03-09-2009, 04:48 AM
INTEGRATION OF DISTRIBUTED GENERATION IN
MEDIUM VOLTAGE GRIDS - PROTECTION ISSUES
Keywords: Distributed Generation; Medium Voltage-grid; Protection;
ABSTRACT
At this moment climate change and CO2 reduction are hot topics in relation to en-
ergy production. Efforts are made to stimulate development of sustainable energy
sources, such as wind turbines and PV-systems. Also the combined heat and power
production is encouraged. This has lead to an increasing penetration of small gen-
erating units in medium voltage (MV) grids. Small generation units, also distrib-
uted generation (DG), can have a significant impact on power flow, voltage profile,
power quality and grid protection. In this article the effect of DG on grid protection
is studied in detail. The paper gives an overview of current protection practices in
MV-grids and shows how these practices are influenced by DG. A procedure is
derived to determine the impact on protection systems in advance which can be
helpful in the planning stage. To illustrate the effects a 3-bus test grid and an exist-
ing 20 kV cable grid including high penetration of DG-units is modelled in profes-
sional software. In the models the grid protection and the protection of the DG-
units are modelled as well. The simulations show that protection problems in com-
pact cables grids are not likely. In weak grids DG can cause a delay of fault clear-
ing which can lead to serious selectivity problems. In the paper it is demonstrated
that the effect of DG on grid protection is minimized by immediate disconnection
of the DG-units.
