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

16 References

f 1 1 Carter, F. W.: Note on losses in cable sheaths: Proceedings of Cambridge Philosophical Society No. 24, pp 65-73: 1927

Dwight, H. B.; Theory for proximity effects in wires, thin tubes and sheaths: AIEE Trans 42: 1923

Skeats, W. F. and Swerdlow, N.: Minimising the magnetic field surrounding isolated phase bus by electrically-continuous enclosures: A1EE Trans. No 62: 1962

Wilson, W. R. and Mankoff, L. L.: Short circuit forces in isolated phase buses: AIEE Trans: 1954

Dwight, H. B.: Electrical coils and conductors: McGraw Hill; 1945

Niemoller, A. B.: Isolated phase bus enclosure currents: Trans. I EEE; August 1968

I EEE: Guide for calculating losses in isolated phase bus: IEEE Paper 298: June 1969

Dwight, H. B., Andrews, G. W. and Tileston Jnr, W.: Temperature rise of busbars: General Electric Review: May 1940

Albright, R. H., Conasla, A., Bates, A. C. and Owens, J. B.; Isolated phase metal-enclosed conductors for large electric generators

Ashdown, K. T. and Swerdlow, N.: Cantilever-loaded insulators for isolated phase bus: AIEE Paper: April 1954

Swerdlow, N. and Buchta, M. A.: Practical solutions of inductive heating problems resulting from high current buses: Trans. AIEE 1960

1 General requirements

1.1Auxiliaries power systems — voltages and fault levels

1.2Switchgear performance

1.3Operational requirements

1.4Control

1.5Environment

2 Types of switchgear

2.1Descriptions

2.2Testing and certification

2.2.1General

2.2.2Certification

2.2.3Type tests

3 Generator voltage switchgear

3.1Required performance

3.2Design and construction

3.2.1General

3.2.2Control

3.2.3Cooling

3.2.4Operating air plant

3.2.5Phase-reversal disconnectors for pumped-storage

schemes

3.2.6 Earthing switches

4 3.3 kV and 11 kV switchgear — circuit-breaker equipment

4.1 Required performance

4.1.1Rated voltage

4.1.2Frequency and number of phases

4.1.3Rated insulation level

4.1.4Rated short-time withstand current of main and earthing circuits

4.1.5Rated peak withstand current of main and earthing

circuits

4_1.6 Rated normal current

4_1.7 Rated short-circuit breaking current (of circuitbreakers}

4.1.8First-pole-to-clear factor

4.1.9Rated short-circuit making current

4.1.10Rated duration of short-circuit

4 1.11 Rated operating sequence

4.2Design and construction

4.2.1General

4.2.2Enclosures

4.2.3Withdrawal/disconnection

4.2.4Electrical interlocks

4.2.5Coded-key devices

4.2.6Identification of conducting parts

4.2.7Earthing of structures

4_2.8 Circuit and busbar earthing 4.2.9 Auxiliary switches

4 2_10 Cabling arrangements 4.2.11 Voltage transformers 4_212 Current transformers

4.2.13Control/selector switches

4.2.14Switchboard/circuit identification

4.2.15Indicating instruments

4.2.16Test devices

4.2.17Circuit-breakers

4.2.18Circuit-breaker operating mechanisms

5 3.3 kV switchgear — fused equipment

5.1 Required performance

5.1.1Rated voltage

5.1.2Frequency and number of phases

5.1.3Rated insulation level

5.1.4Rated short-time current

5.1.5Rated normal current

5.1.6Rated breaking current of switching devices

5.1.7First pole-to-clear factor: 1:5

5.1.8Rated short-circuit making current

5.1.9Rated duration of short-circuit

5.1.10Rated operating sequence

5.1.11Co-ordination of switching device with fuse protection

5.2Design and construction

5.2.1General

5.2.2Duty of switching device and circuit earthing facilities

5.2.3Switching devices

5.2.4Switching device operating mechanisms

5.2.5Main circuit fuselinks

6 Low voltage switchgear, controlgear and fusegear

6.1 Required performance

6.1.1Short-circuit withstand strength of busbar systems

6.1.2Capability required of main circuit making/breaking

devices

6.2 Design and construction

6.2.1General

6.2.2Enclosures

6.2.3Cabling arrangements

6.2.4Electrical clearances and creepage distances

6.2.5Busbar systems

6.2.6Earthing of structures

6.2.7Mechanical interlocks

6.2.8Coded-key devices

6.2.9Protective systems components

6.2.10Current transformers

6.2.11Ammeters and voltmeters

6.2.12Control switches

6.2.13Fuses

6.2.14Circuit-breaker equipments

6.2.15Contactor controlgear

6.2.16Fusegear

6.2.17Specialised switchboards/units

7Fuses

7.1Introduction

7.2Definitions

7.3Required performance

8DC swItchgear

8.1General

8.2System conditions

8.2.1Short-circuit withstand strength of busbar systems

8.2.2Current making! breaking and short-circuit capability of main circuit switching devices

9Construction site electrical supplies equipment

9.1General

9.2Portable substations

9.3Portable distribution units (415/240 V)

9.4Portable distribution units (110 VI

1 General requirements

1.1 Auxiliaries power systems — voltages and fault levels

The philosophy of the design of the systems of power supply to auxiliaries plant is dealt with in Chapter 1. This chapter deals with the operational facilities provided by, and the performance required of, the switchgear and controlgear used in those systems, and also with the switchgear used in schemes in which the main generators are switched at generator voltage. The design features necessary to meet these objectives in CEGB power stations are described.

The presentation is intended to assist the engineer concerned with the application and operation of switchgear and controlgear rather than for the information of the specialist designer. In consequence, technical detail available from text books and such sources as 13ritish and International Standards is included only as necessary to illustrate a particular aspect properly.

The terms and definitions used in this chapter are, in general, in accordance with the British Standard Glossary of Electrotechnical, Power, Telecommunication, Electronics, Lighting and Colour Terms — BS4727: Part 2: Terms Particular to Power Engineering

— Group 06: switchgear and controlgear terminology (including fuse terminology).

The terms and definitions are in close agreement with those of Publication 277 of the International Electrotechnical Commission, where there are corresponding terms and definitions in that Publication. Switchgear and controlgear are defined as follows:

Switchgear and contra/gear A general term covering s witching devices and their combination with associated control, measuring, protective and regulating equipment, and also assemblies of such devices and equipment with associated interconnections, accessories, enclosures and supporting structures.

Switchgear A general term covering switching deices and their combination with associated control, measuring, protective and regulating equipment, and also assemblies of such devices and equipment with associated interconnections, accessories, enclosures and supporting structures, intended in principle for use in

connection with generation, transmission, distribution and conversion of electric power.

Controlgear A general term covering switching devices and their combination with associated control, measuring, protective and regulating equipment, and also assemblies of such devices and equipment with associated interconnections, accessories, enclosures and supporting structures, intended in principle for the control of electric power consuming equipment.

Except where necessary to avoid ambiguity, only the generic term switchgear, is used henceforth.

By the early to mid 1950s, the design of the auxiliaries power systems in power stations in the United Kingdom had become established at the dual voltages of 415 V and 3.3 kV, with short-circuit levels of up to 43.3 kA (31 MVA) and 26.3 kA (150 MVA) respectively. However, the demand on the systems consequent upon the rapid increase in the rating of main generating plant and, in consequence, of its auxiliaries, thereafter necessitated the introduction of a higher voltage — 11 kV — having, initially, a short-circuit level of up to 26.3 kA (500 MVA), but progressing to the present value of 39.4 kA (750 MVA).

Until the introduction of a third voltage, and with the aim of holding auxiliaries to two voltage levels, switchgear having a short-circuit capacity of 43.8 kA (250 MVA) at 3.3 kV was installed in some instances. However this expedient was soon abandoned in favour of the substitution of 6.6 kV, an innovation that was also short-lived as, yet again, auxiliaries power requirements were seen to be increasing beyond the capability of 6.6 kV. Thus, currently, and probably for the foreseeable future, the major power stations in the United Kingdom have 'three-tier' auxiliaries power systems, i.e., systems operating at 415 V, 3.3 kV and 11 kV. Additionally, there are services operating at lower voltages, e.g., 240 V AC, 110 V and 220 V DC.

1.2Switchgear performance

Traditionally, the calculated prospective current likely to flow under three-phase fault conditions in auxiliaries power systems necessitates (for the purpose of determining the short-circuit performance required of circuit-breakers) equipment capable of interrupting a current waveform featuring a transient DC compo-

Electricity Regulations', published by HM Stationery Office. To meet this earthing requirement, all installations of HV switchgear have facilities for the connection to earth of all current carrying parts, i.e., all phase conductors.

The switchgear in the power station plays a vital role in the procedures for work on station plant — particularly electrical equipment. It is, therefore, pertinent to refer to the circumstances giving rise to the design requirements described under the 'Design and Construction' sections which follow, concerning isolation and earthing. A fundamental requirement of the CEGB Safety Rules is that work on the conductors of high voltage equipment may be carried out only when the equipment is isolated from all sources of supply and, except in special circumstances, earthed. The switchgear provides the points of isolation.

The protection of persons working on such apparatus afforded by earthing is dependent upon the combination of:

•The efficiency of the connection of primary earths and their capability to carry the fault current until the electrical protective devices operate.

•The speed of operation of electrical protective devices.

•The system voltage, voltage gradient to the point of earthing and the fault level at the point of work.

The Safety Rules (Electrical and Mechanical) recognise three classifications of 'earth', viz: 'primary earth', 'drain earth' and `metalclad switchgear movable earth'

Primary earth A fixed or portable earthing device applied at a position defined in a safety document. A primary earth must be applied within the isolated zone, and in accordance with the terms of a 'permit for work'.

A path to earth established by closure of a circuitbreaker has, of course, the fault current carrying capability of the circuit-breaker, i.e., an ability to carry a current equal to the rated breaking current of the circuit-breaker for 3 s. Thus a circuit-breaker applied earth may serve as a primary earth anywhere on the system. The principles of application of a primary earth are:

•With the exception of certain work on metalclad switchgear feeder, busbar and voltage transformer spouts (see below under `metalclad switchgear movable earths'), primary earths must remain in position

until the associated permit(s) for work has(ve) been cancelled.

•Where reasonably practicable, primary earths must be applied between the point of work and the point(s) of isolation. Where this is not reasonably practicable, any alternative procedure adopted must have specific approval.

•Where primary earths are applied, all phases must be earthed except where work is to be carried out on phase segregated apparatus. Provided that all three phases of phase segregated apparatus are isolated, work may be carried out on one phase with a primary earth applied to that phase only.

•Where possible, a circuit-breaker or purpose designed earth switch must be used to make the first earth connection.

•When a non-fixed circuit-breaker, i.e., a circuitbreaker comprising a 'removable' or a 'withdrawable' part, is used to apply a primary earth, any automatic trip feature must, unless impracticable, be rendered inoperative before closing. After closing, any means of opening the circuit-breaker must be locked inoperative.

•When a fixed circuit-breaker is used to apply a primary earth, all tripping functions must be ren-

dered inoperative after closing, and the circuitbreaker locked the closed position.

•Whenever reasonably practicable a circuit-breaker used to establish a primary earth should be closed from a remote control station, i.e., closure from local (at switchgear) controls should be avoided. Although the circuit-breaker has a proven full system prospective short-circuit current making capability, the avoidance of local control is considered to be a worthwhile precaution.

Drain earth A fixed or portable earthing device applied for the purpose of protection against induced voltages. Drain earths must be applied under the terms of a 'permit for work' or 'sanction for test' where induced voltages may cause danger at the point(s) of work. They are applied and removed as necessary during the course of the work or testing as specified in an 'earthing schedule'.

Meta!clad switchgear movable earth A portable earth applied to metalclad switchgear spouts before a 'permit for work' on the spouts is issued, which can be removed and replaced one phase at a time during the process of work being done under a 'permit for work'. The term 'spout' is used to describe the contacts in the switchgear enclosure from which a removable or withdrawable circuit-breaker or voltage transformer is disconnected when the circuit-breaker or voltage transformer is disconnected from the busbars or circuit.

The use of the word 'must' without qualification in the earthing procedures described, indicates a mandatory requirement with no discretion permitted and no judgement to be made. Where a statement is qualified by the word 'practicable', a slightly less strict standard is imposed. It means that where it is possible to achieve, in the light of current knowledge and invention, but bearing in mind the hazards associated

on the medium voltage or low voltage side of a transformer.

Note: Under present UK legislation, the terms low voltage' and 'medium voltage' have the following meanin2:

Low voltage A difference of potential between any conductor and earth,

(iv)On the feeder, voltage transformer or busbar spouts on which work is to be carried out, the primary earths must be replaced by metalclad switchgear movable earths.

(v)If there are no other primary earths left on the circuit connected to the spouts being worked on, then while this work is in progress, no other work must be carried out on that circuit. Where the spouts are connected to a circuit on which there is any likelihood of induced voltages occurring, drain earths must, where reasonably practicable, be connected at the nearest point to the point of work where access to the conductors can safely be obtained.

(1,, i) must be attached where applicable on, or adjacent to, live apparatus at the limits of the work area.

(vii)A 'permit for work' must be issued.

(viii)The work may be carried out by a 'competent person'. The earths may be removed one phase at a time to give the necessary access. Each phase earth so removed must be replaced by the competent person before another phase earth is removed.

Work without using meta/clad switchgear movable earths

When work is to be carried out on the busbar spouts of a multi-panel switchboard without using metalclad switchgear movable earths, the following operations must be carried out in strict sequence:

•Operations (a), (b), (c) and (d) as for work using metalclad switchgear movable earths, followed sequentially by operations (f) and (g).

•The work on the busbar spouts must then be carried out under the personal supervision of an 'authorised person', who must prove each spout dead by means of an approved voltage indicator immediately before the spout is worked on. The voltage indicator itself must be tested immediately before and immediately after use.

• If it is necessary to carry out work on the spouts of the panel on which the primary earths have been applied, then after the work on the available busbar spouts has been completed, the permit for work must be cleared and cancelled. The primary earths must be removed and replaced on the busbar spouts of another panel on the isolated section of busbar. Danger notices must be re-affixed, a permit for work issued, and the work as described above carried out under the personal supervision of an 'authorised person'.

Note: An 'authorised person' is a 'competent person who has been nominated by an appropriate officer of the CEGB to carry out duties specified in writing'.

When work is to be carried out on feeder spouts, voltage transformer spouts and single panel busbar spouts without using metalclad switchgear movable earths, the following operations must be carried out in strict sequence:

•As in (i) for work using metalclad switchgear movable earths.

•As in (ii) for work using metalclad switchgear movable earths.

•Primary earths must be applied to the circuit at each point of work and at all points of isolation, except where such a point of isolation is on the medium voltage or low voltage side of a transformer. If reasonably practicable, all primary earths must be locked in the earthed position.

•Where the work to be carried out will involve the removal of the primary earths at the point of work, then before a permit for work is issued alternative primary earths must be applied as close as is reasonably practicable to the point of work. However, if this cannot be achieved, then whilst this work is in progress no other work must be carried out on the circuit connected to the spouts being worked on. Where the spouts are connected to a circuit on which there is any likelihood of induced voltages occurring, drain earths must, where reasonably practicable, be connected at the nearest point to the point of work where access to the conductors can safely be obtained.

•As in (vi) for work using metalclad switchgear movable earths.

•As in (vii) for work using metalclad switchgear mo' - able earths.

•Work on the spouts must then be carried out only under the personal supervision of an authorised person who must prove each spout dead by use of an approved voltage indicator immediately before the spout is worked on. The voltage indicator itself must be tested immediately before and immediately after use.

When a fault making device or a circuit-breaker has been removed from its service position in prepara- ti on for work, it must be immediately electrically discharged to earth. The application of primary or drain earths is then not required. After a fault making

je%ice or a circuit-breaker has been removed from its ,orvice position and electrically discharged to earth, a

•safety document' is unnecessary for the purpose of orkinc on the device or circuit-breaker unless the \t, ork is to be done whilst it is within the confines of

sv,itchroorn or similar place, when a 'limited work

a

certificate' must be issued for the work.

.4 Control

The schemes of control provide for electrical 'close/ open' operation at the switchgear itself and, in most cases, from remote locations. Operation at the switch-

is designated 'local control'. Control circuitry dear is

internal to the switchgear operates at the following °Wages:

Whereas local control operates directly into the cAviichgear 110V circuitry, that from remote locations and schemes of automatic/sequence control is usually at 48 V, via 48/100 V interposing relays mounted in the switchgear. Plant protective interlocks/trips and certain other control functions from external sources are, in general, connected directly into the 110 V circuitry.

Trip circuit supervision is provided as a matter of course on all 3.3 kV and 11 kV switchgear, and on the principal 415 V and lower voltage circuit-breaker and latched contactor equipments. Supervision of 'closing' control circuitry is provided where closure immediately on demand is more essential than usual, i.e., in certain safety circuits. Selection of the mode of operation,

e., local or remote, is made at the switchgear. Also provided at the switchgear are facilities for testing the closing and opening operation of the circuit-breaker Or contactor electrically whilst the main circuit con-

trolled is disconnected (isolated) from the source of

Supply

Because of the heavy power requirement of the closing coils of the majority of solenoid closed circuitbreakers, particularly at 11 kV, it is impracticable T O energise the operating coils directly at the control

,oltage of 110 V. Accordingly, such mechanisms are

'applied at a higher voltage, presently at 220V DC,

via 110 V/220 V auxiliary contactor type relays mounted in the switchgear. The adoption of 220 V DC for this duty is intended to discourage the use of such a supply for purposes other than switchgear operation. Prior to the introduction of this voltage for the duty, 240 V DC (a voltage employed generally for other power station services) was the rule.

The DC supply voltages quoted above are derived from 'float-charged' batteries, normally of the leadacid type. AC supplies are usually obtained from control transformers located in the switchgear and fed from the 415 V main (power) circuit. However, the range of voltage appearing across the terminals of a battery of lead-acid cells — from fully-charged down to the loaded condition unsupported by the charger, at the limit of discharge compatible with the avoidance of damage to the battery — can be appreciably wider than that over which circuit-breaker mechanisms of the solenoid type are guaranteed to function satisfactorily.

As explained later in this chapter, the current British Standards for circuit-breakers are BS4752 for voltages up to, and including, 1000 V AC and 1200 V DC, and BS5311 for AC voltages above 1000 V.

The circuit-breakers in use currently, and for many years, in CEGB power stations are designed and tested basically to an earlier British Standard (8S3659) which specifies that closing mechanisms of the solenoid type shall operate satisfactorily over a voltage range, with operating current flowing, of 80% to 100% of the rated (nominal) value, whereas BS4752 and BS5311 require operation over the range 85% to 105%. Based on the requirements of the latter Standards, the minimum and maximum values of voltage acceptable at the terminals of a 220 V (rated) solenoid coil become:

Minimum 0.85 x 220 V = 187 V

Maximum 1.05 x 220 V = 231 V

To meet a comparable operating voltage range, the rated voltage of a closing solenoid mechanism designed to BS3659 becomes (100/80)187 = 234 V. This value is also the maximum permissible at its terminals, i.e., the 100% value.

For shunt-trip mechanisms, i.e., opening devices operated from a source of voltage separate from the main circuit, the limiting values of voltage at the terminals are 80% to 120% for devices to BS3659, and 70% to 110% for those to BS4752 and BS5311. In recognition of these requirements, control supply systems are designed to observe the following voltage limits at the incoming terminals of the switchboard whilst operating current is flowing:

The minimum value of 190 V for a system of nominal voltage 220 V DC allows for a 'volt drop' of about 3 V within the switchgear, i.e., between the incoming terminals of the switchboard and the terminals of the solenoid coil — the furthermost solenoid in the case of switchboard formations. To allow for voltage drop in circuitry external to the switchgear, e.g., in interlock circuits, it is necessary that 48 V and 110 V control relays be capable of functioning satisfactorily at values of voltage down to 39 V and 88 V, respectively.

Operating supplies in switchboards are provided by buswires, sectionalised as shown in Fig 5.3. In normal operation, the sections are run electrically separate. However, an arrangement of links/fuses enables the sections to oe coupled in the event of loss of a source, but does not allow parallel operation. Parallel operation is precluded to minimise the risk of failure of one source interfering with the functioning of another.

Where operating supplies are derived from 415/ 110 V transformers, two 100°70 rated units per switchboard or per section of switchboard are provided. They are segregated from one another, and located as far apart in the switchboard as is practicable. One pole of the 110 V winding is earthed.

Depending upon the duty of the switchboard, each transformer is rated to supply a proportion of all elec- trically-held contactors in the closed state, together with a proportion of all contactors, both electrically-held and latched, in the process of closing simultaneously.

To ensure proper functioning of AC-operated mechanisms, it is necessary to hold the output voltage of control transformers between 85% and 110°70 of the nominal value whilst operating current is flowing. The frequency is, of course, governed by the limits observed for the main (power) circuit, i.e., 47-51 Hz. Each single-circuit contactor gear unit is provided with a discrete transformer.

Normally, each 110 V and 220 V DC system is earthed through an earth fault relay connected to the mid-point of a resistor across the positive and negative poles, the relay providing indication of faults to earth. 48 V DC systems normally have the positive pole connected directly to earth. Equipment energised from systems having the mid-point earthed through a relay is connected to the positive and negative poles of the supply through a fuse and solid link, respectively. Equipment fed from systems having the positive pole earthed is connected to that pole through a solid link, and to the negative pole through a fuse. Similarly, equipment fed from an AC source, is connected to the earth pole of the supply through a solid link and to the live pole through a fuse.

To minimise the use of repeat relays, auxiliary contacts used for control, indication and alarm circuitry are, wherever possible, driven directly by the operating mechanism of that element of the switchgear to the movement of which the contacts are responsive. Thus, those contacts responsive to the change of state of

the circuit-breaker/contactor, i.e., from open to close, and vice-versa, are activated directly by the switchgear closing mechanism. Likewise, those contacts responsive to the service and disconnected states of the switchgear are driven directly by the mechanism employed to select the service/disconnected conditions. However, the number of auxiliary contacts available so driven by the operating mechanisms of present designs of switchgear is limited. Accordingly, single-pole switching is generally the rule, arranged at present as follows:

•In the connection to the positive pole of systems having the mid-point earthed.

•In the connection to the negative (i.e., live) pole of the supply in control circuits having the positive pole earthed.

•In the connection to the live (i.e., unearthed) pole of the supply in AC control circuits.

Thus is preserved, in control circuitry, the basic convention of fusing and switching in the 'live' lead. In the case of mid-point earthing, the choice of pole to be fused and switched is arbitrary — both poles being at a finite potential with respect to earth. The fusing and switching of alarm and indication circuits is dealt with in Volume F.

1.5 Environment

As a general rule, the switchgear is grouped into multi-circuit switchboard formations, accommodated in purpose built switchrooms. Exceptions are plantmounted items such as the control gear built into some designs of valve and other actuators. The switchrooms should provide an environment in which the ambient temperature is held between + 10 ° C and 40 ° C — the upper limit being subject to an average value not exceeding 35 ° C over a 24-hour period — and the relative humidity should not exceed 70 07o whilst the switchgear is energised. Except for main circuit terminals, i.e., those terminals to which cabling external to the switchgear are connected, the upper limit of 40 ° C permits exploitation of the limits of temperature rise allowed by TEC Standards for switchgear. The temperature rise of main circuit terminals is held to a maximum of 50 ° C out of the necessity to limit the ultimate operating temperature to a value acceptable for elastomeric insulated cables. It is also necessary to ensure that the environment, particularly in the case of air-insulated gear, is substantially free of pollution by dust (especially from concrete surfaces), smoke, corrosive or flammable gases and vapours. Should there be circumstances necessitating the location of switchgear in ambient temperatures higher than cited above, the assigned full-load current rating is reduced to the value necessary to limit the maximum temperature likely to be attained in service, i.e., the average ambient temperature plus the permissible temperature