Proposal for measurement of earth impedance using variable frequency injection

Measurement of the impedance of earthing systems of high voltage installations at power fundamental frequency has been studied extensively, and standards are available which provide guidance on methods of measurement and types of instrumentation. In the UK, the staged-fault test is seldom performed. Therefore, it is necessary to measure earth impedance without any circuit interruption by injecting current into the earthing system using an auxiliary test circuit. This type of 'fall-of-potential' test generally uses low voltage energization and produces test currents of the order of only a few amperes. At this level of current, potential measurements are made difficult due to the presence of relatively high standing voltages in the earthing system. In this paper, a new earth impedance measurement system which is able to overcome the effects of system noise and provide accurate measurements is described. Initial laboratory tests which resulted in optimized instrument settings and subsequent field tests demonstrated successful application at a range of operational electricity installations.