If the IMD circuit does not include a dedicated voltage source, the circuit must rely on the individual AC-line voltage as the excitation. This reliance creates two potential difficulties:

• Resolution at the voltage peak: Accurate monitoring of insulation requires the measurement to be taken near the peak of the waveform, where the signal?to?noise ratio is highest. If the measurement is performed at a zero?crossing, the sensed voltage is essentially zero and the input offset of the op?amp dominates the reading, resulting in large errors. Consequentially, maintaining sensing that occurs very close to the AC peak is a challenge.
• Variable AC amplitude: In a V2H operation the output of the inverter must track the grid voltage, which can vary over a wide range. Since the reference voltage used to judge an insulation failure is derived from the AC line, the threshold for acceptable leakage must adapt to the instantaneous amplitude. Defining a robust, voltage?independent fault criterion under these changing conditions is therefore challenging.