voltage stability by compensating reactive power

1

A REPORT

ON

STABILIZATION OF VOLTAGE BY COMPENSATING REACTIVE

POWER

BY

M.SAI MANOBHIRAM 2012A3PS224H

DURGA RAO GUNDU 2012A3PS255H

MOHITH DEVATHI 2012A3PS166H

UNDER THE SUPERVISION OF

Dr. ALIVELU MANGA PARIMI

ASSISTANT PROFESSOR, EEE DEPARTMENT

BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE, PILANI

HYDERABAD CAMPUS

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ACKNOWLEDGEMENT

We would like to thank the Department of Electrical & Electronics, BITS Pilani

Hyderabad Campus and in particular Dr. Alivelu Manga Parimi, Asst. Professor for giving

us the opportunity to do this project and also for her guidance over the course of completing

this project which helped us immensely. We would also like to thank Mr. Ramachandra and

Gayatri mam for the support and encouragement during the project.

ABSTRACT

Voltage stability problems usually occur in heavily loaded systems. While the

disturbance leading to voltage collapse may be initiated by a different kinds of contingencies,

the underlining problem is an inherent weakness in the power system.

This project will illustrate the basic issues related to voltage instability by considering

the characteristics of transmission system and afterwards examining how we can improve

voltage stability by using reactive power compensation devices. Main consideration in this

project will be focused on delivering the reactive power directly to buses in a distributing

system, by installing sources of reactive power. The reason is that transmission lines can be

operated with varying load and nearly constant voltage at both ends if adequate sources of

reactive power are available at both ends. Before these considerations, there will be the

description of the voltage stability phenomena and ways of improving it, because only with

the description and researches this project will be understandable and complete.

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Table of contents

1. ABSTRACT 03

2. BACKGROUND 04

3. OBJECTIVES 04

4. BASIC CONCEPTS AND REFERENCES 05

5. REACTIVE POWER &VOLTAGE CONTROL 05

6. WHY ONLY REACTIVE POWER? 06

7. ELEMENTS OF THE SYSTEM 07

8. WAYS OF IMPROVING 07

9. SHUNT CAPACITORS 08

10. POWER FACTOR CORRECTION 08

11. ADVANTAGES AND DISADVANTRAGES 09

12. RESULTS 09

13. CALCULATION OF SHUNT CAPACITANCE 10

14. WAVEFORMS 12

15. SVC 13

16. TCR 13

17. SIMULATION AND RESULTS 14

18. ADVANTAGES AND DISADVANTAGES 16

19. CONCLUSIONS 16

20. REFERANCES 16

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Background of the project:

Voltage stability problems usually occur in heavily loaded systems. While the disturbance

leading to voltage collapse may be initiated by a different kinds of contingencies, the underlining

problem is an inherent weakness in the power system.

Loss of power system stability may cause a total blackout of the system. It is the

interconnection of power systems, for reasons of economy and of improved availability of supply

across the broader areas that makes widespread disruptions possible. Current civilization is

susceptible to case of power system blackout, the consequences of systems failure are social and

economic as well. Even short disturbance can be harmful for industrial companies, because

restarting of process might take several hours.

In recent years, voltage instability has been responsible for several major network

collapses.

- New York Power Pool disturbances of September 22, 1970

- Florida system disturbance of December 28,1982

- French system disturbance of December 19, 1978

- Northern Belgium system disturbance of August 4, 1982

- Swedish system disturbance of December 27, 1983

- Japanese system disturbance of July 23, 1987

This project will illustrate the basic issues related to voltage instability by considering the

characteristics of transmission system and afterwards examining how we can improve voltage

stability by using reactive power compensation devices.

Objectives of the project:

Explaining basic principles of equipment used for improving voltage stability. Main

consideration in this project will be focused on delivering the reactive power directly to bus

(in this project we will consider one bus) in a distributing system, by installing sources of

reactive power. The reason is that transmission lines can be operated with varying load and

nearly constant voltage at both ends if adequate sources of reactive power are available at

both ends. Before these considerations, there will be the description of the voltage stability

phenomena and ways of improving it.

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Basic concepts and definitions:

What is Voltage Stability?

Power system stability may be defined as that property of a power system that enables

it to remain in a state of operating equilibrium under normal operating conditions and to

regain an acceptable state of equilibrium after being subjected to a disturbance. Traditionally,

the stability problem has been the rotor angle stability, maintaining the synchronous

operation between two or more interconnected synchronous machines. Instability may also

occur without loss of synchronism, in which case the concern is the control and stability of

voltage. A criterion for voltage stability is that, at a given operating condition for every bus in

the system, the bus voltage magnitude increases as the reactive power injection in the same

bus is increased. A system is voltage unstable if, for at least one bus, the bus voltage

magnitude decreases as the reactive power injection in the same bus is increased.

In other words, power system is voltage stable if voltages after disturbances are close

to voltages at normal operating conditions. A power systems becomes unstable when voltages

uncontrollably decrease due to outage of equipment, increment in load, decrement in

production or in voltage control.

Even though the voltage stability is generally the local problem, the consequences of voltage

instability may have a widespread impact. The result of this impact is voltage collapse, which

results from a sequence of contingencies rather than from one particular disturbance. It leads

to really low profiles of voltage in a major part of power system.

The main factors causing voltage instability are:

• High reactive power consumption at heavy loads • Generating stations are too far from load centres. • Source voltages are too low. • Poor coordination between various control and protective systems • The inability of the power system to meet demands for reactive power in the heavily

stressed system to keep voltage in the desired range.

Reactive Power and Voltage Control: For efficient and reliable operation of power system, the control of voltage and reactive power

should satisfy the following objectives

• Voltages at all terminals of all equipment in the system are within acceptable limits

• System stability is enhanced to maximize utilization of the transmission system

• The reactive power flow is minimized so as to reduce RI2and XI2 losses. This ensures that the

transmission system operates mainly for active power.

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Why only reactive power??

i.e., Voltage drop depends on Q. if reactive power flows over the transmission line, there will

be a voltage drop.

Elements of the system, which are producing and absorbing reactive power

Loads-

A typical load bus supplied by a power system is composed of a large number of

devices. The composition changes depending on the day, season and weather conditions. The

composite characteristics are normally such that a load bus absorbs reactive power. Both

active and reactive powers of the composite loads vary due to voltage magnitudes. Loads at

low-lagging power factors cause excessive voltage drops in the transmission network.

Industrial consumers are charged for reactive power and this convinces them to improve the

load power factor.

Underground cables-

They are always loaded below their natural loads, and hence generate reactive power

under all operating conditions.

Overhead lines-

Depending on the load current, either absorb or supply reactive power. At loads below

the natural load, the lines produce net reactive power, on the contrary, at loads above natural

load lines absorb reactive power.

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Synchronous generators-

Synchronous generators can generate or absorb reactive power depends on the

excitation. When overexcited they supply reactive power, and when under excited they

absorb reactive power.

Compensating devices-

They installed in power system to either supply or absorb reactive power.

Ways of improving voltage stability and control:

Reactive power compensation is often most effective way to improve both power

transfer capability and voltage stability. The control of voltage levels is accomplished by

controlling the production, absorption and flow of reactive power. The generating units

provide the basic means of voltage control, because the automatic voltage regulators control

field excitation to maintain scheduled voltage level at the terminals of the generators. To

control voltage throughout the system we have to use addition devices to compensate reactive

power. Reactive compensation can be divided into series and shunt compensation. It can be

also divided into active and passive compensation.

The devices used for these purposes may be classified as follows:

• Shunt capacitors

• Series capacitors

• Shunt reactors

• Synchronous condensers

• SVC (Static Var Compensator)

• STATCOM (Static synchronous Compensator)

Shunt capacitors and reactors and series capacitors provide passive compensation. They are

either permanently connected to the transmission and distribution system or switched. They

contribute to voltage control by modifying the network characteristics. Synchronous

condensers, SVC and STATCOM provide active compensation. The reactive power absorbed

or supplied by them is automatically adjusted so as to maintain voltages of the buses to which

they are connected. Together with the generating units, they establish voltages at specific

points in the system. Voltages at other locations in the system are determined by active and

reactive power flows through various elements, including the passive compensating devices.

NOTE: But in this project our main concentration is on Shunt compensation (by capacitive

banks) and SVC (by using thyristors and PWM blocks).

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SHUNT CAPACITORS:

Shunt capacitors banks are always connected to the bus rather than to the line. They

are connected either directly to the high voltage bus or to the tertiary winding of the main

transformer. Shunt capacitor banks are breaker-switched either automatically by a voltage

relays or manually. The primary purpose of transmission system shunt compensation near load

areas is voltage control and load stabilization. In other words, shunt capacitors are used to

compensate for XI2 losses in transmission system and to ensure satisfactory voltage levels during

heavy load conditions.

Shunt capacitors are used in power system for power-factor correction. The objective

of power factor correction is to provide reactive power close to point where it is being

consumed, rather than supply it from remote sources.

Switched shunt capacitors are also used for feeder voltage control. They are installed

at appropriate location along the length of the feeder to ensure that voltages at all points

remain the allowable minimum or maximum limits as the loads vary.

For voltage stability, shunt capacitor banks are very useful on allowing nearby generators

to operate near unity power factor

POWER FACTOR CORRECTION

The initial reactive power we have is VA uncorrected. By using shunt compensation we make

power factor to unity. (i.e. phi2 is nearly equal to 0)

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Advantages

Compared to Static Var Compensators, mechanically switched capacitor banks have

the advantage of much lower cost.

Switching speeds can be quite fast.

Following a transmission line outage, capacitor bank energization should be delayed

to allow time for line reclosing.

Disadvantages

The biggest disadvantage of shunt capacitors is that the reactive power output drops

with the voltage squared.

During the severe voltage decays these devices are not efficient enough.

If voltage collapse results in system breakdown, the stable parts of the system may

experience damaging over voltages immediately following separation.

Results (Matlab implementation):

Transmission system without any compensation

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System Data

Electrical Component Parameter Absolute measurement

Voltage Source Type Single phase

Frequency 50Hz

Voltage 132KV(RMS)

Resistance 2.645 Ω

Inductance 70.2mH

Line conductor resistance 5.2Ω

Inductance 138mH

Capacitance 1.934µF

Load Active Power(P) 75MW

Reactive Power 20MVAR

Power Factor 0.96623

Nominal voltage 132KV(RMS)

Nominal frequency 50Hz

Uncompensation Results

Sending end current 506.34A(RMS)

Load Current 526.9.76A(RMS)

Sending end voltage 132KV(RMS)

Load voltage 118.276KV(RMS)

Calculation of calculation of capacitance:

Compensation Capacitance can be calculated by using the formula

Here Q can be calculated by

Q = (reactive power by compensating device(Qc)) + (reactive power drop

across transmission line) + ( a correction factor of 8-9% of the load rated

reactive power)

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E is the compensated voltage

By taking above results into consideration we get

Qc =19323575.1VAR;

Reactive power drop across Transmission line = 11665070.64VAR

Q = 32998645.74VAR

By applying above formulas for calculation of capacitance we get C=6.06µF

Simulation Circuit (transmission system after compensation):

Compensation results:


Load Current 587.74A(RMS)



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Waveforms of voltage and current before and after compensation

respectively

Before compensation

After compensation

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Static Var Compensator (SVC):

Static var compensators are shunt-connected static generators or/and absorbers whose

outputs are varied so as to control specific parameters of an electric power system. SVCs

overcome the limitation of mechanically switched shunt capacitors or reactors. Advantages

include fast, precise regulation of voltage and unrestricted, transient free capacitor switching.

The basic elements of SVCs are capacitor banks or reactors in series with a bidirectional

thyristors.

Basic types of SVCs:

• Saturated reactor (SR)

• Thyristor-controlled reactor (TCR)

• Thyristor-switched capacitor (TSC)

• Thyristor-switched reactor (TSR)

• Thyristor control transformer (TCT)

• Self- or line-commutated converter (SCC/LCC)

NOTE: In this project we considered SVC type as Thyristor-controlled reactor (TCR).

The Thyristor-Controlled reactor (TCR):

An elementary single-phase thyristor-controlled reactor TCR consists of fixed reactor of

inductance L, and a bidirectional thyristor valve.

Thyristor-controlled reactor

The thyristor conducts on alternate half cycles of the supply frequency depending on the firing

angle α. The magnitude of the current in the reactor can be varied continuously by this method

of delay angle control form maximum (α=0) to zero at (α=90). The adjustment of current in

the reactor can take place only once in each half cycle, in the zero to 90o interval.

The ability of the Thyristor-controlled Reactor (TCR) to limit current is a vital part of

controlling power flow. The current is controlled by the firing angle (_), which at 0º the

switch is permanently closed, then slowly limits current as _ increases to 180° where current

is then zero. Limiting the current ultimately limits the reactive current which results in how

much reactive power can be added to or subtracted from the system.With the SVC,when α =0

the current passes fully through the inductor bypassing the capacitor. Yet as α increase to

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180°, current will be forced to slowly pass through the capacitor, and thus raise the lagging

power factor of the system. Therefore the firing angle α may be directly related to how much

power factor correction we need for the system.

Simulation and results

Circuit basically look like

System data

Electrical Component Parameter Absolute measurement

Voltage Source Type Single phase

Frequency 50Hz

Voltage 132KV(RMS)

Resistance 2.645 Ω

Inductance 70.2mH

Line conductor resistance 5.2Ω

Inductance 138mH

Capacitance 1.934µF

Load Active Power(P) 75MW

Reactive Power 20MVAR

Power Factor 0.96623

Nominal voltage 132KV(RMS)

Nominal frequency 50Hz

PID controller Proportional constant 5

Integrational constant 0.01

PWM generator Carrier frequency 300Hz

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Calculation of Inductance

Voltage current Power Power factor

118.276KV 526.976A 75MW 0.96623

Power factor angle = 14.931o

S=VI*= (118.276)526.976L (14.931) = 62328.6134=44443.16396+i (43699.67073)

Imaginary part is XL= 43699.67073

By equating it to XL =2πf*L

We get L=0.2H

The capacitance that we got in the shunt compensation result can be used here

After substituting the above results and compensating using SVC compensator we got the

simulation result as


Load Current 588.02A(RMS)



Current and Voltage waveforms:

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Advantages

• The main advantage of SVCs over simple capacitor bank compensation schemes is

their near-instantaneous response to changes in the system voltage

• Dynamic compensation can be achieved by SVC

Disadvantages:

However, static VAR compensators are more expensive than Capacitor banks

Conclusions:

In this project we have briefly studied the construction and working of various

reactive power compensating devices (ex capacitor banks and SVC) in order to stabilize the

voltage and then the same were implemented in matlab and then simulated. The achievement

of ours in this project is theoretical values and simulated results are closely matched.

To conclude, since there is huge increase in load demand and it is very hard to build

new transmission lines and generation plants, the existing power network has to be utilized

more efficient and hence voltage stability is becoming very important issue. In this project we

tried to introduce methods which can improve utilization of exciting network and voltage

stability.

References

“Power system stability and control” Prabha Kundur ISBN 0-07-035958-X

“Power system voltage stability” Carson W. Taylor ISBN 10-0071137084

“Power system analysis” Hadi Saadat ISBN 0-07-116758-7

“Shunt versus Series compensation in the improvement of Power system performance

by Irinjila Kranti Kiran ,Jaya Laxmi.A” INTERNATIONAL JOURNAL OF APPLIED

ENGINEERING RESEARCH, DINDIGUL Volume 2, No 1, 2011

“A Small Scale Static VAR Compensator for Laboratory Experiment” 2nd IEEE

International Conference on Power and Energy (PECon 08), December 1-3, 2008, Johor

Baharu, Malaysia.

“A New Approach to Reactive Power Calculation of the Static VAr Compensator”

Alexander Osnach Department of Optimization of Electricity Supply Systems, Institute

of Electrodynamics NAS, Kiev, Ukraine.

“Controlled sources of reactive power used for improving voltage stability” Institute of

Energy Technology Electrical Power Systems and High Voltage Engineering Aalborg

University Aalborg