The unprecedented high speed switching of Wide Bandgap (WBG) power devices may change the partial discharge (PD) mechanism inside power modules. However, very few research have been published on PD research methods and PD behaviors under ultra-high dv/dt excitation voltages. To address these problems, a three-step PD study method is proposed in order to comprehensively study PD mechanism inside power module with ultra-high dv/dt excitation voltages. The three-step study consists of PD tests under single pulse, repetitive pulses, and Sinusoidal Pulse Width Modulation (SPWM) pulses. The preliminary results of single pulse tests show influences of the rise time and the pulse width on PD behaviors in power modules. Based on test data, a PD model based on the bubble expansion theory is provided to explain results. The results presented in this paper can provide a better understanding of PD mechanism in power modules under high dv/dt excitations.