Mathematical description of the method for ungrounded AC systems to determine the network insulation

Abstract

For the safety of humans and animals in the vicinity of single-phase earth faults in ungrounded AC systems, the development of methods for monitoring the condition of insulation and protection against electric shocks is an important issue. This paper presents a method for determining insulation parameters in ungrounded AC systems, which provides: (1) Satisfactory accuracy; (2) easy measurements of the voltage modules magnitude; (3) safety when working in electrical installations. The proposed method is based on measuring the zero-sequence voltage module and phase-to-earth voltage module, as well as the angle of phase-to-earth voltage vector and zero-sequence voltage vector before and after connection of additional conductivity between a phase and the earth. In addition, the obtained insulation parameters enable the zero-sequence current to be decomposed into an active and reactive component, thus ensuring accurate compensation of capacitive currents. By monitoring the state of the active conductivity value, the value of the single-phase earth-fault current can be determined at an early stage and the relay protection can thus be correctly set. Consequently, by knowing the insulation condition parameters, the single-phase earth-fault currents can be monitored and reduced more effectively, and thus the step voltage, which determines the degree of safety.