Previous Year Question

Correct answer: B — STATCOM acts as a voltage source behind a reactance and SVC acts as a variable susceptance.

SVC: variable shunt susceptance

A Static Var Compensator (SVC) regulates reactive power using shunt capacitor and reactor branches controlled or switched by thyristors. Its fundamental-frequency equivalent is a controllable susceptance B connected to the bus. With capacitive injection defined as positive:

At the limit of available capacitive susceptance, its maximum capacitive VAR output falls approximately with the square of bus voltage. This is an important limitation during a deep voltage reduction.

STATCOM: controllable voltage behind reactance

A STATCOM uses a voltage-source converter connected to the network through coupling reactance. If its converter voltage E is approximately in phase with bus voltage V, the voltage-magnitude difference drives reactive current:

If E is greater than V, the STATCOM supplies capacitive reactive power to the bus. If E is smaller than V, it absorbs reactive power. The coupling reactance X limits and controls this current exchange.

A STATCOM can maintain approximately its rated reactive current over part of its low-voltage operating range, subject to converter limits. Its reactive power still decreases with voltage because Q is proportional to VI. The decisive distinction in this question is the equivalent model: variable susceptance for SVC; voltage source behind reactance for STATCOM.