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

Established designs, i.e., those evolved during the currency of B5162 and 1353659, are rated 3.3 kV or 11 kV, as appropriate to the system concerned. However, in order to align with IEC Standard values, the presently assigned ratings are 3.6 kV and 12 kV respectively.

4.1.2 Frequency and number of phases

This switeligear is al ■,,ays three-phase, 50 Hz.

4,1.3 Rated insulated level

"I he rated insulation level is the value of the impulse v,ithstand voltage and the value of the power frequency withstand voltage, which together characterise the insulation of the switchgear with regard to its ability to withstand the electric stresses.

The 'classification' of the insulation of established designs — again those designs evolved during the

currency of BSI62 and BS3659 — is based upon the achievement of clearances between phases, and clearances phase-to-earth to the dimensions specified in those Standards for 'Class B', i.e., the higher of the two classes recognised. The 'insulation level' of such designs is determined by a combination of the specified clearances and the ability to withstand, for one minute, the prescribed value of test voltage. This is accepted as equivalent to the power frequency and impulse voltage test philosophy. The 'insulation level' of the newer developments, i,e, designs to B55227 and BS5311 are, of course, proved by test in accordance with Table 1, List 2, of BS6581: 1985 (IEC 694: 1980).

4.1,4 Rated short-time withstand current of main and earthing circuits

The rated short-time withstand current is the RMS value of the current which the switchgear can carry

HJ

It is probable that system requirements in the future will be adjusted to accommodate the IEC rating of 40 kA at both 3.3 kV and Il kV, but for 3 s — the IEC standard value of 1 s being insufficient to permit satisfactory time grading of protection.

4.1.5 Rated peak withstand current of main and earthing circuits

The rated peak withstand current is the peak value of the first major loop of the rated short-time withstand current which the switchgear can carry under prescribed conditions of use and behaviour. The present requirements are

value of the AC component of the rated shorttime current. Whilst the standard value recognised by the IEC and, in consequence, by present British Standards for high voltage switchgear, is 2.5 times the RMS value of the AC component of the shortcircuit current, those Standards concede that higher values may be attained in certain circumstances. Instances of such are installations in which large gas-turbine generators are connected directly into the 11 kV voltage system and/or featuring a heavy motor load.

I I. . 5.22 SMO,..-pole earthing switch, dismounted (13rni,h lirown-Boveri Lid)

1or a specified short-time under prescribed condiuse and behaviour. The present requirements

• 3$ k V rstem equipment — 26.3 kA or 43.8 kA for

3 N corresponding to system short-circuit levels of 150 \IVA and 250 MVA for 3 s, respectively (at 3.3

kV), both levels occurring in CEGB installations.

4.1.6 Rated normal current

Traditionally, the rated current of circuits and busbars is chosen from the ratings specified in BS3659, but, again to align with EEC ratings, they are now selected from the values listed in BS5311. To date, the maximum ratings required are 2500 A at 3,3 kV, and 3150 A at 11 kV. The rated normal current is permitted to take full advantage of the limits of temperature rise of current carrying parts accepted by the IEC. However, these values are, in the main, appreciably higher than those allowed by the superseded British Standards. This being so, to minimise the risk of damage to the insulation of the types of cable now in use, the tem-

4.1.7 Rated short-circuit breaking current

(of circuit-breakers)

• 3.3 system equipment:

(a) Symmetrical — 26.3 kA or 43.8 kA as demanded by the system fault level.

(b) Asymmetrical — as the symmetrical value plus 50% DC component.

• 1/ kV system equipment:

(a)Symmetrical — 39.4 kA.

(b)Asymmetrical — as the symmetrical value

plus 50% DC component.

The value 50% DC specifies the magnitude of the displacement from zero of the horizontal axis of the current waveform at the instant of separation of the circuit-breaker contacts which, added to the RMS value of the AC component of the current at that instant, determines the asymmetrical value.

Figure 5.24 illustrates the method of determination of breaking currents.

A DC component of 50% is the 'traditional' value used in the UK for the specification of asymmetrical breaking current capability. It takes into account the opening time of the circuit-breaker to the extent that the X/R ratio of the system is normally not likely to be so high as to present a more onerous condition at he instant of contact separation. The British Standard currently appropriate to circuit-breakers for power station service is BS5311: 'Specification for AC circuitbreakers of rated voltage above 1 kV' — which itself derives from IEC Publication 56 — specifies values appropriate to the actual opening time of the circuitbreaker, based on a system X/R ratio of 14. However, the BS also recognises that in certain applications, e.g., where the X/R ratio is higher than 14, or if a circuit - breaker is close to a generator, the percentae.. DC component of the system short-circuit current aveform may be higher than the value derived from the 'curve' shown in the Standard. A more detailed treatment of the significance of 'percentage DC component' is given in Clause 6.2 of BS5311: Part 2: 1976.

4.1.8 First-pole-to-clear factor

The first-pole-to-clear factor (of a three-phase system, and at the location of the circuit-breaker) is defined

anti the other two phases during a two-phase shortwhich may or may not involve earth, at the locution or the circuit-breaker, to the phase-to-neutral olla:Ie which would be obtained at the same location

\\ h the short-circuit removed.

F or the purpose of testing, the first-pole-to-clear

r,i,:tor is the ratio of the value of the power frequency rccacrv voltaae appearing across the pole in which

current is first interrupted, to the phase-to-neutral

01t.itc of the test circuit. For power station service, he hkihest attainable ratio, viz 1.5, is used.

41.9 Rated short-circuit making current

•3.i kr system equiptnent 2.55 times the AC compo- nent of the rated short-circuit breaking current.

•//AV .s.s. wern equipment 121 kA. The derivation of

ilie factor 2.55 at 3.3 kV, and the value 121 kA at 11 kV is explained in Section 4.1.5 of this chapter.

4.1.10 Rated duration of short-circuit

This is the maximum length of time for which the switchgear is guaranteed to be capable of carrying a current equivalent to its rated breaking current. It follows, therefore, that such is the maximum time for which protection may be allowed to delay tripping of the circuit-breaker on short-circuit.

4.1.11 Rated operating sequence

This is the sequence (0-t-CO-C-CO) on which the test duties for certification of short-circuit performance are based (see Section 2.2.3 of this chapter).

4.2 Design and construction

4.2.1 General

As indicated in Section 2.1 of this chapter, the inter rupter in 11 kV switchgear is invariably, by definition, a circuit-breaker, whereas at 3.3 kV it is, depending

upon the duty, a circuit-breaker or a fused switching device. However, whether featuring circuit-breaker or fused switching device, the general form of design and construction of the switchgear as a whole, together with the operational facilities offered, are similar at both 3.3 and 11 kV. Those features peculiar to, and the service performance required of, fused switching equipment are dealt with in Sections 6.1 and 6.2 of this chapter, respectively.

The switchgear is of the metalclad type, i.e., switchgear assemblies in metal enclosures intended to be earthed — complete except for external connections

— and in which components are arranged in separate compartments with metal partitions intended to be earthed.

Essentially, metalciad switchgear has separate compartments for:

•Each main circuit switching device, e.g., the circuitbreaker.

•Those components connected to one side of the main circuit switching device, e.g., the feeder circuit.

•Those components connected to the other side of the main circuit switching device, e.g., the busbars.

With few exceptions, the switchgear in service in CEGB power stations features circuit-breakers or fused switching devices of the air-break type, and was designed during the currency of British Standards BS162 and BS3659. However, fused switching equipment incorporating vacuum interrupters is gaining favour — particularly for frequently switched circuits. The exceptions in the circuit-breaker field are the few installations — generally to be found in the older stations

— equipped with oil circuit-breakers, and sometimes themselves of the oil and compound insulated type.

Whilst complying basically with the above Standards, the design overall features, of necessity, a number of operational facilities particular to power station service. However, it should be noted that the principal British Standards presently current in the UK for this class of switchgear are BS5227: Specification for AC metalenclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 72.5 kV, and BS53 11: AC circuit-breakers of rated voltage above 1 kV — Standards based upon LEG Publications 298 and 56, respectively. The introduction of the newer British Standards has had no significant impact upon the performance and operational facilities required. Hence, designs to the superseded Standards are, for all practical purposes, accepted by the CEGB as compliant with the current publications. Nevertheless, to regularise the situation, the technical requirements of purchasing specifications are now couched in terms of the current British Standards.

The switchgear is factory assembled, i.e., built-up into complete switchboard formations in the manufacturer's works, albeit dismantled into transport units for shipment to site. Each switchgear equipment, i.e.,

each circuit in a switchboard, is provided with a control and instrument panel forming an integral part of the switchgear cubicle. Wherever possible the relays required for the protection of the circuit are mounted on the control panel. However, where the space available on the control panel is insufficient to accommodate the total requirement, separate panels are provided on which all relays associated with the circuit concerned are mounted. That is to say, the relays specific to a particular circuit are all mounted either on the switchgear control panel, or on a separate panel — they are not divided between the two. Figures 5.25 to 5.31 depict typical II kV and 3.3 kV switchboard formations.

4.2.2 Enclosures

Enclosures provide, when all doors and covers are closed, protection against the approach of persons to live parts to 'degree of protection' IP3X (see BS5227). Additionally, they are required to afford protection against dripping water, e.g., from condensation on switchroom ceilings. Thus the degree of protection provided overall is not less than IP31 to BS5490. To exclude also the ingress of vermin, the protection must be independent of the fitting of closing plates or other sealing arrangements at the point of entry of cabling into the switchgear. Normally the cable glanding arrangement adopted satisfies this requirement.

Doors and covers, the opening of which gives direct access to main circuit conductors which may be live, are secured by fasteners the removal of which requires the use of tools, e.g., spanners, screwdrivers. Fasteners designed primarily for release by the use of a coin or similar implement are not accepted for this purpose. Alternatively, where for the purpose of attention to other apparatus it is necessary to gain access to compartments containing main circuit conductors, doors and covers are interlocked to prevent such access unless the main circuit conductors are de-energised. Doors and covers for which it is permissible to provide fasteners requiring neither tools for their release, nor involvement in a scheme of interlocking are, where it is desired to prevent unauthorised opening, provided with simple locking facilities — usually padlocking.

In recognition of the stature and physical capability of the average male in the UK, apparatus mounted on switchgear is, wherever practicable, positioned within the following height limitations — measured from the operating floor level:

Door and panel fastenings other than those of relay panels integral with the

switchgear cubicle structure. *300 mm *2000 mm

*Highest and lowest positions reached by an operator's hand.

Fat. 5.25 Typical I I kV switchboard of Reyrolle manufacture

Jea6pwAs A> I. L PueA1ET

luaLudpba

REMOTE LOCAL

SWITCH

SECONDARY ISOLATINC

CONTACTS

CLOSING LEVER

SECONDARY

ISOLATING

CONTACTS

MAIN EARTH

FIXED•CONTACT

FIG. 5.26 Interior of cubicle fitted to the type of switchboard illustrated in Fig 5.25

OPEN CLOSE

VeITCH

CUBICLE

TRIP BUTTON

PRASE

SEPARATION

BARRIER

SECONDARY CONTACTS

CARRIAGE

SYJITCH

SHUTTERS

EARTH

CONNECTIONS

SHUTTER OPERATING MECHANISM

4.2.3 Withdrawal/disconnection

st without exception, the switchgear is of the

A l mo

\%ithdrawable type, i.e., the circuit-breaker is mounted wheeled carriage and hence is removable in Li ron oretv from the switchboard. This arrangement

it cn not only a \.cry simple and effective means of ides

conneetion (isolation) of an individual circuit from itchboard busbars, but also, by removal totally

,‘N

he circuit-breaker from the switchboard structure, permits maintsmance work (on the circuit-breaker) away from the switchboard. This latter feature is of parti- "i a r value in that work may be carried out on the Jircuit-breaker outwith the CEGB Safety Rules aper- ,,iininu to work on high voltage electrical equipment.

Upon removal of a circuit-breaker from its operaonal location it is necessary to cover the contacts by

Il

,,tl ich it connects with the busbar and its circuit in the

,,,. iehboard. This is achieved by shutters closing over hc busbar and circuit contacts. The shutters, which must be of metal construction, are actuated automatically by the process of connection and disconnection of the circuit-breaker. Opening is by positive drive to minimise the risk of short-circuit upon re-entry of the c ircuit-breaker into the 'service', i.e, 'connected' posi-

lion. Closing, if not positive, must be by two independent means — one of which may be gravity —

cach capable of performing the closing operation alone. The use of withdrawable circuit-breakers facilitates

ready access to busbars and circuits for testing purpoNes. To this end, provision is made, upon removal

of a circuit-breaker from the switchboard, for opening and securing open, the shutters protecting the busbar contacts whilst those for the circuit remain closed, and ice- \.ersa. However, the means for securing the shutters in the open position are cancelled automatically and normal operation is restored upon reconnection of the circuit-breaker. Padlocking facilities are provided for lockine the shutters closed. When closed the shutters pro\ide degree of protection IP3X (see BS5227) against to the busbar and circuit contacts. To avoid risk inistake, the shutters are identified in accordance

,\ it h he following code:

LIVE

CABLE

Lemon 3, ci

Viditionally, on busbar sectioning units, the section of husbar to which each group of disconnection contacts is .onnecteci is indicated by a white arrow on the as-

, ociaied shutter, pointing toward the relevant section of bushar.

The means of disconnection of the circuit-breaker from the busbars and feeder circuits comprise off-load plug type contacts of the self-aligning pattern, suitable for use whilst the busbars and/or feeder circuits are live. Whilst in some designs disconnection is effected by withdrawal of the circuit-breaker carriage, in others it is achieved by the operation of off-load selectors. However, in all cases a system of mechanical interlocks ensures that the switchgear is locked positively in the required condition, i.e., connected for service, disconnected, or arranged for circuit earthing or, where appropriate, busbar earthing. Padlocking or coded-key devices are provided to permit enforcement of the required operational condition. Mechanical indication is provided to show when the circuit-breaker is in the 'service' or 'disconnected' position. The indicators are inscribed SERVICE or DISCONNECTED (or ISOLATED) in black lettering on a white background.

Arrangements for isolation of the control and auxiliary circuitry when the circuit-breaker is disconnected (isolated) include:

•Automatic disconnection of those circuits upon disconnection of the circuit-breaker or, alternatively;

•Where such disconnection is not automatic upon disconnection of the circuit-breaker, purpose designed facilities are provided to permit such disconnection, if desired, when the circuit-breaker is disconnected.

However, whichever arrangement is adopted, it must not be possible to restore the circuit-breaker to the service condition, i.e., reconnect, without reconnection of the control and auxiliary circuits.

4.2.4 Electrical interlocks

Electrical interlocks for the prevention of closure of a circuit-breaker are arranged to interrupt the operating supply to the energising contactor of solenoid closing mechanisms, or the release coil of stored energy, e.g., spring closed mechanisms, as appropriate. Mechanical interlocks are required to be preventative, rather than curative, i.e., they are designed to prevent, as close as possible to the point at which manual force may be applied, an action of improper operation, rather than 'correct' the improper action. An example of this philosophy is the interlock provided to prevent connection/disconnection of a circuit-breaker, other than when open. Rather than initiate tripping of a closed circuit-breaker during the execution of the action of connection/disconnection, the interlock blocks the attempt positively.

4.2.5 Coded-key devices

To assist the enforcement of operational and safety procedures, the switchgear is provided, where necessary, with coded key-operated devices whereby:

(a) 1 1 kV switchboard

FIG. 5127 Typical II kV switchboard or GEC manufaciure

Jea6ianuop pue Jeabtowv■ s