The six-pulse rectifier feeding a standard drive draws current in sharp gulps around the voltage peaks. Multiply by fifty drives and the plant’s current waveform looks like a saw blade — that is harmonic distortion, and it is not academic.
What harmonics actually cost
Harmonic currents heat transformers and cables beyond what the kW figures suggest (the 5th and 7th dominate for six-pulse loads), trip breakers mysteriously, and interact badly with power-factor capacitor banks (resonance is a real fire-starter). On generator-fed systems — ships, islands, standby power — the tolerance is far lower: a generator’s source impedance is several times a utility’s, so the same drives distort the voltage much more. Marine rules cap voltage THD for exactly this reason.
The mitigation ladder
- 3 % line reactor or DC-link choke — cheap, drops current THD from ~80 % to ~35–40 %, protects the drive’s own capacitors, and should be near-default on drives above a few kW (many SINAMICS sizes include a DC choke — check, don’t assume).
- 12-pulse or multi-drive DC-bus systems — cancellation across rectifier groups, sensible at system design time.
- Passive harmonic filters — into the low-teens THDi, watch capacitive reactive power at light load.
- Active front end / low-harmonic drives — near-sinusoidal input plus regeneration; the clean answer where rules or generators demand it.
FAQ
Do harmonics hurt the motor? Motor-side effects come from the inverter PWM (a different topic — dV/dt, cable length); input harmonics hurt the supply side.
How do I know what I have? A power-quality logger for a week at the main incomer. Measure before buying filters — plants are regularly sold solutions to problems they do not have.
Zone Otomasyon measures power quality during retrofit surveys — especially on vessels. Book a survey.