Impact of Combined Heat and Power (CHP) generation on the fault current level in urban distribution networks (UDN)

The increasing demand on the urban distribution network (UDN) imposed by distributed generation (DG), such as renewable sources and Combined Heat and Power (CHP), will impact on the operation of the UDN in a number of areas including fault current level and voltage stability. In general most DG connections are small CHP plants employing reciprocating engine or gas turbine as a prime mover directly coupled to synchronous generator with electrical output up to 1MVA and 0.415kV generating voltage are mainly connected to low voltage busbars of 0.415 kV and in some cases to 10.5kV busbars through a transformer. In general, all new connected CHP plants causes some increase in fault level. For significant volumes, connection of CHP that would most likely occur in the UDN, which would lead to increase in fault level issues as the UDN tent to have the lowest fault level headroom. The aim of this paper is to present the consequences and operating limitations of connecting CHP to the UDN. In order to calculate the fault current at a network bus, a simple Thevenin model is used for the UDN.. The application of the methodology is demonstrated and result obtained by using ERAC power system analyzing software on 13 busbars network resembling part of typical UDN where continuity of power supply is very important. A discussion is also included on potential measures available to handle increase in fault current, cost and responsibility of upgrading equipment in the network. The analysis estimates the fault current on each busbar and the Average Current Fault (ACF) due to the addition of the CHP generation plants.