Author(s)

V. Silbermann

Abstract

Many hydro power plants (HPP) in various countries are being operated with double generating units (DGU) where each unit generator-transformer has its own generator circuit breaker (CB) and both units are connected to a high voltage (HV) network via two common CBs installed in an air-insulated switchgear (AIS). Such circuitry was designed to halve the number of rather expensive HV CBs and to save on the space necessary for their installation. The necessary flexibility of HPP is continued as DGUs allow keeping one unit transformer in idle mode and putting into operation a unit generator via generator CB very rapidly if necessary. Long-term experience has demonstrated the high reliability of operation of HPPs with such circuitries, nevertheless due many reasons the AISs in HPPs are replacing now with more compact and convenient in operation gas-insulated switchgears (GIS). This replacement is taking place in many countries, and everywhere, the existing DGUs are connected to GISs via HV cables. It shall also be noted that each GIS circuitry is generally designed identically to previous AIS circuitry. But cases of damage of some unit transformers working in idle mode have been observed in a number of countries after such replacement. This paper reports the results of investigating one of these cases of damage, discusses the possible cause of these incidents and elaborates some measures to prevent such damage in the future.

Keywords

GIS, HV cable, idle mode of transformer, magnetizing characteristic of transformer core, Rayleigh area, resonance in L-R-C circuit