Designing MV networks to meet quality of supply targets

The Norweb MV distribution network consists of some 3000 separate circuits fed from about 500 primary substations (33/11kV, 33/6.6kV, 132/11kV). Details of all faults are recorded in the NAFIRS database. Thus the number of faults on each circuit can be found. Since the number of faults can vary substantially from year to year, the recommended method of choosing a fault rate per km of underground cable or overhead line is to use the average over 5 years for all circuits at a primary. It has been found that for extensive overhead lines many of the recorded MV faults only affect a few customers supplied by a single transformer. These are termed local faults and are omitted when deriving the rate of faults per km ofline. Three examples are given of the use of the GROND power system design software package. The first two examine aspects of strategy, whilst the third illustrates a detailed design study