Feasibility Study of a 6.6kV, 1MW Transformerless BTB-Based Loop Controller

This paper achieves a feasibility study of a 6.6kV, 1MW loop controller that consists of a transformerless back-to-back configuration using two 5-level diode-clamped converters. However, the loop controller requires reducing the zero-sequence current circulating between the two distribution lines below than 0.2 A in rms, in order to avoid malfunction of line-to-ground fault protection relays. Moreover, all the dc voltages across four capacitors in the dc link have to be controlled equally. This paper presents a solution to these problems. Two common-mode chokes are installed at the ac side of each converter to suppress high-frequency zero-sequence currents, while feedback control is applied to eliminate low-frequency zero-sequence currents. Two bidirectional buck-boost dc-dc converters are employed to keep the four capacitor voltages equal. Simulation results verify viability and effectiveness of the loop controller, along with the developed theoretical analysis.