Substations and Protection
Chapter 7 described techniques for the analysis of various types of faults which occur in a power system. The major uses of the results of fault analysis are for the specification of switchgear and the setting of protective relays, both being housed in substations. However, the design of all items in an electrical plant is influenced by knowledge of fault conditions, for example, the bursting forces experienced by underground cables.
Circuit-breaker ratings are determined from the fault currents that may flow at their particular locations and are expressed in kA (fault current flow) or MVA (short circuit level). The maximum circuit-breaker rating is of the order of 5000060 000 MVA, and this is achieved by the use of several interrupter heads in series per phase. Not only has the circuit breaker to extinguish the fault-current arc, with the substation connections it has also to withstand the considerable forces set up by short-circuit currents, which can be very high.
Knowledge of the currents resulting from various types of fault at a location is essential for the effective specification and setting of system protection. Faults on a power system result in high currents with possible loss of sychronism of generators and must be removed very quickly. Automatic means, therefore, are required to detect abnormal currents and voltages and, when detected, to open the appropriate circuit breakers. It is the object of protection to accomplish this. In a large interconnected network, considerable design knowledge and skill is required to remove the faulty part from the network and leave the healthy remainder working intact.
There are many varieties of automatic protective systems, ranging from simple over-current relays to sophisticated systems transmitting high-frequency signals along the power lines. The simplest but extremely effective form of protection is the over-current relay, which closes contacts and hence energizes the circuit-breaker opening mechanisms when currents due to faults pass through the equipment.
Electric Power Systems, Fifth Edition. B.M. Weedy, B.J. Cory, N. Jenkins, J.B. Ekanayake and G. Strbac. © 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd.
The protection used in a network can be looked upon as a form of insurance in which a percentage of the total capital cost (about 5%) is used to safeguard apparatus and ensure continued operation of the power system when faults occur. In a highly industrialized society the maintenance of an uninterrupted supply to consumers is of paramount importance and the adequate provision of protection systems is essential. It is important the protection system discriminates between those items of plant that are faulted and must be removed from the system and those that are sound and should remain in service.
Summarizing, protection and the automatic tripping (opening) of associated circuit breakers has two main functions: (1) to isolate faulty equipment so that the remainder of the system can continue to operate successfully; and (2) to limit damage to equipment caused by overheating and mechanical forces .