itz继电保护中英文翻译(精)

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 and fuses.

In addition to relays the term includes all accessories such as current and voltage

transformers, shunts, 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-