Cascade active power filter based on ZVS DC-link clamping circuit and hierarchical control for medium voltage applications

A cascade active power filter (CS-APF) with additional stages can be directly connected to medium-voltage networks without a transformer. However, the system has a large number of stages and is quite complicated. The system also has a direct current (DC)-link capacitor voltage unbalance. The hierarchical control strategy for CS-APF described in this paper separates the top main function control and the bottom unit control and enlarges the computational time margin for the main control algorithm. The complicated algorithm can still be implemented with quick response. An auxiliary circuit using soft-switch control for DC-link capacitor voltage clamping transfers energy between the capacitors to balance their voltages and ensure system reliability. The auxiliary circuit operates as a zero-voltage switch using phase and mode control. The circuit switching frequency and efficiency are improved and the electromagnetic interference (EMI) noise is suppressed. Both simulations and experiments show that the hierarchical control and the clamping circuit enable stable operation of the medium-voltage CS-APF.