Q11Power Quality and FACTS

Series Compensation Concept

Series compensation involves inserting a capacitor (or a controllable series device such as TCSC/SSSC) in series with a transmission line to reduce its effective inductive reactance. Since the maximum power transfer over a line is inversely proportional to its series reactance, reducing the effective reactance directly increases the power transfer capability of the line for a given sending and receiving end voltage and angle.

Why Mid-Point Location

For a long transmission line, compensation can be placed at the sending end, receiving end, or at the mid-point. Analysis using the transmission line equivalent circuit shows that when a series capacitor is placed at the electrical mid-point of the line, it symmetrically divides the line into two equal halves, each carrying half of the total voltage drop. This symmetric division produces the largest possible increase in the effective power-angle curve compared to any other location for the same amount of compensation, i.e., mid-point compensation gives the maximum improvement in power transfer capability and transient stability margin.

The mid-point voltage also receives maximum support, since compensation forces the voltage profile along the line to bulge upward near the middle rather than sagging as it would in an uncompensated line. This flattening of the voltage profile improves voltage regulation for the connected loads and reduces reactive power demand from the sources at both ends.

Benefits and Practical Use

• Increases steady-state and transient stability limits of the transmission corridor
• Improves voltage profile along the line, reducing need for additional shunt compensation
• Enables better load sharing among parallel transmission lines
• Practically implemented via fixed series capacitors, or dynamically via TCSC/SSSC for continuous control and to damp sub-synchronous resonance and power oscillations