A static var generator (SVG) is a power-electronic device that measures the reactive component of load current and injects an equal, opposite current into the electrical system. The source then supplies mainly active current, improving displacement power factor and reducing unnecessary kVA loading. Unlike a capacitor bank, an SVG can regulate continuously in both capacitive and inductive directions and follow rapidly changing loads.
Quick answer: an SVG uses current transformers, voltage sensing, a digital controller, a DC-link capacitor, IGBT power modules and coupling inductors. The controller transforms three-phase measurements into a rotating reference frame, calculates the compensation command and uses pulse-width modulation to make the converter inject the required current. System response can be within milliseconds, but the guaranteed value must specify current rise, settling time and measurement point.
| Component | Function | Critical design detail |
|---|---|---|
| CTs and voltage sensing | Measure load/source current and bus voltage | Ratio, polarity, phase order and location |
| Digital controller | Calculates reactive, harmonic and unbalance references | Algorithm, priorities and stability |
| DC-link capacitor | Maintains converter DC voltage and transient energy | Voltage ripple, precharge and lifetime |
| IGBT converter | Creates the controlled AC voltage/current | Switching frequency, current and thermal limits |
| Coupling inductor | Controls current slope and filters switching ripple | Inductance, saturation and losses |
| Protection and cooling | Maintains safe operation | Overcurrent, overtemperature, fans and airflow |
A simplified single-line path is: bus → breaker/fuse → coupling inductor → IGBT voltage-source converter → DC-link capacitor. CTs are installed at the load or source side according to the control architecture, and voltage leads provide synchronization and magnitude information. The SVG is connected in parallel, so the load remains supplied if the SVG is isolated for maintenance.
In the common voltage-oriented reference frame, the d-axis is aligned with the grid voltage. Active current is mainly represented on one axis and reactive current on the other. The exact sign convention varies by manufacturer, so commissioning documents should state whether positive kvar means injection or absorption. What matters is that the upstream current after compensation meets the specified power factor or voltage target.
When a load draws lagging reactive power, the SVG injects capacitive current. When the system is leading—perhaps because fixed capacitors remain connected during light load—the SVG absorbs reactive current. This bidirectional behavior is one of the main differences from a conventional capacitor bank. The SVG can move smoothly through zero without waiting for contactors to switch or discharge resistors to reduce capacitor voltage.
The rating applies in both directions only if the datasheet explicitly guarantees a symmetrical range. Some designs allocate capacity differently or derate at high/low bus voltage. Ask for continuous current and kvar capability across the stated voltage and ambient-temperature range, not only a nominal ±kvar value.
Manufacturers may describe SVG response as below 5 ms, below 10 ms or within one cycle. These numbers may refer to detection time, converter-current rise or settling of the measured power factor. A plant-level result also includes CT filtering, communication, transformer impedance and grid dynamics. A useful specification defines an initial and final kvar, allowable overshoot, settling band and the point where the result is measured.
For a welding line, crane or press, the cycle can change faster than a stepped capacitor bank. An SVG should be tested against the actual repetitive duty, including thermal accumulation. A fast controller is not useful if the converter reaches current limit during every peak. Trend the compensated and uncompensated current together to verify capacity.
A three-wire SVG connects to the three phase conductors and is suitable for balanced or unbalanced three-phase compensation where neutral-current control is not required. A four-wire SVG includes a neutral connection and a converter topology able to control zero-sequence current. It can compensate phase imbalance and neutral current from single-phase loads, subject to its per-phase and neutral ratings.
Do not select four-wire equipment merely because the distribution system has a neutral. Measure the zero-sequence and neutral current and define the target. Triplen harmonic current, fundamental imbalance and neutral displacement are distinct components. Confirm whether the product rating is total three-phase current, per-phase current or includes a separate neutral limit.
| Requirement | SVG | Capacitor bank |
|---|---|---|
| Changing load | Continuous millisecond control | Discrete stages and switching delay |
| Leading and lagging | Bidirectional | Primarily capacitive unless reactors are controlled |
| Light-load overcompensation | Can regulate near zero kvar | Minimum step may be too large |
| Harmonic environment | No resonant capacitor stages, but compatibility study required | Detuning/resonance study essential |
| Cost per kvar | Higher power-electronic cost | Usually lower for stable bulk kvar |
| Maintenance | Fans, capacitors, electronics | Contactors, capacitors, reactors |
Many facilities benefit from a hybrid arrangement: capacitor stages provide stable base kvar and the SVG corrects the fast residual. This can reduce converter rating while avoiding excessive switching. The control must coordinate both devices and prevent leading power factor after a production line stops. See our hybrid SVG and capacitor sizing guide.
A 400 V workshop records 600 kW at 0.78 displacement power factor during peak production. The initial reactive power is approximately P × tan(cos⁻¹0.78), or 481 kvar. To reach 0.98, the final reactive power is about 122 kvar. The theoretical compensation is therefore roughly 359 kvar. A 400 kvar SVG may appear suitable, but this single point is not enough.
The engineer should plot kvar over complete shifts, include leading conditions during idle periods, verify voltage range and allow for a future load. If short peaks reach 460 kvar for two seconds, check the SVG overload curve rather than increasing the continuous rating automatically. If 250 kvar is stable and only 110 kvar changes quickly, a 250 kvar detuned capacitor bank plus a 150 kvar SVG may provide a lower-cost hybrid solution.
Commission with the SVG disabled first. Verify voltage, CT ratio, direction, phase sequence and the analyzer’s kvar sign. Enable a limited output, confirm the source kvar moves toward the target, then increase capacity and test leading as well as lagging conditions. Record alarms, thermal response and behavior after communication loss or CT failure.
Low-voltage SVGs correct industrial power factor close to loads. Medium-voltage systems scale the voltage-source-converter concept for renewable plants, substations, mining and rapidly changing industrial processes. At transmission scale, STATCOMs support voltage and stability. CIGRE provides technical knowledge on power-electronic and power-system applications; project requirements should also follow the applicable grid code and utility specification.
It exchanges mainly reactive power, but it draws active power to cover semiconductor, reactor, DC-link, fan and control losses. Include those losses in lifecycle cost and ventilation design.
A standard SVG primarily controls fundamental reactive current. Some advanced SVG products also compensate selected harmonics and imbalance. Confirm the supported orders, current allocation and residual guarantee instead of assuming AHF performance.
Ratings may be stated in kvar or amperes at nominal voltage. Check continuous current, capacitive and inductive range, voltage derating, ambient-temperature derating, overload duration and whether functions share the same current capacity.
Elumotive supplies 400 V advanced SVG, 440 V ASVG and 480 V ASVG platforms. To size a static var generator, send the voltage, CT arrangement, kvar trend, power-factor target, load cycle and harmonic measurements.
