47

CHAPTER 4

STATCOM BASED LOAD VOLTAGE STABILITY OF SEIG

4.1 INTRODUCTION

STATCOM is a voltage-source converter based device, which converts a

DC input voltage into an AC output voltage in order to compensate the active and

reactive needs of the system. STATCOM has better characteristics than SVC; when

the system voltage drops sufficiently to force the STATCOM output to its ceiling,

its maximum reactive power output will not be affected by the voltage magnitude.

Therefore, it exhibits constant current characteristics when the voltage is low under

the limit.

Srinivas.P and Devakumar.M.L.S (2010) dealt with the Optimization of

Power Factor and Energy Management in Wind Energy Station. PraneshRao, M. L.

Crow, and Zhiping Yang (2000) dealt with the STATCOM Control for Power

System Voltage Control Applications. P. W. Lehn, and M. R. Iravani (1998) dealt

with the Experimental evaluation of STATCOM closed loop dynamics.Raimondset

al (2011) dealt with the static synchronous compensator for reactive power

compensation under distorted mains voltage conditions. Sidhartha Panda and

PadhyN.P (2007) dealt with the power electronics based FACTS controller for

stability improvement of a wind energy embedded distribution system.

Kuang Li et al (2007) dealt with the strategies and operating point

optimization of STATCOM control for voltage unbalance mitigation in three-phase

three-wire systems. Jianye Chen and et al. (2006) dealt with the analysis and

implementation of thyristor based STATCOM. Zhiping,and et al. (2000) dealt with

the improved STATCOM model for power flow analysis. Shaheen H.I, RashedG.

I.andChengS. J (2008) d

static synchronous compen

4.2 PROBLEM STA

An Approach to m

based STATCOM Contr

steady state performance

(VSC) by which the stator

proposed.

In this approach th

excitation reference frame

of q-axis current on the

controlled by d-q equivale

axis current of SEIG is co

The proposed STATCOM

supplies the required react

4.3 BASIC MODEL

Figu

dealt with the nonlinear optimal predictive

pensator (STATCOM).

TATEMENT

maintain the load voltage using d-q equivalent

ntroller and basic STATCOM Controller is

e of STATCOM based on six pulse voltage so

tor flux oriented vector control of terminal volta

the total flux is aligned to the d-axis of the sta

me. A decoupling signal is also generated to ca

e d-axis flux or total flux. The load voltage o

alent model using STATCOM controller. The tr

compared with the reference voltage of VSC o

OM eliminates the harmonics, provides load

active power to the load and the generator.

L OF A STATCOM

ure 4.1 Basic Model of the STATCOM

48

ve controller for

t model of SEIG

s proposed. The

source converter

ltage for SEIG is

stator flux in the

cancel the effect

e of the SEIG is

transformed d-q

of STATCOM.

d balancing and

4.4 BASIC STATCO

Figure 4

4.5 STATCOM BAS

A static synchrono

alternating current elect

electronics voltage-source

AC power to an electrici

provide active AC power

STATCOM is installed to

and often poor voltage reg

load voltage stability.

COM CONTROL SCHEME

4.2: Basic STATCOM Control Scheme

ASED LOAD VOLTAGE STABILITY O

nous compensator (STATCOM) is a regulating

ctricity transmission networks. It is based

ce converter and can act as either a source or s

icity network. If connected to a source of pow

er. It is a member of the FACTS family of dev

to support electricity networks that have a poo

regulation. Hence, the STATCOM is proposed

49

OF SEIG

g device used on

ed on a power

r sink of reactive

ower it can also

evices Usually a

oor power factor

ed to enhance the

50

4.6 DESIGN OF BASIC STATCOM CONTROLLER FOR THE

LOAD VOLTAGE STABILITY OF SEIG

To enhance the load voltage stability, the STATCOM is proposed as the

active VAR supporter. Figure illustrates the STATCOM compensated load voltage

stability system. Even though, the primary purpose of the STATCOM is to support

the load voltage by injecting or absorbing reactive power, it is also capable of

improving the voltage stability. This thesis investigates the performance of

STATCOM based on six pulse voltage sourced converter.

Figure 4.3: STATCOM Controller

STATCOM

SEIG

Load

51

4.7 STATCOM CONTROLLER TO ENHANCE THE TERMINAL

VOLTAGE STABILITY USING D-Q EQUIVALENT MODEL

Figure 4.4: STATCOM controller to enhance the terminal voltage stability using

D-Q equivalent model

52

In general, electrical loads are linear in nature. However, due to the

extensive use of solid-state controllers in different appliances, they draw harmonic

current from AC sources and behave as non-linear loads. Adjustable speed drives

used in pumps, compressors, air conditioner and other domestic appliances such as

TV’s, computers, SMPS and UPS consist of either three-phase or single-phase

rectifiers at the front-end. These non-linear loads draw non-sinusoidal currents from

the generating system, therefore injecting harmonics into the system. An SEIG is an

isolated system, which is small in size and the injected harmonics pollute the

generated voltage.

A dynamic model of an SEIG-STATCOM system with the ability to

simulate varying loads has been developed using a stationary d-q axes reference

frame. This enables to predict the behavior of the system under transient conditions.

The simulated results show that by using a STATCOM based voltage regulator the

SEIG terminal voltage can be maintained constant and free from harmonics under

linear and nonlinear loads.

4.8 SYSTEM CONFIGURATION AND CONTROL SCHEME

The schematic diagram of an SEIG with excitation capacitor, STATCOM,

load and control scheme is shown in Fig. Excitation capacitors are selected such that

the SEIG generates rated voltage at rated speed under no load. The additional demand

for reactive power is fulfilled using the STATCOM under varying loads. The

STATCOM acts as a source of lagging or leading current to maintain the constant

terminal voltage despite variations in load. The STATCOM consists of a three-phase

IGBT based current controlled voltage source inverter, a DC bus capacitor and AC

inductors. The output of the inverter is connected through the AC filtering inductor to

the SEIG terminals. The DC bus capacitor is used as an energy storage device and

provides the self-supporting DC bus of the STATCOM.

53

The control technique is used to regulate the terminal voltage of the SEIG is

based on the generation of source currents. (They have two components, in-phase and

quadrature, with AC voltage.) The in-phase unit vectors (ua, ub and uc) are three-phase

sinusoidal functions, computed by dividing the AC voltages va, vb and vc by their

amplitude Vt. Another set of quadrature unit vectors (wa, wb and wc) are sinusoidal

functions obtained from in-phase vectors (ua, ub and uc). To regulate the AC terminal

voltage (Vt), it is sensed and compared with the reference voltage. The voltage error is

processed in the PI controller. The output of the PI controller (Ismq*) for the AC

voltage control loop determines the amplitude of the reactive current to be generated

by the STATCOM. Multiplication of quadrature unit vectors (wa, wb and wc) with the

output of the PI based AC voltage controller (Ismq*) yields the quadrature component

of the reference source currents (isaq*, isbq* and iscq*). To provide a self-supporting DC

bus for STATCOM, its DC bus voltage is sensed and compared with the DC

reference voltage. The error voltage is processed in another PI controller. The output

of the PI controller (Ismd*) determines the amplitude of the active current.

Multiplication of in-phase unit vectors (ua, ub and uc) with the output of the PI

controller (Ismd*) yields the in-phase component of the reference source currents (isad*,

isbd*and iscd*). The instantaneous sum of quadrature and in-phase components gives

the reference source currents (isa*, isb* and isc*), which are compared with the sensed

line current (isa, isb and isc). These current error signals are amplified and compared

with the triangular carrier wave. If the amplified current error signal is equal to or

greater than the triangular carrier wave, the lower device of the inverter phase is

turned on and the upper device turned off. If the amplified current error signal is equal

to or less than the triangular carrier wave the lower device of the inverter phase is

turned off and the upper device turned on. A non-linear load draws non-sinusoidal

currents which causes harmonics to be injected into the generating system. Under

unbalanced load conditions, SEIG currents may be unbalanced which may cause the

machine to be derated. STATCOM is able to filter out the harmonics and balance the

unbalanced load resulting in balanced and sinusoidal currents and voltages in the

generator.

54

4.9 MODELING OF CONTROL SCHEME OF STATCOM

Different components of the SEIG-STATCOM system shown in Fig. 1 are

modeled as follows. From the three-phase voltages at the SEIG terminals (va, vb and

vc), their amplitude (Vt) is computed as:

Vt={(2/3)(Va2+Vb

2+Vc

2) (4.1)

It (Vt) is filtered to eliminate ripples if there are any present.

The unit vector in phase with va, vb and vc are derived as:

ua=va/Vt; ub=vb/Vt; uc=vc/Vt (4.2)

The unit vectors in quadrature with va, vb and vc may be derived using a quadrature

transformation of the in-phase unit vectors ua, ub and uc as:

Wa=-ub/ 3+uc/ 3 (4.3)

Wb= 3ua+(ub-uc)/2 3 (4.4)

Wc=- 3ua+(ub-uc) )/2 3 (4.5)

Figure 4.5: Simulation model for the system with STATCOM

55

4.10 QUADRATURE COMPONENT OF REFERENCE SOURCE

CURRENTS

The AC voltage error Ver(n) at the nth

sampling instant is:

Ver(n) = Vtref(n)– Vt(n) (4.6)

Where Vtref(n) is the amplitude of the reference AC terminal voltage and Vt(n) is the

amplitude of the sensed three-phase AC voltage at the SEIG terminals at the nth

instant. The output of the PI controller (I*smq(n)) for maintaining constant AC terminal

voltage at the nth sampling instant is expressed as:

I*smq(n) = I*smq(n-1) + Kpa { Ver(n) – Ver(n-1) } + KiaVer(n) (4.7)

Where Kpa and Kia are the proportional and integral gain constants of the proportional

integral (PI) controller. Ver (n) and Ver(n-1) are the voltage errors at the nth and (n-

1)th

instant and I*smq(n-1) is the amplitude of the quadrature component of the reference

source current at the (n-1)th

instant. The quadrature components of the reference

source currents are computed as:

i*saq = I*smqwa; i*sbq = I*smqWb; i*scq = I*smqWc (4.8)

4.11 IN-PHASE COMPONENT OF REFERENCE SOURCE

CURRENTS

The error in the DC bus voltage of the STATCOM (Vdcer(n)) at the nth

sampling

instant is:

Vdcer(n) = Vdcref(n)– Vdc(n) (4.9)

Where Vdcref(n) is the reference DC voltage and Vdc(n) is the sensed DC link voltage of

the STATCOM. The output of the PI controller for maintaining the DC bus voltage of

the STATCOM at the nth

sampling instant, is expressed as:

I*smd(n) = I*smd(n-1) +Kpd{Vdcer(n)–V dcer(n-1)}+KidVdcer(n) (4.10)

I*smd(n) is considered to be the amplitude of the active source current. Kpd and Kid are

56

the proportional and integral gain constants of the DC bus PI voltage controller. The

in-phase components of the reference source currents are computed as:

i*sad= I*smdua; i*sbd = I*smdub ; i*scd= I*smduc (4.11)

4.12 TOTAL REFERENCE SOURCE CURRENTS

The total reference source currents are the sum of the in-phase and quadrature

components of the reference source currents as:

i*sa = i*saq +i*sad (4.12)

i*sb = i*sbq +i*sbd (4.13)

i*sc = i*scq +i*scd (4.14)

4.13 PWM CURRENT CONTROLLER

The total reference currents (i*sa, i*sb and i*sc) are compared with the sensed

source currents (isa, isb and isc). The ON/OFF switching patterns of the gate drive

signals to the IGBTs are generated from the PWM current controller. The current

errors are computed as:

isaerr = i*sa – isa (4.15)

isberr = i*sb – isb (4.16)

iscerr = i*sc – isc (4.17)

These current error signals are amplified and then compared with the triangular

carrier wave. If the amplified current error signal is greater than the triangular wave

signal switch S4is ON and switch S1 is OFF, and the value of the switching function

SA is set to 0. If the amplified current error signal corresponding to isaerr is less than

the triangular wave signal, switch S1 is ON and switch S4 is OFF, and the value of SA

is set to 1. Similar logic applies to the other phases.

57

4.14 MODELING OF STATCOM

The STATCOM is a current controlled VSI and is modeled as follows:

The derivative of its DC bus voltage is defined as:

pvdc = (i caSA + icb SB + i ccSC)/ Cdc (4.18)

Where SA, SB and SC are the switching functions for the ON/OFF positions of the

VSI bridge switches S1-S6.The DC bus voltage reflects the output of the inverter in

the form of the three-phase PWM AC line voltage eab, ebc and eca. These voltages may

be expressed as:

eab= vdc (SA- SB) (4.19)

ebc= vdc (SB-SC) (4.20)

eca = vdc, (SC-SA) (4.21)

The volt-amp equations for the output of the voltage source inverter

(STATCOM) are:

va=Rfica+Lfpica+eab-Rficb-Lficb (4.22)

vb=Rficb+Lfpicb+ebc-Rficc-Lfpicc (4.23)

ica+icb+icc=0 (4.24)

The value of icc from eqn (24) is substituted into eqn. (23) which results in:

vb=Rficb+Lfpicb+ebc+Rfica+Lfpica+Rficb+Lfpicb (4.25)

Rearranging the eqn (4.22) and eqn (4.25) results in:

Lfpica-Lfpicb=va-eab-Rfica+Rficb (4.26)

Lfpica+2Lfpicb=vb-ebc-Rfica-2Rficb (4.27)

58

Hence, the STATCOM current derivatives are obtained by solving eqns. (4.26) and

(4.27) as:

pica={(vb-ebc)+2(va-eab)-3Rfica}/(3Lf) (4.28)

picb={(vb-ebc)-( va-eab)-3Rficb}/(3Lf) (4.29)

4.15 MODELING OF SEIG

The dynamic model of the three-phase SEIG is developed a using stationary d-

q axes references frame, whose voltage-ampere equations with usual notation are,

[v]=[R][i]+[L]p[i]+wg[G][i] (4.30)

from which, the current derivatives can be expressed as:

P[i] = [�]��{[v]-[R][i]-wg[G][i]} (4.31)

where [v] = [vdsvqsvdrvqr] T; [i] = [idsiqsidriqr] T

[R]=diag[Rs Rs RrRr]

[L]=

LrrLm

LrrLm

LmLss

LmLss

00

00

00

00

; [G]=

−

00

00

0000

0000

LrrLm

LrrLm (4.32)

Where Lss=L1s+Lm and Lff=L1r+Lm

The electromagnetic torque balance equation of the SEIG is defined as:

Tshaft=Te+J(2/P)p wg (4.33)

The derivative of the rotor speed of the SEIG from eqn. (33) is:

Pwg={p/(2J)}(Tshaft-Te) (4.34)

where the developed electromagnetic torque of the SEIG is expressed as :

Te=(3p/4)Lm(iqsidr-idsiqr) (4.35)

The shaft torque of the prime mover is considered a function of speed as:

Tshaft=(K1-K2wg) (4.36)

Where Tshaft is the shaft torque which is decided by the drooping characteristic of the

59

prime-mover. Constants k1(3370) and k2 (10) are fixed for a particular type of prime

mover. The SEIG operates in the saturation region and its magenetizing characteristic

is non-linear in nature. Therefore, the magnetizing current should be calculated at

each step of integration in terms of the stator and the rotor dq axes currents as:

Im={(�� + ���) + ��� + ���� }�� /√2 (4.37)

Magnetizing inductance is calculated from the magnetization characteristic

expressed using the curve between Lm and Im. The relation between Lm and Im is

obtained by a synchronous speed test for the SEIG under test [13] and can be written

as:

Lm=0.205+0.0053Im-0.0023 Im2+0.0001Im

3 (4.38)

60

Figure 4.6: Schematic of the proposed STATCOM system

61

Figure 4.7: STATCOM based SEIG

62

4.16 RESULTS

It is observed that SEIG terminal voltage remains constant in spite of

application and removal of loads. Generator current increases and decreases with

application and removal of three-phase load respectively to provide active power to

the load. The STATCOM supplies the reactive power to the load as well as generator

and balances the SEIG system. Therefore STATCOM current increases and decreases

with application and removal of loads. An under-shoot in DC bus voltage at

application of load is observed which shows instantaneously the energy transfer

from STATCOM DC bus to SEIG to maintain the terminal voltage constant and an

over-shoot in DC bus voltage at removal of load is also observed.

Figure 4.8: Output Voltage, Torque, rotor speed versus time in X axis

63

Figure 4.9: Stator flux, Stator Current, Torque Power versus time in X axis

64

Figure 4.10: Signal Builder

65

Figure 4.11: Dynamic Response of STATCOM Voltage

66

Figure 4.12: Initial input mean value 1, initial input mean value2 and voltage

67

4.17 CONCLUSION

STATCOM is proposed to enhance the load voltage stability from the results it

is clear that the load voltage variation is within the tolerable limit. Load voltage

stability using STATCOM d-q axis method and Basic STATCOM Controller is

obtained. The STATCOM is proposed as the active VAR supporter.

A mathematical model of three-phase SEIG with STATCOM based voltage

regulator under resistive and reactive loads. It is concluded from the simulated results

that the STATCOM acts as an ideal voltage regulator and load- balancing device,

which maintains the SEIG voltage constant and balances the SEIG system at varying

balanced and unbalanced loads.

.