Urban (Town or City) Systems
Very heavy loadings (up to 100MW/mile2 or 40MW/km2) are usual, especially where high-rise buildings predominate. Extensive heating and air-conditioning loads as well as many small motors predominate. Fluorescent lighting reduces the power factor and leads to some waveform distortion, but computer and TV loads and power electronic motor drives now cause considerable harmonics on all types of network.
Again, a basic L.V. grid, reinforced by extensions to the H.V. network, as required, produces minimum costs overall. The H.V. network is usually in the form of a ring main fed from two separate sections of a double busbar substation where 10-60 MVA transformers provide main supply from the transmission system (see Figure 1.22). The H.V. network is sectionalized to contain short-circuit levels and to ease protection grading. A high security of supply is possible by
Figure 1.21 Principle of suburban distribution system
overlapping H.V. rings so that the same L.V. grid is fed from several transformers supplied over different routes. Failure of one portion of the H.V. system does not affect consumers, who are then supported by the L.V. network from adjacent H.V. supplies.
In the UK transformers of 500 or 1000 kVA rating are now standard, with one H.V. circuit breaker or high rupturing capacity (HRC) fuse-switch and two isolators either side to enable the associated H.V. cable to be isolated manually in the event of failure. The average H.V. feeder length is less than 1 mile and restoration of H.V. supplies is usually obtainable in under 1 h. Problems may arise due to back-feeding of faults on the H.V. system by the L.V. system, and in some instances reverse power relay protection is necessary.
In new urban developments it is essential to acquire space for transformer chambers and cable access before plans are finalized. High-rise buildings may require substations situated on convenient floors as well as in the basement.
Apart from the supply of new industrial and housing estates or the electrification of towns and villages, a large part of the work of a planning engineer
Figure 1.22 Typical arrangement of supply to an urban network-British practice
is involved with the up-rating of existing supplies. This requires good load forecasting over a period of 2-3 years to enable equipment to be ordered and access to sites to be established. One method of forecasting is to survey the demand on transformers by means of a maximum-demand indicator and to treat any transformer which has an average load factor of 70% or above as requiring up-rating over the next planning period. Another method is to analyze consumers' bills, sectionalized into areas and distributors and, by surveys and computer analysis, to relate energy consumed to maximum demand. This method can be particularly useful and quite economic where computerized billing is used. The development of the Smart Grid is leading to much greater monitoring being installed on the 400 V network and control on the 11 kV system.
In practice, good planning requires sufficient data on load demands, energy growth, equipment characteristics, and protection settings. All this information can be stored and updated from computer files at periodic intervals and provides the basis for the installation of adequate equipment to meet credible future demands without unnecessary load shedding or dangerous overloading.
