Earthing of previously unearthed medium voltage networks ANTE MARUŠIĆ Department of Power Systems Faculty of Electrical Engineering and Computing Unska 3, Zagreb CROATIA http://www.fer.hr/~marusic DUBRAVKO FRANKOVIĆ Department of Electrical Engineering Faculty of Engineering Vukovarska 58, Rijeka CROATIA Abstract: A large number of public utilities adopted the concept of isolated medium network operation. Increased cabling of previously overhead lines and construction of new cable feeders resulted in capacitive earth-fault currents increase. High earth-fault currents cause unacceptable touch potentials in vicinity of fault location and inadmissible potentials of neutral conductor in low voltage installations. Resonant earthing of isolated medium voltage networks has become an attractive way of earthing isolated networks. In fact, resonant earthed networks retain all advantages of isolated networks, especially, continuity of operation under sustained earth-fault conditions. Due to the fact that an adjustable inductance is connected to main HV/MV transformer’s neutral point, through fault location only a small active component of the earth-fault current flows, thus causing no thermal stress to faulted equipment and preventing restriking faults i.e. intermittent overvoltages. The "tunable" inductance is adjusted to compensate network's ground capacitance. In this paper isolated and resonant earthed networks will be compared, and expressions for normal operating conditions and fault conditions will be derived. Key-Words: medium voltage network, isolated network, resonant earthing, earth-fault, recovery voltage 1 Introduction Medium voltage networks (MV), located between transmission lines and customers at low voltage, need to be as available as possible and as simple as possible to maintain. A vast number of public utilities adopted, during past decades, the concept of isolated network operation i.e. neutral points of all transformers and eventual generators were held isolated from earth. Increased cabling of previously overhead lines and construction of new cable feeders resulted in overall network capacitance increase, therefore capacitive earth- fault current increase. High earth-fault currents cause unacceptable touch potentials in vicinity of fault location and inadmissible potentials of neutral conductor in low voltage (LV) installations. Resonant earthing of isolated medium voltage networks has become an attractive way of earthing isolated networks. In fact, resonant earthed networks retain all advantages of isolated networks, especially, continuity of operation under sustained earth-fault conditions. Due to the fact that an adjustable inductance is connected to main HV/MV transformer’s neutral point the capacitive earth- fault current is compensated by coil's inductive current. Therefore through earth-fault location only a small active component flows, thus causing no thermal stress to faulted equipment. A direct consequence of small current at fault location is absence of restriking faults i.e. intermittent overvoltages of faulted phase and sound phases. In this paper isolated and resonant earthed networks will be compared, and expressions for normal operating conditions as well as fault conditions will be derived. 2. Isolated networks Isolated networks have no intentional connection with earth, i.e. all neutral points of all transformers or generators are isolated from earth, see Fig. 1. Single phase faults are by far (80%) the most frequent type of faults on overhead lines [3]. Proc. of the 5th WSEAS/IASME Int. Conf. on Electric Power Systems, High Voltages, Electric Machines, Tenerife, Spain, December 16-18, 2005 (pp273-279)