A novel combined approach to voltage sag ride-through and current waveform improvement for adjustable-speed drives

A combined sub-circuit with voltage compensation and harmonic modulation is proposed to improve the voltage sag ride-through capability of adjustable-speed drives (ASD) with improved AC mains current waveform. The voltage compensation and harmonic modulation circuits share the same Y₀/Δ connected interface transformer. The harmonic modulation module injects a calculated amount of third harmonic current through the neutral at the primary side of the interface transformer and improves line current waveform. Upon detection of a voltage sag, the voltage compensation rectifier, supplied from the secondary of the interface transformer, outputs a controlled DC voltage in series with the reduced main rectifier´s output and thus keeps the DC-bus voltage at the desired level. Operating principle of the combined sub-circuit is presented. The theoretical analysis is verified by simulation using SABER and PSIM software. Some experimental results on a 12.5 kW prototype converter with third harmonic modulation are presented