Electric Power Distribution - Distribution Network Configurations

Distribution Network Configurations

Distribution networks are typically of two types, radial or interconnected (see spot network). A radial network leaves the station and passes through the network area with no normal connection to any other supply. This is typical of long rural lines with isolated load areas. An interconnected network is generally found in more urban areas and will have multiple connections to other points of supply. These points of connection are normally open but allow various configurations by the operating utility by closing and opening switches. Operation of these switches may be by remote control from a control center or by a lineman. The benefit of the interconnected model is that in the event of a fault or required maintenance a small area of network can be isolated and the remainder kept on supply.

Within these networks there may be a mix of overhead line construction utilizing traditional utility poles and wires and, increasingly, underground construction with cables and indoor or cabinet substations. However, underground distribution is significantly more expensive than overhead construction. In part to reduce this cost, underground power lines are sometimes co-located with other utility lines in what are called common utility ducts. Distribution feeders emanating from a substation are generally controlled by a circuit breaker which will open when a fault is detected. Automatic circuit reclosers may be installed to further segregate the feeder thus minimizing the impact of faults.

Long feeders experience voltage drop requiring capacitors or voltage regulators to be installed.

Characteristics of the supply given to customers are generally mandated by contract between the supplier and customer. Variables of the supply include:

• AC or DC - Virtually all public electricity supplies are AC today. Users of large amounts of DC power such as some electric railways, telephone exchanges and industrial processes such as aluminium smelting usually either operate their own or have adjacent dedicated generating equipment, or use rectifiers to derive DC from the public AC supply
• Nominal voltage, and tolerance (for example, +/- 5 per cent)
• Frequency, commonly 50 or 60 Hz, 16.6 Hz and 25 Hz for some railways and, in a few older industrial and mining locations, 25 Hz.
• Phase configuration (single-phase, polyphase including two-phase and three-phase)
• Maximum demand (usually measured as the largest amount of power delivered within a 15 or 30 minute period during a billing period)
• Load factor, expressed as a ratio of average load to peak load over a period of time. Load factor indicates the degree of effective utilization of equipment (and capital investment) of distribution line or system.
• Power factor of connected load
• Earthing systems - TT, TN-S, TN-C-S or TN-C
• Prospective short circuit current
• Maximum level and frequency of occurrence of transients