Reactive Power Compensation in Power Systems: Capacitor Banks, SVC and SVG Technology Compared

Introduction

Reactive power is essential for maintaining voltage stability and efficient power transfer in AC systems, but excessive reactive power flow increases line losses, reduces available capacity and degrades power factor. Reactive power compensation aims to balance the reactive power demand at the point of use, improving power factor, stabilizing voltage and reducing electricity costs. This article compares the main compensation technologies and provides selection guidance.

Why Reactive Power Compensation Matters

Inductive loads such as motors, transformers and welding equipment consume reactive power, causing the power factor to lag. A low power factor means that for the same active power, higher current flows, increasing losses in conductors and transformers and reducing the system's ability to deliver active power. Utilities typically impose power factor penalties, and improving the power factor from 0.8 to 0.95 can reduce losses by roughly 30% and release significant system capacity.

Fixed and Automatic Capacitor Banks

Capacitor banks are the most widely used compensation devices due to their low cost and simple construction. Fixed capacitor banks provide a constant amount of compensation and are suitable for loads with stable reactive power demand. Automatic switching capacitor banks use contactors or thyristor switches controlled by a reactive power controller to connect or disconnect capacitor steps according to the measured power factor, matching compensation to load variations.

Capacitor banks must be protected against overvoltage, overcurrent and harmonic overloading, and discharge devices must be provided for safety. Series reactors are often installed with capacitor banks to limit inrush current and detune the bank from dominant harmonic frequencies, preventing harmonic resonance.

SVC: Static Var Compensator

A Static Var Compensator (SVC) provides fast, continuously variable reactive power control using thyristor-controlled reactors (TCR) and thyristor-switched capacitors (TSC). SVCs respond within one cycle, making them suitable for voltage stabilization at transmission level, arc furnace compensation and rapidly fluctuating loads. The main advantages of SVC are mature technology, high reliability and proven performance, while the disadvantages include relatively large footprint, harmonic generation from the TCR and continuous losses in the reactor branch.

SVG: Static Var Generator

A Static Var Generator (SVG), also known as a static synchronous compensator (STATCOM), uses a voltage-source converter based on IGBT power electronics to inject or absorb reactive power with extremely fast response, typically within milliseconds. SVG can provide both capacitive and inductive compensation smoothly, handles unbalanced loads and offers excellent harmonic mitigation capability when configured with active filtering functions. Its advantages include compact size, fast response and low harmonic output, while its higher cost and more complex maintenance requirements are the main trade-offs.

Selection Guidance

For general industrial and commercial facilities with moderate load variation, automatic capacitor banks with a reactive power controller offer the best cost-effectiveness. For facilities with rapidly fluctuating loads, such as welding plants and arc furnaces, SVC or SVG is required to provide dynamic compensation and flicker mitigation. For harmonic-heavy environments, SVG with active filtering or capacitor banks with appropriate series reactor detuning should be selected. At transmission level, SVC has been the standard choice for voltage support, with SVG increasingly adopted for new installations due to its superior dynamic performance.

Conclusion

The choice of reactive power compensation technology depends on load characteristics, required response speed, harmonic environment, budget and maintenance capability. A well-designed compensation system improves power quality, reduces energy costs and enhances the reliability and efficiency of the entire power network.