Three-loop digital control strategy combining PI and quasi-PR controller for high tracking precision power supply used in ZnO characteristics testing

In the zinc oxide (ZnO) varistor characteristics testing, high tracking precision and stability are required both for the output voltage and load current, while the load is severely nonlinear. In this paper, a three-loop control strategy combining PI and Quasi-PR compensator is presented. Firstly, the inner-loop controls the inductor current using a PI controller to suppress the bias direct current (DC) flowing through the transformer. Secondly, the middle-loop controls the output voltage combining PI and Quasi-PR compensator to improve the tracking precision for sinusoidal waveform control. Finally, the outer-loop is a DC component of ZnO current feedback loop applying a PI compensator to achieve high control performance of the ZnO current. The proposed control strategy was verified in a 10kW testing power supply. Experimental results show that the tracking precision is nearly 1% and the THD is 0.8% under the proposed three-loop control strategy.