Earth-fault protection challenges for modern distribution power systems

Modern distribution power systems are characterized by extensive use of power cables and often have unusual grounding impedance arrangements. Typical examples are offshore MV windfarm collector systems. In the paper, the basic physics behind the earth-faults in high impedance grounded networks will be explained first. When and how earth-fault protection relays based on steady-state 3Io and 3Uo quantities can be used will be clarified. Then, a more universal transient based earth-fault protection method will be explained. Its determination of the earth-fault direction is based on the short-term built-up transient at the beginning of the fault. This transient is, to a large extent, independent of the neutral point treatment; thus, the same principles can be applied without any modification in all types of high-impedance grounded or isolated power systems. Restriking or intermittent earth-faults will also be successfully detected and cleared by such earth-fault protection.