Protection Devices

Unit 3 继电保护 Protection Devices

Part A Protective Devices

Electrical wires, cables, devices, equipment and apparatus are rated ac cording to the normal value of voltage which can be applied to them and the current which they can safely carry. If other than the safe value of voltage were applied to a conductor or piece of equipment, or if higher than rated current were to flow, the conductor or piece of equipment would be seriously damaged or completely destroyed1. In addition, a general condition of hazard to personnel would result. To protect conductors and equipment against abnormal operating conditions and their consequences, a special group of devices are used in electrical systems. These devices include fuses, circuit breakers and relays and are commonly called “protective devices”.

Fuses: A fuse is the simplest, best known and most widely used device for opening an electrical circuit when excessive current flows due to overload or short circuit. Basically, a fuse is a series circuit device which will conduct the current for which the circuit（and the fuse） is rated；but it has an internal element, called a “link” which will melt and open the circuit when excessive current flows.

There are many types and sizes of fuses used to provide overload protection for both circuits and apparatus. Enclosed fuses range in capacity up to 600 amps at 250 and 600 volts. Fuses for high voltage applications（above 600 volts）are available with current capacities up to 400 amps and voltage ratings to 132 000 volts. Fuses will carry their maximum rated current continuously but will blow in from 1 to 5 minutes if current exceeds a 15 per cent rise over rated capacity.

Plug Fuses: These are screw-in type fuses. The Edison-base plug fuse is familiar to everyone from its wide application as a branch circuit protector in residential electrical systems.

An Edison-base plug fuse consists of a wire or strip of fusible metal in a small porcelain case with a mica disk set in the top making the fuse link visible. The screw base corresponds to the base on a standard medium－base incandescent lamp. These fuses are confined to use on circuits rated at a maximum of 125 volts. They are made in various current ratings：3, 6, 10, 12, 15, 20, 25 and 30 amps. Those fuses rated at 15 amps or less have an hexagonal opening in the cap, through which the mica shows, or they may have an hexagonal recess or projection in the top, or the top itself may be hexagonal in form. Those fuses rated from 16 to 30 amps have a round window in the top. The purpose of this is to make the lower rated fuses easily distinguishable from the higher rated fuses to minimize accidental interchange of the fuses.

Plug fuses and fuse holders classified as type S are similar in general construction to the Edison-base plug fuse and its holder. Types fuses, however, are tamper resisting, i. e., fuses of the 0 to 15 amp sizes cannot be interchanged with fuses of the 16 to 30-amp sizes. This arrangement eliminates the possible hazards of overfused (too large a fuse) circuits, in which the current may exceed the rating of the circuit conductor and load devices but does not exceed the rating of the fuse.

The use of tamper-resisting plug fuses is widely recommended. These fuses are available with the same ratings as Edison base plug fuses. The tamper-resisting characteristic is accomplished by using a special fuse construction and adapter. The adapter is first screwed on the fuse base. Then the combination is screwed into a regular Edison-base cutout or holder. if the fuse should then be removed from the bother, the adapter stays in the holder, and only the same size

Type S fuse can be replaced in the holder. An ordinary plug fuse cannot be inserted in the holder, and a Type S fuse of the wrong group rating cannot be inserted.

Times-delay plug fuses are fuses which allow the abnormal circuit operating condition to persist for a longer time before blowing than ordinary fuses. Also called “time－lag” fuses, they are available in regular Edison-base type plug and tamper resisting screw base up to 30 amps. These are one-time（non-renewable）fuses. The Edison-base type and tamper-resting base type are identical except for the base construction. Again the tamper-resisting type needs an adapter before it can be screwed into an Edison plug fuse receptacle. Adapters are made in three common sizes: a 15-amp adapter which will take any size fuse up to 15 amps; a 20-amp adapter for a 20-amp fuse；and a 30-amp adapter a 20-, 25-, or 30-amp fuse.

The time-delay type fuse is widely used on motor circuits in which the starting current inrush is much higher than the continuous current. The time-lag fuse w ill not open on the short duration of high starting current. If, however, the high current should persist past the high current should persist past the delay time of the f use, it will blow just as if a short circuit or heavy overload current had developed.

Cartridge Fuses: A typical cartridge fuse consists of a tube of stiff, treated paper, fiber or some similar material, with the fuse element within the tube. A contact piece of some type is mounted on each end of the tube. The fuse element is connected between these metallic contact pieces inside the tube.

In a nonrenewable（one-time）cartridge fuse, the fuse element consists of a fuse wire stretched through the tube in an insulating compound between copper ferrule terminals on the ends of the tube. When the fuse blows, the compound quenches any possible arc.

Renewable cartridge fuses contain fuse links which can be replaced when blown. This type of cartridge fuse can be readily taken apart for simple and easy replacement of a blown link. Renewable fuses are made in types with copper ferrule terminals at the tube ends and with knife－blade contacts.

Cartridge fuses have ratings and dimensions as follows: 0 to 250 volts, 61 to 60 0 amps-up to 11 inches long, 2 1/2 inches in diameter; 251 to 600 volts, 0to 600 amps-5 inches to 14 inches long, 3 inches in diameter.

New Words and Expressions

abnormal a. 反常的, 不正常的 hexagonal a. 六角(边)形的 cartridge n. 拾音器心座 quench v. 淬熄；扑灭 ferrule n. 套圈；箍 circuit breaker 断路器

Notes

1. If other than the safe value of voltage were applied to a conductor or piece of equipment, or if higher than rated current were to flow, the conductor or piece of equipment would be seriously damaged or completely destroyed.

本句含有两个非真实条件状语从句，分别由if other than… 和or if…引出，主句也为现在时态虚拟语气。全句译为：如果施于导体或某设备的不是安全电压，或者其所通过的电 流高于额定电流，则该导体或设备将严重受损或完全被破坏。 Exercises

I．Translate the following words or expressions into English 1. 2 3. 4 5. 6. 7. 8. 9. 10. II． Fill in the blanks according to the passage.

I．In power plants convert the primary energy to mechanical energy, and convert mechanical energy to electric energy．

2．The generator voltages are usually stepped up to the transmission voltage level by 3．Residential and commercial customers are supplied by the secondary distribution feeders at 4．In USA，the voltages of the transmission system are typically ． 5. 6. 7. 8. 9. 10.

III．Answer the following questions according to the passage. 1．What components is a modern power system comprised Of? 2．What is the function of the distribution system? 3．What is the function of the transmission system? 4．What is the function of the subtransmission system?

5．What is the characteristics of a modern power system in network structure? Ⅳ. Retell the elements of a power system based on Fig. 1.

Part B Relays

The protection of a power system is planned by dividing it (on paper) into regions or zones that can be separated from the rest of the system (physically) by the placement of circuit breakers. The action of the circuit breakers is controlled by current information about the state of the system, especially the state of a zone. The general idea is to define the zones so that (a) the isolation of the faulted zone has minimum effect on the remainder of the system, (b) the expensive, vulnerable equipment is protected from permanent damage that might be caused by the excessive currents, and (c) the faulted zone can be restored to normal service as quickly as possible.

The state of the system is monitored by real time samples of the currents and voltages in the system， usually with the aid of current transformers (CT's) and potential transformers (PT's). Often, the currents and voltages so sensed are fed to relays, which in turn close or open the trip circuits on the circuit breakers.

A relay is defined as a device that responds to the sampled system quantities (i.e, voltage and /or current) to indicate or isolate an abnormal operating condition. At present there are two main categories of relays, the traditional electromagnetic type, and the semiconductor (static) type. There are four basic types of electromagnetic relays: the plunger, the hinged armature (or clapper type), the induction disk, and the induction cup. In the semiconductor category, there are many circuits involving transistors and thyristors that can process the system variables in prescribed ways and then activate a tripping circuit if the processed information suggests that this should be done.

The time required to activate a relay is an important quantity to be considered in the particular application of relays. The trade jargon employs the word “instantaneous” to refer to those relays for which no time delay is deliberately introduced. When such fast-acting relays are required, the type employed is the plunger, hinged armature, or induction cup (or a semiconductor if very fast response is required) .The induction disk type is used for deliberate time delay. The time required to activate a relay is proportional to some inverse power of the system quantity (e. g, the current). A graph of the time to close vs. system quantity thus has a form similar to the ones shown in Fig. 12.1. The names are more trade jargon.

图

FIG.12.1 ILLUSTRATION OF TYPES OF RELAYS ACCORDING TO THE TIME TO CLOSE VERSUSACTIVATING PARAMETER.

Relays have a minimum current to activate at all, and a minimum time to activate with any current. According to the details of how the fields and forces are made to interact in a specific relay design, the relay may respond to product of voltage and current (i. e power) or to the ratio of voltage to current (i.e. impedance). The impedance at a point on the line depends on how far away the fault is located from the relay, which means that the impedance relay is sensitive to the distance away from the fault. This property is useful in zone protection schemes.

Relays can be made to act on the basis of a voltage-to-current ratio by a two-coil construction in which the coils act in opposition. For example， one-coil is energized with the sampled current, and may act in such a direction as to “pick up ,”or close, the relay contacts, while the other coil is energized in proportion to the sampled voltage, and the second coil acts in opposition (or reset) to the current-sensitive coil. The device can be designed and set so that it is just on the verge of picking up at a specified value of the ratio of I/V or V/I; appropriately, this type of relay is known as an impedance relay. It picks up when I is large and V is small. It is helpful to diagram an operating characteristic for such a relay. Conventionally, this is done in an impedance plane chart.

图

FIG. 12.2 IMPEDANCE OR MHO-TYPE RELAY DIAGRAMS

The constant ratio of V/I=Z is a circle, so that the region inside the circle is the pick-up

region. The radius of the circle is adjustable.

The basic construction can be altered also so as to displace the center of the pick-up region. Such devices are often called mho-type relays.

Another generic type of very useful relay is known as the differential relay. Sensing elements are placed on two sides of the equipment or zone to be protected as in Fig. 12.3.

图

FIG.12.3 DIFFERENTIAL RELAYS TO INDICATE THAT THE FAULT LIES BETWEEN THEM.

Both of these latter types of relays (and many others) are useful in providing zones of protection for indicating the location and type of failure or malfunction, and for providing back-up relaying in the event of a malfunction of the primary relaying mechanism. Back-up protection is required to guard against such things as failure of the samplers (CT's or PT's, for example) providing the information, the relay mechanism itself, the triggering circuits for the breaker, or the breaker mechanism itself.

Back-up relays are those for which a deliberate time delay is introduced. Their operation must be delayed until sufficient time has elapsed for the primary relays to operate.

As pointed out above, system protection usually proceeds by dividing it into zones of protection, such that all circuit breakers in a given zone and only those in that zone, trip when a fault occurs in that zone. Ideally, zones of protection each overlap a circuit breaker, which makes it possible to isolate a faulty zone with the minimum number of circuit breakers tripping.

New Words and Expressions

zone n. 区，带 isolation n. 隔离 fault v. 故障

potential a. 电压的，电位的 potential transformer 电压互感器 trip circuit 跳闸电路 sampled a. 抽样的 static a. 静电的

plunger n. 插棒式继电器 hinged armature 枢轴衔铁 clapper type 抬刀式，铃锤式 transistor n. 晶体管

the trade jargon 本行业术语 deliberate a. 故意的 inverse a. 逆的 inverse power 逆能 graph n. 曲线图 interact v. 相互作用 ratio n. 比率，比值 property n. 特征

scheme n. 设计，系统 reset v. 重新调整，复原 diagram v. 用图表示 plane chart 平面图

radius n. 半径距离，活动界限 displace v. 变位，移动 mho n. 姆（欧）（电导电位） generic a. 通用的

differential relay 差动继电器 malfunction n. 故障 back-up n. 后备 trip v. 跳闸，断开

triggering circuit 触发电路 overlap v. 覆盖，重叠

elapse v. （时间）过去，经过；

Exercises:

ⅠTranslate the underlined sentences in t he passage.

ⅡAnswer the following questions according to the passage. 1.举例说明保护设备 2.什么是保险丝

3.什么是继电器，有哪些类型 4,. 电磁继电器有哪些类型 5. 简述如何使电网安全运行