Control Strategy and Stability Analysis of Impedance Isolated Medium Voltage UPS in Off-grid Operation

The impedance isolated medium voltage UPS proposed by ABB has superior performance, but the load power will be switched on and off at the megawatt level in off-grid operation, which will adversely affect the output power quality and stability of the system. At present, the above issue has not been discussed in relevant papers. In this paper, considering the voltage change of the leakage inductance of the step-up transformer caused by the load switching, a mathematical model of impedance isolated medium voltage UPS in off-grid operation is established, and its double closed-loop control strategy is proposed. The output impedance and input impedance of the system are deduced in detail. Moreover, based on the impedance ratio criterion method, the influence of the controller parameters and the equivalent leakage inductance of the step-up transformer on the system stability is analyzed. Finally, a simulation model of medium-voltage UPS in off-grid operation was built on PSCAD/EMTDC to verify the effectiveness and feasibility of the studied control strategy and stability analysis.