There is one major issue that differentiates the characteristics of a network – its urban or rural nature. Urban networks are usually short and underground, and rural networks are usually long, but not necessarily overhead.
!!! In some parts of Germany and in Electrabel (Belgium) they are mostly underground, and we see a trend in all utilities to increase the part of underground in rural networks. Many characteristics of networks can be explained when considering the urban or rural feature.
Two remarks should be made about this chapter:
This document reflects the state of the art in network structures, and some changes might occur in the future.
All the utilities contributing to this survey use MV networks without the distribution of the neutral wire. Some conclusions don’t apply to networks of the “distributed neutral” kind (very common in the United States and Canada).
Structure and Operation
The structure of the MV networks is mainly meshable, so that feeders can be backed by adjacent lines. Urban networks are almost completely meshable, but that is not the case of rural networks for obvious reasons of cost. The percentage of rural networks that can be backfeed varies between 50% and 90% on the utilities studied.
LV networks are much less meshable than MV networks. Usually, only some urban networks can be backed. The percentage varies between 0% and 30%.
!!! MV and LV networks are almost always operated radially. No utility have plans to loop networks in the future.
Primary substations (HV/MV substations) have generally from 1 to 4 power transformers. 2 transformers are the most typical value. The average number of MV feeders by substation varies between 4 and 20, with most typical values being around 10.
Usually, we find differences in the typical numbers of feeders of rural and urban substations, but the trends are different from one utility to another. The tendencies regarding the total number of primary substations are not the same around Europe:
In Finland, the number is decreasing, in order to diminish the maintenance costs.
In Austria, the number is increasing in order to cope with new demand.
In EDF and ENEL the number is increasing in order to improve the quality of supply.
Fault Management in Electrical Distribution Systems - Final Report of CIRED Working Group