原文:
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 fused. 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.
译文:
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。
