Application of zig-zag transformers in a three-wire three-phase dynamic sag corrector system

The industrial and commercial sector is increasingly dependent on sensitive electronic and power electronic process controls and experiences severe productivity losses from power quality events. The single phase DySC (dynamic sag corrector) provides protection against 92% of all power quality events, operating on a line-neutral basis. A three-phase DySC system consists of three single phase DySC modules connected one per phase, each of which use the neutral point as a reference for the voltage regulator as well as for maintaining charge on the DC bus capacitors. The zig-zag transformer is a very cost effective method of forming this neutral point. During an unsymmetrical sag event, there is a significant zero-sequence component at the input of the DySC, and the zig-zag transformer which has low impedance for zero-sequence components, will source most of the current to the rectifier. The voltage distortion at the formed neutral point and at the DySC input terminals depends on the zero sequence impedance of the transformer and the input current The harmonic content of the formed neutral voltage has to be limited to be within the bandwidth of the inverter so that it can be canceled out by the action of the inverter voltage regulator. A detailed mathematical analysis of the zig-zag transformer is performed to obtain its impedance characteristics. The variation of zero sequence impedance with the turns ratio is also studied for zig-zag transformers wound using E-cores and symmetric cores. Simulation results that show the behavior of the neutral point and the current sourced by the zig-zag in a three-wire DySC system are included along with experimental plots that validate the simulations. This study provides a detailed understanding of the zig-zag transformer impedance characteristics, its effects when applied in a three-wire DySC system and provides a method to specify and design the transformer.