Method for insulation monitoring and insulation monitoring device with network balancing in an ungrounded direct voltage system

An insulation monitoring device having network balancing for operation in an ungrounded direct voltage system having two active conductors (L+, L−), the insulation monitoring device comprising

• voltage-measuring devices for measuring conductor-to-ground voltages (UDC+, PE, UDC−, PE) between each of the active conductors (L+, L−) and ground (PE),
• an insulation monitoring function according to standard IEC 61557-8 and using conductor-specific computation of insulation resistances (Rf+, Rf−) and having a control circuit (4) for compensating an asymmetrical voltage between the conductor-to-ground voltages (UDC+, PE, UDC−, PE),
• wherein
• the control circuit is implemented as a digital control (4) comprising the following functional blocks:
• a multiplier (12) for multiplying a specified asymmetrical factor (D) by a conductor-to-conductor voltage (UDC+, UDC−) to generate a weighted conductor-to-conductor voltage (UDC+, UDC−) as a reference variable (w(t));
• a subtractor (14) which determines a control error by subtracting one of the conductor-to-ground voltages (UDC+, PE, UDC−, PE) from the weighted conductor-to-conductor voltage (w(t));
• a controller (16) which computes a resistance setting value as a manipulated variable (u(t)) by means of a discrete-time control algorithm;
• a variable resistance (Rst1, Rst2) which is disposed between each of the active conductors (L+, L−) and ground (PE) and is selectively set in such a manner via the manipulated variable (u(t)) that the voltage balancing of the conductor-to-ground voltages (UDC+, PE, UDC−, PE) is continuously compensated;
• an insulation-resistance computation for determining the conductor-relevant insulation resistances (Rf+, Rf−) from the variable resistances (Rst1, Rst2) resulting as a function of the asymmetrical factor (D) at two different specified asymmetrical factors (D1, D2).