Variable-speed drives distort the ac line feeding the drive. The distortion is described as harmonic content.
Distortion comes mainly from rectifying the incoming 3-phase ac to dc using a bridge rectifier. Bridges producing a very low dc voltage have high harmonics. The bridge on an ac drive runs at its maximum voltage so this source of harmonics is at a minimum with ac drives compared with dc drives.
A bridge feeding a capacitor draws square-wave current from the ac input which has a high harmonic content. All AC drives have capacitors on the dc link.
There are limits on the harmonic content permitted as established by IEEE Std 519. This becomes an issue if a large portion of a plant's electrical load is consumed by variable speed drives.
To comply with IEEE 519, several techniques have been introduced:
- Harmonic cancellation – feed part of the plant with Wye connected transformers and the other part with delta connected transformers. This is useful for canceling 5th and 7th harmonics. To be really effective, the transformers must be equally loaded, have equal impedance and equal output voltages.
- Reactors may be used to reduce harmonics
- Multi-step rectifiers will reduce harmonics.
- Many drives have IEEE 519 compliant options that rely on filtering capacitors and inductors, maximum lead lengths and specified switching frequencies. Contact your drive vendor.