A high performance DSP controller for three-phase PWM rectifiers with ultra low input current THD under unbalanced and distorted input voltage

Although in recent years three-phase PWM rectifiers have become increasingly popular, in most practical cases the line unbalance and/or the presence of distortion can seriously affect their performances. Line unbalance and distortion add unwanted low frequency harmonics to the input current, leading to an increase in the total harmonic distortion (THD). At high power level, the switching frequency is restricted to a few kilo hertz and therefore an LC filter is needed to eliminate the switching ripple of the current. The resulting structure is an LCL filter that causes instabilities and oscillations. This paper details how to implement a high performance multi-loop digital controller suitable for a boost type PWM rectifier, leading to an extremely low THD content even with non-ideal input voltage conditions. The multi-loop method uses only two capacitor filter voltages and inverter currents to estimate the line voltages and currents. Positive and negative sequence controllers are used to regulate the fundamental currents while resonant controllers tuned to each harmonic from third to thirteenth remove the low frequency components due to input voltage unbalance and/or distortion. The proposed approach is implemented on a 16-bit fixed point DSP (ADMC401) and tested on a three-phase 20 kW IGBT-based PWM rectifier.