Method and monitoring device for determining a partial insulation resistance and a partial system leakage capacitance in a branched ungrounded power supply system

A method for selectively determining a partial system leakage capacitance Cx of an outgoing line (4) in a branched ungrounded power supply system (2) which is monitored using an insulation monitoring device (10) which actively superposes a measuring alternating voltage Um on the ungrounded power supply system (2), the method comprising the following steps:

• detecting a partial insulation resistance Rx of the outgoing line (4), calculating partial system leakage capacitance Cx according to Cx=ic(t)/(due(t)/dt),
• with a leakage current ic(t) flowing via partial system leakage capacitance Cx and a temporal sequence of measured non-stationary conductor-to-ground voltage ue(t) which drops via the outgoing line (4) and whose temporal derivative is due(t)/dt, leakage current ic(t) being calculated from ic(t)=ix(t)−ue(t)/Rx with a temporal sequence of non-stationary partial current ix(t) measured in the outgoing line (4) and detected partial insulation resistance Rx, meaning partial system leakage capacitance Cx is calculated from Cx=(ix(t)−(ue(t)/Rx))/(due(t)/dt).

A monitoring device (20) for selectively determining a partial insulation resistance Rx and a partial system leakage capacitance Cx of an outgoing line (4) in a branched ungrounded power supply system (2), having an insulation monitoring device (10) which is configured to actively superpose a measuring alternating voltage Um on the power supply system (2),

• wherein an AC/DC measuring current transformer (14) for measuring a stationary partial current Ixstat and a non-stationary partial current ix(t) in each outgoing line (4) to be tested,
• a voltage measuring device (22) which is switched between an active conductor of the ungrounded power supply system (2) and ground (PE) for measuring the temporal sequence of non-stationary conductor-to-ground voltage ue(t), and an evaluation unit (24) which is connected to the AC/DC measuring current transformer (14) and to the insulation monitoring device (10) and is configured for calculating partial insulation resistance Rx and partial system leakage capacitance Cx.