Power Factor Correction: Capacitor Bank or Static Var Generator?

Why the power factor matters commercially

A low power factor means more current for the same useful power, which means larger cables, bigger switchgear and higher losses across the whole installation. On top of that, most utilities in export markets price it: either through a penalty on the bill, or through a specified minimum power factor that has to be maintained at the point of common coupling.

This is why compensation is usually a commercial requirement rather than an engineering preference — and why the equipment decision is often driven by the utility's tariff, not by the load itself.

Capacitor bank: the simple answer

A capacitor bank is switched in steps. Each step brings a fixed amount of reactive power, so the correction is coarse — the power factor is stepped rather than continuously held. At steady load this works well and costs very little: at steady state a capacitor bank has essentially no losses.

The weakness is transition. A stepped bank cannot follow a load that varies continuously. It will over-compensate when the load drops, which pushes the power factor below target and can trigger a penalty — the classic complaint about capacitor installations.

SVG: the fast answer

A static var generator injects or absorbs reactive power continuously, in milliseconds, by switching its own semiconductor output against the grid voltage. It holds the target power factor through a load change instead of stepping towards it.

The cost is losses — roughly 0.5% to 1% of rated output, from the switching and conduction losses in the converter — and a higher purchase price. For a load that swings continuously, the operating saving usually outweighs both.

Choosing between them

Use a capacitor bank where the load is steady over long periods — lighting, ventilation, a constant process load, a bank of pumps at fixed output. Here the low losses and low cost dominate, and the stepping behaviour never becomes visible.

Use an SVG where the load fluctuates fast: welding machines, rolling mills, hoists, and inverter-driven loads such as photovoltaic plants, where the reactive output moves with the available solar power.

Use both where a large steady base load dominates and a varying component rides on top — a stepped bank covers the base, and an SVG handles the fluctuation around it.

Harmonics and detuned reactors

Capacitors in an inductive network can resonate. Where the load contains significant harmonics — rectifiers, variable frequency drives, welding equipment — detuned reactors are fitted at 5.6%, 7% or 14% tuning to move the resonance point away from the harmonics present in the network.

The tuning percentage has to be matched to the harmonic spectrum of the load. Over-specifying the detuning reduces the usable kvar output of the bank, so this is a calculation, not a default.

What to send us for a quotation

To size compensation properly we need the load profile rather than the connected capacity: the typical and peak demand, the power factor at each point, the number of operating hours, and a list of the significant loads with their type. If harmonic measurements are available, those are the most useful input of all.

With that we can propose the kvar rating, the switching mode, the detuning and the step structure, and the control method — local, or integrated with the building management system or the plant SCADA.