LFAC at 8Hz

International Journal of Science, Engineering and Technology Research (IJSETR), Volume 3, Issue 7, July 2014

ISSN: 2278 – 7798 All Rights Reserved © 2014 IJSETR 1940

Simulation of Dc-Link Power Converter for

Integrating Offshore Wind Turbine Generator to Grid Chaitanya Krishna Jambotkar #1, Prof. Uttam S Satpute #2

#1Department of Electronics and Communication Engineering, KLS’s VDRIT, Haliyal-581329, India.

#2Department of Electrical and Electronics Engineering, KLS’s VDRIT, Haliyal-581329, India.

Abstract—In the recent years Off-shore wind power plants

have gained interest due to the fact that, there is an ample space

available for its installation. Moreover it offers other advantages

such as; high wind energy potential, turbine blades can be larger

(no transportation and construction problems), also the blades

can be allowed to rotate at higher speed (no noise constrain)

thereby increasing the rated power. The most important

requirements of Off-shore wind power plants are less weight, less

maintenance and less cost; and these requirements can be met by

eliminating or reducing gear size. So, Direct-drive Permanent

Magnet Synchronous generator concept is found most attractive.

In such scheme, the frequency of power generation depends upon

the number of generator poles and the rotor speed. The rotor

speed depends upon the wind speed, which is fixed for a site. So

the frequency of power generation depends upon the number of

poles of generator. In this project the concept of Fractional

frequency Transmission system (FTTS) is introduced. This

project deals with optimal selection of frequency for power

converter such that the total cost of generation-transmission is

minimum with maximum converter efficiency. This study

requires the analysis on power quality of power converter which

is integrated to grid. SVPWM based three phase controlled DC-

Link power converter integrated to grid is implemented on

MATLAB/Simulink platform (7.11 R2011b version) where in the

power converter is simulated for lower frequencies such as 50/2,

50/3, 50/4, 50/5, 50/6, 50/7, 50/8 further all the results such as

%THD, harmonic analysis, efficiency of converter etc are noted

based on which we conclude the optimal frequency for power

converter. Based on the obtained results we can conclude that f/6

i.e. 8.33 Hz is the optimal frequency for the power converter

which is integrated to grid via FFTS. The generator and

transmission lines are also expected to present the similar

benefits. But the cost of power generation increases with the

decrease in frequency. So systematic methodology has to be

adopted in optimal selection of frequency for complete system design.

Keywords- AC-DC-AC power converter, FFTS, PLL, SVPWM.

I. INTRODUCTION

In recent years, the electrical power generation from

renewable energy sources, such as wind is increasingly

attraction interest because of environmental problem and

shortage of traditional energy source in the near future.

Nowadays, the extraction of power from the wind on large

scale became a recognized industry. It holds great potential

showing that future will become undisputed number one

choice form of renewable source of energy. The force that

pushes this technology is the simple economics and clean

energy. As a consequence of rising fossil fuel price and

advanced technology, more and more homes business have

been installing small wind turbines for the purposes of cutting

energy bills and carbon dioxide emissions and are even selling

extra electricity back to the national grid.

The kinetic energy in the wind is converted into mechanical

energy by the turbine by way of shaft and gear box

arrangement because of the different operating speed ranges

of the wind turbine rotor and generator. The generator

converts this mechanical energy into electrical energy.

However, as wind is an intermittent renewable source, the

wind source extracted by a wind turbine is therefore not

constant. For this reason, the fluctuation of wind power results

in fluctuated power output from wind turbine generator. From

the point of view of utilities, due to the fluctuation of generator output, it’s not appropriate for the generator to be

directly connected to power grid. In order to achieve the

condition that the generator output power is suitable for grid

connection, it is necessary to use a controller to manage the

output produced by the wind turbine generator.

At present the transmission technologies such as HVAC,

MVAC, HVDC and FACTS are giving their level best. Each technology has got its own importance when evaluated on

different parameters. Basically the fundamental issue that

made the rise for various technologies was the power quality.

The power quality is given by

Pmax = V2

X =

V2

2пfL ----------- (1)

%∆V = QX

V2 × 100 = 2пfLQ

V2 × 100 ----------- (2)

From equation (1) we can say that power transmission

capacity increases by the factor of decrease in frequency and

equation (2) says that voltage fluctuation will decrease by the

factor of increase in transmission efficiency.

Power converter is required at the grid side to convert the

fractional frequency of transmission into a standard frequency

50Hz before integrating to grid. We have implemented



AC-DC-AC converter i.e. a pair of rectifier and inverter as

power converter in our project. SVPWM technique is used to

produce an appropriate firing angle to the switching device so

that the output of the inverter circuit is always maintained at

50Hz at appropriate voltage level. SVPWM is used ahead of

other PWM techniques as it provides less THD, switching losses, and a bit boost in voltage level compared to other

PWM techniques.

II. SYSTEM DESCRIPTION

The power converter system is analyzed by giving various lower frequencies as input to it such as f/2, f/3, f/4, f/5, f/6,

f/7, f/8 at standard transmission voltage level. The block

diagram implementation of power converter integrated to grid

is shown in the figure 1.

Fig 1 Block diagram of the power converter integrated to

grid

It is assumed that the power is transmitted at the fractional

frequency and given as input to the controlled rectifier which

converts a fractional frequency AC voltage to a DC voltage

which acts as input to the controlled inverter the output of the

controlled inverter is produced with reference to the grid

voltage and frequency. Then the output of controlled inverter

is compared with the grid code as defind by IEC61727 [3] and

conditions for synchronization such as voltage magnitude, phase and frequency. If the incoming line (output of inverter)

satisfies all these conditions then the control signal will be

provided to the circuit breaker to integrate the incoming line

to the grid.

The following figure 2 shows 2 Electrical scheme of a wind

turbine equipped with a direct-drive PMSG.

Fig 2 Electrical scheme of a wind turbine equipped with a

direct-drive PMSG.

III. MATLAB/SIMULINK IMPLEMENTATION

A. SVPWM Modeling

The proposed approach is based on the instantaneous values

of the reference voltages of a, b and c phases only and the

actual switching times for each inverter leg are deduced

directly. The obtained load current is converted from three

phase Iabc to two phase components Id and Iq respectively.

The two phase currents are then compared with the reference

values of the two phase components and the obtained output is

again converted back to three phase components.

The obtained three phase components are used to obtain the

Uα and Uβ by using the transformation as stated below .

Uα = 2

3 Ia + Ib cos

2π

3+ Ic cos

2π

3 ---------- (3)

Uβ = 2

3 Ib sin

2π

3− Ic cos

2π

3 ---------- (4)

Fig. 3 The sinusoidal wave of three phases

The Figure 3 illustrates the characteristic of sinusoidal waves

of a 3-phase system. The period of sinusoidal waves of 3-

phase is considered as 6 sectors, which are divided at the

critical points of 1.0472 (π/3), 2.0944 (2π/3), 3.1416 (π),

4.1888 (4π/3), 5.236 (5π/3) and 6.2832 (2π) radians. The blue,

green and red lines indicate A – neutral, B – neutral and C –

neutral voltages in a 3-phase balanced system.

Analysis of the figure reveals the following table (the digit 1

is representative of the corresponding phase amplitude being

greater than or equal to 0, the digit 0 expressing the amplitude

is less than 0.).



For the neutral voltage of Phase A, B and C, in this

interpretation of the logical relationship an alternative method

of expressing the relationship in algebra is given by ,

N = A + 2B + 4C ------------ (4)

Where N represents the sector mapping Based on the above

equation the above table can be modified as follows

Fig 4 Vector synthesis schematic in sector 1

The Figure 4 illustrates the vectors located in sector, assuming

Tz is the unit of time. According to the Figure 4 the time T1 and T2 are obtained as follows:

Iref

sin 2π

3

= I1T1

sin π

3−α

------------ (5)

Simplifying the above equations we get the equations for T1,

T2 and T0 as follows [2],

T1 = Iref

I1

sin

π3 − α

sin 2π3

= M sin π

3− α

T2 = Iref

I1

sin α

sin 2π3

= M sin α

T0 = Tz − T1 − T2 ------------- (6)

Where M is the ratio of modulation, T0 is the dead time of the

inverter.

M = 2

√3

Iref

Idc ------------- (7)

The generated times are the pulse width modulated based on

the sector and the time periods obtained based on the

following table.

B. Rectifier

The bridge configuration is used to implement controlled

rectifier unit in MATLAB/Simulink platform. The

switching device used is IGBT (Insulated gate bipolar transistor) because of its low on state voltage and less

switching time.

The below figure 5 shows the Simulink model of

SVPWM based three phase controlled rectifier.

Fig 5 SVPWM based controlled Rectifier



C. Inverter

The bridge configuration is used to implement controlled inverter unit in MATLAB/Simulink platform. The switching

device used is IGBT (Insulated gate bipolar transistor)

because of its low on state voltage and less switching time.

The below figure 6 shows the Simulink model of SVPWM

based three phase controlled inverter.

Fig 6 SVPWM Based controlled Inverter Integrated to

grid

D. PLL Technique

Phase – Locked Loop (PLL) is a phase tracking algorithm

widely applied in communication technology, being able to

provide an output signal synchronized with its reference input

in both frequency and phase. Here, the PLL technique is

utilized to extract the phase angle of the grid voltages. The

PLL is implemented in dq synchronous reference frame. This

structure needs the coordinate transformation from abc to dq

and the lock is realized by setting the reference to zero. A PI controller is used to control the variable. This structure can

provide both the frequency of grid as well as the grid voltage

angle.

IV. RESULTS

The following table provides the THD values for voltage and current at lower fractional frequency simulated for controlled

rectifier circuit.

Table 4

THD values for voltage and current at fractional frequencies

for controlled rectifier

FREQUENCY

(Hz)

Vdc

(V)

Idc

(A)

(Vdc)

THD

(Idc)

THD

f

(50)

478.2 95.7 8.439

e+005

7.808

e+005

f/2

(25)

485.9 97.2 3.852 4.427

f/3

(16.67)

486.4 97.3 1.569 1.617

f/4

(12.5)

493.7 98.8 0.6255 0.618

f/5

(10)

493.9 98.7 0.1734 0.1716

f/6

(8.33)

489.6 97.9 0.1301 0.1295

f/7

(7.142)

500.7 100 0.5134 0.5043

f/8

(6.25)

484.4 96.9 0.3461 0.3634

The following figure 7 shows output voltage waveform of the

controlled rectifier circuit.

Fig 7 Output voltage waveform of the controlled Rectifier

circuit.



The following figure 8 shows output current waveform of the

controlled rectifier circuit.

Fig 8 Output current waveform of the controlled rectifier

circuit.

The following table provides the THD values for voltage and

current at lower fractional frequency simulated for controlled

inverter circuit integrated to grid with synchronization

frequency maintained at 50Hz constantly for all fractional

frequency given as input to the controlled inverter circuit.

Table 5

THD values for voltage and current at fractional frequencies

for controlled inverter integrated to grid

FRQUE

NCY

(Hz)

Vdc

(V)

Vl

(V)

Vph

(V)

Fsy

n

(Hz)

(Vout)

THD

(Iout)

THD

f

(50)

478.2 385 234 50 0.249 0.2712

f/2

(25)

485.9 392 238 50 0.249 0.2712

f/3

(16.67)

486.4 392 239 50 0.249 0.2712

f/4

(12.5)

493.7 398 242 50 0.249 0.2709

f/5

(10)

493.9 398 242 50 0.249 0.2709

f/6

(8.33)

489.6 395 240 50 0.249 0.2711

f/7

(7.142)

500.7 404 246 50 0.249 0.2704

f/8

(6.25)

484.4 391 238 50 0.249 0.2712

The following figure 9 shows the control signal to produce

firing angle using SVPWM Technique.

Fig 9 Control signal to produce firing angle using SVPWM

Technique.

The following figure 10 shows the line voltages of the

controlled inverter.

Fig 10 Line voltages of the controlled inverter.

The following figure 11 shows the phase voltages of the

controlled inverter.

Fig 11 Phase voltages of the controlled inverter.



The following figure 12 shows the output voltages of the

controlled inverter after filter circuit.

Fig 12 Output voltages of the controlled inverter after

filter circuit.

The following figure 13 shows waveforms for Grid voltages

and currents.

Fig 13 Waveforms for Grid voltages and currents.

The below figure 14 shows the graph for efficiency of power

converter. From graph, we can say that f/6 is the optimal

frequency for power converter which is same as illustrated

using power quality of power quality of power converter in

the tabular column of both rectifier and inverter.

Fig 14 The efficiency plot of power converter.

V. CONCLUSION

The paper presents a novel idea of integrating Dc-Link power converter to grid via FFTS. Complete design of system

requires frequency response characteristics of different

components involved in. The paper has attempted to obtain

the frequency response characteristics of power converter.

From the results following different points are concluded;

SVPWM method of producing PWM is beneficial as

it produces boost in output voltage compared to other

PWM techniques and uses input voltage to better

extent.

f/6 i.e. 8.33 Hz is optimal frequency for power

converter as %THD and harmonic variations are less for this frequency value than other frequencies and

power converter efficiency is also high at this

frequency.

Lower frequency operation of system is beneficial.

REFERENCES

[1] X. Wang, "The Fractional Frequency Transmission System" in

Proc, Inst., Elect, Eng., Japan, Power System, Tokyo, Japan, Jul.

1994.

[2] Wang Xifan, Cao Chengjun, Zhou Zhichao, “Experiment on

Fractional Frequency Transmission System” IEEE Trans on Power

System, vol. 21, no.1, Feb 2006.

[3] Uttam S Satpute, S H Jangamshetti, D R Joshi, “Feasibility Study

of Fractional Frequency Transmission System” IEEE Trans on

Power Electronics, Drives and Energy Systems (PEDES) & 2010

Power India, 2010.

[4] Sandhya Rani , A.Appaprao, “A Space Vector PWM Scheme for

Three level Inverters Based on Two-Level Space Vector

PWM” International Journal Of Power System Operation and

Energy Management (IJPSOEM) Volume-1, Issue-1, 2011,

pp 6 – 10.

[5] P.Tripura, Y.S.Kishore Babu, Y.R.Tagor, “Space Vector Pulse

Width Modulation Schemes for Two-Level Voltage Source

Inverter” ACEEE Int. J. on Control System and Instrumentation,

Vol. 02, No. 03, October 2011 pp 34 – 38.

[6] M. H. Rashid, “Power Electronics Handbook”, New York 2001.

[7] B. K. Bose, “Modern Power Electronics and AC Drives”,

Englewood Cliffs, NJPrentice-Hall, 2002.

[8] Ned Mohan, “Power Electronics”, Willey Publications.

LFAC at 8Hz

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