i

ACKNOWLEDGEMENT

At first, our humble honor goes to our supervisor Prof. Dr. Indraman Tamrakar for his perspective

guidance which helped us greatly to strive in right direction with this project.

We would like to express our gratitude to the department of electrical engineering for providing us the

opportunity to do a final year project on the topic of interest.

Our thankfulness also goes to Associate Prof. Kumudini Koirala, head of department of electrical

engineering.

At last, we’d like to express our deep appreciation to our lectures and friends, for endless support

and help from the inception of this project till date, whether directly or indirectly.

ii

TABLE OF CONTENT

1. Introduction……………………………………………………………………………..……...1

2. Literature Review…………………………………………………………………………..…..2

2.1 FACTs……………………………………………………………………………..2

2.1.1 Series Compensators…………………………………………….2

2.1.2 Shunt Compensators…………………………………………….2

2.1.3 STATCOM……………………………………………………….4

2.1.3.1 Operating Principle…………………………………4

2.1.3.2 Control Strategy………………………………….…5

2.1.3.3 STATCOM with hysteresis band current control....6

3. Objectives……………………………………………………………………………………….9

4. Methodologies…………………………………………………………………………………..9

5. Progress till date……………………………………………………………………………….10

5.1. Simulation Model of the Proposed System………………………………………………10

5.1.1 Iq Generator……………………………………………………...10

5.1.1.1 Introduction………………………………………...10

5.1.1.2 Operation…………………………………………...10

5.1.2 Id Generator………………………………………………………10

5.1.2.1 Introduction…………………………………………10

5.1.2.2 Operation……………………………………………10

5.1.3 Iref Generator……………………………………………………...11

5.1.3.1 Introduction………………………………………….11

5.1.3.2 Operation…………………………………………….11

5.1.4 Iref (Phase) Generator……………………………………………...12

5.1.4.1 Introduction…………………………………………..12

5.1.4.2 Operation……………………………………………..12

5.1.5 Hysteresis Band Generator and Comparator…………………….13

5.1.5.1 Introduction………………...………………………...13

5.1.5.2 Operation…...……………...…………………………13

5.1.6 Voltage Source Inverter……………………………………………14

5.1.6.1 Introduction…...………………………………………14

5.1.6.2 Operation……………………………………………...14

5.1.7 Whole System Model…...………………………………………….15

5.2. Simulation Result…………………………………………………………………………….17

6. Future work…………………………………………………………………………………………...20

References…………………………………………………………………………………………………………….21

1

1. INTRODUCTION

Electric power is the rate at which electric energy is transferred by an electric circuit. In case of AC, this

power is of two types as: active power (real power) and reactive power (imaginary power).

i.e. Apparent power = √( ) ( ) ………………………1.1

Active power is used to perform the real work and reactive power is responsible to create electric field in

the capacitor and magnetic field in a coil. The transmission line consists of inductive and capacitive

elements; hence to transfer active power, some reactive power needs to be supplied by the generator. In

addition to transmission line, the reactive load also consumes the reactive power. Every unit that comes in

a transmission system like conductors, transformers, etc. has a maximum apparent power carrying

capacity. Thus, if reactive power flow requirement is lowered as much as possible, then flow of large

amount of active power through the same transmission line can be achieved. To measure this

effectiveness, a term power factor is used, which is defined as

Power factor (pf)

………………………………….…………………………..1.2

Major loads are inductive, which needs to develop magnetic field. For example, motors. Thus, inductors

are called sink and capacitors are called source of reactive power. If the reactive power needed by the

inductive load is supplied using a capacitor bank placed just nearby, the cost of using high capacity units

for the flow of power from the source (generator) can be reduced. This is called reactive power

compensation. But the use of capacitor is not that flexible. Thus, this project aims to build up such a

compensator, not by using a capacitor bank but using power electronics devices. The device is called

STATCOM (Static Compensator). It is shunt compensator. This device uses inverter which transfers the

dc power of a capacitor into ac power which is controlled by a controller.

2

2. LITERATURE REVIEW

2.1 FACTs

Flexible AC Transmission System (FACTS) is a new emerging technology and its principal role is to

enhance controllability and power transfer capability in ac systems. The philosophy of FACTS is to use

power electronics for controlling flow in a transmission network, thereby allowing the transmission line

to be loaded to its full capability. All the systems under FACTS are mainly classified into two groups as:

shunt compensation and series compensation.

2.1.1 Series Compensators

A series compensator, in principle injects a voltage, in series with the line. Even variable impedance

multiplied by the current flown though it represents an applied series voltage in the line. As long as the

voltage is in phase quadrature (900 lead or lag) with the line current, the series compensator supplies or

consumes variable reactive power only. Therefore, a series compensator could be a variable impedance

(such as capacitor or a reactor), which could be an electronic-based device.

2.1.2 Shunt Compensators

In the shunt compensation, a current is injected into the system at the point of connection. This can be

implemented by varying shunt impedance, a voltage source or a current source. As long as the injected

current is in phase quadrature with line voltage, the shunt compensator only supplies or consumes

variable reactive power. There are different types of shunt compensators which are described as follows:

i. Thyristor Controlled Reactor

It consists of a fixed reactor of inductance L and a bidirectional Thyristor switch as shown in

fig. 2.1

YL(𝞪)=

(

)…………….2.1

The compensator can vary the impedance (varying the admittance)

by changing firing angle 𝞪 as shown in above equation and hence

compensating the current.

L

Fig 2.1 Thyristor Controlled Reactor

3

ii. Thyristor Switched Capacitor

It consists of a fixed capacitor, a bidirectional

Thyristor switch and a relatively small surge limiting

reactor L. This compensator’s compensating current

can be varied by changing the firing angle.

iii. Static VAR Compensator

It consists of both capacitor and reactor bank which are

controlled using a bidirectional switch. A controller is used

which senses the voltage using PT and compares with the reference signal. Thus, obtained error is

processed to get required value of firing angle for the control of compensating current.

iv. STATCOM

STATCOM is similar to that of ideal synchronous condenser which can generate or absorbs reactive

power by varying the excitation in the field winding. It is essentially a voltage source converter as shown

in fig.2.3. This device also makes the exchange of both active and reactive power with the system

possible.

Fig 2.3 Advanced Shunt Static VAR Compensator(STATCOM)

PT

Coupling

Transforme

Controller

Voltage Source

Converter

+ Vc -

Vs Vr

Fig 2.2 Thyristor switched Capacitor

C

L

4

2.1.3 STATCOM

2.1.3.1 Operating Principle

STATCOM is a switching converter type static Var generator, which generates or absorbs reactive power

without using capacitor and inductor bank, by various switching pattern within its converter.

Functionally, the operation of STASTCOM is similar to that of an ideal synchronous condenser which

can generates or absorbs the reactive power by varying the excitation in the field winding.

Fig. 2.4 shows a basic configuration of a synchronous condenser. If the dc excitation current If is

increased, magnitude of E will increase and if |E| becomes greater than the |Vs|, then the synchronous

machine will draw the leading current and supplies reactive power to the system. On the other hand, if the

dc excitation current If is decreased, magnitude of E will decrease and if |E| becomes less than the |Vs|,

then the synchronous machine will draw the lagging current and consumes reactive power from the

system. In the similar manner, the STATCOM system shown in Fig.2.5, can generate or consume reactive

power. From the dc voltage source across the dc capacitor Cs, the converter of the STATCOM system

produces a set of controllable single-phase output voltage (V0) with frequency equal to frequency of ac

system voltage. By varying the magnitude of the inverter output voltage (V0), the reactive power

exchange between the inverter and the ac system is achieved in a similar manner to that of the

synchronous condenser.

Exciter

If

I0 X0 = Total reactance of

Tr + machine

Coupling Tr

Vs

E (a,b,c)

Load

Fig 2.4 Synchronous Condenser

+ Vdc -

(Pf)Ref

DC –AC

Converter

Cs

Coupling Tr

Vs

I0 X0 =Reactance of

Tr

V00 (a,b,c)

Load

Pf

Sensor

Controller

(gate signal Gen)

Fig 2.5 STATCOM

5

i) If |V0| > | VS| , then the inverter generates the reactive power

ii) If |V0| < | VS| , then the inverter consumes the reactive power

iii) If |V0| = | VS| , then there is no exchange of reactive power

2.1.3.2 Control Strategy

The reactive power generated by the STATCOM can be varied by changing the inverter output voltage

(V0). Referring to the fig. 2.6, whenever reactive power drawn by the load changes, the voltage across the

terminal of the load also changes. Thus, this change in voltage and dc voltage across the capacitor can be

compared with the reference value of voltage to generate the reference current by the controller at each

instant of time. The reference current is the reactive component of the load current. The microcontroller

now can generate the control signals to control the switching pattern of power IGBTs inside the inverter

so that actual current from the STATCOM i0 tracks the reference current iref generate by the controller.

The actual current from the STATCOM is maintained within a limited hysteresis band.

Fig 2.6 Inverter

Fig 2.7 STATCOM with hysteresis band current control PWM inverter

VS I IL

0

I0

Vdc

PWM

inverter Load Z0

Voltage Sensor

Gate signal generator + VRef

+

Vdc(Ref)

+

r1 PI-2

r2 -

V00

PI-1

-

-

Q1 Q3

Q2 Q4

6

When switch Q1 & Q4 are ON and Q2 &Q3 are OFF, the inverter output current rises. When the current

reaches the upper limit of the reference current, the microcontroller turns OFF Q1 & Q4 and turns ON Q2

& Q3. On doing so, the output current i0 of STATCOM decays. When the current i0 crosses the lower

limit of the reference current, microcontroller turns ON Q1 &Q4 and turns OFF Q2 &Q3 and so on. Thus,

the output current i0 of the STATCOM tracks the reference current evaluated by the microcontroller to

satisfy the reactive power generation and consumption by the load.

2.1.3.3 STATCOM with hysteresis band current control

Hysteresis band current control technique is basically an instantaneous feedback current control method

of PWM, where the actual current through the STATCOM branch continuously tracks the reference

current within a limited hysteresis band. The controller generates the reference current by sensing the ac

terminal voltage and dc voltage across the capacitor in the dc side of the inverter.

When, the reactive power of the load increases, the reactive component of the load current increases. This

increase in reactive current is sensed to obtain the magnitude of the q-axis component (Iq) of the reference

current iref. The dc voltage across the capacitor is sensed and compared with the reference value. The error

signal thus obtained is passed through a PI controller to obtain the magnitude of the d-axis component (Id)

of the reference current iref. The hysteresis band current controller compares the actual currents through

the STATCOM branch with the reference currents and generates the gate signals to turn on and off the

switch pairs T1-T2, T3-T4 and T5-T6 several times in a cycle so that the actual inverter current i0 (actual)

tracks the reference current iref within a limited hysteresis band. Fig.2.8 shows the switching instants and

waveform of the current through the inverter branch along with the reference current for a phase.

7

The reference lagging current required by load is generated by the controller, described by:

Iref =Im Sin (t-900-0) …………………………………………………………………….2.2

Where, 2qI 2

dImI …………………………………………………………………2.3

(-900 -0) = Angle of lag of Iref with respect to VS

The upper and lower limit currents are given by:

Iref (upper) =Im Sin (t-900 -0) + HB…………………….…………………………………………2.4

Iref (lower) =Im Sin (t-900 -0) - HB ……….………………………………………………2.5

When the switch T1 and T4 of the bridge inverter is turned on keeping T2 and T3 off, the inverter output

voltage is Vc and the inverter current (i0) rises up satisfying the following equation.

dt

di0

000L .R i

svVc ……………………………………………………………………2.6

Fig 2.8 Switching instants and waveforms of hysteresis band current control PWM

inverter

Im

V0 = +0.5 Vdc

0

+HB

-HB

ia(Ref) =Im Sin (t-900 -0) Im Sin (t-900 -0) + HB

Im Sin (t-900 -0) - HB

Actual Current (i0 )

V0 = -0.5 Vdc

0 1 2 3

8

When the actual current exceeds a prescribed upper hysteresis band, the upper switch T1 and T4 are turned

off and T2 and T3 are turned on. As a result, the output voltage transits from +Vc to – Vc, and the current

starts to decay satisfying the following equation.

dt

di0

000L .R i

svVc ….……………………………………………………………2.7

When the current crosses the lower band limit, the lower switch T2 and T3 are turned off and the upper

switch T1 and T4 are turned on. The actual current wave is thus forced to track the sine reference wave

within the hysteresis band by sequential switching of the upper and lower switches as shown in Fig.2.5.

The inverter then essentially becomes a current source with peak-to peak current ripple, which is

controlled within the hysteresis band irrespective of Vc fluctuation.

9

3. OBJECTIVES:

To develop and study the single phase STATCOM for reactive power compensation.

4. METHODOLOGIES:

The whole project work consists of simulation, software and hardware work. The works to be

performed are listed in a chronological order as follows:

Step 1: Literature review of the STATCOM and development of strategy to meet the objective,

Step 2: Simulation development of proposed system in MATLAB,

Step 3: Simulation of proposed hardware model in PROTEUS with microcontroller programming

using MPLAB,

Step 4: Hardware fabrication,

Step 5: Testing and debugging.

Step 6: Final documentation of the project.

10

5. PROGRESS TILL DATE:

The literature review of the STATCOM, development of strategy to meet the objective and simulation

development of proposed system in MATLAB has been completed till date.

5.1 Simulation Model of the Proposed System

5.1.1 Iq Generator

5.1.1.1 Introduction

This block is responsible to calculate the amount of reactive

component of load current which is utilized by the Iref generator

block to generate Iref. The sub-system diagram is as shown in

figure 5.1.

5.1.1.2 Operation

This block senses the load current and load voltage. Using these

parameter it calculates the reactive power and rms value of load

voltage. This gives the reactive component of load current using

the equation as follows:

Iq= I SinФ =

…………………………………………5.1

5.1.2 Id Generator

5.1.2.1 Introduction

This block is responsible to calculate the amount of active component of current drawn by the

STATCOM which is utilized by the Iref generator block to generate Iref. The sub-system diagram is as

shown in figure 5.2.

5.1.2.2 Operation

The voltage across the capacitor Vdc is sensed and compared with the reference voltage (Here 440V dc) to

generate the error signal. This error signal is fed to PI controller. The PI controller generates active

Fig 5.1 Iq generator

11

component of current drawn by the STATCOM. This output current is fed to the reference current

generator.

5.1.3 Iref Generator

5.1.3.1 Introduction

This block generates wave of the reference current that has to be supplied by the STATCOM. Hence, a

control system has to be designed to make the output current from the STATCOM track this reference

current with proper switching. The fig 5.3 shows the Iref generator.

5.1.3.2 Operation

The Iref generator has rms value of Iq from the Iq generator and rms value of Id from Id generator as input.

These values are utilized to generate the reference current that has to be supplied by STATCOM. The

generator first generates the rms value of the reference current by the following equation.

Iref (rms) = √ ( ) ( ) ……………………………………………………5.2

This rms value is multiplied by √ to get the peak value of reference current.

Im = Iref(rms) X √ …………………………………………………………………….5.3

The Iref(phase) block generates sine wave with the required phase angle for the Iref. The working of the

Iref (phase) is explained in the following section 5.1.4. Let the phase angle between the reference current

that STATCOM has to supply and the current drawn from the source be Ɵ.

Thus, the reference current is:

Iref = Im Sin(ωt-900-Ɵ)………………………….………………………………….5.4

This is the current that STATCOM has to supply to the load.

Fig 5.2 Id Generator

12

5.1.4 Iref (Phase) Generator

5.1.4.1 Introduction

This block generates the sine wave with the required phase angle of Iref and unit magnitude. The block has

Iq (rms) and Id (rms) as input. The output of the Iref(phase) generator is multiplied with the output of the

Iref generator to get the actual sine wave of the reference current. The fig 5.4 shows the Iref(phase)

generator.

5.1.4.2 Operation

The ratio of Id and Iq is first evaluated. The arc tan of this ratio gives the phase angle Ɵ.

Ɵ = tan-1(

)

……………………………………………………………………………5.5

The ouput of the Iref(phase) generator is:

1.sin (ωt-900-Ɵ)

To evaluate above expression following equation is used:

Sin (ωt-900-Ɵ) = Sin (ωt-90

0).CosƟ + Cos (ωt-90

0). SinƟ

= Sin (ωt-900).CosƟ + Sinωt.SinƟ …………………………………………….5.6

It can be clearly seen that the above expression is identical to the fig 3.4.

Fig 5.3 Iref generator

13

5.1.5 Hysteresis Band Generator and Comparator

5.1.5.1 Introduction

This block generates the gate signals for the inverter controlling the switching pattern of the IGBTs so as

to generate the required current. The block has actual output current from the inverter (I0) and reference

current generated (Iref) as input. The hysteresis band of the current is created. The block checks if actual

current from inverter is within the limit of the band. If not, then switching is changed so as to bring it

back to the band. The block is shown in fig 5.5.

5.1.5.2 Operation

The difference of the reference current (Iref) and actual current (I0) is fed to the relay. The relay is

configured with the switch on point as 0.5 and switch off point as -0.5. Hence, the relay creates a

hysteresis band and checks if the actual current is within the band. Thus, the relay generates gate signals

in following pattern:

When Iref – I0 ≤ -0.5, output of relay=1

When Iref - Io ≥ 0.5, output of relay =0

Fig 5.4 Iref(phase) generator

14

5.1.6 Voltage Source Inverter

5.1.6.1 Introduction

The inverter converts the dc voltage of capacitor into ac voltage. It has a dc capacitor as source. The

inverter receives the gate signals from the hysteresis band generator and comparator. It has 2 pairs of

IGBTs. It generates the reactive power for the load. However, due to internal losses, it too consumes the

active power. Due to its active power consumption we have introduced the Id current calculation in Id

generator. The more the switching frequency the more is the active power consumed by it. This active

power is the loss in the STATCOM. The fig 5.6 shows the voltage source inverter.

5.1.6.2 Operation

There are two pairs of IGBTs. The IGBT_1 and IGBT_3 form one pair and remaining form the other pair

as shown in fig 5.6. When the first pair is ON, the voltage at output is positive and hence the current from

the inverter rises. And reversely, when the second pair is ON, the voltage at the output of inverter is

negative. Thus, the current drops. When the current of the inverter rises, the capacitor discharges and

when it falls, the capacitor charges. As it is observed, when one pair is on and the other pair has to be off.

Thus, the signal to one pair is inverted for the other pair using NOT gate.

For gate signal = 1: IGBT_1&_3 Turns ON and IGBT_2&_4 Turns OFF, V0 = +Vc.

and for gate signal = 0: IGBT_1&_3 Turns OFF and IGBT_2&_4 Turns ON, V0 = -Vc.

Fig 5.5 Hysteresis band generator and comparator

15

5.1.7 Whole System Model

The fig 5.7 shows the whole system model of the project.

Fig 5.6 Voltage Source Inverter

16

Fig

5.7

Wh

ole

Sys

tem

Mo

del

17

5.2 Simulation Result

Fig 5.8 Voltage(Vs) and Current(Is) from source (upper) and current (I0) from STATCOM and Hysteresis

band of reference current(Iref) (lower)

Fig 5.9 magnified section of the source voltage and current (upper) and current from STATCOM with

hysteresis band (lower) (load is changing so is the current)

18

Fig 5.10 Magnified section of current from the STATCOM (I0) with hysteresis band and the reference

current (Iref)

The above fig 5.8 shows the current and voltage from the source. It can be clearly seen that the current

from the source is in phase with the voltage. So, from this it can be concluded that the use of the

STATCOM has resulted in unity power factor from the source. The figure too shows the actual current

delivered by the STATCOM and the hysteresis band of the reference current. The magnified picture is

shown in fig 5.9 and fig 5.10. It can be seen that the switching of the IGBTs of the inverter has caused

the actual current from the STATCOM to oscillate within the hysteresis band. The fig shows that the

actual current tracks the reference current. The current drawn by the load changes after t=0.4 sec. Thus, it

can be seen that the actual current supplied by the STATCOM and the reference current changes at t=0.4

sec such that the new current from the source is again in phase with the source voltage.

Fig 5.11 Active and reactive power delivered by the source (top), active and reactive power delivered by

the STATCOM (middle) and active and reactive power consumed by the load (bottom)

19

The above fig 5.11 shows the power supplied by the source (top), the power supplied by the STATCOM

(middle) and the power consumed by the load (bottom). The top fig shows that at the start the source

supplies both active and reactive power. However, after short period, the reactive power delivered by the

source falls to zero. It only supplies the active power. When the load changes, the source provides this

changed active power. However, the reactive power delivered by the source is again zero. The middle

graph shows the active power and reactive power by the STATCOM. It can be observed that the active

power consumed by STATCOM is nearly zero. It consumes small amount of active power indicating the

active power losses in the STATCOM. The STATCOM initially draws high amount of active power from

the source. At steady state, this active power consumes drops near to zero (with some losses). Whenever

the load changes, the change in reactive power is taken by the STATCOM not the source. The bottom

figure shows the power consumed by the load. The load consumes both active and reactive power. Thus,

the active power for the load is provided by the source while the reactive power is supplied by the

STATCOM.

Fig 5.12 Voltage across the capacitor

The fig 5.12 shows the capacitor voltage. Initially, the capacitor is fully discharged. Thus when power is

switched on, the capacitor voltage builds up. The switching of the IGBTs causes slow rise and fall in the

voltage level. For ideal case, the voltage across the capacitor is constant.

20

6. FUTURE WORK

The remaining works to be accomplished are to:

- Develop the simulation of proposed hardware model in PROTEUS,

- Microcontroller programming using MPLAB,

- Hardware fabrication,

- Testing and debugging and

- Final Documentation.

21

REFERENCES

[1] Hingorani, Narain G., Understanding FACTS: concepts and technology of flexible AC transmission systems, IEEE press, 1999

[2] Muhammad H. Rashid, Power Electronics Circuits, Devices and Application, Pearson/Prentice Hall 3

rd edition, 2004 [3] http://www.google.com

[4]http://en.wikipedia.org/wiki/STATCOM