig. 19, the reference DC link voltage (202) will be

combined with the actual DC link voltage by a comparator.

The difference (205) between these two values is send to a PI

(Proportional Integral) (201) to output a DC link error power

signal (206). Another comparator will then receive this signal

and the feed forward power, which is the difference between

the power supplied to the generator-side converter (207) and

the power transferred by the grid-side converter (208), to

operate a duty ratio and determine the total power to be

dissipated. Duty ratio is the percentage of time period when

the chopper circuit is activated in one cycle; it is usually equal

zero under normal conditions, and when there is imbalance

power, this value is from 0 to 1. For example, when the duty

ratio is 0.7, the chopper circuit is only turned on for 70% of

the time in one duty cycle. Similarly, when the duty ratio has a

value of 0.15, the power dissipating unit will be only activated

for 15% of duty cycle.

The advantage of duty ratio is that only the excess power in

the wind turbine is dissipated in a sufficient time; therefore, it

leaves less stress on the load dump, and thus the maximum

power delivered to the grid is warrantied. Another advantage

of this invention is the feed forward power which gives the

fast activation of the chopper circuit in case of imbalance

power; therefore the oscillation of the wind turbines due to the

wind gust can be avoided.

In case of grid faults, the rotor’s speed will rapidly increases

due to the excessive aero-dynamical power. This may cause

serious oscillation of the wind turbine, and a large amount of

power generated by generator will suddenly enter to the power

converter, which is not desired. Therefore, the aero-dynamical

power should be suppressed during the occurrence of grid

faults. One way to do it is to trip offline wind turbine

generator. However, the Low voltage ride through (LVRT)

requires the wind turbine generator stay connected, thus this

method is not applicable. With other ideas, pitching the blades

of wind turbine seems to have more advantages and because it

maintains the rotor speed under an over speed strip limit,

hence it can adhere to the LVRT grid code. Issued US Patent

Fig. 18. Dump load arrangement in accordance of with an embodiment of

the invention [17]

Fig. 19. Control algorithm of chopper circuit in accordance of with an

embodiment of the invention [17]

Fig. 20. Method to return to the operational setting of normal grid code of

wind turbine in accordance of with an embodiment of the invention [18]

OREGON TECH. CLASS REE511, PROJECT 1: STATE-OF-THE-ART PATENT SEARCHING.

8487462 discloses method comprising four steps provides an

effective method to operate a wind turbine with the

operational setting of normal grid code. Fig. 20 illustrates

those four steps of the invented control method.

The normal operation block (0) will detect a utility grid

fault event. If it detects a fault, the method will go to the first

state: initial control (1). The purpose of second stage is to

pitch one or more rotor blades out of the wind immediately for

avoiding the over speed of the rotor due to excessive aero-

dynamical power. If the fault is suitably short, a short dip is

detected and it will resume the normal operation condition for

wind turbine. If the fault lasts too long, in one embodiment of

invention, the blades are pitched until it reaches the “no

acceleration pitch angle” (NOPA) where the acceleration of

generator is approximately equal zero, and the algorithm will

go to second state (2): intermediate control. According to

another embodiment, when the gird recovers from the fault

before NOPA is reached, the invented algorithm will still go to

the second state since the sudden jump back to the normal

operation condition will cause negative impact on the wind

turbine.

The basic task of the second state is to keep the wind

turbine operating under the control of wind turbine controller

and staying connected to the electric grid. If the NOPA is

reached, the wind turbine controller will keep the rotational

speed of the rotor constant at a level above the nominal speed.

If the grid recovers before reaching NOPA, the second state

will be initiated and the pre-defined control parameters are re-

obtained (cited). All the control sequences just mentioned will

stay in state 2 until the recovery of the grid occurs. At the last

state (3): final control, the turbine blades are pitched back to

their operation position by reapplying thrust based on the

means of oscillation sensor. If the means present, the pitch is

ramped back to normal operation condition with the maximum

pitch velocity in counter phase to oscillation; otherwise, the

pitch value will be re-obtained with pre-defined control

parameters.

V. CURRENT AND FUTURE DIRECTION

The search results showed that most of the new patents are

in the following two categories: control of WTS and FRT

capability for WTS. Controlling of wind generator to behave

as good as conventional generator without compromising for

less energy harvested or higher cost with added battery are

among the most attractive topics right now. Besides, many

patents focus on new GC conformity which is an essential

outcome to manage the high level of wind energy penetration

to the utility grid. The evolution of GC and WTS will continue

to generate more patents as the wind energy penetration grows

higher.

The main research drivers for future development of WTS

are always reduction in cost and increased in reliability.

The evolution of modern wind turbines is an impressive

story involves scientific, engineering along with strong

entrepreneurial spirit. The story is far from finished. Many

technical and economic challenges are waiting to be solved;

more impressive achievements will comes.

VI. CONCLUSION

In this paper, we have particularly reviewed the circuit

design topology, the control techniques and the fault-ride-

through capabilities of full scale wind turbine system over

fourteen patents and patent applications. It has shown the wind

power is on its rapid growth with many supporting

technologies. The paper also includes a current and future

direction of full scale wind turbine system based on further

research.

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