Provided is a method for performing a testing procedure of an electrical power system for a wind turbine by means of a power supply unit, wherein the method includes connecting the power supply unit to a low voltage distribution system of the wind turbine. The method further includes closing a low voltage circuit breaker so that electrical connection is provided between the low voltage distribution system and an auxiliary transformer. Electrical power to a power converter is provided from the power supply unit via the low voltage distribution system thereby energizing a direct current link of the power converter. The power converter is synchronized with an electrical grid, and a main transformer switchgear unit is closed, such that electrical connection is provided between a main transformer and the electrical grid.