A PRESENTATION ON REACTIVE POWER COMPENSATION

BY

K.S.S VENKATA SATYA NAVEEN

S.MANOJ KUMAR

CONTENTS

Introduction Reactive power Need for reactive power Need for reactive power compensation Shunt compensation Series compensation Static VAR compensators (SVC) Static compensators (STATCOM) Synchronous Condensor Conclusion References

INTRODUCTION

Reactive power (VAR) compensation is defined as the management of reactive power to improve the performance of ac systems. There are two aspects:-

a) Load Compensation – The main objectives are to :-

i) increase the power factor of the system

ii) to balance the real power drawn from the system

iii) compensate voltage regulation

iv) to eliminate current harmonics.

b) Voltage Support – The main purpose is to decrease the voltage fluctuation at a given terminal of transmission line.

Therefore the VAR compensation improves the stability of ac system by increasing the maximum active power that can be transmitted.

WHAT IS REACTIVE POWER ?

Power is referred as the product of voltage and current

i.e. power = V x I The portion of electricity that establishes and sustains the

electric and magnetic fields of alternating-current equipment. Reactive power must be supplied to most types of magnetic equipment, such as motors and transformers.

In an ac transmission, when the voltage and current go up and down at the same time, only real power is transmitted and when there is a time shift between voltage and current both active and reactive power are transmitted.

ANALOGY OF REACTIVE POWERWhy an analogy? Reactive Power is an essential aspect of the electricity system, but one that is difficult to comprehend by a lay man. The horse and the boat analogy best describe the Reactive Power aspect. Visualize a boat on a canal, pulled by a horse on the bank of the canal.

In the horse and boat analogy, the horse’s objective (real power) is to move the boat straightly.

The fact that the rope is being pulled from the flank of the horse and not straight behind it, limits the horse’s capacity to deliver real work of moving straightly.

Therefore, the power required to keep the boat steady in navigating straightly is delivered by the rudder movement (reactive power).

Without reactive power there can be no transfer of real power, likewise without the support of rudder, the boat cannot move in a straight line.

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POWER TRIANGLE

Power factor = cosø

= real power / apparent power

= kW/ kVA Whenever there is a phase shift

between V and I we have:-

a) real power (kW)

b) reactive (imaginary ) power (kVAR)

c) The combination is a complex or

apparent power (kVA)=√(kW)² + (kVAR)²

WHY DO WE NEED REACTIVE POWER?

In resistive loads the current produces the heat energy which produces the desired output but incase of inductive loads the current creates the magnetic field which further produces the desired work. Therefore reactive power is the non working power caused by the magnetic current to operate and sustain magnetism in the device .

Reactive power (vars) is required to maintain the voltage to deliver active power (watts)through transmission lines. When there is not enough reactive power the voltage sags down and it is not possible to deliver the required power to load through the lines.

Need for Reactive Power Compensation

Reactive power generated by the ac power source is stored in a capacitor or a reactor during a quarter of a cycle and in the next quarter of the cycle it is sent back to the power source. Therefore the reactive power oscillates between the ac source and the capacitor or reactor at a frequency equals to two times the rated value (50 or 60 Hz). So to avoid the circulation between the load and source it needs to be compensated .

Also to regulate the power factor of the system and maintain the voltage stability we need to compensate reactive power .

Methods of Reactive Power Compensation

Shunt compensation Series compensation Synchronous condensers Static VAR compensators Static compensators

Shunt compensation

The device that is connected in parallel with the transmission line is called the shunt compensator. A shunt compensator is always connected in the middle of the transmission line. It can be provided by either a current source, voltage source or a capacitor.

An ideal shunt compensator provides the reactive power to the system.

Shunt-connected reactors are used to reduce the line over-voltages by consuming the reactive power, while shunt-connected capacitors are used to maintain the voltage levels by compensating the reactive power to transmission line.

Transmission line with shunt compensation

Series compensation

When a device is connected in series with the transmission line it is called a series compensator. A series compensator can be connected anywhere in the line.

There are two modes of operation – capacitive mode of operation and inductive mode of operation.

A simplified model of a transmission system with series compensation is shown in Figure .The voltage magnitudes of the two buses are assumed equal as V, and the phase angle between them is δ.

Transmission line with series compensation

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Static VAR compensators

A static VAR compensator (or SVC) is an electrical device for providing  reactive power on transmission networks. The term "static" refers to the fact that the SVC has no moving parts (other than circuit breakers and disconnects, which do not move under normal SVC operation).

The SVC is an automated impedance matching device, designed to bring the system closer to unity power factor. If the power system's reactive load is capacitive(leading), the SVC will use reactors (usually in the form of thyristor-Controlled Reactors) to consume vars from the system, lowering the system voltage.

Under inductive (lagging) conditions, the capacitor banks are automatically switched in, thus providing a higher system voltage.

ADVANTAGES

a) Static VAR compensation is not done at line voltage; a bank of transformers steps the transmission voltage (for example, 230 kV) down to a much lower level (for example, 9.5 kV).This reduces the size and number of components.

b) They are more reliable .

c) Faster in operation .

d) Smoother control and more flexibility can be provided with the help of thyristors.

Static Compensator

 The devices use synchronous voltage sources for generating or absorbing reactive power. A synchronous voltage source (SVS) is constructed using a voltage source converter (VSC). Such a shunt compensating device is called static compensator or STATCOM .

A STATCOM usually contains an SVS that is driven from a dc storage capacitor and the SVS is connected to the ac system bus through an interface transformer. The transformer steps the ac system voltage down such that the voltage rating of the SVS switches are within specified limit.

Structure of STATCOM

Basically, the STATCOM system is comprised of

Power converters,

Set of coupling reactors or a step-up transformer,

Controller

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COMPARISON OF VI CHARACTERISTICS OF SVC AND

STATCOM

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Advantages of STATCOM

The reactive components used in the STATCOM are much smaller than those in the SVC.

The characteristics of STATCOM are superior. The output current of STATCOM can be controlled up to

the rated maximum capacitive or inductive range. Reduction of the capacity of semiconductor power

converter and capacitor bank to one half of those for the conventional SVC.

Better transient response of the order of quarter cycle. Reduction of harmonic filter capacity. Reduction of size of high value air-cored reactor. Reduction of equipment volume and foot-print.

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Synchronous Condensor

A device whose main function is the improvement of pf of the electrical system is known as the synchronous condensor. It is installed at the receiving end of the line .

When a synchronous condensor is introduced it supplies the kVAR to the system , and hence the current is reduced .

Therefore the losses are reduced and provides a better efficiency . Hence more power can be delivered to the load and improves the pf of the system.

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CONCLUSION

From all the previous discussion we can conclude reactive power compensation is a must for improving the performance of the ac system. By reactive power compensation we can control the power factor and reduce the consumption of electricity.

References

[1] Juan Dixon and Luis Moran -“ Reactive Power Compensation Technologies”.

[2] Yongan Deng -“ Reactive Power Compensation Of Transmission Lines”.

[3] Hong Chan- “ Practices of Reactive Power Management and Compensation”.

T.J Millen- “ Reactive Power Control in Electrical Systems.”

Canadian Electrical Association-Static Compensators For Reactive Power Control.

THANK YOU!!