Electric power conversion device, system using same, and diagnostic method for same

Provided is an electric power conversion device having a function of detecting, at low cost and at an early stage, a sign of insulation failure of a transformer or a rotary machine. This electric power conversion device is provided with an inverter circuit and a PWM signal generation unit that makes a comparison between a carrier signal and a voltage command value and generates a PWM signal for driving the inverter circuit. The electric power conversion device feeds and receives electric power through connection to a transformer or a rotary machine provided with a winding wire. The electric power conversion device is provided with: an electric current sensor that detects an electric current to be fed to or received from the transformer or the rotary machine; and a diagnosis unit that diagnoses insulation degradation of the transformer or the rotary machine. The diagnosis unit is provided with: an electric current sampling unit that acquires an electric current detection value by detecting an electric current