Use of shunt & series capacitors in transmission lines

Name: H.M. GunasekaraIndex No.: 070149cField: ElectricalDate of per: 23/09/2009Date of sub: 07/10/2009

INSTRUCTED BY: Mrs. Selvarasa

CALCULATIONS

Per Unit compensation of the line =

Sample calculation

Series capacitance = 6uFSeries capacitive reactance =

= (because =2 f )

= 530.79 ΩInductive reactance = Lω

= 0.15×

= 47.1 Ω

Per Unit compensation of the line =530.79/47.1=11.27

series capacitance (uF) Power (W)capacitive reactance (Ω) PU value

6 5 530.79 11.2712 15 265.39 5.6318 25 176.93 3.7624 20 132.70 2.8230 25 106.16 2.25

2

RESULTS

Power received Vs. series capacitance

3

Power received Vs. shunt capacitance

4

Power received Vs. PU compensation of the line

5

DISCUSSION

1. Effect of power factor on the power system.

Power factor is the ratio between real power and apparent power. A low power factor tells you that the system is demanding more power than the amount it consumes to operate. This causes reactive energy to flow back and forth wasting energy as losses in transmission lines. Therefore having a power factor close to unity is desirable.

In industrial usage electricity is charged separately for the power factor. If the system has a lower pf it will require a bigger capacity energy supply. Therefore the charge will be high. So industrial consumers are advised to maintain a pf higher than 90%.

2. Usefulness of shunt capacitors in improving power factor of load.

Most of the time industrial loads are inductive. Therefore when a lot of inductive loads are coupled the power factor becomes undesirably low. Inductive loads make the current lag the voltage. Capacitive loads make the current lead the voltage. To compensate for the lagging current, a capacitor bank is connected in parallel to the load. The capacitor bank adjusts to make the power factor of the system close to unity as the load varies. This will lower the energy loss due to reactive energy and will also minimize the industrial users’ energy bill by lowering the maximum demand value.

3. Effects of series and shunt capacitance.

Shunt capacitance

This is the normal method of adding capacitors to the system. The rating of the capacitor will be a little higher than the voltage.

Series capacitance

Even though this method gives a good voltage regulation it has many disadvantages. For a certain capacitor value the system could resonate. In such conditions the impedance of the system becomes very low. Very high currents can pass through the system and it could damage the equipment connected.

In short circuit conditions the capacitor should be able to withstand high currents. This will not be as economical as using shunt capacitors.

6

4. Other methods to improve power factor.

Synchronous motor

An unloaded synchronous motor connected to the grid will act like a variable capacitor. It can be adjusted to the required amount of correction.

Filters

Some loads cannot be directly connected to capacitors because harmonics exist in the wave form. Filters are used to cut out the unwanted frequencies and then only the capacitors are connected. It is possible to achieve near unity power factors through this method.

7