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:

Q_SVC = V²B

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:

I = (E − V)/(jX)

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 .

Quick Trick

SVC → B (susceptance); STATCOM → V-source.

Why Other Options Are Wrong

A reverses the usual dynamic-performance comparison. C is incorrect because STATCOM is not the slower device. D is also opposite to the usual comparison; converter switching does create harmonics, but modern STATCOMs are not characterized simply as having ‘higher harmonics’ than SVCs as a defining advantage/disadvantage.

Common Mistake

Assuming both devices are just variable capacitors. Their circuit models and low-voltage VAR characteristics are fundamentally different.

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:

Q_SVC = V²B

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:

I = (E − V)/(jX)

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 .

Quick Trick

SVC → B (susceptance); STATCOM → V-source.

Why Other Options Are Wrong

A reverses the usual dynamic-performance comparison. C is incorrect because STATCOM is not the slower device. D is also opposite to the usual comparison; converter switching does create harmonics, but modern STATCOMs are not characterized simply as having ‘higher harmonics’ than SVCs as a defining advantage/disadvantage.

Common Mistake

Assuming both devices are just variable capacitors. Their circuit models and low-voltage VAR characteristics are fundamentally different.

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:

Q_SVC = V²B

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:

I = (E − V)/(jX)

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 .

Quick Trick

SVC → B (susceptance); STATCOM → V-source.

Why Other Options Are Wrong

A reverses the usual dynamic-performance comparison. C is incorrect because STATCOM is not the slower device. D is also opposite to the usual comparison; converter switching does create harmonics, but modern STATCOMs are not characterized simply as having ‘higher harmonics’ than SVCs as a defining advantage/disadvantage.

Common Mistake

Assuming both devices are just variable capacitors. Their circuit models and low-voltage VAR characteristics are fundamentally different.