Static VAR Compensator (SVC)

The core variable inductance element of an SVC. A thyristor valve phase-controls the current through a fixed air-core reactor.

• Firing Angle Control: Varies conduction angle from 90° (full conduction) to 180° (zero conduction)
• Response Time: Typically 1-2 cycles of system frequency (16-33 ms at 60 Hz)
• Harmonic Generation: Produces odd-order current harmonics requiring passive filtering
• Continuous Range: Provides infinitely variable inductive compensation from zero to rated MVAR

A shunt capacitor bank switched by anti-parallel thyristor pairs, providing stepped capacitive reactive power without transients.

• Zero-Voltage Switching: Thyristors gate at the instant capacitor voltage equals system voltage, eliminating inrush current
• Stepwise Control: Discrete capacitive steps, unlike the continuous TCR range
• No Harmonic Generation: Pure fundamental-frequency current when fully conducting
• Transient-Free Operation: Can be switched multiple times per second without discharge cycles

The steady-state terminal voltage versus reactive current output defines the SVC regulation envelope.

• Slope Reactance: A deliberate 1-5% droop setting prevents hunting between adjacent voltage controllers
• Linear Control Range: Between inductive and capacitive limits, voltage is maintained per the droop characteristic
• Susceptance Limits: Beyond rated capacity, the SVC behaves as a fixed capacitor or reactor
• Overload Capability: Short-term overload typically 1.5-2.0 pu for transient stability support

SVC installations inherently include passive filters that serve dual purposes.

• TCR Harmonics: 5th, 7th, 11th, and 13th harmonic currents are absorbed by tuned shunt filters
• Capacitive Compensation: Filters provide a fixed capacitive MVAR base at fundamental frequency
• Tuning Configurations: Single-tuned, double-tuned, and high-pass damped filter topologies
• Total Harmonic Distortion: Designed to meet IEEE 519 limits at the point of common coupling

SVCs are the primary solution for suppressing rapid voltage fluctuations caused by arc furnaces and other fluctuating industrial loads.

• Flicker Frequency Range: Effectively compensates fluctuations from 0.5 to 25 Hz
• Short-Circuit Voltage Depression (Pst): Reduces perceptibility index below the IEC 61000-4-15 planning level of 1.0
• Feed-Forward Control: Uses measured furnace current to anticipate reactive power demand before voltage deviation occurs
• Asymmetric Control: Individual phase control can balance negative-sequence currents

SVCs inject maximum capacitive reactive power during fault-induced voltage depressions to support generator synchronizing torque.

• First Swing Stability: Maintains voltage magnitude during the critical post-fault period
• Voltage Recovery: Accelerates post-fault voltage restoration, reducing risk of induction motor stalling
• Power Transfer Limit: Increases the steady-state and dynamic power transfer capability of long transmission corridors
• Subsynchronous Resonance: SVC controls can be augmented with supplemental damping to mitigate torsional interactions

The core variable inductance element within an SVC. A thyristor valve phase-controls the current through a fixed shunt reactor, allowing for continuous and rapid variation of inductive reactive power absorption. The firing angle delay from 90° (full conduction) to 180° (zero conduction) provides the smooth variability that distinguishes SVCs from mechanically switched devices.

A capacitor bank switched by anti-parallel thyristors that provides stepped capacitive reactive power injection. Unlike the continuous TCR, TSCs operate in a binary on/off state to avoid harmonics and transients. Multiple TSC branches are often combined with a TCR to provide a wide, continuously variable reactive power range from net inductive to net capacitive.

SVCs provide dynamic voltage support by injecting capacitive reactive power during voltage sags and absorbing inductive reactive power during voltage swells. This sub-cycle response (typically 1-2 cycles) prevents voltage collapse in weak transmission corridors. Key applications include:

• Mitigating flicker from electric arc furnaces
• Stabilizing HVDC converter terminal voltages
• Supporting long radial transmission lines with poor regulation

The phase-controlled TCR inherently generates odd-order harmonic currents (3rd, 5th, 7th, 11th) that must be trapped to prevent distortion of the voltage waveform. SVC installations include tuned passive filters that serve a dual purpose: providing fixed capacitive reactive power at the fundamental frequency while presenting a low-impedance path for specific harmonic frequencies to ground.

While both provide dynamic reactive power, the Static Synchronous Compensator (STATCOM) uses voltage-source converter technology rather than thyristor-switched passive elements. Key distinctions:

• SVC: Lower cost, higher losses, physically larger, requires harmonic filters
• STATCOM: Superior low-voltage performance, smaller footprint, faster response, higher capital cost
• SVC output current decreases linearly with voltage; STATCOM maintains rated current down to very low voltages

Beyond steady-state voltage regulation, SVCs can be equipped with a Power Oscillation Damper (POD) controller. This supplementary control loop modulates the SVC's reactive power output in response to detected low-frequency electromechanical oscillations (0.1-2.0 Hz) on the transmission system, providing positive damping to inter-area modes that could otherwise limit power transfer capability.