Series Compensation of Transmission Line

As power demand increases in many parts of the world, power transmission needs to be developed as well. The building of more power lines may not be the best way, however, as transmission lines cost a lot of money, take considerable time to      construct, and are subject to severe environmental constraints.

With series compensation, the power transmission capability of existing, long lines can be increased considerably. Series compensation of AC transmission systems has been utilized for many years with excellent results. The usefulness of this concept can be illustrated as fellow:

Power transfer over uncompensated transmission line is given by

 

Power transfer over uncompensated transmission line is given by

 

By means of series compensation, the overall reactance between the line ends is reduced, hence the flow of active power is increased without any increment in the angular separation of the end voltages.

Similarly it is also clear that by introducing a capacitive reactance it is possible to achieve a decrease of the angular separation with power transmission capability unaffected, i.e. an increase of the angular stability of the link.

The influencing of transmission reactance by means of series compensation also opens up for optimizing of load sharing between parallel circuits, thereby bringing about an increase in overall power transmission capacity again.

With the reactance of the capacitive element, i.e. the series capacitor equal to X_C and the inductive reactance of the line equal to X_L, we can introduce a measure of the degree of series compensation, k:

k = X_C / X_L

In power transmission applications, the degree of compensation is usually chosen within the range 0.3 ≤k ≤0.7. Substituting X_C by k, we get

 

Series compensation of power transmission circuits enables several benefits:

 

1. Improves voltage profile of the lines or Reduces line voltage drops: When the  line  has high value  of reactance to resistance ratio, the  inductive  reactance of the  transmission line  can be  decreased  by introducing  series capacitors which results in  low  voltage drop. When a  load  with lagging  power factor is connected at the  end, voltage drop  in the  line  is

                        VD = I(R cos? + X_L sin?) volts

Note: Above equation is approximation for small voltage drop.

If a  capacitance ‘C’ with reactance X_C is connected  in series with the  line,  then the  reactance  will be  reduced  to  (X_L - ­X_C) and  hence the  voltage drop is reduced.  Further  the  reactive  power  taken by the  line  is also reduced.

 

2. Optimises power flow between parallel lines.

3. Increases power transmission capacity over the circuit without violating angular or voltage stability.

 

 

 

where K is degree of compensation. The economic degree of compensation lies in the range of 30-70% (K < 1, i.e. 0.3-0.7)

 

4. A reduction of the number of required EHV transmission lines.

5. Less installation time: The installation time of the series capacitor is smaller (2 years approx) as compared to installation time of the parallel circuit line (5 years approx).

6. Increases system stability: For same amount of power transfer and same value of E and V, the δ in the case of series compensated line is less than that of uncompensated line.

 

 

A lower δ means better system stability. Series compensation offers most economic solution for system stability as compared to other methods (reducing generator & transformer reactance, use of bundled conductors, increasing no. of parallel circuits) Reduces significantly the need for power generation or transfer line investments.

 

Disadvantages:

1. The series capacitor adversely affects the short circuit rating of circuit breaker and / or fuses in case of system encounters a fault as a series capacitor reduces the reactance of line and increase the fault current level.

2. A series capacitor changes the natural resonance frequency of the transmission system. In case of disturbances if frequency of disturbance is equal to subsynchronous frequency a phenomenon called subsynchronous resonance occurs. The mechanical failure of generator shaft may occur if the frequency is close to resonant frequency. 

3. A series capacitor may cause faulty operation of distance relays if degree of compensation and location of capacitor is not proper.

4. When a fault occurs on a line carrying the series capacitors then the fault current flows through the series capacitor which increases voltage across capacitor to high value which may damage the series capacitor and break the continuity of the supply. Hence the protection circuitry should be installed.

5. When a series capacitor is installed in EHV line between a transformer and source of supply a phenomenon known as ferroresonance may occur. Under this condition a high voltage may appear across the capacitor terminals which may damage capacitor itself. Similarly it may cause severe over voltage problems in associated network.