Method and device for monitoring the insulation of ungrounded DC and AC voltage networks

A method for monitoring the insulation of an unearthed DC voltage network or an unearthed AC voltage network, or an unearthed DC voltage network and an unearthed DC-AC voltage network, comprising the steps of:

• (a) generating a DC measurement voltage Umess= connected on one side to ground for a predeterminable measurement period tvar;
• (b) generating and superpositioning at least one AC measurement voltage Umess˜ connected to ground with a periodic time tcons onto the DC measurement voltage Umess= for the purpose of forming a total measurement voltage Umess;
• (c) introducing the total measurement voltage Umess into the voltage network to be monitored;
• and
• (d) determining an insulation resistance R= from the DC measurement voltage Umess= and an insulation impedance Z˜ from the AC measurement voltage Umess˜;
• (e) measuring a total measured current Imess generated by the total measurement voltage Umess, when determining the insulation resistance R= and the insulation impedance Z˜;
• (f) dividing the total measured current Imess into a DC measured current Imess= and an AC measured current Imess˜;
• (g) determining the insulation resistance R= based on the DC measured current Imess= and determining the insulation impedance Z˜ based on the AC measured current Imess˜;
• (h) determining an insulation resistance R˜ and a leakage capacitance C˜ from the insulation impedance Z˜;
• and
• (i) outputting the insulation resistance R˜ as an insulation resistance of the voltage network when there is a voltage network-specific small leakage capacitance and outputting the insulation resistance R= as an insulation resistance of the voltage network, when there is a large leakage capacitance.