Fault current level issues for urban distribution network with high penetration of distributed generation

Electrical power production from distributed generation (DG) is playing an increasing role in the supply of electricity in liberalised electricity markets. The popularity of DG is on the rise due to a number of reasons that include: deregulation of power system, increasing difficulty faced in installing new transmission and distribution infrastructure and recent technological advance in the area of DG energy sources. Currently DG attracted to both distribution utilities and electricity users, as it can provide meaningful advantages to both. The increasing demand on the urban distribution network (UDN) imposed by new DG, such as renewable sources and Combined Heat Power (CHP), will impact on the operation of the UDN in a number of areas including fault current levels and voltage levels. In general most DG connections are small CHP plants employing reciprocating engine or gas turbine as a prime mover directly coupled to synchronous generators with electrical output up to 1MVA and with a 0.415kV generating voltage are mainly connected to low voltage busbars of 0.415kV and in some cases to 10.5kV busbars through a transformer. In general, all new connected DG causes some increase in fault level. For significant volumes, connection of DG that would most likely occur in the UDN, which would lead to increase in fault level issues as the UDN tend to have the lowest fault level headroom. The aim of this paper is to present the consequences and operating limitations of connecting DG to the UDN. The methodology used is based on the latest edition of IEC 60909 standard to calculate the maximum fault level in UDN with DG connected at MV/LV levels. The application of the methodology is demonstrated using ERAC power system analysing software on fictitious UDN, resembling part of a typical UDN where continuity of power supply is very important. A discussion is also included on potential measures available to reduce the fault level.