Active or Passive Harmonic Filter? How to Choose the Right Solution for Power Quality

Harmonics form when nonlinear loads such as variable frequency drives (VFDs), UPS systems, rectifiers and arc furnaces draw non-sinusoidal current and inject harmonic currents into the network. These currents contain components at integer multiples of the 50 Hz fundamental frequency, most often the 5th, 7th, 11th and 13th orders in industrial plants.

Harmonics lead to transformer overheating, capacitor damage and failures of sensitive equipment. Compensation capacitors can also resonate with the network, which shows up as blown fuses, swollen capacitors and premature failure. Knowing which orders dominate, and how they change with the load, is the starting point for choosing a filter.

Filter selection starts with a measurement because the right filter depends on which harmonic orders are present, how large they are and how they change with the load. A filter chosen from rules of thumb can be undersized or tuned to the wrong order, and in the worst case it can create a new resonance with the plant's capacitors and the network.

A useful measurement records the harmonic spectrum order by order across all load conditions, typically over at least one week. The existing capacitor banks and the transformer and network short-circuit data are reviewed at the same time, because together they determine where a resonance point could fall.

The limits a filter has to meet come from the IEC 61000 series and IEEE 519; keeping total harmonic distortion (THD) below 5% is a widely accepted industry target. These limits are explained in our separate guide, What Is Total Harmonic Distortion (THD)? Measurement and Limits.

The load profile decides. Passive filters suit plants with a stable harmonic profile and one or two dominant orders, while active filters suit plants with variable loads and a broad harmonic spectrum.

Passive filters are tuned reactor-capacitor combinations that target a dominant order such as the 5th. They are economical and robust, but they are fixed to their tuning, and because they also supply reactive power at the fundamental frequency, they have to be designed together with the plant's compensation. Line or DC chokes at drive inputs are a simple first step to reduce current distortion.

Active harmonic filters inject harmonic currents in antiphase in real time and suppress a wide range of orders dynamically. They adapt as the load changes, which makes them the most flexible option for plants with many different harmonic sources, at a higher cost than passive filters. In some plants the best answer is a combination of the two.

Miratek brings plants' power quality into line with IEC standards using active and passive harmonic filter solutions, selected according to the measured harmonic spectrum. We deliver complete engineering for industrial plants across Türkiye, from on-site measurements to filter design, harmonic filter panel manufacturing, installation and commissioning. Because harmonics and reactive power interact, we design harmonic mitigation and compensation together to avoid resonance.