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Question

LINE PROTECTION WITH OVERCURRENT RELAYS
Lines are protected by overcurrent-, distance-, or pilot-relaying equipment, depending on the requirements. Overcurrent relaying is the simplest and cheapest, the most difficult to apply, and the quickest to need readjustment or even replacement as a system changes. It is generally used for phase- and ground-fault protection on station-service and distribution circuits in electric utility and in industrial systems, and on some subtransmission lines where the cost of distance relaying cannot be justified. It is used for primary ground-fault protection on most transmission lines where distance relays are used for phase faults, and for ground back-up protection on most lines having pilot relaying for primary protection. However, distance relaying for ground-fault primary and back-up protection of transmission lines is slowly replacing overcurrent relaying. Overcurrent relaying is used extensively also at power-transformer locations for externa-fault back-up protection, but here, also, there is a trend toward replacing overcurrent with distance relays.

It is generally the practice to use a set of two or three overcurrent relays for protection against interphase faults and a separate overcurrent relay for single-phase-to-ground faults. Separate ground relays are generally favored because they can be adjusted to provide faster and more sensitive protection for single-phase-to-ground faults than the phase relays can provide. However, the phase relays alone are sometimes relied on for protection against all types of faults. On the other hand, the phase relays must sometimes be made to be inoperative on the zero-phase-sequence component of ground-fault current. These subjects will be treated in more detail later.

Overcurrent relaying is well suited to distribution-system protection for several reasons. Not only is overcurrent relaying basically simple and inexpensive but also these advantages are realized in the greatest degree in many distribution circuits. Very often, the relays do not need to be directional, and then no a-c voltage source is required. Also, two phase relays and one ground relay are permissible. And finally, tripping reactor or capacitor tripping (described elsewhere) may be used.

In electric-utility distribution-circuit protection, the greatest advantage can be taken of the inverse-time characteristic because the fault-current magnitude depends mostly on the fault location and is practically unaffected by changes in generation or in the high-voltage transmission system. Not only may relays with extremely inverse curves be used for this reason but also such relays provide the best selectivity with fuses and reclosers. However, if ground-fault-current magnitude is severely limited by neutral-grounding impedance, as is often true in industrial circuits, there is little or no advantage to be gained from the inverse characteristic of a ground relay.

Answer 1

线路过流保护
Lines are protected by overcurrent-, distance-, or pilot-relaying equipment, depending on the requirements. Overcurrent relaying is the simplest and cheapest, the most difficult to apply, and the quickest to need readjustment or even replacement as a system changes.
根据需要，线路保护可使用过电流保护，距离保护，导引线保护等。过流中继是最简单和最便宜。但是根据电力系统的变化，它要调整。它通常用于相和接地故障对变电站和电力公司配电线路及工业系统，也用在一些哪些认为距离保护价格太贵的中压系统中。
It is generally used for phase- and ground-fault protection on station-service and distribution circuits in electric utility and in industrial systems, and on some subtransmission lines where the cost of distance relaying cannot be justified.
线路用相距离保护作为主保护，接地过流保护作为后备保护。也有相间故障采用导引线保护作为主保护，接地过流保护作为后备保护。
However, distance relaying for ground-fault primary and back-up protection of transmission lines is slowly replacing overcurrent relaying. Overcurrent relaying is used extensively also at power-transformer locations for externa-fault back-up protection, but here, also, there is a trend toward replacing overcurrent with distance relays.然而，线路保护中接地距离保护有取代过流保护的趋势。过流保护广泛应用于变压器外部故障的后备保护，但是，现在也有被距离保护取代的趋势。

相间故障由2个或3个过流继电器来反映，接地保护由一个过流继电器来反映。采用独立的接地过流保护是有优点的，它可以很灵敏的反映接地故障。有时，相过流保护可以保护各种类型的故障（相间故障或接地故障）。有时，相过流保护只是反映相间故障，接地故障由接地过流保护来反映。

Answer 2

线路保护功能过流继电器线路过电流保护的要求，，距离，或试点，中继设备，这取决于。过流中继是最简单和最便宜，最困难的申请，并最快需要调整，甚至作为一个系统的变化更换。它通常用于相和接地故障对车站服务和电力公司配电线路及工业系统，以及在一些subtransmission那里的中继距离的成本是没有道理的保障线。它主要是用于接地故障的最传送距离继电器用线相故障保护，地面上最有试点的主要保护中继线路的后备保护。然而，对于地面中继故障距离小学和后勤保障的输电线路正在逐渐取代过电流中继。过流中继外耳道广泛用于电力变压器的地点也是在故障后备保护，但在这里，也有一对距离继电器趋势与更换过流。这是一般的做法是使用过电流设置两个或三个继电器对相间故障和单相对独立的过流继电器接地故障保护。独立的接地继电器的普遍青睐，因为他们可以进行调整，以提供更快速，更敏感，比相继电器可提供单相位对接地故障保护。然而，相继电器有时仅依靠对故障的各种保障。另一方面，相继电器有时必须作出建立在接地故障电流零相序分量不起作用。这些科目均会在以后详细讨论。过流保护适合中继系统保护的几个原因分销。不仅是过流中继基本上简单和廉价，而且这些优势是体现在许多配电线路最大程度。很多时候，继电器不必为定向，然后没有交流电压源是必需的。此外，两相继电器和一个地面中继是允许的。最后，脱扣器或电容器跳闸（介绍其他地方）也可使用。在电动工具分布短路保护，最大的优势特点，可采取的反时限，因为故障电流大小和主要取决于断层的位置实际上是在一代的变化或在高压输电系统不受影响。不仅可以极逆曲线继电器用于此原因，但也提供了这样的继电器保险丝和重合的最佳选择。但是，如果接地故障电流的幅度是非常有限的中性接地阻抗，因为往往是在工业电路真实，几乎没有或根本没有优势，从一个地面中继逆特征上涨。

参考资料：lingoes