技术
Protective gear
This is a collective term which covers all the equipment used for detecting,locating and initiating the removal of a fault from the power system. Relays are extensively used for major protective functions, but the term also covers direct-acting a.c.trips and fuses.
In addition to relays the term includes all accessories such as current and voltage transformers, shunts, d.c.and a.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 services, such as the station battery and any other equipment required to secure opera- tion of the circuit breaker.
In order to fulfil the requirements of discriminative protection with the optimum speed for the many different configurations, operating conditions and construction features of power systems, it has been necessary to develop many types of relay which respond to various functions of the power system quantities. For example, observation simply of the magnitude of the fault current suffices in some cases but measurement of power or impedance may be necessary in others. Relays frequently measure complex functions of the system quantities, which are only readily expressible by mathematical or graphical means.
In many cases it is not feasible to protect against all hazards with any one relay. Use is then made of a combination of different types of relay which individually protect against different risks. Each individual protective arrangement is known as a 'protection system', while the whole co- ordinated combination of relays is called a 'protection scheme'.
1.1
Reliablity：
The need for a high degress of reliablity is discussed in Section1.1. Incorrect operation can be attributed to one of the following classifications:
a. Incorrect design.
b. Incorrect installation.
c. Deterioration.
Design：
This is of the highest importance. The nature of the power system condition which is being guarded against must be thoroughly understood in order to make an adequate protection design. Comprehensive testing is just as import- ant, and this testing should cover all aspects of the protec- tion, as well as reproducing operational and environmental conditions as closely as possible. For many protective systems, it is necessary to test the complete assembly of relays, current transformers and other ancillary items, and the tests must simulate fault conditions realistically.
Installation：
The need for correct installation of protective equipment is obvious, but the complexity of the interconnections of many systems and their relationship to the remainder of the station may make difficult the checking of such correct- ness. Testing is therefore necessary; since it will be difficult to reproduce all fault conditions correctly, these tests must be directed to proving the installation. No.attempt should be made to 'type test' the equipment or to establish complex aspects of its technical performance;
1.1.3
Deterioration in service：
After a piece of equipment has been installed in perfect condition, deterioration may take place which, in time, could interfere with correct functioning. For example. contacts may become rough or burnt owing to frequent operation, or tarnished owing to atmospheric contamination; coils and other circuits may be open-circuited, auxiliary components may fail, and mechanical parts may become clogged with dirt or corroded to an extent that may interfere with movement.
One of the particular difficulties of protective relays is that the time between operations may be measured in years, during which period defects may have developed unnoticed until revealed by the failure of the protection to respond to a power system fault. For this reason. relays should be given simple basic tests at suitable intervals in order to check that their ability to operate has not deteriorated.
Testing should be carried out without disturbing permanent connections. This can be achieved by the provision of test blocks or switches. Draw-out relays inherently provide this facility; a test plug can be inserted between the relay and case contacts giving access to all relay input circuits for injection. When temporary disconnection of panel wiring is necessary, mistakes in correct restoration of connections can be avoided by using identity tags on leads and terminals, clip-on leads for injection supplies, and easily visible double-ended clip-on leads where 'jumper connections' are required.
1.2
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 select and trip only the neareat circuit breakers. This property of selective tripping is also called 'discrimination' and is achieved by two general methods:
a Time graded systems.
Protective systems in successive zones are arranged to operate in times which are graded through the sequence of equipments so that upon the occurrence of a fault, although a number of protective equipments respond, only those relevant to the faulty zone complete the tripping functiopn. The others make incomplete operations and then reset.
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 classed as unit protection.
Whichever method is used, it must be kept in mind that selectivity 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 settings, taking into account the possible range of such variables as fault currents. maximum load current, system impedances and other related factors, where appropriate.
1.3
Zones of protection：
Ideally, the zones of protection mentioned in Section 1.4 The circuit breaker being included in both zones.
This principle of assessment gives an accurate evaluation of the protection of the system as a whole, but it is severe 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.
Complete reliability is unlikely ever to be achieved by further improvements in construction. A very big step, however, can be taken by providing duplication of equipment or 'redundancy'. Two complete sets of equipment are provided, and arranged so that either by itself 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.
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 cases, 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.
These two features can be obtained together by adopting a 'two-out-of-three' arrangement in which three basic systems are used and are interconnected so that the operation of any two will complete the tripping function. Such schemes have already been used to a limited extent and application of the principle will undoubtedly increase. Probability 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 separate protection systems be completely independent; any common factors, such as common current transformers or tripping batteries, will reduce the overall perfor
1.4
Stability:
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 essentially a term which is applicable to unit systems; the term 'discrimination' is the equivalent expression applicable to non-unit systems.
1.5
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 phase displacements between the voltages at different points and therefore increases the probability that synchronism will be lost when the system is disturbed by a fault. Figure 1.5 shows typical relations between system loading and fault clearance times for various types of fault. It will be noted that phase faults have a more marked effect on the stability of the system than does a simple earth fault and therefore require faster clearance.
It is not enough to maintain stability; unnecessary consequential damage must also be avoided. The destructive power of a fault are carrying a high current is very great; Even away from the fault arc itself, heavy fault currents can cause damage to plant if they continue for more than a few seconds.
It will be seen that protective gear must operate as quickly as possible; speed, however, must be weighed against economy. For this reason, distribution circuits for which the requirements for fast operation are not very severe are usually protected by time-graded systems, but generating plant and EHV systems require protective gear of the highest attainable speed; the only limiting factor will be the necessity tor correct operation.
1.6
Primary and back-up protection:
The reliability of a power system has been discussed in earlier sections. Many factors may cause protection failure and there is always some possibility of a circuit breaker failure. For this reason, it is usual to supplement primary protection with other systems to 'back-up' the operation of the main system and to minimize the possibility of failure to clear a fault from the system.
Bak-up protection may be obtained automatically as an inherent feature of the main protection scheme, or separately by means of additional equipment. Time graded schemes such as overcurrent or distance protection schemes are examples of those providing inherent back-up protection; the faulty section is normally isolated discriminatively by the time grading, but if the appropriate relay fails or the circuit breaker fails to trip, the next relay in the grading sequence will complete its operation and trip the associated circuit breaker, thereby interrupting the fault circuit one section further back. Where the system interconnection is more complex, the above operation will be repeated so that all parallel infeeds are tripped.
If the power system is protected mainly by unit schemes, automatic back-up protection is not obtained, and it is then normal to supplement the main protection with time graded overcurrent protection, which will provide local back-up cover if the main protective relays have failed, and will trip further back in the event of circuit breaker failure.
Such back-up protection is inherently slower than the main protection and, depending on the power system con- figuration, may be less discriminative. For the most important circuits the performance may not be good enouugh, even as a back-up protection, or, in some cases, not even possible, owing to the effect of multiple infeeds. In these cases duplicate high speed protective systems may be installed. These provide excellent mutual back-up cover against failure of the protective equipment, but either no remote back-up protection against circuit breaker failure or, at best, time delayed cover.
Breaker fail protection can be obtained by checkina that fault current ceases within a brief time interval from the operation of the main protection. If this does not occur, all other connections to the busbar section are interrupted, the condition being necessarily treated as a busdar fault. This provides the required back-up protection with the minimum of time delay, and confines the tripping operation to the one station, as compared with the alternative of tripping the remote ends of all the relevant circults.
The extent and type of back-up protection which is applied will naturally be related to the failure risks and relative economic importance of the system. For distribution systems where fault clearance times are not critical, time delayed remote back-up protection is adequate but for EHV systems, where system stability is at risk unless a fault is cleared quickly, local back-up, as described above, should be chosen.
保护装置
保护装置是一个集合术语，它包括了所有用来检测、定位和触发切除电力系统故障的设备。继电器作为主要的保护功能元件而广泛使用。但保护装置还包括直接动作的交流脱扣器和熔丝。
除了继电器，保护装置包括所有附件，诸如电流互感器、电压互感器、分流器、直流线圈、交流线圈以及其它与继电器有关的设备。
一般来说，虽然开关设备也能起到基本保护的作用，但它不算是保护装置。公用设备，如电站蓄电池和其它用来保证开关动作的设备，也不属于保护装置。
为了满足许多不同配置、运行方式和建设性能的电力系统以最佳速度辨别保护的要求，必须开发多种类型的继电器以反映电力系统量。比如，在某些情况下，仅仅测量故障电流量就可以了，但在某些情况下，就有可能要测量功率和阻抗。继电器通常要测量许多复杂的系统量，这些量只有通过数学或图表方式才能方便地表达出来。
1.1
可靠性：
在1.1节中我们已讨论了，它需要一个很高的可靠性。操作错误可以归因于下列原因：:
a.不合理的设计
b.不正确的安装
c.设备老化
设计：
这是最重要的因素。要想取得完善的设计就必须对被保护的电力系统状态特性有透彻的了解。同样重要的是综合测试，测试时应尽可能模拟运行和环境状态，检查保护装置的各方面，对于多数继电器，必须测试继电器的所有组件、电流互感器和其它配件。
1.1.2
安装：
正确安装保护装置的必要性是显而易见的。但在系统复杂的内部连接和与电厂的关系致使很难检查正确与否。因此有必要采取试验手段。由于正确模拟所有故障状态是一件比较困难的事情，所以试验的目的是检验安装质量。这就是现场试验的作用，这种简单直接的试验可检验接线正确与否，并避免设备损坏，但不涉及设备故障类型试验和证实其技术性能复杂的部分。
1.1.3
运行导致的设备老化：
设备虽经正确安装，但老化可导致其正常功能受到影响。比如：频繁动作可使触点变得粗糙或烧毛，大气污染可造成触点生锈，线圈或其它回路开路，附件故障，机械部件因脏物造成卡涩，或一定程度的腐蚀而影响其动作。
保护装置的运行时间可以年为单位来衡量，麻烦的是装置缺陷可能在运行期间已经发展生成，但要等到电力系统发生故障，而保护拒动时才能发觉有缺陷存在。因此要每隔一段适当的时间对继电器进行简单、基本的试验，以检验其动作性能并未恶化。
试验应在无固定接线干扰的条件下进行，这可以通过试验插座和开关等装置达到。抽出式继电器本身就提供了这种功能，一个试验插头可插入继电器和连接点之间，使继电器所有输入回路可以通流升压。当有必要临时解开盘内接线时，将线头、线尾做上标记可防止恢复接线时弄错。
1.2
选择性：
保护是分区域布置的，这样整个电力系统都得到了保护，而不存在保护死区。当故障发生时，保护应有选择地动作，跳开距离故障点最近的开关。选择性跳闸也称为“鉴别”，一般可通过以下两种方法实现：
a 分时限保护
按序分区的保护装置被设计成分时限动作，这样当故障发生时，虽然有多套装置响应，但只有那些与故障区域有关的装置实现跳闸功能，另一些不完全动作然后复归。
b 单元保护
可以将保护装置设计成只响应某一特定区域的故障，这种“单元保护”或“限制保护”可在整个电力系统内使用。由于它不带延时，相对来说可快速响应动作。单元保护通常以比较区域边界量来实现。某些保护装置由于电力系统的布置而具有“限制”特性的，也可称为单元保护。
无论采取何种方法，必须注意选择性不仅仅依靠继电器设计，它还同时依靠适当整定的继电器与电流互感器的正确配合。要考虑到以下变量的变化范围：如故障电流、最大负荷电流、系统阻抗和其它相应的因素。
1.3
保护区域：
理论上第1.4节提到的分区域保护通过断路器重叠，断路器同时包含在两个区域内。
评价的原则对于系统的整套保护给出了一个精确的公式，但它取决于继电器的动作，每一个系统故障需要许多继电器动作来切除，只有所有的继电器都正确动作才能算做一次正确动作。有基于此，标准的技术设备能达到94%的正确动作率。即使在施工时做了进一步改进，要达到百分之百的可靠性也是不可能的。双重化配置可大大提高可靠性，两套装置中的任何一套均可达到所要求的功能。若一套装置的故障风险为x/套，采用双重化配置后，风险为x2，由于x很小，x2可忽略不计。
实际上母线保护早就采用了双重化配置。两套装置均动作以完成跳闸操作，这就是“2取
以上两个功能可以相互结合，生成“3取
电力系统的保护通常根据电流互感器的位置相应地划分区域。如果保护是单元型的，那么保护范围是一明确区分的闭合回路。
另一方面，保护区域也可不受限制。定义好保护起点，而保护终点根据系统量的测量来决定，并由于系统状态的变化和测量误差导致变化。
1.4
稳定性：
保护装置的稳定性与电网的稳定性概念不同。它是指所有负荷状态和区外故障都不会使装置动作。它实际上是相对单元保护而言的，而对非单元保护则以“识别率”来表示。
1.5
速度：
自动保护的作用就是在非常短的时间内切除电力系统故障，而该时间若使用人工手段则不可能实现，就算采用大量的人员监视也不能。其目的是使每一个扰动在导致失步扩散甚至造成设备停机前就消除它，以保证供电不中断。
故障在系统中允许存在的时间越短，系统可带的负荷越大。图1.5表明了不同故障类型下系统负荷和故障切除时间的典型关系。请注意，相对于单相接地故障，相间故障对系统稳定的影响更大，因此要求更快地切除。
仅仅追求可靠性是不够的，应避免可靠性带来的不必要的损失。故障放电产生的大电流具有及大的破坏力，它能在非常短的时间内烧坏铜导体，并将变压器或电机中的铁心叠片熔焊在一起。即使远离故障放电点，如果巨大的电流持续几秒钟也能造成设备损坏。
可见保护装置应尽可能快的动作，而速度又受到经济的限制。因此，对于那些快速动作要求不太严格的配电回路可采用分时限保护，而对于发电厂或超高压设备，在保证正确动作的前提下，保护装置应具有最快的响应速度。
1.6
主保护和后备保护：
前面章节已讨论了保护装置的可靠性，有许多因素可导致保护拒动，而断路器失灵也时有发生。有基于此，除了装设主保护，还配备其它装置作为主保护的后备，而使切除系统故障失败的可能性降到最低程度。
后备保护可以作为主保护内部的一部分，也可以通过附加设备独立出来。过流保护或距离保护可作为前一种方案的例子，通常采用分时限区别来隔离故障段，如果相应的继电器拒动或断路器拒跳，按时限下一个继电器动作跳开有关的断路器，这样就断开了故障回路的下一段。采用这种方法就获得了完整的后备保护。虽然要多断开一段回路，但在断路器失灵的情况下这是不可避免的。
当系统内部连接更复杂时，上述动作将重复执行以保证所有并联回路都跳开。
如果电力系统主要采用单元型保护，不能获得自动后备保护，后备保护通常以带时限过流保护作为主保护的补充，这样当主保护继电器拒动时，后备保护提供近后备，如果断路器失灵将跳开上一级开关。
这种后备保护速度要比主保护慢，而且根据系统布置，辨别力较弱。对于最重要的回路来说，就算是作为后备保护，这种性能也是不够好的，同时在某些情况下，由于并联回路的影响，还达不到这种性能。在这种情况下，可采用快速保护装置额定双重化配置，以使其一个装置故障时，两套装置能互为备用，但两者都不能实现断路器失灵的远后备保护，最好加上延时。
断路器失灵保护可通过检查主保护动作后一小断÷段时间内故障电流是否依然存在而实现。如果失灵保护未起动而母线上其它所有断路器断开，这种情况应视为母线故障。
很显然，后备保护的范围和类型与故障危害和系统的经济价值有关。对于配电系统，由于对故障切除时间的要求不是十分苛刻，因此采用延时远后备保护就足够了，而对于超高压系统，故障若不能迅速切除将会影响系统稳定，因此应选择上述的近后备保护。
摘自：http://www.digbbs.com 数字电气网
