A Hybrid 18-Pulse Rectification Scheme for Diode Front End Variable Frequency Drives

Overview:

Diode rectifier with large DC bus capacitors, used in the front ends of Variable Frequency Drives (VFDs), draw discontinuous current from the power system resulting in current distortion and hence voltage distortion. Typically, the power system can handle current distortion without showing signs of voltage distortion. However, when the majority of the load on a distribution feeder is made up of VFDs, current distortion becomes an important issue. Multi-pulse techniques to reduce input harmonics are popular because they do not interfere with the existing power system either from higher conducted EMI when active techniques are used or from possible resonance, when capacitor based filters are employed.

In this paper, a new 18-pulse topology is proposed that has two six-pulse rectifiers powered via a phase-shifting isolation transformer, while the third six-pulse rectifier is fed directly from the AC source via a matching-impedance. This idea relies on harmonic current cancellation strategy rather than the flux cancellation method and results in lower overall harmonics. It is also seen to be smaller in size and weight, and lower in cost compared to an isolation transformer. Experimental results are given to validate the concept.