Analysis of imbalanced voltage on 35kV side after single phase replacement of 500kV transformer

Abstract

After the B-phase of the 500kV transformer was replaced, the three-phase voltage on the 35kV low-voltage side was imbalanced and the neutral point displacement voltage was relatively high during the start-up test. For this problem, firstly, the calculation method of neutral point displacement voltage is given through the method of vector analysis and equivalent circuit, and the reasons for the imbalance of the three-phase voltage are analyzed. According to the measured parameters, the PSCAD/EMTDC power system simulation software is used to establish a model, and the steady-state operating characteristics of the system are simulated and calculated. Theoretical analysis and simulation calculation have determined that the cause of the imbalance of the three-phase voltage is the inconsistency between the ground capacitance of the low-voltage winding and the capacitance between the high-voltage and low-voltage windings of the B-phase transformer than that of the A-phase and C-phase. The impact of the capacitance between the high-voltage and low-voltage windings of the transformer is far greater than the capacitance of the low-voltage windings to ground on the imbalance of three-phase voltage to ground on the low-voltage side of the transformer.