Planning of Low Voltage Distribution Network

A realistic view of the power distribution systems should be based on "gathering" functions rather than "distributing" them since the size and locations of the customer demands are not determined by the distribution engineer but by the customers. Customers install all types of energy-consuming devices which can be connected in every conceivable combination and at times chosen by the customers. The concept of distribution starts with the individual customers and loads, and proceeds through several gathering stages where each stage includes various groups of increasing numbers of customers and their loads. Ultimately the generating stations themselves are reached through services, secondaries, distribution transformers, primary feeders, distribution substation, subtransmission networks, bulk power stations and transmission network as will be discussed in the next section.

In designing a system, distribution engineers may find a conflict between fulfilling the requirements of the electrical considerations and the economical considerations in the same time. A good distribution system is the one compromising both considerations in the same time as much as possible. An example of this conflict is the voltage drop on the feeders. For achieving good performance of the system, voltage drop should be eliminated in order to have a flat voltage profile. To achieve this we use cables of larger cross sectional area (c.s.a) in order to have smaller resistance. On the other hand, the economical considerations in some cases permits a certain range of voltage drop so as to fully use the used cables. Yet if the conflict between electrical requirements and economical requirements can't be solved; the priority is always for the electrical requirements since they represent the safe operation which is the main aim of the distribution engineer.

Another example on the conflict between electrical and economical requirements is to increase the service reliability for the critical loads, e.g. hospitals, computer and control centers, critical industrial loads. To do this some back-up systems such as emergency generators and/or batteries with automatic switching devices are used in such places. These extra equipments cost more money, yet the reliability of the service is more important in this case than any money.

In their system design decisions of the secondary distribution network, distribution engineers are primarily motivated by the considerations of economy, coppers losses in the transformer and the secondary circuit, permissible voltage drops and voltage flickers of the system. Of course, there are some other engineering and economic factors affecting the selection of the distribution transformer and the secondary configuration, such as permissible transformer loading, balanced phase loads for the primary system, investment costs of the various secondary system components, cost of labor, capital cost, inflation rates and other factors.