In this topic, you study Series Compensation – Definition, Theory, Diagram, Advantages, & Applications.
The purpose of series compensation is to cancel out part of the series inductive reactance of the line using series capacitors. As shown in Figure 1, the circuit diagram when series capacitor is connected on a transmission line. Figure 2 shows the Phasor diagram corresponding to the circuit shown in Figure 1.
where,
VR = Receiving end voltage without series capacitor;
VS = Sending end voltage;
= Receiving end voltage with series capacitor.
Figure 1 A transmission line with series-capacitor-compensation applied.
Due to the effect of series capacitor the receiving end voltage will be instead of VR as seen from the phasor diagram (Figure 2). Thus with series capacitor in the circuit the voltage drop in the line is reduced and receiving end voltage on full load is improved. Series capacitors improve voltage profile.
Figure 2 Phasor diagram of transmission line with series compensation.
Power transfer with Series Compensation
Series capacitors also improve the power transfer ability. The power transferred with series Compensation as
where, is the phase angle between VS and VR;
Hence capacitors in series are used for long EHV transmission system to improve power transfer ability (stability limit). These are installed in sending end, receiving end and intermediate sub-stations of long EHV AC and UHV AC transmission lines.
Advantages with Series Compensation
- Increase of maximum power transfer
- Reduction in power angle for a given amount of power transfer
- Increased loading
- Control of voltage.
Applications of Series Compensation
Series capacitors are used in transmission systems to modify the load division between parallel lines. If a new transmission line with large power transfer capacity is to be connected in parallel with an already existing line, it may be difficult to load the new line without overloading the old line. But the reduction of series reactance by series compensation ensures proper load division among these parallel lines for maximum power transfer.
Limitation of Series Compensation
The recommended value of degree of compensation is 25 to 75 %. Hundred percent series compensation of long EHV AC transmission line never considered. At 100 % compensation, the effective line reactance would be zero, and the line current and power would be extremely sensitive to changes in the relative angles of terminal voltages.
Installation of series capacitor
Series capacitors are installed either at both ends of the EHV and UHV transmission line i.e. at sending end and receiving end sub-station or in an intermediate compensating switching sub-station.
