Q3Power System - I

Series compensation: series capacitor banks are connected in series with the transmission line conductors (typically at an intermediate switching station along a long line), directly cancelling a portion of the line's series inductive reactance. Since the maximum power transfer capability of a line (for a lossless line, approximately Pmax = V²/X) is inversely proportional to the net series reactance X, reducing the effective reactance by series capacitive compensation directly increases the power transfer limit and improves the voltage regulation/profile along the line; series compensation also improves transient and steady-state stability margins by effectively 'electrically shortening' the line. However, series capacitors can give rise to a phenomenon called sub-synchronous resonance (SSR), where the compensated line's electrical resonant frequency can interact adversely with the mechanical torsional modes of nearby turbine-generator shafts, requiring careful study and sometimes additional SSR-damping equipment.

Shunt compensation: shunt reactors or capacitors (or dynamic devices like SVC/STATCOM) are connected in parallel (shunt) with the line, typically at the line ends or intermediate substations, to control the net reactive power balance of the line under varying loading conditions. Shunt reactors are switched in during light-load conditions to absorb the line's own excess charging reactive power (preventing the Ferranti-effect overvoltage that would otherwise occur at the receiving end of a lightly loaded or open-circuited long EHV line); shunt capacitors (or SVC/STATCOM in dynamic mode) are switched in during heavy-load conditions to supply the additional reactive power the line consumes due to its series reactance under load, supporting the voltage profile and increasing the effective power transfer capacity for a given acceptable voltage drop.

Comparative summary: series compensation primarily targets increasing power transfer capability and improving stability by reducing effective line reactance, and its effect is proportional to the line current (self-regulating with loading); shunt compensation primarily targets voltage control and reactive power balance at specific points (nodes) in the network, and its effect depends on the connected bus voltage rather than directly on line current. In practice, large transmission systems use a combination of both series and shunt compensation, often supplemented with modern FACTS devices (SVC, STATCOM, TCSC — Thyristor Controlled Series Capacitor), to achieve the desired balance of increased power transfer capacity, improved voltage regulation, and enhanced system stability across the full range of operating conditions.