Series Compensation

Series compensation is the practice of inserting capacitors in series with a long transmission line to cancel part of its inductive reactance. Lowering the line's effective impedance increases the power it can carry for a given angle across it, improves voltage regulation and strengthens stability margins. It is one of the most cost-effective ways to raise the transfer capability of long, stability-limited AC corridors.

Fixed series capacitors are installed on insulated platforms at line potential, usually at one end or at the midpoint of the line, with a degree of compensation typically between 25 and 70 percent of the line reactance. Protection equipment, including metal oxide varistors, triggered spark gaps and bypass breakers, shields the capacitors from fault currents. Thyristor-controlled series capacitors add the ability to vary compensation continuously, which helps damp power oscillations and manage flows in real time.

The main technical risk is subsynchronous resonance, an interaction between the compensated line and the shafts of nearby thermal generators that can damage turbines. Similar subsynchronous control interactions have affected wind farms connected close to series-compensated lines. Planners screen for these risks in interconnection studies, and new inverter-based plants may need specific damping controls or other mitigation.

Series compensation is widely used in North and South America, India, China, Scandinavia and other regions with long distances between generation and load. As grids add remote renewable zones, it remains an attractive alternative or complement to new lines, though it is now often evaluated against HVDC and other FACTS options. Upgrading existing capacitor banks, or converting them to thyristor control, is also a common way to extend the life and capability of older corridors.