继电保护中英文翻译

Fundamentals of protection practice

The purpose 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 these two requirements are largely opposed, 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 itself, 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.

The greatest threat to the security of a supply system is the short circuit,which imposes a sudden and sometimes violent change on system operation. The large current which then flows, accompanied by the localized release of a considerable quantity of energy, can cause 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 caused to the system.

A power system represents 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 users 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 service 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 closely 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 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. Reliablity

The performance of the protection applied to large power systems is frequently assessed numerically. For this purpose each system fault is classed as an incident and those which are cleared by the tripping of the correct circuit breakers and only those, are classed as 'correct'. The percentage of correct clearances can then be determined.

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. On this basis, a performance of 94% is obtainable by standard techniques.

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 performance. 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.

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.

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. 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 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.

SENSITIVITY

Sensitivity is a term frequently used when referring to the minimum operating current of a complete protective system. A protective system is said to be sensitive if the primary operating current is low.

When the term is applied to an individual relay, it does not reter to a current or voltage setting but to the volt-ampere consumption at the minimum operating current.

A given type of relay element can usually be wound for a wide range of setting currents; the coil will have an impedance which is inversely proportional to the square of the setting current value, so that the volt-ampere product at any setting is constant. This is the true measure of the input requirements of the relay, and so also of the sensitivity. Relay power factor has some significance in the matter of transient performance .For d.c. relays the VA input also represents power consumption, and the burden is therefore frequently quoted in watts.

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.

Back-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. In this way complete back- up cover is obtained; one more section is isolated than is desirable but this is inevitable in the event of the failure of circuit breaker. 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.

Ideal back-up protection would be completely indepen_ dent of the main protection. Current transformers, voltage transformers, auxiliary tripping relays, trip coils and d.c. supplies would be duplicated. This ideal is rarely attained in practice. The following compromises are typical:

a. Separate current transformers (cores and secondary windings only) are used for each protective system, as this involves little extra cost or accommodation compared with the use of common current transformers which would have to be larger because of the combined burden.

b. Common voltage transformers are used because duplication would involve a considerable increase in cost, because of the voltage transformers themselves, and also because of the increased accommodation which would have to be provided. Since security of the VT output is vital, it is desirable that the supply to each protection should be separately fused and also continuously supervised by a relay which wil1 give an alarm on failure of the supply and, where appropriate, prevent an unwanted operation of the protection.

c. Trip supplies to the two protections should be separately fuse

d. Duplication of tripping batteries and of tripplng coils on circuit breakers is sometimes provided. Trip circuits

should be continuously supervised.

d. It is desirable that the main and back-up protections (or duplicate main protections) should operate on different princlples, so that unusual events that may cause failure of the one will be less likely to affect the other.

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继电保护原理

发电并将电力供应给用户这就是电力系统的作用。系统要通过设计、组织，以使电力能够可靠、经济地送到用户2端。由于可靠和经济是两个相互对立的要求，因此系统的可靠性和其所花费用要综合考虑，

一方面，系统设计时必须保证可靠性。而另一方面，不应该不计费用的盲目追求高可靠性，应该结合各方面因素综合考虑。

改进设备设计可以更好地提高供电可靠性，比如增加备用容量裕度和安排多回路供电。将系统分段，有带保护装置的开关控制，这样可以在正常运行时有多种运行方式可供选择，而在故障时可将损失减少到最低程度。

对供电系统最大的威胁就是短路，它往往使系统运行发生突然巨大的变化，巨大能量的局部释放产生的大电流，可导致故障点起火和贯穿系统的机械损伤，特别是对于电机和变压器线圈绕组。迅速断开离故障点最近的开关可减少损失并防止系统瓦解。

一个电力系统就意味着非常巨大的投资。为最大限度地收回成本，系统应尽可能多带负荷。因此不仅要使系统在AB范围内（图1.1〕运行以吸引潜在的用户，还要尽可能保证系统持续地满负荷运行，只有这样才能给用户带来最好的服务，供电方才能获得最大收益。不能保证设备和电网绝对不发生故障。虽然故障的危害对于单一元件是轻微的，但经过大电网中密切相关的众多元件，故障的危害就扩大了，这就是说任何一个故障有可能影响到整个电网。对于一个大电网，故障发生的几率和故障带来的扰动是相当大的，如果没有切除故障的设施，电网是不允许运行的。没有相应的故障切除装置，就算安装了很多开关，电网的作用也不能实现，还必须根据电力系统的特性和要求配备不同的保护装置来控制开关。一个电力系统如果没有相应的保护，它就不能算是合理的设计和组织。

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。可靠性

为使大型电力系统的继电保护装置动作能以数字来评价，每一个系统故障定义为一次事故，故障由且仅由正确的断路器来切除的，称为正确动作，由此可得到正确动作率。

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2”配置。在另一些情况下，重要回路的主保护采用双重化配置，每一套装置能独立完成跳闸操作，这就是“2取1”配置。前一种配置是为了避免误动，后一种配置是为了避免拒动。

以上两个功能可以相互结合，生成“3取2”配置，也就是联合使用三套同样的装置，任何两套装置动作均可实现跳闸功能。这种设计方案已在一小范围内使用，而该原理的应用将日渐广泛。如果一个电网采取这种设计方案，用概率论可以得出这样一个结论，即保护正确动作率可达99.8%。这种正确动作率指标要求单独的保护装置完全独立，任何公用因素，如公用的电流互感器、跳闸用蓄电池，将降低正确动作率。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。

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。

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。

系统负荷使不同点电压之间发生相位移，这使系统故障时增加了失步的可能性。一个故障在系统中允许存在的时间越短，系统可带的负荷越大。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。

灵敏性

灵敏性通常是指一套完整的保护装置的最小动作电流。如果一个装置的一次动作电流是低的，就称该装置是灵敏的。

对于一个单独的继电器，灵敏性与电流或电压的整定值无关，而是根据最低动作电流下的伏安消耗量来定的。

一个定型的继电器元件通常根据整定电流可大范围变化来绕制的，线圈阻抗与电流整定值的平方成正比，因此在任一整定值下伏安数是恒定的。这是继电器输入要求和灵敏度的真实尺度。继电器功率因数对于暂态性能有重要意义，这个问题将在第五章讨论。

对于直流继电器，输入伏安数以功率代替，所带负载量也以功率表示。

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。

理想的后备保护完全独立于主保护，即电流互感器、电压互感器、辅助跳闸继电器、跳闸线圈和直流电源应双重配置。可在实际应用中很少采用，而往往采用以下几种方案：

a.每一套保护装置采用独立电流互感器(仅铁心和二次绕组)，与要带很多负载的公用电流互感器相比，只增加了很少的费用。

b.由于电压互感器自身及相应配置的设备大大增加了双重化配置的费用，因此采用电压互感器公用的方式。由于电压互感器的输出可靠是极其重要的，因此每套保护的电压回路应装设独立的熔丝，并用一个继电器持续监视，当熔丝熔断时发信，以防

止保护误动。

c.两套保护的跳闸电源应装设独立熔丝。有时候跳闸蓄电池和跳闸线圈也采用双重化配置。跳闸回路应持续监视。

d.主保护和后备保护(或主保护双重化配置)应采用不同的动作原理，这样万一有情况导致一套保护故障时，不会影响到另一套保护。

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