Reactive Power Compensation

Reactive power (Q) represents the energy oscillating between the magnetic field of an inductive load and the electric field of a capacitive source at twice the system frequency. Unlike active power (P), which performs net work, VARs do not propagate efficiently over long distances. The relationship is defined by the power triangle: S² = P² + Q², where S is apparent power. Compensating Q locally reduces the magnitude of S, directly lowering the I²R losses in conductors and transformers. This is governed by the lagging (inductive) or leading (capacitive) phase angle between voltage and current.

The most ubiquitous and cost-effective method for injecting reactive power. A shunt capacitor bank provides a leading current that partially cancels the lagging inductive current drawn by motors and transformers. Key characteristics include:

• Fixed Banks: Energized continuously, suitable for base-load compensation of transformers during light-load periods to prevent leading power factor.
• Switched Banks: Activated via circuit breakers or switches based on time-of-day, temperature, or voltage setpoints.
• Step Size: The discrete nature of capacitor steps can cause voltage flicker if the inrush current is not managed with pre-insertion resistors or zero-crossing closing.

A synchronous condenser is a wound-field synchronous machine operating without a mechanical prime mover. By adjusting the DC field excitation current, the machine can smoothly transition from absorbing reactive power (under-excited, inductive behavior) to injecting reactive power (over-excited, capacitive behavior). This provides dynamic VAR reserve with high inertia, contributing to system short-circuit strength and frequency stability. Unlike power electronics, synchronous condensers provide genuine fault current contribution, making them critical for weak grids with high renewable penetration.

An SVC combines thyristor-controlled reactors (TCR) with fixed or mechanically switched capacitors to provide rapid, continuously variable reactive power control. The TCR varies the inductive current by phase-angle control of the thyristor valves, effectively creating a variable inductance. Key subsystems:

• TCR: Provides smooth inductive range.
• TSC (Thyristor-Switched Capacitor): Provides stepped capacitive range without transients.
• Filter Banks: Mitigate the characteristic harmonics generated by the thyristor switching. SVCs respond within 1-2 cycles, making them suitable for flicker mitigation in arc furnace applications.

A STATCOM is a voltage-source converter (VSC) based shunt device that synthesizes a voltage waveform from a DC capacitor bank. It behaves as a fully controllable current source behind a coupling reactance. If the converter output voltage magnitude is greater than the system voltage, it injects capacitive current (leading); if lower, it injects inductive current (lagging). Advantages over SVCs include:

• Superior low-voltage performance: Output current capability is independent of system voltage magnitude.
• Smaller footprint: No large passive reactors or capacitor banks required.
• Active filtering capability: Can inject harmonic currents to cancel distortion.

The practical objective of reactive power compensation is to improve the displacement power factor (DPF) from a lagging value (e.g., 0.7) toward unity (1.0). This is achieved by supplying the magnetizing VARs locally rather than drawing them from the utility source. Benefits include:

• Released System Capacity: Reducing kVA demand frees up thermal headroom on transformers and feeders.
• Reduced Losses: A power factor improvement from 0.7 to 0.95 reduces line current by approximately 26%, which reduces resistive losses by roughly 45%.
• Avoided Penalties: Utilities impose reactive demand charges or kVA billing to incentivize high power factor at the point of common coupling (PCC).

A centralized or distributed control strategy that coordinates voltage regulators and reactive power sources to minimize system losses and energy consumption while maintaining voltage within ANSI C84.1 limits. VVO represents the overarching framework within which reactive power compensation operates, using real-time sensor data and optimization algorithms to determine the optimal setpoints for capacitor banks, load tap changers, and smart inverters.

The automated switching of shunt capacitor units based on time-of-day schedules, temperature, or real-time voltage measurements to inject reactive power and boost local voltage profiles. Modern controllers use peer-to-peer communication to coordinate multiple banks on a feeder, preventing hunting and overvoltage conditions. Key control modes include:

• Time-based: Fixed schedule switching
• Voltage-based: Local measurement threshold
• Temperature-based: Correlated with load
• VAR-based: Reactive power demand following

A demand-side management technique that intentionally lowers service voltage to the lower bound of the allowable range (typically 114V on a 120V base) to reduce energy consumption without requiring customer action. The effectiveness is quantified by the CVR factor (CVRf)—the percentage reduction in active power per one-percent voltage reduction. Reactive power compensation enables CVR by flattening the voltage profile, allowing operators to lower the source voltage without violating minimum voltage constraints at the feeder end.

The capability of a grid-tied photovoltaic inverter to dynamically modulate its reactive power output to regulate voltage, as mandated by IEEE 1547-2018. Key control modes include:

• Volt-VAR control: Piecewise linear curve defining Q injection/absorption based on terminal voltage
• Volt-Watt control: Active power curtailment during overvoltage
• Fixed power factor: Constant reactive power ratio
• Dynamic VAR reserve: Holding reactive capacity for contingency response

These inverters transform distributed generation from a voltage problem into a voltage solution.

A power electronics device combining thyristor-controlled reactors (TCR) and fixed capacitors to provide fast-acting, continuously variable reactive power injection for dynamic voltage support. Unlike mechanically switched capacitors, an SVC can respond within 1-2 cycles to voltage disturbances, making it essential for:

• Flicker mitigation from arc furnaces
• Transient stability support
• Subsynchronous resonance damping

SVCs represent the high-speed, high-power end of the reactive compensation spectrum.

A voltage-source converter (VSC)-based shunt device that injects a balanced, sinusoidal current to mitigate voltage flicker, correct power factor, and balance load currents at the distribution level. Unlike SVCs, DSTATCOMs use IGBT-based switching for superior harmonic performance and can supply full rated reactive current even at depressed voltages. They are the preferred solution for:

• Unbalanced load compensation
• Voltage sag ride-through
• Renewable integration at weak grid points