SOLUTION

The change in the electrical characteristics of a transmission line in order to increase its power transmission capability is known as line compensation.

Reactive compensation is used to improve the performance of the transmission line. Generally, there are two types of compensation:

1. Shunt
2. Series

Series capacitive compensation on EHV transmission lines is used to improve the stability of the system.

Series Compensation: In this type of compensation, the capacitor bank is connected in series with the transmission line. As a result, the net reactance of the line is reduced. The series capacitive compensation decrease the overall effective series transmission impedance from the sending end to the receiving end. Hence it increases the power system stability. It also reduces the net reactive voltage drop of the line. Thus, the performance of the line is improved. The power transfer capability of a transmission line can be written as

$P = \frac{{{V_S}.{V_R}.\sin \delta }}{{{X_L}}}$

Where

V_S = Sending end voltage

V_R = Receiving end voltage

δ = Angle between two voltage vectors

X_L = Line reactance

Advantages of series compensation

1. A series compensated capacitor is an effective tool for line reactance compensation. The equivalent line reactance in this case will be X_L −  X_C. 50% compensation is generally used.
2. Increase in transmission capacity.
3. More power can be transferred from one end to another by double circuit (parallel) lines. But, instead, a series-compensated line is cheaper as one can transmit more power through a single line.
4. Better voltage regulation and improved voltage profile are possible at the load end as a series capacitor can supply reactive power during heavy load periods.
5. The stability limit is improved.
6. Series capacitors are inherently self-regulating and a control system is not required.
7. For the same performance, series capacitors are often less costly than SVCs and losses are very low.
8. For voltage stability, series capacitors lower the critical or collapse voltage.
9. Series capacitors possess the adequate time-overload capability.
10. Series capacitors and switched series capacitors can be used to control the loading of parallel lines to minimize active and reactive losses.

Disadvantages of series compensation:

1. Series capacitors are line connected and compensation is removed for outages and capacitors in parallel lines may be overloaded.
2. During heavy loading, the voltage on one side of the series capacitor may be out of range.
3. Shunt reactors may be needed for light load compensation.
4. Subsynchronous resonance may call for expensive countermeasures.

Shunt Compensation: In this type of compensation, either shunt inductors or shunt capacitors are used. They are connected between each line and neutral to reduce the effect of shunt susceptance of the line.

Shunt inductors are also known as shunt reactors. It absorbs the reactive power from the lines and controls the voltage of the line when the line is at no load or is lightly loaded. They are connected at both the ends of transmission lines.

With the help of shunt capacitor banks, reactive power is injected into the line to maintain the voltage within the limits at heavy loads. The shunt capacitors are connected near the load terminals.