Fundamentals of protection practice
The purpo of an electrical power system is to generate and supply electrical energy to consumers. The system should be designed and managed to deliver this energy to the utilization points with both reliability and economy. As the two requirements are largely oppod, it is instructive to look at the reliability of a system and its cost and value to the consumer.
One hand ,The diagram mast make sure the reliability in system design,. On the other hand, high reliability should not be pursued as an end in itlf, regardless of cost, but should rather be balanced against economy,taking.
Security of supply can be bettered by improving plant design, increasing the spare capacity margin and arranging alternative circuits to supply loads. Sub-division of the system into zones. each controlled by switchgear in association with protective gear. provides flexibility during normal operation and ensures a minimum of dislocation following a breakdown.
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The greatest threat to the curity of a supply system is the short circuit,which impos a sudden and sometimes violent change on system operation. The large current which then flows, accompanied by the localized relea of a considerable quantity of energy, can cau fire at the fault location, and mechanical damage throughout the system, particularly to machine and transformer windings. Rapid isolation of the fault by the nearest switchgear will minimize the damage and disruption caud to the system.
A power system reprents a very large capital investment. To maximize the return on this outlay. the system must be loaded as much as possible. For this reason it is necessary not only to provide a supply of energy which is attractive to prospective urs by operating the system ,but also to keep the system in full operation as far as possible continuously, so that it may give the best rvice to the consumer, and earn the most revenue for the supply authority. Absolute freedom from failure of the plant and system network cannot be guaran- teed. The risk of a fault occurring, however slight for each item, is multiplied by the number of such items which are cloly associated in an extensive system, as any fault produces repercussions throughout the network. When the
system is large, the chance of a fault occurring and the disturbance that a fault would bring are both so great that without equipment to remove faults the system will become, in practical terms, inoperable. The object of the system will be defeated if adequate provision for fault clearance is not made. Nor is the installation of switchgear alone sufficient; discriminative protective gear, designed according to the characteristics and requirements of the power system. must be provided to control the switchgear. A system is not properly designed and managed if it is not adequately protected.
Protective gear
This is a collective term which covers all the equipment ud for detecting,locating and initiating the removal of a fault from the power system. Relays are extensively ud for major protective functions, but the term also covers ips and fus.
In addition to relays the term includes all accessories such as current and voltage transformers, shunts, d.c. wiring and any other devices relating to the protective relays.
In general, the main switchgear, although fundamentally protective in its function, is excluded from the term protective gear, as are also common rvices, such as the station battery and any other equipment required to cure opera- tion of the circuit breaker.
Reliablity
The performance of the protection applied to large power systems is frequently assd numerically. For this purpo each system fault is clasd as an incident and tho which are cleared by the tripping of the correct circuit breakers and only tho, are clasd as 'correct'. The percentage of correct clearances can then be determined.
This principle of asssment gives an accurate evaluation of the protection of the system as a whole, but it is vere in its judgement of relay performance, in that many relays are called into operation for each system fault, and all must behave correctly for a correct clearance to be recorded. On this basis, a performance of 94% is obtainable by standard techniques.
男士皮鞋>鸡胗热量Complete reliability is unlikely ever to be achieved by further improvements in constructi
on. A very big step, however, can be taken by providing duplication of equipment or 'redundancy'. Two complete ts of equipment are provided, and arranged so that either by itlf can carry out the required function. If the risk of an equipment failing is x/unit. the resultant risk, allowing for redundancy, is x2. Where x is small the resultant risk (x2) may be negligible.
泥字开头的成语 It has long been the practice to apply duplicate protective systems to busbars, both being required to operate to complete a tripping operation, that is, a 'two-out-of-two' arrangement. In other cas, important circuits have been provided with duplicate main protection schemes, either being able to trip independently, that is, a 'one-out-of- two' arrangement. The former arrangement guards against unwanted operation, the latter against failure to operate. 生活习惯的英文
The two features can be obtained together by adopting a 'two-out-of-three' arrangement in which three basic systems are ud and are interconnected so that the operation of any two will complete the tripping function. Such schemes have already been ud to a limited extent and application of the principle will undoubtedly increa. Pr
obability theory suggests that if a power network were protected throughout on this basis, a protection performance of 99.98% should be attainable. This performance figure requires that the parate protection systems be completely independent; any common factors, such as common current transformers or tripping batteries, will reduce the overall performance.
辞职怎么写SELECTIVITY
Protection is arranged in zones, which should cover the power system completely, leaving no part unprotected. When a fault occurs the protection is required to lect and trip only the neareat circuit breakers. This property of lective tripping is also called 'discrimination' and is achieved by two general methods:
a Time graded systems想好好爱一个人
Protective systems in successive zones are arranged to operate in times which are graded through the quence of equipments so that upon the occurrence of a fault, altho
ugh a number of protective equipments respond, only tho relevant to the faulty zone complete the tripping functiopn. The others make incomplete operations and then ret.
b Unit systems
It is possible to design protective systems which respond only to fault conditions lying within a clearly defined zone. This 'unit protection' or 'restricted protection' can be applied throughout a power system and, since it does not involve time grading, can be relatively fast in operation.
Unit protection is usually achieved by means of a comparison of quantities at the boundaries of the zone. Certain protective systems derive their 'restricted' property from the configuration of the power system and may also be clasd as unit protection.
Whichever method is ud, it must be kept in mind that lectivity is not merely a matter of relay design. It also depends on the correct co-ordination of current transformers and relays with a suitable choice of relay ttings, taking into account the possible range of such variables as fault currents. maximum load current, system impedances and other related factors, where appropriate.
STABILITY
酒店前台工作总结This term, applied to protection as distinct from power networks, refers to the ability of the system to remain inert to all load conditions and faults external to the relevant zone. It is esntially a term which is applicable to unit systems; the term 'discrimination' is the equivalent expression applicable to non-unit systems.
SPEED
The function of automatic protection is to isolate faults from the power system in a very much shorter time than could be achieved manually, even with a great deal of personal supervision. The object is to safeguard continuity of supply by removing each disturbance before it leads to widespread loss of synchronism, which would necessitate the shutting down of plant.
Loading the system produces pha displacements between the voltages at different points and therefore increas the probability that synchronism will be lost when the system is disturbed by a fault. The shorter the time a fault is allowed to remain in the system, the greater can be the loading of the system. Figure 1.5 shows typical relations b
etween system loading and fault clearance times for various types of fault. It will be noted that pha faults have a more marked effect on the stability of the system than does a simple earth fault and therefore require faster clearance.
