USE OF SERIES AND SHUNT CAPACITORS

IN

TRANSMISSION LINES

Name : K.A.P. Chathuranga

Index No : 050059G

Date of Per : 24.05.2007

Date of Sub : 06.06.2007

Objective

1. To investigate the effect of series capacitors on the limits of power transfer over a

transmission line with its terminal voltages fixed.

2. To investigate the effect of shunt capacitors on a transmission line with its terminal voltage

fixed.

Apparatus

3 Inductors - (0.15 H, 3A)

1 Wattmeter

1 Capacitor Bank

2 Ammeters - (0-3 A)

2 Voltmeters - (0-150-300 V)

1 Variac - (500Ω, 2A)

Rheostats - (200 Ω, 3A)

Calculations

Series Capacitive ReactancePer Unit Compensation of the Line =

Inductive Reactance

Per Unit Compensation of the Line =

Sample Calculation

C = 6 µF

L = 0.15 mH

f = 50 Hz

Series Capacitive Reactance =

Inductive Reactance =

Per Unit Compensation of the Line =

(Series Capacitive

Reactance)/(-i)Inductive Reactance/i

Per Unit Compensation

of the Line

Power Received

(W)

530.3 47.14 11.25 9

265.2 47.14 5.62 21

176.8 47.14 3.75 30

132.6 47.14 2.81 44

Discussion

Effect of Power Factor on the Power Systems

The power factor of an AC electric power system is defined as the ratio of the real power

to the apparent power, and is a number between 0 and 1. Real power is the capacity of the circuit

for performing work in a particular time. Apparent power is the product of the current and

voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a

non-linear load that distorts the wave shape of the current drawn from the source, the apparent

power can be greater than the real power. Low-power-factor loads increase losses in a power

distribution system and result in increased energy costs.

A power factor of unity is the goal of any electric utility company because if the power

factor is less than one, they have to supply more current to the user for a given amount of power

use. To do so, they incur more line losses. They also must have larger capacity equipment in

place than would be otherwise necessary. As a result, an industrial facility will be charged a

penalty if its power factor is much different from 1.

Utilities typically charge additional costs from the industrial users who have a power

factor below some limit, which is typically 90 to 95%. So it is necessary to have a power factor

close to unity to reduce additional costs in electricity payments.

Usefulness of Shunt Capacitors in Improving the Power Factor of the Load

Inductive components in a power system such as ballasts, motors and heaters, draw

Lagging Reactive Power from the supply. It lags behind the Active Power by 90 degrees. If a

capacitor is connected across the supply, it will draw Leading Reactive Power, which leads the

Active Power by 90 degrees. The direction of the Capacitive (i.e. Leading) Reactive Power is

opposite to the direction of the Inductive (i.e. Lagging) Reactive Power.

Industrial facilities tend to have a lagging power factor, where the current lags the voltage

because of having a lot of electric induction motors. This will lead to the consumption of

Lagging Reactive Power. To minimize this effect we should either consume Leading Reactive

Power or Supply Lagging Reactive Power within the system. This can be accomplished by

adding Shunt Capacitors to the system. Some industrial sites will have large banks of capacitors

strictly for the purpose of correcting the power factor back toward one to save on utility company

charges.

Effect of series and shunt capacitance

Shunt Connection:

This is the most popular method of connection. The capacitor is connected in parallel to

the unit. The voltage rating of the capacitor is usually the same as or a little higher than the

system voltage.

Series Connection:

This method of connection is not much common. Even though the voltage regulation is

much high in this method, it has many disadvantages. One is that because of the series

connection, in a short circuit condition the capacitor should be able to withstand the high current.

The other is that due to the series connection due to the inductivity of the line there can be a

resonance occurring at a certain capacitive value. This will lead to very low impedance and may

cause very high currents to flow through the lines.

Other Methods Available to Improve the Power Factor

Synchronous Motor

For power factor correction, instead of using a capacitor, it is possible to use an unloaded

synchronous motor. This is referred to as a synchronous condenser. It is started and connected to

the electrical network. It operates at full leading power factor and puts VARs onto the network as

required to support a system’s voltage or to maintain the system power factor at a specified level.

The condenser’s installation and operation are identical to large electric motors. The reactive

power drawn by the synchronous motor is a function of its field excitation. Its principal

advantage is the ease with which the amount of correction can be adjusted; it behaves like an

electrically variable capacitor.

Filters

There are certain situations where capacitors are not connected directly to the supply

lines. The reason for this is the presence of harmonics in the waveform caused by switched mode

power supplies. The simplest way to control the harmonic current is to use a filter. It is possible

to design a filter that passes current only at line frequency (e.g. 50 or 60 Hz). This filter kills the

harmonic current, which means that the non-linear device now looks like a linear load. At this

point the power factor can be brought to near unity, using capacitors or inductors as required.

This filter requires large-value high-current inductors, however, which are bulky and expensive.