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PQ Improvement of Grid Connected WECS using FACTS Devices )

MEA Engg. Coege! Perint"#m#nn# Dep#rtment of EEE

In the event of increasing grid disturbance, a battery energy storage system for

wind energy generating system is generally required to compensate the fluctuation

generated by wind turbine. ' (T'T)$* based control technology has been proposed

for improving the power quality which can technically manages the power level

associates with the commercial wind turbines. The proposed (T'T)$* control scheme

for grid connected wind energy generation for power quality improvement has following

ob+ectives.

-nity power factor at the source side.

eactive power support only from (T'T)$* to wind /enerator and 0oad.

(imple 1I controller for (T'T)$* to achieve fast dynamic response.

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PQ Improvement of Grid Connected WECS using FACTS Devices *


C$APTE% )

P'WE% Q(A+IT, IMP%'-EME&T

).1 P'WE% Q(A+IT, STA&DA%DS

Intern#tion# Eectro Tec"nic# Commission Guideines

The guidelines are provided for measurement of power quality of wind turbine. The

International standards are developed by the working group of Technical )ommittee!!

of the International 2lectro technical )ommission 3I2)4, I2) standard 567""6,

describes the procedure for determining the power quality characteristics of the wind

turbine. The standard norms are specified.

I2) 567""68 &ind turbine generating system, part6. *easurement and

'ssessment of power quality characteristic of grid connected wind turbine.

I2) 567""698 &ind Turbine:measuring procedure in determining the power

behavior.

I2) 567""9;8 'ssessment of emission limits for fluctuating load I2) 567""68

&ind Turbine performance. The data sheet with electrical characteristic of wind

turbine provides the base for the utility assessment regarding a grid connection.

).) P'WE% Q(A+IT, ISS(ES

).).1 -ot#ge -#ri#tion

The voltage variation issue results from the wind velocity and generator torque. The

voltage variation is directly related to real and reactive power variations. The voltage

variation is commonly classified as under8

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PQ Improvement of Grid Connected WECS using FACTS Devices


Table 6 )ategories of 1? problems

).).) -ot#ge Fic/er

The voltage flicker issue describes dynamic variations in the network caused by

wind turbine or by varying loads. Thus the power fluctuation from wind turbine occurs

during continuous operation. The amplitude of voltage fluctuation depends on grid

strength, network impedance, and phaseangle and power factor of the wind turbines. It is

defined as a fluctuation of voltage in a frequency 6"@9A #z. The I2) 567""76A

specifies a flicker meter that can be used to measure flicker directly.

).).) $#rmonics

The harmonic results due to the operation of power electronic converters. The

harmonic voltage and current should be limited to the acceptable level at the point of

wind turbine connection to the network. To ensure the harmonic voltage within limit,

each source of harmonic current can allow only a limited contribution, as per the standard

I2)567""95 guideline. The rapid switching gives a large reduction in lower order

harmonic current compared to the line commutated converter, but the output current will

have high frequency current and can be easily filterout.

).).* Wind Tur0ine +oc#tion in Poer S2stem

The way of connecting the wind generating system into the power system highly

influences the power quality. Thus the operation and its influence on power system

depend on the structure of the ad+oining power network.

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PQ Improvement of Grid Connected WECS using FACTS Devices 3


).). Sef4E5cit#tion of Wind Tur0ine Gener#ting S2stem

The selfexcitation of wind turbine generating system 3&T/(4 with an

asynchronous generator takes place after disconnection of wind turbine generating system

3&T/(4 with local load. The risk of selfexcitation arises especially when &T/( is

equipped with compensating capacitor. The capacitor connected to induction generator

provides reactive power compensation. #owever the voltage and frequency are

determined by the balancing of the system. The disadvantages of selfexcitation are the

safety aspect and balance between real and reactive power.

).* C'&SEQ(E&CES 'F T$E ISS(ES

The voltage variation, flicker, harmonics causes the malfunction of equipmentBs

namely microprocessor based control system, programmable logic controller% ad+ustable

speed drives, flickering of light and screen. It may lead to tripping of contractors, tripping

of protection devices, stoppage of sensitive equipmentBs like personal computer,

programmable logic control system and may stop the process and even can damage of

sensitive equipments. Thus it degrades the power quality in the grid.

). G%ID C''%DI&ATI'& %(+E

/rid )odes are issued by grid companies, spelling out the rules that apply for anyone

who wishes to connect a wind farm to the grid. *ain requirements involve8

eactive power supply

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The grid quality characteristics and limits are given for references that the

customer and the utility grid may expect. 'ccording to 2nergy2conomic 0aw, the

operator of transmission grid is responsible for the organization and operation of

interconnected system.

-ot#ge %ise 7u8

The voltage rise at the point of common coupling can be approximated as a

function of maximum apparent power (max of the turbine, the grid impedances and D

at the point of common coupling and the phase angle E,

&here Fu is the voltage rise, (max is the maximum apparent power, E is the phase

difference and - is the nominal voltage of grid. The 0imiting voltage rise value is G H

-ot#ge Dips 7d8

The voltage dips is due to startup of wind turbine and it causes a sudden reduction

of voltage. It is the relative H voltage change due to switching operation of wind turbine.

The decrease of nominal voltage change is given as

&here > is relative voltage change, (n is rated apparent power, (k is short circuit

apparent power, and u is sudden voltage reduction factor. The acceptable voltage dips

limiting value is G 9H.

Fic/er

The measurements are made for maximum number of specified switching

operation of wind turbine with 6"min period and h period are specified, as given as,

&here 1lt is the 0ong term flicker, )3Jk 4 is the Clicker coefficient calculated from

ayleigh distribution of the wind speed. The 0imiting

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$#rmonics

The harmonic distortion is assessed for variable speed turbine with an electronic

power converter at the point of common connection. The total harmonic voltage

distortion of voltage is given as,

&here of current and limit for 69 < is G .AH.

Grid Fre:uenc2

The grid frequency in India is specified in the range of 7;[email protected] #z, for wind

farm connection. It shall be able to withstand change in frequency up to ".A #z=s.

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C$APTE% *

FACTS

C')T( make up a family of high power devices that are applied in power

systems in shunt and=or in series. C')T( solutions are particularly +ustifiable in

applications requiring rapid dynamic response, ability for frequent variations in output,

and=or smoothly ad+ustable output. -nder such conditions, C')T( is a highly useful

option for enabling or improving the utilization of power systems. C')T( devices can

basically be subdivided into three categories8

(hunt devices such as (ynamic voltage control, to enable limiting of overvoltage over long, lightly

loaded lines and cable systems, as well as prevent voltage depressions or even

collapses in heavily loaded or faulty systems. Increased power transmission capability and stability, without any need to build

new lines. This is a highly attractive option, costing less than new lines, with less

time expenditure or environmental impact.

Cacilitating integration of renewable power by maintaining grid stability and

fulfilling grid codes, as well as making room for this additional power in existing

grids.

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C$APTE%

T'P'+'G, F'% P'WE% Q(A+IT, IMP%'-EME&T

The (T'T)$* based current control voltage source inverter in+ects the current

into the grid in such a way that the source current are harmonic free and their phaseangle

with respect to source voltage has a desired value. The in+ected current will cancel out the

reactive part and harmonic part of the load and induction generator current, thus it

improves the power factor and the power quality. To accomplish these goals, the grid

voltages are sensed and are synchronized in generating the current command for the

inverter. The proposed grid connected system is implemented for power quality

improvement at point of common coupling 31))4. The grid connected system, consists of

the wind energy generation system and battery energy storage system with (T'T)$*.

Cig.7.6 /rid connected system for power quality improvement

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PQ Improvement of Grid Connected WECS using FACTS Devices 1=


.1 WI&D E&E%G, GE&E%ATI&G S,STEM

In this configuration, wind generations are based on constant speed topologies with pitch

control turbine. The induction generator is used in the proposed scheme because of its

simplicity, it does not require a separate field circuit, it can accept constant and variable

loads, and has natural protection against short circuit. The available power of wind

energy system is presented as

Pind > =.3?A-*ind

&here K 3kg=m4 is the air density, ' 3m4 is the area swept out by turbine blade and < wind

is the wind speed in m=s. It is not possible to extract all kinetic energy of wind, thus it

extract a fraction of power in wind, called power coefficient of the wind turbine and is

given as,

Pmec" > Pind

&here is the power coefficient, depends on type and operating condition of wind

turbine. This coefficient can be expressed as a function of tip speed ratio and pitch angle.

The mechanical power produced by wind turbine is given as,

Pmec" > =.3?@% )-

*indCp

&here, is the radius of the blade in m.

.) STATC'M STATIC S,&C$%'&'(S C'MPE&SAT'%

The (T'T)$* 3or (()4 is a shuntconnected reactivepower compensation

device that is capable of generating and= or absorbing reactive power and in which the

output can be varied to control the specific parameters of an electric power system. It is

in general a solidstate switching converter capable of generating or absorbing

independently controllable real and reactive power at its output terminals when it is fed

from an energy source or energystorage device at its input terminals.

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PQ Improvement of Grid Connected WECS using FACTS Devices 1)


The fluctuating power due to the variation of wind cannot be smoothened by

reactive power control using a (T'T)$*, because it has no active power control ability.

To overcome this problem, Lattery 2nergy (torage system 3L2((4 has been incorporated

with (T'T)$* which has both real and reactive power control ability. Curthermore, a

(T'T)$* does the following8

6. It occupies a small footprint, for it replaces passive banks of circuit elements by

compact electronic converters.

. It offers modular, factorybuilt equipment, thereby reducing site work and

)ommissioning time.

9. It uses encapsulated electronic converters, thereby minimizing its environmental

impact.

' (T'T)$* is analogous to an ideal synchronous machine, which generates a

balanced set of three sinusoidal voltages at the fundamental frequency with controllable

amplitude and phase angle. This ideal machine has no inertia, is practically instantaneous,

does not significantly alter the existing system impedance, and can internally generate

reactive 3both capacitive and inductive4 power.

.* 'PE%ATI'& SC$EMEIt is possible to improve the (T'T)$* response by employing the 1I> control method.

'pplication of the 1I> involves choosing the 1, I and > that provide satisfactory

closedloop performance. Lut the main method is based on trial and error, although timeconsuming. To achieve equilibrium among range control parameters, response speed,

settling time, and proper overshoot rate, all of which guarantee the system stability, the

1I> is employed. PID controer designing$ne of the most common controlling devices in the

market is the 1I> controller. The order of types of

controller positioning is depicted in Cig. .

1I> control is a linear control methodology with a verysimple control structure 3see Cig. 4. This type of

controller operates directly on the error signal, which is

the difference between the desired output and the actual

output, and generates the actuation signal that drives the

plant. 1I> controllers have three basic terms8

proportional action, in which the actuation signal is

proportional to the error signal, integral action, where

the actuation signal is proportional to the time integral

of the error signal, and derivative action, where the

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PQ Improvement of Grid Connected WECS using FACTS Devices 1*


actuation signal is proportional to the time derivative of

the error signal.

Cig.7.9 (ystem operational scheme in grid system.

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C$APTE% 3

S,STEM PE%F'%MA&CE

The proposed control scheme is simulated using (I*-0IM in power system block set.

The system parameter for given system is given Table I.

(.M 1arameters atings

6 /rid voltage 9phase,76A

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Cig.A.6 wind turbine model output voltage

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Cig.A. &ind Turbine *odel $utput current

Cig.A.9 51ulse (T'T)$* $-T1-T

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Cig.A.7 CCT analysis without )ontroller 3T#>N7.Q5H4

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PQ Improvement of Grid Connected WECS using FACTS Devices 1;


Cig.A.A CCT analysis with )ontroller 3T#>N".76H4

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PQ Improvement of Grid Connected WECS using FACTS Devices 1<


C'&C+(SI'&

In this paper we present the C')T( device 3(T'T)$*4 based control scheme

for power quality improvement in grid connected wind generating system and with

nonlinear load. The power quality issues and its consequences on the consumer andelectric utility are presented. The operation of the control system developed for the

(T'T)$* in *'T0'L=(I*-0IM for maintaining the power quality is to be

simulated. It has a capability to cancel out the harmonic parts of the load current. It

maintains the source voltage and current inphase and support the reactive power demand

for the wind generator and load at 1)) in the grid system, thus it gives an opportunity to

enhance the utilization factor of transmission line. The integrated wind generation and

C')T( device with L2(( have shown the outstanding performance. Thus the proposed

scheme in the grid connected system fulfills the power quality norms as per the I2)

standard 567""6.

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PQ Improvement of Grid Connected WECS using FACTS Devices )=

MEA E C P i " D f EEE

%EFE%E&CES

(T'T)$*T*T R > )orporation C')T( for /rid Integration of &ind 1ower% olf /rSnbaum, (enior *ember,

I222

$02 $C C')T( >2 2M2&'L02 2M2/ ((T2*% >r. (.*. 'li6 L..1rusty

*..>ash (. 1. *ishra% Uournal of 2ngineering esearch and (tudies

1ower quality enhancement using custom power devices% 'rindam /hosh

/erard 0edwich 0-&2 ')'>2*I) 1-L0I(#2(% Loston = >ordrecht = 0ondon

Improving /rid 1ower ?uality with C')T( >evice on Integration of &ind

2nergy (ystem% uvara+ eepa, ' 1 oger ozario, *adhusudan umar%"66 Cifth 'sia *odelling (ymposium

C')T( R 1ower ?uality Issues%*'MU-0' M'I R (IM>#-.*.% >21T $C

222% '*IT' ()#$$0 $C 2M/IM22IM/, )$I*L'T$2

1ower2lectronic (ystems for the /rid Integration of enewable 2nergy (ources% Vbigniew 0eonowicz, &roclaw Institute of Tchnology

$ptimized 1erformance of (T'T)$* with 1I> )ontroller Lased on /enetic

'lgorithm

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