SVG Sizing for Power Factor Correction

SVG sizing for power factor correction cannot be estimated from transformer capacity or a motor nameplate alone. Those figures describe how much load a site could connect, not how much reactive power the load actually exchanges with the supply at the point of common coupling (PCC). A Static Var Generator chosen from nameplate data will either leave the power factor short of target or spend on capacity the load never demanded. The correct rating comes from a measured load profile, a defined target, and a network checked for harmonics, not from an assumed one.

The Cause: Transformer kVA and Motor Nameplate kW Hide Reactive Demand

Three quantities describe the same load. Active power (kW) performs useful work; apparent power (kVA) is what the transformer, cables and switchgear must carry; kvar is the reactive component exchanged with inductive equipment without doing work. They are linked by kVA² = kW² + kvar², and power factor is kW ÷ kVA.

A transformer rating is a thermal capacity limit, not a measure of reactive demand, and a motor nameplate shows shaft output rather than the magnetising current drawn at part load. A 1000 kVA transformer can serve a site needing 150 kvar of correction or one needing 500 kvar; the nameplate cannot tell the two apart.

The Consequence: Undersizing, Oversizing and Resonance Risk

When the compensator is sized from the wrong input, the failure appears in one of three forms.

• Undersized: power factor stays below the contractual target, current and losses stay high, and the shortfall grows as loads are added.
• Oversized: capital cost rises for kvar the site does not use, and a capacitive system can drift into leading power factor at light load.
• Resonance: fixed capacitors in a harmonic-rich network form a parallel resonance with the supply impedance, which can amplify harmonic current and overheat equipment.

Sizing here meets the harmonic study: if the network already carries significant distortion, capacitive compensation must be checked against the harmonic spectrum before it is added.

Measurement First: What to Record at the PCC

Correct sizing starts with a power quality survey at the PCC. Current transformers (CTs) on the load side of the connection record the true load current, and a logger captures the load profile, meaning how kW, kvar and power factor move through a normal cycle rather than in a single spot reading.

CT placement is critical: the CT should measure the load current the compensator is meant to correct, so the SVG's own output is excluded from the feedback signal. If the CT includes the SVG current, the closed control loop fights its own injection and the power factor never settles at target.

Where no long record exists, a power quality report can still supply these values if it was taken at the right point. The note on reading a power quality report explains how survey columns map onto sizing inputs.

From Measurement to kvar: The Sizing Calculation

The compensated reactive power is the difference between the reactive power at the measured power factor and at the target:

Required kvar = kW × (tan φ1 − tan φ2)

Here φ1 is the angle of the measured power factor and φ2 the angle of the target. A worked case: at 500 kW, corrected from 0.75 to 0.95, the sum is 500 × (0.882 − 0.329) ≈ 277 kvar, so the next standard rating at or above that figure is selected, with margin if the profile shows growth or fast swings.

A fixed, step-switched source offsets a steady inductive (lagging) demand, while a dynamic source such as an SVG injects leading reactive power when the load is inductive and absorbs it when the load turns capacitive. That difference between inductive and capacitive compensation is why a static bank and a fast compensator are not interchangeable on a variable load.

How an SVG Delivers the Compensation

An SVG is a voltage-source converter connected in parallel with the load. Its power stage uses a multi-level (3-level) topology with an IGBT bridge switching at high frequency, and the coupling inductor and filter shape the current so the injected waveform stays close to a clean sinusoid. Reading load current through the CT, the controller calculates the reactive current in real time and the bridge injects or absorbs it within a few cycles, correcting power factor dynamically rather than in discrete steps.

Because the bridge handles steep voltage transitions, the design controls dv/dt through the filter and magnetics. Thermal design matters as much as the electrical rating: SVG datasheets typically define the continuous rating at 45 °C full-load ambient, and usable output must be derated above that limit.

Choosing the Right Device: SVG, Hybrid or AHF

Sizing decides how many kvar; the network decides which technology.

The boundary is straightforward: an AHF targets harmonic current and does not correct fundamental displacement power factor to a meaningful degree, while an SVG corrects displacement and does not remove harmonic distortion. Where both limits are exceeded the two functions are combined. For a closer comparison, see the guide to active versus passive harmonic filters.

Limitations: What SVG Sizing Cannot Fix

A correctly sized SVG improves displacement power factor and reactive power balance, but it is not a universal remedy. It does not cancel harmonic current, so a distorted network still needs an AHF; it depends on accurate CT measurement and correct CT placement, and a wrong feedback signal produces a wrong result whatever the rating. Its continuous output is limited by ambient temperature and derating, and it does not replace protection devices or compensate DC offset and severe unbalance on its own.

Sizing also cannot promise a fixed saving or payback. The reduction in current, losses and any utility charge depends on the real load profile, tariff and how consistently the target is held; losses and capacitor banks also interact with harmonics in ways a single estimate cannot capture. The note on capacitor bank overheating after adding VFDs shows how resonance risk appears in practice. Good sizing yields a compensator matched to measured demand, not a guaranteed percentage.

Frequently Asked Questions

How do you estimate the SVG capacity for a site?

From measured kW and power factor, not the transformer plate. Record the load profile at the PCC, take the worst-case kW and the lowest sustained power factor, set the target, then apply kvar = kW × (tan φ1 − tan φ2) and select the next standard rating at or above the result.

Where should the CT for an SVG be installed?

Where it measures the load current the SVG is meant to correct, so the SVG's own output is excluded from the feedback loop. A CT that includes the compensator current makes the controller regulate against its own injection.

Do I need both an SVG and an AHF?

Only if both problems exist. Use an SVG when the issue is displacement power factor or kvar, and an AHF when the issue is harmonic current and THDi. When both are out of limits they are combined, sometimes inside one hybrid or multi-function unit.

What maintenance and payback should be expected?

An SVG has no moving parts and no step contactors, so routine work is mainly cooling-path checks, filter and fan inspection and CT wiring verification. Payback is site-specific and cannot be fixed in advance, so any single ROI figure should be treated with caution.

Next Step: Size Against Your Own Measurement

The reliable route to a correct SVG rating is a survey of the actual site, starting from measured kW, power factor and load profile. The guide to power factor and reactive power covers the underlying economics, and the Static Var Generator SVG range shows the module ratings available for low-voltage correction. Send your survey data (kW, power factor, target and harmonic level) and YT Electric can review the sizing basis for your project.