HARMONICS
Harmonic mitigation needs to match the loads producing the distortion and the network supplying them. Distorted currents, overheating, neutral loading or compatibility issues may call for filtering. Active and passive filters offer different approaches, which must be assessed at the intended connection point and under actual operating conditions. THDI needs to be read alongside actual current, the harmonic spectrum and voltage distortion.
These measure currents and simultaneously inject compensating components at several harmonic frequencies, providing broadband compensation within the limits of the device. Compensation can respond within a few milliseconds, depending on the model and control mode. Measurement location, CT arrangement, available current and priorities between any additional functions affect the result.
These are networks of reactors, capacitors and, where needed, resistors. Once the filter is connected, compensation acts instantaneously, without waiting for an electronic controller; the circuit still has its own transients. Tuned filters normally act on one or more harmonic orders and frequencies close to their tuning points. Wider-band configurations and filters designed for particular converters are also available. Selection must consider resonance, harmonic loading from the supply and operation at different load levels.
Preliminary study of the electrical installation and targeted measurements to define the filter requirements and functional details. Support includes review of existing recordings, comparison of central filtering and measures at individual loads, and post-installation measurements to verify operation of the installed system and the results achieved.
General guidance by solution type, using a 400 V low-voltage supply as reference. Ranges summarise the ratings considered and do not imply that all combinations are available. Connection, rating and actual performance must be checked for the project.
| Configuration | Ratings and supply | Features | Checks required |
|---|---|---|---|
| Active filter | 20–400 A; three-phase 400 V supply | Multi-frequency compensation; installation and modularity defined for the project | Available current, bandwidth and compensation priorities |
| Passive converter filter | 1.1–250 kW; three-phase 400 V supply | Circuits selected for the converter and harmonic spectrum | Load, supply impedance and performance at the relevant frequencies |
The stated current is filter current, not total installation current. Residual distortion targets depend on the load, supply and verification conditions.
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