20

CHAPTER 2

CONVENTIONAL STATCOM

2.1 COMPENSATION PRINCIPLE OF STATCOM

STATCOM is a converter type FACTS device, which generally

provides superior performance characteristics when compared with

conventional compensation methods employing TSCs and TCRs.

STATCOM based on VSC topology utilize either GTO or IGBT devices. A

functional model of a STATCOM is shown in Figure 2.1. In its simplest

form, the STATCOM is made up of a coupling transformer, a VSC, and a

DC energy storage device. The energy storage device is a relatively small

DC capacitor, and hence the STATCOM is capable of only reactive power

exchange with the transmission system.

Figure 2.1 Functional model of a STATCOM

21

If a DC storage battery or other DC voltage source were used to

replace the DC capacitor, the controller can exchange real and reactive

power with the transmission system, extending its region of operation from

two to four quadrants.

Inorder to understand the compensation principle of

STATCOM, two sources V1 with a phase angle of and V2 with a phase

angle of 0° connected together by means of an inductive link of impedance

(R +j X) ohms as shown in Figure 2.2 are considered. In the STATCOM

principle, the source V1 is the power system voltage at the bus where the

STATCOM is connected, V2 is the AC voltage generated by the

STATCOM inverter, X is the reactance in the line, R is the total loss

resistance in the link comprising of the winding losses in the link inductor,

interface magnetic, the inverter switches and snubber etc. Assuming is

small and R << X, if V2 represents the STATCOM condition and if the

active power flowing into the source V2 is constrained to be zero, the power

delivered by the source V1 and the reactive power delivered to the link by

the source V2 will be given by the following equations .

Figure 2.2 Single line diagram

Active power delivered by V1 is

WR

VP 22

1 (2.1)

22

Reactive power delivered by V2 is

varRVVQ 21 (2.2)

Also,

varX

)VV(VQ 122 (2.3)

where the powers P, Q and voltages V1, V2 have phase values.

These relations can be used upto about 20° for . Active power drawn from

the source V1 is independent of sign of phase angle (V1 supply losses only

in R and not at V2) whereas the reactive power delivered by V2 is directly

proportional to the phase angle. The phase angle of V1 with respect to V2 is

varV

VVXR

1

12 (2.4)

Thus Q is proportional to or equivalently to (V2-V1). In the

STATCOM, the required AC voltage source V2 is generated by inverting

the DC capacitor voltage. By making the output voltages of the converter

lag the AC system voltages by a small angle, the converter absorbs a small

amount of active power from the AC system to balance the losses in the

converter. But if the active power which goes into the inverter from the

source is kept zero, the initially charged capacitor will soon discharge

down to zero due to supply of active power losses in the inverter. The DC

side voltage will remain constant if the power drawn from source is just

enough to supply all the losses which take place everywhere due to the

flow of demanded reactive current.

23

The DC side capacitor voltage is,

RX1

kVV 1

dc (2.5)

where V1 is the rms phase voltage of AC mains, k is a constant,

which also absorbs the modulation index of PWM process in the inverter.

Through appropriate switching sequence, VSC transforms DC voltage at its

DC terminals into an AC voltage of controllable frequency, magnitude and

phase angle at its terminals. The output voltage could be fixed or variable,

at a fixed or variable frequency. For FACTS application purposes, it is

always assumed that the output voltage waveform has a fixed frequency

equal to the fundamental frequency of a power system to which the

converter is connected, as high voltage and power harmonics could create

many problems.

2.1.1 Power Exchange in STATCOM Application

The amount and type of power exchange between the

STATCOM and the PCC can be adjusted by controlling the magnitude of

STATCOM output voltage with respect to the system voltage as shown in

Figure 2.3. The STATCOM is connected at the PCC where Vo is the

magnitude of STATCOM output voltage, pccV is the magnitude of the

system voltage at PCC and Xs is the equivalent reactance between

STATCOM and the system. The reactive power supplied by the

STATCOM is given by:

pccs

pcco VX

VVQ (2.6)

24

STATCOM supplies or absorbs reactive power based on the Q

value either positive or negative as shown in Figure 2.3, A STATCOM can

provide full capacitive output current at any system voltage, practically

down to zero (Sidhu et al. 2005).

Figure 2.3 Operating modes of STATCOM

The maximum capacitive or inductive reactance generated by

STATCOM decreases linearly with voltage with constant current.

Therefore, the capacitor does not change its voltage and this shows that the

capacitor does not play any role in the reactive power generation

(Blaabjerg et al. 2004).

Figure 2.4 (a) shows the STATCOM output current and voltage

diagram where phasors IQ and IP represent the AC current Iac components

that are in quadrature and in phase with the AC system voltage Vac,

respectively. The DC current Idc and voltage Vdc are shown in

Figure 2.4 (b). If the losses in the STATCOM circuit are neglected and it is

assumed that real power exchange with the AC system is zero, then the

25

active current component IP and DC current Idc are equal to zero and the

AC current Iac is equal to the reactive component IQ (Gyugyi 1994).

Assuming that the AC current flows from the STATCOM to the AC

system the AC current magnitude can be calculated as:

XVVI acout

ac (2.7)

where Vout and Vac are the magnitudes of the STATCOM output

voltage and AC system voltage respectively, while X represents the

coupling transformer leakage reactance.

(a) at AC terminals

(b) at DC terminals

Figure 2.4 STATCOM phasor diagrams (Gyugyi 1994)

26

In Figure 2.4 (a), if both real and reactive power flows are

positive, then the power is absorbed by the STATCOM while negative

means that the power is injected by the STATCOM. The corresponding

reactive power exchanged can be expressed as follows:

XcosVVVQ acout

2out (2.8)

where the angle is the angle between the AC system bus

voltage Vac and the STATCOM output voltage Vout. Figure 2.5 (a) shows

the case when the angle is equal to zero and there is no real power

transfer between the STATCOM and AC system. The STATCOM absorbs

real power from the AC system, if the STATCOM output voltage is made

to lag the AC system voltage, as shown in Figure 2.5 (b). The amount of

exchanged real power is very small in steady state; hence, the angle is

also small which is approximately equal to 2°. The real power exchange

between the VSC and the AC system can be calculated using:

XsinVV

P outac (2.9)

The energy going into the converter at its AC terminals has to

be equal to the energy lost due to switching, plus the energy stored in DC

capacitor and vice versa, i.e.,

dclossesac PPP (2.10)

27

(a) steady-state operation (b) charging of DC capacitor

(c) discharging of DC capacitor

Figure 2.5 STATCOM phasor diagrams

The converter draws a rippled current from the DC capacitor

because the converter output voltage waveform is not perfectly sinusoidal;

the amount of harmonics depends on the modulation strategy used to

obtain the output waveforms. Since the voltage waveform is not perfectly

sinusoidal, the instantaneous power going in/out of converter contains

ripples, resulting in a rippled DC current. While the average value of the

DC current is zero in steady state, during transients this current follows the

changes in the output waveforms to maintain the desired mean DC voltage.

2.2 STATCOM CONTROL STRATEGY

To take effect of the bidirectional flow of reactive power, the

STATCOM output voltage should be varied according to requirement of

the reactive power compensation, and this can be accomplished in two

28

ways: by changing the switching angles of the power electronic devices

(i.e. varying the modulation index) while maintaining constant DC

capacitor voltage (direct control) or keeping the switching angles fixed and

varying the DC capacitors voltages (indirect control) (Schauder and Mehta

1993, Tin and Wang 1997). The variation of DC capacitors voltages is

simply achieved by varying the active power transfer between STATCOM

and power system and by adjusting phase angle difference between the

inverter and the line. All these control schemes have their own merits and

demerits. An important condition for an active operation is a constant DC

link voltage, Vdc.

In general, direct control is preferred where very fast voltage

control is required (absence of capacitor dynamics) makes the response fast

but THD of converter voltage varies with modulation index, thereby

producing more harmonic distortion in the voltage at low modulation

index. On the other hand, indirect control operation is slow as AC output

voltage of STATCOM varies according to variation of DC capacitor

voltages (presence of capacitor dynamics make the response slow) but

harmonic injection in the power system bus voltage can be kept at a very

low level by operating the inverter at a high modulation index where THD

of converter voltage is least.

In the proposed work, a PWM based control technique is

investigated since that real power flow in and out of the STATCOM can be

controlled by controlling the DC bus voltage, while the reactive power

generation/absorption is directly proportional to the magnitude of the

STATCOM output voltage. In this chapter, a new technique based on

interpolation firing control scheme is analyzed and implemented.

29

2.2.1 Conventional PWM Control Technique

The aim of the PWM control scheme is to maintain constant

voltage magnitude at the PCC, under system disturbances. The control

system only measures the rms voltages at the PCC, i.e., no reactive power

measurements are required. With these converters, the AC output voltage

can be controlled by varying the width of the voltage pulses. With PWM

technique, the output of each converter pole is switched several times

during a fundamental cycle between the positive and negative terminals of

the DC source. PWM requires a considerable increase in the number of

switch operations, thereby it generally increases the switching losses of the

converter. However, the always increasing switching frequency of modern

solid-state power switches used in FACTS controllers made possible the

use of PWM in high power applications.

The implementation and the design for the PWM controller are

simpler than for phase control, due to the easy separation of the active and

reactive components of the STATCOM output current without a need for a

d-q decomposition. For a STATCOM with PWM-based controller, the

fundamental component of converter output voltage can be easily

controlled from the maximum value to zero, independently in each phase.

Therefore, it is possible for the PWM converter to control each phase

current independently. This is not possible for phase-control based

STATCOM due to fact that all three phases are directly proportional to the

DC voltage, without possibility to adjust them independently. The basic

structure of a STATCOM with PWM-based voltage controls is depicted in

Figure 2.6 (Schauder et al. 1995).

30

Figure 2.6 Block diagram of a STATCOM with PWM

voltage control

In the block diagram, the Phase Locked Loop (PLL) provides

the basic synchronizing signal which is the phase angle of the bus voltage,

. It is obtained from the zero crossing of the bus voltage. In the case of a

sudden change in the power system, such as cyclic loads, it takes about half

a cycle of voltage for the PLL to be synchronized with the new voltage

phase angle, plus the signal processing delay. During this time the

STATCOM operates at the previous phase angle, while the bus voltage

phase has changed. Depending on the amount of phase angle change and

whether it is increased or decreased an uncontrolled real power, and

therefore reactive power exchange would occur between the STATCOM

and the transmission line during this inherent PLL delay. PWM controls

are becoming a more practical option for transmission system applications

of VSC-based controllers.

31

2.3 INTERPOLATION FIRING CONTROL SCHEME

Power electronic circuits present a special problem for

electromagnetic transient’s simulation (Gole et al. 1997). A power

electronic switch may open and close several times in a cycle. Finite time-

step Electro Magnetic Transient DC (EMTDC) type programs usually

allow such switching to occur on integral multiples of the time-step which

results in slower simulation process. On the other hand, the proposed finite

time-step approach usually results in faster simulation compared to

programs using a variable time step. Interpolation is a means for accurately

modeling the exact switching instant in transient and simulation programs

without using a time step and allows for an exact representation of the

switching event, without having to use smaller time step.

2.3.1 Generation of Interpolation Firing Pulses

Interpolation can be used in the generation of the firing pulses

for power electronic circuits as shown in Figure 2.7.

Figure 2.7 Firing angle measurement with and

without interpolation

32

The firing angle ( ), the time from the zero crossing of the

forward biasing voltage to the time at which the firing pulse is issued, is

generated by the intersection of a timing ramp with the firing angle order

signal. The firing pulse forwarded to the EMTP type algorithm consists of

the pulse itself which can only be synchronous with a time step and the

interpolation correction t2. The additional correction t2 is then passed to

the EMTDC type algorithm so that the above described changes to the

solution process can be made. Similarly, linear interpolation allows for a

much more accurate measurement of by using the additional correction

timestamp t1 as shown in Figure 2.7.

Although interpolation requires a few additional computations

in the algorithm, it has been shown that this only introduces a small

computation time overhead. Using the proposed interpolation approach it is

possible to get very precise switching times without response to a small

time-step and large processing time. The advantages of interpolation

include

Allows simulation to be run with a larger time step without

affecting accuracy

Results in correct theoretical harmonics generated by switching

devices like FACTS controllers because each switch device fires

at the correct instant

Avoids voltage fluctuations in STATCOM and VSC circuits due

to incorrect back-diode turn on times. Snubber circuits are

required to control these spikes in fixed time step programs

33

Avoids numerical instabilities that can occur due to

arrangements of multiple switching devices in close proximity

Results in more accurate models of non-linear devices like surge

arresters, especially in energy calculations

Models accurately the low frequency damping and harmonics of

switching devices interacting with Sub-Synchronous Resonance

(SSR) effects in machines

2.4 STATCOM MODELING

The control blocks should be modeled in great detail,

representing all necessary firing pulses for each of the valves. The models

should be simplified to reduce computational time and should accurately

capture the controller behavior at the desired fundamental frequency

(Jovcic et al. 1999). The controls should be represented with all functions

that are relevant to the proposed study. Several authors have demonstrated

the importance of realistic modeling of FACTS controllers for steady state

and transient stability studies (Nelson et al. 1995). The STATCOM have

been typically modeled as ideal VSC or Current Source Converter (CSC)

without operating and control limits, i.e. capable of generating or absorbing

unlimited amounts of reactive power. More accurate dynamic and steady

state models, similar to the ones presented in this thesis have been

proposed based on power balance principles between the AC and DC sides

of the VSC. However, these models have been discussed at a theoretical

level and only tested with conventional PWM. The STATCOM proposed

in this chapter are tested in a more realistic power system environment,

which clearly identify the need for representing properly the operating and

control limits.

34

Schauder and Mehta (1993), provide a mathematical modeling

and control approach for a VSC connected to a stiff utility system.

However, in the proposed system, for accurate analysis of the interactions

of the VSC with the utility system, network dynamics and dynamics of the

PLL need to be taken into account and included in the system model.

Vc

Vb

Va

Vsa

Vsb

Vsc

VSCVdc

C

idc

ia

ib

ic

Figure 2.8 Equivalent circuit of STATCOM

Figure 2.8 shows the simplified equivalent circuit of the

STATCOM. In this work, the VSC is modelled as a 6-pulse VSC with a

DC link capacitor. It contains a DC link capacitor, a VSC, and series

inductances in the three lines connecting to the system bus.

These inductances account for the leakage inductances of the power

transformer, as well as the inductances used for filtering the STATCOM

AC side currents. The circuit also includes resistance in series with the AC

lines to represent the VSC and transformer conduction losses.

Under balanced conditions, the AC side circuit equations in

Figure 2.8 can be written in a synchronously rotating reference frame using

the d-q transformation (Kundur 1994).

35

sddqsdd V

L1V

L1ii

LR

dtdi (2.11)

sqqdsqq V

L1V

L1ii

LR

dtdi

(2.12)

where R is the equivalent resistance representing the power

losses, is the synchronously rotating angle speed, (id, iq), (vd, vq), and

(vsd, vsq) are the dq components of (ia, ib, ic), (va, vb, vc), and (vsa, vsb, vsc),

respectively. Neglecting the harmonics due to switching and the losses in

the VSC and the transformer, the power balance between the AC and DC

sides of the VSC is given by

dcdcsqsqdsd iV)iViV(23 (2.13)

With correct alignment of the reference frame, the Vsq term is

zero, and hence, the following equation can be written to relate the DC link

voltage to the d-axis current id and d axis component of the power line

voltage Vsd. Assuming that the DC link voltages are balanced, their

dynamics can be represented using Vdc.

dc

dsddc

CV2iV3

dtdV

(2.14)

where C is the total dc capacitance,Vdc is the equivalent total DC

link voltage.

The dynamics of the VSC can be described by a set of first order

Ordinary Differential Equations (ODEs). Since Equation (2.14) is a

nonlinear equation of the DC link voltage, this set of ODEs represents a

36

nonlinear model to be controlled. In terms of the instantaneous variables,

the reactive power balance at the PCC is given by

qqdd iViV23P (2.15)

qddq iViV23Q (2.16)

Aligning the d-axis of the reference frame along the grid voltage

position, qV = 0

ddiV23P (2.17)

qdiV23Q (2.18)

From the above equations, it can be seen that Q can be

controlled through iq and Vdc can be controlled through id.

2.4.1 STATCOM Transient Stability Model

In transient stability modeling, it is typically assumed that the

converter output voltage is a balanced and harmonic free waveform at

fundamental frequency. The VSC can be accurately represented as a

sinusoidal voltage source operating at fundamental frequency. In order to

develop a fundamental frequency balanced model of the STATCOM, a

power balance technique, similar to the one used in (Schauder et al. 1993)

for developing d-q axis controls of VSC based static compensators is used

here. The proposed model allows the representation of different types of

controls and contains all relevant physical variables required to simulate

37

various PWM control strategies. For the STATCOM, instantaneous power

flowing into the converter from the AC bus, when neglecting transformer

losses, may be represented by :

sinX

VVa3P acsh (2.19)

Figure 2.9 Transient stability model of a STATCOM

Assuming balance fundamental frequency voltages, the

controller can be accurately represented in transient stability studies using

the model shown in Figure 2.9. The p.u Differential - Algebraic Equations

(DAE) corresponding to this model can be summarized as follows:

ref,dcref,acdcaccc

c

V,V,V,V,k,,Xfk

x(2.20)

dcC

acdc V

CR1sin

XCVka3V (2.21)

38

C

2dc

acac RV-)-cos(IV30 (2.22)

cosX

VVkaX

VQ

sinX

VVkaP0

acdc2

ac

acdc

(2.23)

where Xc and fc stand for the internal control variables and

equations respectively and vary depending on which STATCOM internal

structure is used.

2.4.2 STATCOM Steady State Model

The steady state or power flow model can be obtained from the

stability model equations by replacing the corresponding equations with

the steady state equations for the DC voltage and the steady state V-I

characteristics. From Figure 2.9, the steady state equations are

acSLrefac,ac IXV-V0

dcC

ac VCR

1sinCX

Vka30

C

dcac

RVsin

XVka30

cosX

VVkaX

VQ

sinX

VVkaP0

acdc2

ac

acdc

(2.24)

39

The above equations define the real and reactive power that

STATCOM exchanges with the AC system. During steady state, the

system operation can be described using the phasor diagram shown in

Figure 2.10.

Figure 2.10 Phasor diagram of a PWM converter

The real and reactive power are represented by

LE-cosV3EQ

LVsin3EP

SS

S

(2.25)

Equations (2.24) and (2.25) suggest that a VSC can generate a

desired, fixed valued reactive power when the power system network

supplies the variable real power demanded by any load. As shown in

Figure 2.10, this can be done by keeping “V cos ” constant and varying

“V sin ”. Thus by controlling the magnitude and phase voltage V, the

steady state control of active and reactive power is possible. However, the

40

equations fail to explain simultaneous control over real and reactive power,

which is required for a dynamic operation of a STATCOM.

2.4.3 Dynamic d-q Modeling of STATCOM

A variable DC link will introduce undesirable fluctuations in the

magnitude and phase of PWM voltages generated by the converter. It will

cause the active and reactive currents drawn by the converter to vary from

the desired values. This will further introduce additional noise in the DC

link voltage, since these line currents charge and discharge the DC

capacitor. To solve this non-linearity and to achieve fast dynamic response,

an effective dynamic control scheme is needed. The rotating reference

frame d-q theory is first used to obtain a dynamic d-q model of the

converter. General transformation techniques based on the Park’s

transformation are widely used for different types of power system studies.

Assuming that the system has no zero sequence components, all currents

and voltages can be uniquely transformed into the synchronously rotating

d-q reference frame.

Instantaneous voltages and currents on the three phase

coordinates can be transformed into the quadrature , coordinates by

Clarke transformation (Paul C. Krause 1986) as shown in Figure 2.11.

Since in a three-phase three-wire system neutral current is zero, the zero

sequence current does not exist. Hence using Clarke’s transformation, the

voltages and currents in the reference frame can be expressed as

B

Y

R

1

vvv

T

v

vv

(2.26)

41

where T1 is the transformation matrix .With this three phase to

two phase transformation the instantaneous powers in both coordinate

systems must remain the same i.e.

Pdq = PRYB (2.27)

Figure 2.11 Three phase to two phase transformation

To obtain this condition the transformation matrix T1 is

multiplied by 32 .

21

21

21

23

230

21

21-1

32T1

(2.28)

The above system model is still complex and response is slow.

Paul C. Krause (1986) proposed a transformation from stationary to a

fictitious rotary reference frame with a speed of rotation r. Rotating

reference frame is used because it offers higher accuracy than stationary

frame-based techniques (Molina and Mercado 2006). Hence the voltages in

42

reference frame can further be transformed into rotating d-q reference

frame as in Figure 2.12 where r is the angular velocity of the d-q

reference frame.

With this transformation

VV

cossinsin-cos

VV

rr

rr

q

d (2.29)

whererr

rr2 cossin

sin-cosT (2.30)

The above equations represent the dynamic d-q model of the

VSC in a reference frame rotating at a speed of r. The current components

in the d-q reference frame can be similarly obtained using the to d-q

transformation matrix. The STATCOM uses the voltage (Vabc) and the

current (Iabc) measurements to check the stability of the system. These two

variables have been transformed into a synchronously rotating reference

frame by using Clarke (V and I ) and Park (Vdq and Idq) transformations.

Figure 2.12 to d-q transformation

43

To transfer the abc variables to a dqo frame a transformation

matrix is with d and q currents and voltages components of STATCOM

proportional to its real and reactive power components respectively.

Thus the control of each current component regulates the corresponding

power component.

c

b

a

iii

34sin3

2sin03

4cos32cos1

32

ii

(2.31)

ii

cossin-sincos

ii

q

d (2.32)

cs

bs

as

ds

qs

iii

23

23-0

21-2

1-0

32

i

i(2.33)

The measured variables are compared with the reference

variables ( ,dqV ref), ( ,dqI ref) and the variations are supplied to the system.

By comparing the measured voltage with the reference value, the necessary

reactive current ( ,qi ref) is supplied to the system to recover the system

voltage drop. The reactive current injected is controlled by a control

scheme to obtain the specified voltage at the grid side. The DC link voltage

Vdc is compared with its reference value and the variation is used to supply

the necessary active current ( ,di ref) to the system to compensate for the

STATCOM losses.

44

2.5 MITIGATION TECHNIQUE REALIZATION IN PSCAD/

EMTDC

To validate the performance of the STATCOM under PWM

control, a test system, introduced has been implemented in the EMTP.

In this chapter, PSCAD (Power System Computer Aided Design)/EMTDC

software is used to simulate and analyze the mitigation technique.

The basic electronic block of the proposed system shown in Figure 2.13 is

a VSC that converts a DC voltage at its input terminals into a three-phase

set of AC voltages at fundamental frequency with controllable magnitude

and phase angle using interpolation firing scheme.

Figure 2.13 Functional model of the proposed system

The three-phase 2-level six pulse converter is connected by an

appropriate magnetic circuit into a power system with inductive loads to

meet practical harmonic, current and voltage rating requirements.

45

2.5.1 STATCOM Control in Simulation Program

The construction of the control blocks for the control circuit of

voltage control loop in PSCAD is shown in Figure 2.14.

Figure 2.14 Control scheme implemented in PSCAD / EMTDC

In the operation of the control circuit, the error amplifier used is

a PI controller, and the gain of this PI controller is adjusted by the slider

error ramp. In this scheme the measured reactive power and the rated

reactive power values are given to the divider component along with the

measured p.u voltage. The voltage output signal is given a droop of 3% to

pass through the filter circuit. At the beginning of simulation the reference

voltage is ramped and compared with the filtered voltage through a

summing junction. The comparator output signal is fed to the PI controller

and whose output is the angle order which represents the required shift

46

between system voltage and the voltage generated by STATCOM.

The shift also determines the direction and amount of real power flow.

Figure 2.15 (a) Generation of triangular pulses

Figure 2.15 (b) Generation of sine wave pulses

47

PWM control is shown in two parts in which the part 1 is;

Generation of triangular waveforms synchronized with system ac voltage

and the part 2 is; Generation of reference waveforms synchronized with

system ac voltage and shifted by the angle order. The modulating angle is

applied to the PWM generators in phase A. The angles for phases B and C

are shifted by 240 and 120 degrees respectively. The generation of

triangular wave forms synchronized with system AC voltage is obtained

using PWM control scheme. The PSCAD/EMTDC circuit model

developed for this PWM technique is shown in Figure 2.15 (a) and

Figure 2.15 (b).

In this scheme of control the parameters of PI controller is given

to the PLL block which generates a ramp signal that varies between 0 and

360°, synchronized or locked in phase, to the input voltage. The gains of

the PLL are controlled using sliders named GpPLL and GiPLL in the

control panel with user interface. The carrier wave is generated by using

PLL block.

The value is multiplied by the carrier frequency along with a

modular factor. Using straight line approximation by a non-linear transfer

characteristic component the triangular waveform signals TrgOn and

TrgOff are generated which are to be synchronized with system AC

voltage.

The triggering pulses for the GTOs used in the VSC are

generated by the Sinusoidal Pulse Width Modulation (SPWM) technique.

The output of the control circuit is added with the output of another PLL

block and the resultant is applied to a sine block which operates on

6-dimensional arrays to get the sinusoidal reference wave signals

RSgnOn and RSgnOff. Both these triangular signals and reference signals

48

are compared to generate the triggering pulses for the power electronic

devices used in the power circuit with interpolation. Two sets of reference

and triangular signals are needed, one set for turning on and the second

one for turning off as shown in Figure 2.15 (a) and Figure 2.15 (b).

Two signals are being sent to each switch, the first signal turns on or off,

the second signal determines an exact moment of switching and is used by

interpolation procedure which allows for switching between time steps.

2.5.2 Interpolation Firing Circuit

EMTDC uses an interpolation algorithm to find the exact instant

of the event if it occurs between time steps. The logic and control blocks

used for construction of the interpolation firing circuit is shown in

Figure 2.16. The process of incorporating interpolation into logic

components is obtained as shown in Figure 2.17.

Figure 2.16 Logic and control blocks in interpolation

49

Firstly the pulse itself which by necessity must be synchronous

with a time step and secondly the time stamp t. The implementation of

logic gates should propagate properly.

Figure 2.17 Interpolation firing component

The interpolation firing component returns the firing pulse and

the interpolation time required for an interpolated turn-ON (or turn-OFF)

of devices, in the form of a two-element array. The first output element is

a 0 or 1 and represents the actual gate control pulse. The second is

information regarding the interpolated switching time. The output is based

on a comparison of High (H) and Low (L) input signals. The L input is

normally a firing angle order and the H input is from a Phase-Locked

Oscillator (PLO). An input signal comparison is provided to the electronic

device for the OFF signal as well.

50

2.6 IMPLEMENTATION OF VSC BASED STATCOM

IN PSCAD

To demonstrate the effect of the power system on the

STATCOM stability, a circuit model for STATCOM is developed using

PSCAD simulation software and the corresponding results are presented.

In simulations such as VSCs or other FACTS devices, care must be taken

in ensuring that the observed losses are realistic. The interpolation

algorithm is automatically invoked during all naturally commutated turns

ON and turns OFF events in order to calculate the exact instant of

switching. However the device turns ON or OFF using a gate signal is not

interpolated unless specifically selected in the input parameters. The user is

provided a choice to interpolate the incoming gate signal. The capacitance

value remains fixed throughout the simulation.

The simulation circuit component is a compact representation of

a DC converter, which includes a built in 6-pulse converter, an internal

PLO, firing and valve blocking controls and firing angle measurement.

It also includes built in RC snubber circuits for each thyristor.

The transformer component is the equivalent of three, 1-phase, 2-winding

transformer connected in a 3-phase bank, where the user can select the

winding interconnections to be Y or on either side. Modeling of

STATOM power circuit and control circuit is done in PSCAD simulation

package in order to verify the simulation results. An improved solution

method of PSCAD is obtained by incorporating instantaneous switching

into the interpolated switching algorithm. The test system is simulated with

and without STATCOM in PSCAD / EMTDC simulation environment and

the performance waveforms are compared as shown from Figure 2.18 to

Figure 2.25.

51

Figure 2.18 Load voltage wave form without STATCOM

Figure 2.19 Load voltage wave form with STATCOM

Figure 2.20 Real power consumption without STATCOM

Vol

tage

(kV

)

(a) Time(s)

(b)Time(s)

Vol

tage

(kV

)R

ealP

ower

(MW

)

Time(s)

52

Figure 2.21 Real power consumption with STATCOM

Figure 2.22 Reactive power consumption with STATCOM

Figure 2.23 Reactive power consumption without STATCOM

Rea

lPow

er(M

W)

Time(s)

Rea

ctiv

ePo

wer

(Mva

r)

Time(s)

Rea

ctiv

ePo

wer

(Mva

r)

Time(s)

53

Figure 2.24 Capacitor voltage wave form

Figure 2.25 Reactive power injected by STATCOM

2.7 DISCUSSION ON RESULTS

The conventional STATCOM is analyzed and the circuit model

for the system is developed using the blocks available in PSCAD.

The VSC STATCOM is simulated with RL loads and the results are

presented. The dynamic performance of the shunt VSC connected to the

system proves that the voltage profile is transiently increased causing a

change of the VSC operating point from reactive power absorption to

generation.

Vdc

(kV

)

Time(s)

Time(s)

Rea

ctiv

ePo

wer

(Mva

r)

54

The transient response is fast and the voltage returns to its rated

value after about 0.1 sec, when load voltage is very close to the

reference value i.e. 80% of its rated value (1 p.u)

The STATCOM absorbs reactive power in order to get the

voltage back to reference value

The regulated rms voltage shows a reasonably smooth profile

where the magnitude of the transient overshoots is kept with in

10% with respect to the reference voltage. These transients last

for two to three cycles

In general the voltage sources take a few seconds to stabilize the

line voltage and after that it reaches the steady value

The reactive power supplied by STATCOM goes from leading

to lagging resulting in a limiting of the over voltage

From the waveforms of power flow in the line it can be

observed that the power transfer capability of the line is also

improved

When VSTAT > Vac, STATCOM injects reactive power, so the

current flows from STATCOM to line

When VSTAT < Vac , STATCOM absorbs reactive power ,current

flows from STATCOM to line

When VSTAT= Vac, STATCOM does not work

55

2.8 CONCLUSION

In this chapter a complete transient stability and steady state

model for the conventional VSC based STATCOM has been implemented

and the simulation results are verified and found to be satisfactory.

A six-pulse STATCOM, using a conventional 2-level VSC is successfully

modeled in the EMTP, including a detailed representation of the control

blocks. Also, the PWM based STATCOM is modeled in the EMTP using

one VSC. The presented time-domain simulations in EMTP verify the

adequate operation of the designed controller, demonstrating successful

application of the PWM controller with interpolation firing scheme. Also it

has been observed that the power transfer capability in the line and the

voltage profile are improved by incorporation of STATCOM in the line.

This system suffers from the drawback of the usage of bulky transformers,

requirement of filter connections, low EMI susceptibility and output

harmonics due to conventional STATCOM and PWM modulation

technique. To solve these problems a modular CMC controller topology is

proposed and analyzed in detail in the next chapter.