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

is installed be capable of withstanding, at the circuit prospective current, the values of 'cut-off current and 'energy' 0 2 0 let-through commensurate with the higher rating, e.g., 100 A in the example.

Cartridge fuselinks of UK manufacture and com-

pliant with the appropriate British/DEF Standard are precision devices which have proved highly reliable in service when applied properly, i.e., selected, in the

•of motor circuits, with due regard to:

•The ratio of the starting/running current.

•The duration of the starting period.

•The number of sequential starts to be allowed for, i.e., successive starts without intervals between starts long enough to permit cooling.

Hie principles of the selection of fuses in schemes protection is described in Chapter 11.

8 DC switchgear

8.1 General

At the beginning of electricity supply, the sources of

such supply — dynamo charged batteries of secondary cells — produced unidirectional current (DC). A first

essential of any system of supply is means for establishment and interruption of current flow, i.e.,

switching on and off. The most elementary means are, of course, the coming together, and parting, in ambient atmosphere (air) of contacts of conducting material. The earliest devices for circuit 'making' and 'breaking' were as simple as this.

All practicable forms of interruption of an electric current involve the production of an arc, which must be extinguished to complete the interruption. The arc, an intensely hot column of conductive gas, is extinguished by cooling and lengthening to the point where the potential difference (voltage) across the contact gap, as it widens, becomes incapable of sustaining the arc. At full opening of the contacts, the dielectric strength of the gap must withstand the voltage across it.

The build-up of dielectric strength across the contact gap takes place similarly in the interruption of AC. However, the process of interruption in an AC circuit is assisted markedly by the occurrence of natural current zeros at every half cycle — arc extinction taking place at or near a current zero. The absence of naturally occurring current zeros in the case of DC thus

renders the interruption of such currents a more difficult process.

From the simplest of devices initially, switching current in air, the switchgear evolved into open-type, i.e., unenclosed, air-break contact systems, at first operated manually, mounted on vertical panels of insulation material, such as slate. As systems developed, arc control became necessary to achieve satisfactorily reliable interruption. Development along these lines led, in due course, to the circuit-breaker concept as presently understood (see Section 2.1 of this chapter for definition of a circuit-breaker.)

Whilst development and general acceptance of the metal-enclosed concept followed rapidly upon the appearance, in the early part of this century, of the oil circuit-breaker in the AC field, DC switchgear continued firmly in the open-type tradition for several decades. However, for many years now, the bulk of DC switchgear in UK power stations has been at least metal-enclosed and, wherever possible, metalclad. A definition of `metalclad' is given in Section 4.2 of this chapter.

Broadly, the several types of switchgear equipment comprising a DC system installation, e.g., circuitbreakers, contactor controlgear, fusegear, are two-pole versions of their three-phase AC counterparts.

8.2 System conditions

The usual system voltages and fault levels are:

To meet these system conditions, the basic capability required of the major components of a DC switchgear installation is described in the paragraphs that follow.

8.2.1 Short-circuit withstand strength of busbar systems

•The full fault current level of the system for 3 s, where the busbar protective device is a circuitbreaker.

•The full fault current level of the system, as limited in magnitude and duration by the 'cut-off' characteristic of the fuses, where the busbar protective device features fuses.

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

Circuit- breaker equipment

Rated short-circuit making and breaking current — 40 kA at a time-constant appropriate to the system.

Fhe equipment comprises:

• Portable substations.

Pori,ible distribulion units 4.15/240 V.

•

Portab!.... distribulion units 110 V.

•

supply scheme is shown in Fig 5.64.

Construction site electrical supplies equipment

9.2 Portable substations

The substations provide supplies at 415/240 V, derived from an ii kV source. Figure 5.65 depicts a typical arrangement.

The transformer in the substation has a rating of 1000 kVA at a primary voltage of 11 kV. The high

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the development of HBC fuses and as a result a viable alternative known as the moulded case circuit-breaker ( MCCB) was developed and subsequently introduced to the UK.

The moulded case circuit-breaker, as its name implies, uses moulded insulation materials, permitting space and weight reduction. in some instances the filler in the moulding material is claimed to assist in the arc extinguishing process by producing gases favourable to current interruption. Current-limiting properties, similar to those of HBC fuses, can also be utilised to reduce fault damage and a regime of rating steps (as for fuses) must also be followed to ensure discrimination. As an alternative to 'traditional' fuse switchgear, the MCCB can also fulfil a role within motor starter circuits. MCCBs now available on the UK market have found limited favour with some users but have yet to make their mark in UK power station applications. With a specified service life for power station plant MCCBs in 'non-maintainable' form would not be acceptable.

The maintenance of power station switchgear at service capability is based upon the programmed overhaul/repair of equipment installed for the station designed life. Whilst 'maintainable' moulded-case circuitbreakers may have a place, the 'non-maintainable' form is as yet unacceptable by virtue of the difficulty of determining the performance capability after a period of service, particularly if fault clearances have been a feature of that service.

10.4 Air- blast switchgear

Exploitation of the improved 'dielectric withstand of air subjected to pressures above atmospheric level has resulted in present day designs of air-blast switchgear. Designs of air-blast interrupter employing compressed air stored at pressures of the order of 30 bar, with sophisticated gas flow technology applied to the nozzles and contacts in the arc region, are capable of achieving the highest short-circuit breaking capacities demanded today. These units therefore find ready application as generator circuit-breakers for the largest steam turbinegenerator units available and can be forced cooled, where necessary, to match generator load currents.

The high cost of air-blast switchgear which must also include ancillary compressor and air storage plant, plus the noise accompanying each switch opening, precludes the wider application of these units to power station general auxiliary systems.

Similar air-blast switchgear was also developed for distribution and transmission systems up to highest EHV levels. However, with the advent of new arcinterruption technology and more environmentally acceptable low maintenance switchgear, the air-blast circuit-breaker has declined rapidly in favour during the last decade and now worldwide is being applied only under special circumstances. This trend can be expected to influence future application of air-blast

technology to power station generator switchgear where alternatives are now available for all but the very highest short-circuit breaking capacities.

10.5 Vacuum switchgear

Although researched in the 1920s, vacuum arc-inter- rupting devices did not achieve commercial viability until the 1960s. The factory sealed vacuum interrupter has subsequently introduced to the switchgear field a unit which is claimed by some to approach the 'ideal' circuit-breaker of electrical theory. However, the aura which tends to surround all 'black-box' components has also given rise to some concern over their performance and reliability, particularly in power station applications, in their behaviour when switching motors and the possibility of loss of vacuum.

Satisfactory service experience with vacuum switchgear during the last decade has dispelled most of the doubts. Furthermore, research in the laboratory and in the field has tended to confirm that some phenomena are not unique to vacuum interrupters and that for critical service conditions some special precautions may be appropriate to both established and innovative designs.

The success of any electric arc-interrupting device lies in its contact geometry and, particularly with the vacuum unit, the chemistry of the contact materials is also of great significance. Whilst the reader wishing to study the contact materials technology in detail can refer to the many learned papers now published on the subject, it is sufficient to note here that it is the precise composition of the basic contact metal, copper, together with controlled additives and the exclusion of impurities, which dictate performance.

Vacuum interrupter development has produced units for circuit-breaker application, with its demands for high short-circuit breaking capacity and moderate number of switching operations, and for contactor application, with moderate short-circuit breaking capacity and high number of switching operations. A basic advantage of the contactor interrupter, as manufactured in the UK, is that it provides a 'soft' operating characteristic particularly suited to regular switching of load currents including high reactance transformers and small motors, etc. Compared with the contactor type, the circuit-breaker interrupter has a relatively 'hard' characteristic. This is presently unavoidable, being an inherent feature of interrupters designed specifically for very high currents, i.e., short-circuit currents. Briefly, a 'soft' interrupter is one with a negligible propensity to 'current chop', and thereby produce overvoltage likely to damage plant insulation. Conversely, a 'hard' interrupter is one with a tendency towards such behaviour. Current chopping is the term used to describe the sudden reduction to zero, during the process of interruption, of an alternating current at a ti me other than the instant of a natural zero. Theoretically, an interrupter 'breaks' an alternating current