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.