reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

rise, to values not exceeding those prescribed for the materials used in the construction of the switchgear.

2 Types of switchgear

2.1Descriptions

The types of switchgear used on auxiliaries power systems in power stations in the UK comprise, broadly, circuit-breaker equipment, fused contactor controlgear and fusegear.

For the purpose of this chapter, the following definitions have been adopted:

Circutt-breaker A mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions, and also of making, carrying for a specified time and breaking currents under specified abnormal circuit conditions, such as those of short-circuit.

Contactor (electrically-held) A mechanical switching device having only one position of rest, operated otherwise than by hand, capable of making, carrying and breaking currents under normal conditions, including operating overload conditions.

Contactor (latched) A mechanical switching device operated otherwise than by hand, capable of making, carrying and breaking currents under normal conditions, including operating overload conditions, and fitted with a latching device. The latching device prevents the contactor from opening when the means of closure is de-energised. Thus a latched contactor is deemed to have two positions of rest. The contactor is opened by release of the latching mechanism electrically.

Switch A mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions, which may include specified operating overload conditions, and also of carrying for a specified time currents under specified abnormal circuit conditions, such as those of short-circuit. It may also be capable of making, but not breaking, short-circuit currents.

ascontiector (isolator) A mechanical switching device which provides, in the open position, an isolating distance in accordance with specified requirements. A disconnector is capable of opening and closing a circuit when either negligible current is broken or made, or when no significant change in the voltage across the terminals of each of the poles of the discornector occurs. It is also capable of carrying currents under normal circuit conditions and of carrying for a specified time currents under abnormal conditions,

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such as those of short-circuit. 'Negligible currents' imply currents such as the capacitance currents of bushings, busbars, connections, very short lengths of cables and currents of voltage transformers and dividers. 'No significant change in voltage' refers to applications such as the by-passing of induction voltage regulators or circuit-breakers.

Fuse-switch A switch in which a fuselink or fuse carrier with fuselink forms the moving contact, of the switch. However, an essential requirement is that the fuselink shall be disconnected on both sides when the switch is open. Thus arrangements in which the fuselink remains stationary during operation of the switch, but in so doing are disconnected on both sides when the switch is open, are generally accepted as qualifying as fuse-switches. Indeed, there is merit is not subjecting the fuselink to the shock of rapid acceleration and deceleration as is the case when mounted on the moving element.

Almost without exception, the II kV switchgear is of the circuit-breaker type. At 3.3 kV, the practice is a little different. Whilst circuit-breaker gear is the rule generally for switchboard incoming supplies, busbar sectioning/interconnector and transformer feeders, the bulk of motor circuits, i.e., drives of up to the order of 1000 kW, have, since circa the mid-I960s, been handled by fused controlgear — known widely as 'motor switching devices' (MSD). However, more recently, such fused controlgear is, on occasion, used for transformer circuits up to 1000 kVA. Motors of rating above 1000 kW are usually controlled by circuitbreakers.

At lower voltages, i.e., 415 V and below, circuitbreakers are normally employed for incoming and busbar section/interconnection duty on switchboards deriving supply from higher voltage sources; fused contactor gear and fusegear being the rule generally for motor control and distribution respectively. Where the scheme of protection allows, switchboards further 'downstream' may be fed via fuses — usually mounted in fuseswitches — switches or disconnectors (isolators).

The reader wishing to research circuit-breaker theory and design in depth is recommended to study 'Power Circuit Breaker Theory and Design' published by Peter Peregrinus Ltd.

2.2 Testing and certification

2.2.1 General

All switchgear and associated equipment is type and routine tested basically to the appropriate British Standard, varied and/or augmented where necessary to satisfy a particular service requirement.

Type tests are performed on one switchgear equipment of each type and rating, erected for the service

al Ca S:

•Short-circuit withstand.

•The making and breaking of current under both normal and short-circuit conditions.

•Temperature rise in normal operation.

•Mechanical/electrical endurance.

•Dielectric (insulation) properties.

switchgear in nuclear safety related ,.stems, testing is carried out to determine the ability to withstand prescribed levels of seismic event, i.e.,

earthquakes. Essentially, the switchgear must be demonstrated to be capable of withstanding, for a defined

period — presently of the order of 10 s — horizontal and vertical 'floor' motion whilst performing any duty,

c.v., an opening or closing operation on command, maintaining an open or closed condition, required in the course of a reactor safe shutdown procedure.

5.4 and 5.5 show typical response spectra which

2.2.2 Certification

Wherever possible, types tests are carried out in accordance with the procedure defined in the relevant Standard. However, the values of the test quantities (e.g., current and voltage) may be varied

the Standard to satisfy a particular performance requirement. A point to be noted in this respect concerns the issue by Testing Authorities such as ASIA Certification Services in the UK, and the KEMA Organisation in The Netherlands, of certification of performance. The necessary documentation is normally provided in the form of a Certificate (of rating) or a Report of Performance. An example of the front sheet of a Certificate of Short Circuit Rating issued by the ASTA is shown in Fig 5.6.

Certificate

A Certificate comprises a record of type tests strictly in accordance with a recognised national or international Standard. lt certifies that the equipment tested

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6g

4 g

so 4

04 05 06 0 7 08 09 0

FREQUENCY, Hz

2 5g

Ftc. 5.5 Vertical response spectra

,,

,

,

1

,

has satisfied the requirements of the Standard and, by so doing, the ratings assigned by the manufacturer are endorsed by the Testing Authority.

Report of Performance

A Report of Performance comprises a record of tests carried out in accordance with the instructions of the client — usually the equipment manufacturer. Reports of Performance are commonly issued to cover tests which, whilst they may have been carried out in accordance with the procedure prescribed in a recognised Standard, featured values of test quantities differing from those specified in the Standard.

2.2.3 Type tests

A particular departure from the requirements for type testing specified in the relevant Standard for 3.3 kV and 11 kV circuit-breakers (i.e., BS5311) concerns the number of operations to be performed during the 'mechanical' test. Here, the number of operating cycles specified presently is 2000, as opposed to the Standard requirement for 1000. It is expected that an even higher level of 'endurance' will be achieved by the designs of vacuum and SF6 circuit-breakers now under de-

velopment. Essentially, the number of cycles of operations demonstrated for the test is taken as indication of the frequency with which routine servicing should be undertaken.

The type testing of circuit-breakers for electrical performance, i.e., current making and breaking capability, centres around the 'basic short-circuit test duties' and 'critical current tests' specified in BS5311 in the case of high voltage equipment, and the tests for 'verification of rated short-circuit making and breaking capacities' specified in BS4752 for lower voltage gear.

The object of the testing is to prove the current making, breaking and carrying capability at any value of current up to and including the rated capacity at prescribed values of power factor, applied voltage, transient recovery voltage (TRV) and power frequency recovery voltage. Obviously the severity of the shortcircuit and normal conditions of the system in which the switchgear is to be installed must fall within the proven capability.

Short-circuit test series

For the purpose of defining the short-circuit capability, and hence the suitability of a particular design in a

it) Record Of Proving

(2) Our ograrns,tt Nos

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Total of 28 itemized or. sheet No. 1.

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given application, circuit-breakers are assigned a 'rated operating sequence'. BS5311 recognises two, viz:

•Breaking operations — breaking current: 100% of the rated breaking current, i.e, the RMS value of the symmetrical rated breaking current.

(a)0-t-CO-t '-CO

(b)CO-t "-CO

where 0 represents an opening operation, CO represents a closing operation followed immediately (i.e., without any intentional time delay) by an opening operation, 1, I and I " are time intervals between successive operations.

The intervals 1, t' and t " are indicative of the period of time which should be allowed to elapse before a circuit-breaker is called upon to repeat the clearance of short-circuit current. Similarly, the assigned rated operating sequence is indicative of the capability of the circuit-breaker to clear successfully repeated shortcircuits without inspection/maintenance.

If, in sequence 0-t-CO-r-CO, the time intervals are not specified, then:

•t = 3 min for circuit-breakers not intended for rapid auto-reclosing.

•t = 0.3 s for circuit-breakers intended for rapid auto-reclosing (dead time).

•t' ,= 3 min.

In sequence CO-t "-CO, t" = 15 s for circuit-breakers not intended for rapid auto-reclosing.

The rated operating sequence deemed appropriate for power station switchgear is 0-t-CO-r-CO, with time intervals t and t' both nominally 3 min but not less than 2 min. In general, there is no requirement for auto-reclosure, faults of short-circuit magnitude on power station systems are unlikely to be of a transient nature.

The basic short-circuit test series to BS53I 1 consists of the following test duties:

Test duty 1 Test duty 1 consists of the rated operating sequence confined to breaking operations only at 10% of the rated short-circuit breaking current with a DC component of less than 20%.

Test duty 2 Test duty 2 consists of the rated operating sequence confined to breaking operations only at 30% of the rated short-circuit breaking current with a DC component of less than 20%.

Test duty 3 Test duty 3 consists of the rated operating sequence confined to breaking operations only at 60% of the rated short-circuit breaking current with a DC component of less than 20%.

Test duty 4 Test duty 4 consists of the rated operating sequence with the following characteristic quantities:

•Making operations — making current: 100% of the rated short-circuit making current.

For this test duty the percentage DC component must not exceed 20% of the AC component.

When the characteristics of the test plant are such that it is impossible to carry out test duty 4 within the specified limits of applied voltage, making current, breaking current, and transient and power frequency recovery voltage, taking account also of the limits apertaining to the time interval t between tests, the making and breaking tests in test duty 4 may be made separately as follows:

•Test duty 4a : making tests. C-t-C where the rated operating sequence is 0-t-CO-V-CO, at 100 0TQ of the rated short-circuit making current.

•Test duty 4b : breaking tests. 0-t-O-t-0 where the rated operating sequence is 0-t-CO-r-CO, at 100% of the rated short-circuit breaking current.

When test duty 4 is performed as 4a and 4b it is necessary, additionally, to demonstrate a 'make break' (CO) capability at values of voltage and current as near to the rated values as is practicable for the test plant.

Test duty 5 Test duty 5 consists of the rated operating sequence confined to breaking operations only at 100% of the rated short-circuit breaking current, with a specified percentage DC component (see Fig 5.24). As stated previously, a DC component of 50% is specified for switchgear for use in CEGB power stations.

Critical current tests

The critical current of a switching device is a value of breaking current, less than the rated short-circuit breaking current, at which the arcing time is a maximum, and is significantly longer than that at the rated short-circuit breaking current. The manifestation of critical current, to a greater or lesser extent, is a feature of switching devices in which the efficacy of the arc extinguishing mechanism is a function of the value of the current interrupted, i.e, the efficacy increases as the current increases, and vice-versa. Circuit-breakers of the air-break type, i.e, circuit-breakers in which interruption of the arc takes place in air nominally at atmospheric pressure, are particularly prone to this behaviour. The problem in air circuit-breakers stems from the inability of arcs of relatively low current value to rise properly in the arc chutes, for reasons of thermal and/or magnetic effect. The situation is markedly improved by the expedient of directing a 'puff' of relatively low pressure air into the arc chute, beneath the arc; this 'puff' of air itself being derived

ally from the circuit-breaker moving contacts

Tests for critical current are made on circuit-breakers likely to exhibit such characteristic at values of cur-

rent less than ION or the rated short-circuit breaking current. It is assumed that this is so if the average of

i he arcing times in test duty 1 is significantly greater than that in test duty 2

Single-phase lion -circuit tests

In addition to the above tests for short-circuit current making and breaking capability, for designs of circuitbreaker in which the contact systems of the three poles are coupled mechanically and provided with a common opening release, it must be demonstrated that the circuit-breaker is capable of breaking in an outer pole a current of not less than 100% of the rated break- ing current. This test is necessary to show that the operation is not affected adversely by the unbalanced forces produced.

Short-time current test

A short-time current test is carried out to demonstrate the capability of the switchgear to carry, for its 'rated duration of short-circuit', a current of not less than its rated breaking current; the peak value of the first major loop of the test current being not less than that of the rated making current.

The principal type tests applied to the designs of Fused equipment covered in Section 6 of this chapter, are described in the following paragraphs.

Switching device in combination with fuselinks — Test duties 1, 2 and 3 in accordance with Publication No 22 of the Association of Short-Circuit Testing Authorities in the UK, an Organisation now incorporated under the name of ASTA Certification Services.

This test duty is an 0-t-CO sequence in a three-phase circuit having prospective symmetrical and peak currents not less than 100 0/o of the rated short-circuit values of the switching device in comhination with its fuselinks — the latter being fitted in all three phases. This test is carried out to verify that the complete switchgear assembly is capable of withstanding the cut-off current of the fuselinks, and that the striker pins incorporated in the fuselinks initiate Opening of the switching device correctly.

This test duty is an 0-t-00 sequence in a three-phase circuit, with fuselinks fitted in all phases, ha%.ine, a prospective current approximating to that Producing maximum arc energy within the fuselinks. This test is carried out to demonstrate that the complete switchgear assembly is capable of withstanding

the maximum energy (I 2 t) let-through of the fuselinks.

This test duty comprises one 0-operation on an outer pole, and repeated on the other outer pole, with fuselinks fitted in all poles, in a singlephase circuit having a prospective current not less than the rated short-circuit breaking current of the switching device in combination with its fuselinks. The test is performed to further verify that the complete switchgear assembly is capable of withstanding the cut-off current of the fuselinks, and that the striker pins incorporated in the fuselinks initiate opening of the switching device correctly.

For the purpose of demonstrating the rated making and breaking current capability of the switching device in high voltage fused equipment the switching device itself is subjected to the series of short-circuit type tests applied to 3.3 and 11 kV circuit-breakers. This procedure proves:

•The adequacy of the current making and breaking capability, having regard to the value at which the series-connected fuselinks 'take-over' the clearance of fault current.

•The capability of the device to function as a circuitbreaker in a system, the fault level of which is within the rated breaking current capacity of the device, i.e., back-up fuse protection is not necessary.

Additionally, the switching device is subjected to the tests for verification of rated making and breaking currents prescribed in BS775: Part 2: 1974: Clause 8.2.4 for Utilisation Category AC4. However, the value of the test current specified in the BS to prove the minimum rated breaking current — a value equal to 20 0/o of the maximum rated breaking current — may well not be low enough to demonstrate satisfactorily the behaviour if required to interrupt the load current of the smaller motors when 'running light'. Accordingly, the 25 opening operations specified in the BS to prove the rated minimum breaking current are carried out at a value of between 5 and 10 A.

Earthing switches are type tested in accordance with BS5253 to prove the rated short-circuit current making and carrying capability; also the specified mechanical endurance. There is no breaking current requirement. Except for the circuit earthing device built into fused equipment in 3.3 kV systems, the short-circuit current making and carrying capability, i.e., the short-time rating, must be not less than that of the system switchgear. The circuit earthing device in fused equipment in 3.3 kV systems is required to be capable of making a current of not less than the maximum peak value of the short-circuit current of the system, but with the ti me element of the short-time current reduced from the 'standard' 3 s to 0.2 s.

Switchgear and controlgear at all operational voltages is subjected to temperature rise tests to prove the rated normal current capability. Additionally, in the case of fused switching device equipment operating in 3.3 kV systems, a test is performed to demonstrate

that the switchgear is capable of carrying for two minutes, without damage, a current equivalent to six ti mes the rated normal current. This test is applied to ensure that the switchgear has sufficient thermal capacity to handle the starting current of direct-on-line started motors for the longest run-up time likely to be encountered.

As has been indicated, type testing is carried out on the switchgear assembled substantially as it will appear in service, albeit without the connection of service cabling, Thus the individual component parts, particularly busbar systems and main circuit elements, e.g., the circuit switching device and circuit isolating facility, are tested so assembled.

With few exceptions to date, all low voltage circuitbreaker equipment, particularly in 415 V systems, is designed, constructed and type tested basically to BS3659: Specification for heavy duty air-break circuitbreakers for AC systems. Consequent upon the publication of BS4752: Specification for switchgear and controlgear for voltages up to and including 1000 V AC and 1200 V DC Part 1: 1977: Circuit-breakers, which Standard is itself identical to EEC Publication 157-1: 1973, BS3659 was withdrawn.

The requirements of BS3659 are, in certain respects, more appropriate in the case of equipment for power station applications. However, it is policy to work to current Standards wherever possible. Thus, new developments in this field are subjected basically to the electrical and mechanical type tests prescribed in BS4752: Part 1, augmented by the following:

•A three-phase short-circuit breaking test comprising an 04-0-1-0 sequence at a value of current not less than 100% of the rated symmetrical breaking current, plus a DC component in one phase at the instant of contact separation of not less than 50% of the peak value of the rated symmetrical breaking current. The method of determination of breaking currents is illustrated in Fig 5.24. Demonstration of this capability is necessary because the waveform of short-circuit current close to the source of generation can exhibit significant asymmetry for a number of cycles after fault inception.

•A single break test at a value of current not less than 100€!0 of the rated symmetrical breaking current, at the appropriate phase-to-neutral voltage, applied to an outer pole. This demonstrates that the operation of the circuit-breaker is not affected adversely by the unbalanced forces produced.

Main circuit contactors in contactor controlgear assemblies are, in addition to the type tests prescribed in BS5424: Part 1, subjected to the second test detailed in Clause 8.2.7 of BS5419, i.e., a current making test, for the specified prospective short-circuit current. For this test, and for the through-fault test prescribed for the contactor in BS5486: Part 1: Clause 8.2.3.2.3,

elding of the contacts is not deemed a failure pro-

vided that no flashover occurs. This demonstrates the capability of the circuit assembly (functional unit) to close onto, and carry until operation of the circuit short-circuit protective device — usually fuselinks — any value of fault current consequent upon short-circuit anywhere on the load side of the circuit short-circuit protective device.

Similarly, the through-fault and making current tests prescribed in BS5419 for the circuit (functional unit) disconnecting (isolating) device — usually a fuseswitch — demonstrates the capability of that device to close onto, and to carry until operation of the circuit short-circuit protective device, any value of fault current arising from short-circuit anywhere on the load side of the circuit short-circuit protective device.

The busbar systems of switchboard formations of low voltage switchgear are tested for short-circuit withstand strength generally in accordance with the procedures prescribed in BS5486.

3 Generator voltage switchgear

3.1 Required performance

By the late 1940s, the 'unit' principle had become the accepted pattern of the electromechanical design of power stations in the UK, where the main generator is connected directly to the lower voltage terminals of a generator transformer. Thus, for operational purposes, the generator and generator transformer operate as an electrically inseparable entity.

Whilst station transformers are supplied at 132 kV from the National Grid system wherever practicable, this voltage level is not available at all power station sites. The high cost of switchgear and transformers at the alternative voltages of 275 kV and 400 kV has encouraged consideration of arrangements which, whilst preserving the essential features of 'unit' operation, would reduce the requirement for plant at these higher voltages. An obvious approach is the abandonment of conventional station transformers, together with their attendant switchgear, and the allocation of their duty to a combination of the Unit and Generator Transformer associated with each main generator. This concept, illustrated in Fig 5.7, necessitates the provision of means for disconnecting the generator at a point between its terminals and the unit transformer(s) tee-off connections. However, apart from issues of cost, the provision of such a disconnection facility can offer significant advantages operationally, particularly in nuclear installations.

The performance required of the means of disconnection is governed by operational needs. The minimum capability of practical usefulness is synchronising the generator, the interruption of full-load current and the provision of an isolating facility — a duty calling for a switch disconnector. In essence, a switch dis-

TRANSMISSION

SYSTEM

GENERATOR

TRANSFORMER

bility of a switch disconnector as outlined above, the ability to interrupt fault currents of short-circuit magnitude.

Wherever possible, the relevant design accords with the principles of British Standards 5311 and 5227.

The first installations within the CEGB of generator voltage switchgear were those at its Hartlepool and Heysham / nuclear power stations, and its Dinorwig pumped-storage project. The concept has also been accepted for Heysham 2 and by the South of Scotland Electricity Board (SSEB) for the Torness and Inverkip power stations.

To meet the system electrical parameters at the CEGB stations, the switchgear is rated as follows:

UNIT

TRANSFORMERS

FIG. 5.7 The configuration of unit/generator

transformers and generator circuit-breaker used as an alternative to the station transformer arrangement

Dinorwig (maximum capability, when generating, 313 MW)

•Nlaking, breaking and carrying continuously any ■.alue of current at any value of power factor up to the maximum load capability of the generator, e, full - load, plus a specified overload, if required.

•Carrying for a specified period of time, e.g., one

second, a specified value of fault (short-circuit)

the generator, including the making, but not breaking, of fault current arising from closure under out-of-phase conditions.

!le ■■ ner, present thinking within the CEGB tends :051ards the use of a circuit-breaker, as this permits

bc clearance of electrical faults in the generator with- out .iiszurbance of power supply to the unit auxiliaries.

circuit-breaker used to switch a generator at generator terminal voltage has, in addition to the capa-

In addition to the basic performance outlined above, it is necessary to demonstrate the ability of the switchgear to deal satisfactorily with out-of-phase current switching and generator pole-slipping situations: also, that it does not cause unacceptable over. oltaee v. hen switch ing capacitive and transformer magnetising currents.

It will be appreciated that the rated operational voltages listed above are the values for which the short -

circuit performance of the switchgear is valid. However, to meet the standard of integrity required of the insulation of the switchgear/generator main connections combination, the following levels are specified:

3.2 Design and construction

3.2.1 General

The switchgear is of indoor-type construction intended specifically for connection directly into phase-isolated systems of generator main connections in a manner designed to preserve the principle of phase isolation described in Chapter 4: Generator Main Connections. Each pole of each three-phase assembly comprises an interrupter system connected in series with a disconnector (isolator). The series disconnector is necessary to provide the switchgear 'open' condition as, in the quiescent state, the interrupter system remains closed

— opening, and remaining so, only for so long as is necessary to complete the process of arc extinction when interrupting the circuit, and to ensure against establishing current flow by the disconnector when closing the circuit. The disconnector also provides an isolating distance in free air, when open. The switchgear may, therefore, be used to isolate the generator electrically, provided that precautions are taken in the design of the scheme of control to safeguard against closure inadvertently when so employed.

As already indicated, the interrupter in each pole comprises, essentially, a system of current making and breaking contacts, housed in an arcing chamber and connected in series with a disconnector. In its most basic form, the unit has a current breaking capability limiting its application to that of a switch disconnector, i.e., capable of load switching but not fault clearance, To increase the capability to that of a circuit breaker (the present requirement in CEGB installations), low ohmic value resistors and auxiliary interrupters are connected across the main interrupter to produce a two or three stage interruption process. The principle is illustrated in Fig 5.8. The process of interruption takes place sequentially as follows:

(a)Main interrupter opens and an axial blast of air extinguishes the arc.

LOY4

RESiSTOR

ASSEMBLv

FIG. 5.8 Diagrammatic arrangement of a multistae interruption process

(b)Greatly reduced current flows through the loks value resistor assembly, the resistors damping the transient recovery voltage (TRV) which appears across the main interrupter.

(c)The current through the resistors is interrupted by the auxiliary interrupters.

Note: Whether or not the auxiliary interrupters (1) and (2) open together (two stage interruption), or (I) before (2) (three stage interruption), depends upon the magnitude of the recovery voltage across the main interrupter. With low values of recovery voltage, the auxiliary interrupters open simultaneously; with high values, (1) precedes (2), the recovery voltage across the circuit-breaker being damped by the low value resistor assembly. A high value non-linear resistor connected across auxiliary interrupter (2) damps any overvoltage arising from the interruption of low value inductive current.

(d)The series disconnector opens, interrupting any residual current flowing through the non-linear resistor.

(e)The main and auxiliary interrupters close.

The circuit-breaker is now open.

Closure of the circuit-breaker involves the following sequence:

•Main and auxiliary interrupters open.

•Series disconnector closes.

•Main and auxiliary interrupters close.

In the matter of mechanical design, the main interrupters, which at rest take up the closed position, are