Abstract: A primary current injection test was carried out on the 400kV main transformer using a diesel generator. Theoretical calculations and analyses based on actual value and per-unit value principles were performed respectively under three test wiring configurations: injecting current on the high-voltage side with the low-voltage side short-circuited, injecting current on the low-voltage side with the high-voltage side short-circuited, and injecting current on Low-Voltage Side 1 with Low-Voltage Side 2 short-circuited. Actual on-site testing of the project verified the correctness of the secondary current circuits for protection, metering and measurement on all sides of the main transformer. In addition to the phase-separated differential protection of the main transformer, the secondary current circuits for zero-sequence differential protection on both the high-voltage and low-voltage sides of the main transformer were also validated. It provides a technical reference for hand-over-acceptance tests and safe-stable operation of 400?kV main transformers.