Propagation of PD pulses through ring-main-units and substations

Online partial discharge (PD) monitoring systems are traditionally installed at a single mediumvoltage (MV) cable connection between two ring-main-units (RMUs). It is more efficient to monitor two or more consecutive cables using a single monitoring system. Moreover, practical experience with the PD-OL system, shows that for substations, with many parallel MV cables, and RMUs installing the inductive sensor may be hampered or even impossible. In this paper the influence of RMUs and substations on the propagation of PDs is studied. An RMU or substation can be modeled as a combination of complex impedances representing switchgear, transformer and MV cables. A PD pulse from a cable encounters a load impedance that does not match the cable´s characteristic impedance, resulting in partial reflection and partial transmission transmission to other cables. Models for RMUs and substations are proposed and verified by measurements. Feasible options for online PD monitoring through RMUs or substations are determined.