2022年电气系统的保护 .pdf

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1、Protection of Electrical System All electrical systems have the common purpose of providing electrical energy to the utilization equipment as safely and reliably as is economically feasible .The utilization equipment then converts the electrical energy to other forms, such as mechanical, light, and

2、heat energy .The design of the electrical energy to transmit the electrical energy to the utilization equipment must focus on two basic requirements. First, the system must be adequate to deliver to each piece of equipment the necessary energy on a continuous basis under normal conditions. Second, t

3、he system must be designed to minimize power outages and damage in the event that abnormal conditions occur on the system. The following is a list of abnormal conditions that can occur on a system and for which corrective action should be taken: 1. Overloads 2. Short circuits 3. Under voltage 4. Sin

4、gle phasing of three phase systems 5. Over-voltages and transient surges 6. Incorrect synchronizing of frequencies 7. Incorrect phase sequence 8. Reverse power flow Next in order of priorities is the desirability of keeping damage to the electrical equipment to the absolute minimum so that normal op

5、eration can resume as quickly as possible .Finally it makes economic sense, especially with large system can continue to operate normally .This requirement for the least amount of shutdown of the system involves coordination of the protective devices. The protective device has two major functions:(1

6、) to detect an abnormal condition on that portion of the system that it is protecting and (2) to automatically and safely disconnect the faulted portion from the balance of the system .Protective devices such as fuses and most low-voltage circuit breakers combine both the detection unit and the disc

7、onnecting means in the one unit. Other types of protective 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 1 页，共 7 页 - - - - - - - - - devices separate the two functions. For example, medium-and high-voltage circuit breakers normally only perform the disconnecting fun

8、ction. They must be used in conjunction with separate protective relays that detect the abnormal conditions and then initiate the tripping of the circuit breaker. It must be emphasized again that protective devices cannot prevent faults from occurring on the system, but can only minimize their effec

9、ts. Protective devices are rated for the following: 1. Maximum continuous voltage: This is the maximum voltage that can be continuously applied to the device without eventually causing the insulation to fail. 2. Maximum continuous current: This is the maximum load current that the device can carry c

10、ontinuously without the contacts or other current-carrying parts overheating. 3. Interrupting rating: This is the maximum current that the device can safely interrupt at the specified voltage. 4. Short-time current ratings: (a)Momentary: This is the maximum rms current that the device can withstand

11、with regard to mechanical stressing. The maximum stressing occur one half-cycle after the fault starts. This rating is necessary to ensure that the device is not physically damaged before it can operate to disconnect the faulted part of the system. (b) Specified time: This is the maximum rms current

12、 that the device can withstand for a specified time 0.5% with regard to thermal stressing .In the case of breakers, it is sometimes necessary under severe short circuits to delay their opening for a very short period of time in order to coordinate with other devices .This rating is necessary to ensu

13、re that the breaker is not damaged by heat before it can operate to disconnect the faulted part of the system. Protective Relaying Protective relaying is that area of power system design concerned with minimizing service interruption and limiting damage to equipment when failures occur. The function

14、 of protective relaying is to cause the prompt removal of a 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 2 页，共 7 页 - - - - - - - - - defective element from a power system .The defective element may have a short circuit or it may be operating in an abnormal manner.

15、Protective relaying systems are designed to detect such failures or abnormal conditions quickly (commensurate with system requirements) and to open a minimum of circuit breakers to isolate the defective element. The effect of quick isolate is threefold: (1) it minimizes or prevents damage to the def

16、ective element, thus reducing the time and expense of repairs and permitting quicker restoration of the element to service; (2) it minimizes the power system; and (3) it. maximizes the power that can be transferred on power system. The second and third points are of particular significance because t

17、hey indicate the important role protective relaying plays in assuring maximum service reliability and in system design. The power that can be transmitted across system without the loss of synchronism is the function of fault clearing times. It is apparent that fast fault clearing times permit a high

18、er power transfer than longer clearing times. High-speed clearing of faults can often provide a means for achieving higher power transfers and thereby defer investment in additional transmission facilities. A protective relaying system is based on detecting fault conditions by continuously monitorin

19、g the power system variable such as current, voltage, power, frequency, and impedance. Measuring of currents and voltage is performed by instrument transformers of the potential type (PT) or current type (CT). Instrument transformers feed the measured variables to the relay system, which in return,

20、upon detecting a fault, commands circuit breaker (CB) to disconnect the faulted section of the system. An electric relaying system is divided into several protective zones for generators, transformers, buses, transmission and distribution circuits, and motors. The division is such that zones are giv

21、en adequate protection while keeping service interruption to a minimum. It is to be noted that each zone is overlapped to avoid unprotected (blind) areas. The connections of current transformers achieve the overlapping. The general philosophy of relay application is divide the power system into zone

22、s that can be adequately protected by suitable protective equipment and can be disconnected from the power system in a minimum amount of time and with the 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 3 页，共 7 页 - - - - - - - - - least effect on the remainder of the

23、power system. The protective relaying provided for each zone is divided into two categories: (1) primary relaying and (2) backup relaying. Primary relaying is the first line of defense when failures occur, and is connected to trip only the protective relays will operate to trip all of the breakers w

24、ithin that zone. If a breaker is omitted between two adjacent elements, both elements will be disconnected for a failure in either one. This latter arrangement is illustrated by the unit generator-transformer connection in the power plant. On bulk power generating and transmission systems, primary p

25、rotection is designed to operate at high speed for all faults. Slower protection may be used in less important system areas but, in general, any system area will benefit by the fastest possible primary relaying. If the fault is not cleared by the primary protection, backup relaying operates to clear

26、 the fault from the system. In general, backup relaying disconnects a greater portion of the system to isolate the fault. Backup protection is provided for possible failure in the primary both relay backup as well as breaker backup. Ideally, the backup protection should be arranged so that anything

27、that may cause the primary protection to fail will not also cause failure of the backup protection .Moreover, the backup protection must not operate until the primary protection has been given an opportunity to function. As a result, there is time delay associated with any backup operation. When a s

28、hort circuit occurs, both the primary and the backup protection start to operate. If the primary protection clears the fault, the backup protection will reset without completing its function. If the fault is not cleared by the primary protection, the backup relaying will time out and trip the necess

29、ary breakers to clear the fault from the system. There are two forms of backup protection in common use on power system. They are remote backup and local backup. (1)Remote backup. In remote backup relaying, faults are cleared from the system one station away from where the failure has occurred. (2)L

30、ocal backup. In local backup relaying, faults are cleared locally in the same station where the failure has occurred. For faults on the protected line, both the primary and the backup relays will operate to prepare 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 4 页，共

31、 7 页 - - - - - - - - - tripping the line breaker. Relay backup may be just as fast as the front line relays. When either of these relays operates to initiate tripping of the line breaker, it also energizes a timer to start the breaker backup function. If the breaker fails to cleat the fault, the lin

32、e relays will remain picked up, permitting the timer to time out and trip the necessary other breaker on the associated bus section. Microcomputer-based Relaying A new development in the field of power system protection is the use of computers (usually microcomputers) for relaying. Although computer

33、s provide the same protection as that supplied by conventional relays, there are some advantages to the use of computer-based relaying is much greater than for electromechanical devices. Computer-based relaying samples the values of the current, voltage, and other items covered in the protection sch

34、eme several times a second, and by use of A/D converters, change these analog values to digital form and then send them to the computer. In the event of a fault, the computer can calculate the fault s current values and characteristics, and settings can be changed merely by reprogramming. Computer-b

35、ased relaying are also capable of locating faults, checking features can be built in and sequence of events information can be downloaded to remote computers for fast analysis of relaying operations. Computer-based relaying system consists of subsystems with well defined functions. Although a specif

36、ic subsystem may be different in some of its details, these subsystems are most likely to be incorporated in its design in some form. The processor is the center of its organization. It is responsible for the execution of relaying programs, maintenance of various timing functions, and communicating

37、with its peripheral equipment. The Random Access Memory (ROM) or Programmable Read Only Memory (PROM) is used to store the programs permanently. In some cases the programs may execute directly from the ROM if its read time is short enough. If this is not the case, the programs must be copied from th

38、e ROM into the PAM during an initialization stage, and then the real-time execution would take place 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 5 页，共 7 页 - - - - - - - - - from the RAM. The Erasable PROM (EPROM) is needed for storing certain parameters (such as t

39、he relaying settings) which may be changed from time to time, but once it is set it must remain fixed even if the power supply to the computer is interrupted. The relaying inputs are currents and voltages - or, to a lesser extent digital signals indicating contact status. The analog signals must be

40、converted to voltage signals suitable for conversion to digital form. The current and voltage signals obtained from current and volts. The current inputs must be converted to voltages by resistive shunts. As the normal current transformer secondary currents may be as high as hundreds of amperes, shu

41、nts of Digital Converter (ADC). An alternative arrangement would be to use an auxiliary current another function; that of providing electrical isolation between the main CT secondary and the computer input system. Since the digital computer can be programmed to perform several functions as long as i

42、t has the input and output signals needed for those functions. It is a simple matter to the relaying computer to do many other substation task, for example , measuring and monitoring flows and voltages in transformers and transmission lines, controlling the opening and closing of circuit breakers an

43、d switches, providing backup for other devices that have failed, are all functions that can be taken over by the relaying computer. With the program ability and communication capability, the computer-based relaying computer offers yet another possible advantage that is not easily realizable in a con

44、ventional system. This is the ability to change the relay characteristics (settings) as the system conditions warrant it. With reasonable prospects of having affordable computer-based relaying which can be dedicated to a single protection function, attention soon turned to the opportunities offered

45、by computer-based relaying to integrate them into a substation, perhaps even a system-wide network. Integrated computer systems for substations which handle relaying, monitoring, and control tasks offer novel opportunities for improving overall system performance. 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 6 页，共 7 页 - - - - - - - - - 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 7 页，共 7 页 - - - - - - - - -