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

CONNECT (DNS

CCE X C ITA T ION

CONNECTIONS

FIELD

SUPPRESSION

SI.NITCH

F1(.. 4.11 Exciter busbar arrangement

NEUTRAL GENERATOR

ENCLOSURE FRAhviE

J J 4

MAIN ExCITEw,'

Ac CONNECTIONS

EXCITER

RECTIFIER

CUBICLE

Li

UNIT

TRANSFORMER

MAIN

CONNECTIONS

EARTH

BAR

many bends. It is essential that sound fixing is provided to prevent the force produced by the fault current from straightening out these bends.

When testing a main connections design, it is usual to include the earth bar, thereby testing a complete system.

6 Setting out the specification

Having described a typical main connections Installation in the previous sections, consideration is now given to the design specification of that system, assuming natural air cooling. Section 7 of this chapter ‘vill look at the component parts.

The first consideration is the particular application

— the environment, voltage and current-carrying requirements. As standard a design as possible should be specified, so that testing need not be necessarily repeated for each application. The parameters that must be specified are:

•The maximum system voltage, frequency and power factor.

•The output of the generator, and hence the line current.

•The variation allowed on the above during abnormal conditions.

•The fault contribution from the generator.

•The fault contribution from the external grid network.

In a tee-off busbar, there is a combined fault infeed from the generator and the external grid. This specifies the maximum fault level of the system for which the installation must be designed, including the earth bar.

The maximum system voltage dictates the impulse level for which the system must be designed and the consequential clearances between the conductor and the enclosure. However, it should be remembered that the impulse and switching overvoltages entering from the HV side of the generator transformer are not transferred according to the winding ratio of the transformer but rather to the ratio of the capacitances of the HV and LV sides. Depending on the steepness of the incoming wave, the percentage overvoltage on the generator side may be higher than the HV side. This overvoltage may be reduced by the capacitance of the generator and the generator main connections. Economics and workability dictate the choice of material for the conductor and the enclosure; aluminium is the most probable, bearing in mind that the electrical resistance should be low but the inherent mechanical strength adequate to withstand the forces produced during fault. The phase configuration, i.e., flat in-line or trefoil, and probably the spacing, will be determined by obstructions on the proposed route. The number of bends should be kept to a minimum and those selected should be of a tested design.

The enclosure insulation level from earth must be specified. This level must be high enough to allow for degradation caused by dust during the periods between cleaning. At present the CEGB specifies 3.6 kV.

The temperature rises permitted during maximum continuous current flow are then specified which, for

CEGB installations, are as stated in Section 3 of this chapter.

7 Component parts of an 1PB system

7.1 Conductor and enclosures

These items have been dealt with in detail in the preceding sections.

7.2 Equipment enclosures

At interfaces with plant, for example a transformer, the physical enclosure of the conductor must include access for maintenance purposes but still insulate the enclosure system from the connected plant. Non-con- ducting bellows, discussed later in this chapter, connect the enclosure to, say, the transformer, the tank of which is earthed separately. This equipment enclosure may have viewing ports (see Section 7.12 of this chapter) in order to inspect the flexible connections making up the conductor interface, and an access cover to allow the application of a portable earth if necessary (see Section 12.2 of this chapter). The enclosure may be a bolted assembly, removable for maintenance purposes, and using bonding strips to ensure that all parts are earthed positively and that no reliance is put on the bolted construction for earthing. To prevent circulation currents occurring, larger access covers should be insulated from the rest of the enclosure, except for a single earth connection on each individual cover. For the same reason, hinges should be bridged, using a flexible connector. Designs should offer the facility of taking a transformer out for maintenance, while allowing the main connections to be re-energised safely. This latter requirement may be achieved by the removal of links or a short section of conductor, and the fitting of caps over the busbar ends in a manner which maintains the insulation of the system.

7.3 Insulators

7.3.1 Post insulators

These support the conductor within the enclosure to maintain the air clearance needed for the highest system voltage. They are made either of epoxy resin or of porcelain; if porcelain, it must be thoroughly vitrified, so that the glaze is not depended upon for insulation. The strength of the insulators should be such that when they are supporting the maximum short-circuit loads, the factor of safety is not less than 2.5. The number of supporting insulators at each point and the spacing between them is confirmed by short-circuit testing. Typical arrangements are shown in Fig 4.13.

Stresses due to expansion and contraction in any • part of the insulator and its fixings must not lead to

t h e development of defects. The fixings, which should be non-magnetic, should be such as to allow movement

of the conductor when centralising forces occur during short-circuit. The post insulator is usually mounted on plate which bolts externally onto a flange fitted to the enclosure, as shown in Fig 4.14, thereby permitting easy removal of the insulator.

7.3.2 Foot insulators (including enclosure supports)

These are plate-type insulators which insulate the support side of the enclosure from the structural steelwork. An insulation level of 3.6 kV is generally specified for this device, which allows for some dust build-up during the operation of the plant. The support structure itself should allow relative movement

T OF BUSBAR

NTLiCTOR

CONDUCTOR SUPPORT FOOT

POST INSULATOR

INSULATOR BASE PLATE

ASSEMBLY

REMOVABLE

11(.. 4.14 A typical insulator support assembly

between it and the enclosure to accommodate thermal expansion. Where supporting structures suffer excessive vibration, installations may require anti-vibration pads to be incorporated in the design, although the CEGB does not generally use them. The design of the connections mountings should be such that the natural frequency of any part of the busbar structure and its supporting metalwork does not lie between ± 30 07o of the frequency of the applied electro-magnetic force, i.e., static and dynamic loadings for the power frequency specified should be taken into consideration.

7.3.3 Disc bushings

Disc bushing seals are fitted at the end of busbar runs adjacent to plant housings to provide an airtight seal, thereby preventing hydrogen from the generator, or oil from the transformers, passing along the isolated phase-runs in the event of leakage. The isolated phaserun is then fed with conditioned air, as discussed later. A bushing seal must also be provided in the VT teeoff, allowing the on-load maintenance of the VTs without affecting the conditioning air system. Careful attention is given to the bushing profile to ensure that moisture, which would reduce the creepage path of the bushing, cannot be trapped in it.

7.3.4 Wall seals

Wall seals are provided where the enclosure passes through a wall (see Fig 4.15). The individual enclosures are connected to the wall seal using the bellows arrangement described in the next section. A similar seal is fitted where the installation passes through a floor.

Also, to prevent transformer noise causing annoyance in residential areas, the generator transformer may be fitted with a noise enclosure. A wall seal will then be required for the busbar to pass through that enclosure.

7.3.5 Bellows

The bellows maintain the insulation of the main connection enclosures from the connected plant (see Fig

Ho, 4.15 An example of a wall seal

4,16). They allow relative movement due to expansion and vibration, and cater, to a limited extent, for any installation misfit due to the worst combination of allowable tolerances. When bellows are installed on a length of busbar at, for example, a conductor expansion point, they must be bridged using flexible aluminium

laminae. Alternatively, aluminium bellows may be used. The bellows are usually of synthetic rubber, completely weatherproof and airtight and must withstand at least twice the design working pressure of the conditioning air system within the enclosure. When bellows are located outside buildings in direct sunlight, the material

must be chosen to ensure that degradation does not

Occur.

7.4 Conductor and enclosure expansion joints

Allowance for the expansion and contraction of the conductor (Fig 4.17) and enclosure (Fig 4.18) is provided on the longer busbar runs. This typically comprises a 'cage' arrangement, where the expansion gap is bridged by aluminium laminae. The connection must have adequate current-carrying capacity and correct current-sharing among the laminae is ensured by using a symmetrical array of laminae.

7.5 Flexible connectors

Flexible connectors can be laminae or braid, the choice depending on the type of relative movement of the parts being connected. When removed, they provide isolation of plant. The significant difference between them is that laminae only allow relative movement in two dimensions, whereas braid gives full three-dimensional movement.

7,5.1 Flexible laminae connectors

hese are constructed from thin strips of aluminium laid one above the other, with aluminium palms welded Ott each end (Fig 4.19). Suitable holes in the palm 3110w the connector to be bolted to the conductor

palm. Their applications are limited to sections of main runs (for example, to allow the insertion of

CTs) at expansion joints, connections to generator voltage switchgear, and earth connections to some items of plant.

7.5.2 Braided flexible connectors

These are made up of tinned-copper braid or braids (Fig 4.20) with ferrules fitted at each end which are

FERRULE TUBE

THICKNESS

ALUMkNIUM

LAMINAE

WELD

SOLID ALUMINIUM TERMINAL PALM

Fin. 4.19 Construction of a flexible laminae connector

LAYERS

{THREE IN THIS CASE)

CRIMPED

SIDE FACE

THICKNESS

OF

COMPRESSED

FERRULE

---11%.

Fin. 4.20 Construction of a braided flexible connector

drilled to allow a bolted connection onto the terminal palms of external plant. It is essential that the ferrules are crimped onto the braid, since sweated assemblies have been found to suffer from mechanical creep problems, with failure resulting after a period of time. Clearly, whilst the braids overcome the problem of relative movement of component parts of the system, they introduced a dissimilar metal interface when connected to the aluminium conductor palm.

It has been found preferable to remove tinning from the ferrule at the joint interface surface. Methods of overcoming problems associated with dissimilar metals are discussed later. Careful selection of the braids is necessary to ensure that there is adequate current-carrying capacity and that they are capable of operating continuously at the maximum specified temperature.

Consideration should be given to:

The length of braid compared with the gap, to en-

•sure that there is adequate flexibility (ideally there should be 25-35 mm free play).

The number, size and position of the fixing holes

•on the crimped ferrule (which affects the clamping pressure on the joint and hence its current-carrying

capability).

•The ambient air temperature within the enclosure.

7.6 Painting

The outside of the conductors and the inside of the enclosures are normally painted with matt black heatresistant paint to improve heat transfer.

7.7 Conditioned air

Bolted inspection covers, portable earth access covers and the insulator base fixing-plates are provided with Easkets, and the ends of busbar runs incorporate a disc bushing. These measures prevent the leakage of moist or polluted air into the enclosure. It is usual to pressurise the enclosure so that leakage is outwards io atmosphere. The leakage rates are approximately

Ocro by volume of the busbar enclosure per hour. This airflow, though quite small, purges any ionised oases which may have accumulated inside the enclosure and prevents condensation forming, particularly during periods of shutdown. The cooling effect of this air is not taken into account in the design of the busbar and therefore failure of the associated equipment does not necessitate shutdown of the unit.

All the main connections within the confines of the bushing seals are fed with dry conditioned air at 12.5 mbar(gauge). To prevent condensation, the design is based on a dewpoint of - 25 ° C. A typical system comprises one air compressor and its associated receiver and drying equipment per unit. Alternatively, if available, air supplies can be obtained from the Station Instrument Air System. Either source is acceptable, though sizing and rating problems can occur with instrument air sources. It is usually preferable to provide an independent compressor. Some air is fed around the disc bushings to vent to atmosphere after passing through the equipment housings.

7.8 Voltage transformers

Voltage transformers (VTs) are mounted within a cubicle (see Section 7.16 of this chapter) which is designed so that the transformer can be safely removed for maintenance without requiring access to live parts, should that be required during operation. A typical VT weighs in the region of 100 kg. It is generally a single-phase cast-resin transformer connected in a star arrangement with a ratio of 22 kV/110 V, the

Component parts of an IPB system

nearest preferred ratio to the present generator voltage with accuracy maintained for 0-100 07o rated output. The primary sides of the VTs are earthed at one end to a common earth, which is then connected to the generator stator earth. They are fed from the voltage transformer tee-off busbar via fuses rated to discriminate against the fuse located at the tee-off from the main busbar. In the secondary circuits, any earth connections are made to the Station earth, as that is the earth which is used in instrumentation circuits. Some manufacturers have traditionally provided tertiary windings on their voltage transformers connected in delta and used to prevent the occurrence of neutral inversion and voltage transformer ferro-resonance. The delta winding is closed solidly or through a loading resistor, depending on the X/R ratio needed to prevent ferro-resonance. Neutral inversion is the displacement of the neutral due to abnormal system conditions, such as open-circuits in one or more phases of systems possessing inductance and capacitance. Ferro-resonance can occur when the magnitude of the inductance of the VT compared to the capacitance of the circuit to which it is connected is equal and opposite. When not required, the tertiary windings are left open-circuited with one open-end earthed; this is usual with neutral earthing methods now used. The reader is referred to Chapter 3 (Transformers) and Chapter 11 (Protection), for a more detailed discussion of these issues and the problems they cause.

7.9 Current transformers

Current transformers (CTs) are located in various parts of the installation, depending upon the protection scheme employed. Typically, for the scheme shown in Fig 4.1, CTs would be installed in the following locations (shown in Fig 4.8).

At the neutral end of the stator winding, between the terminal plate and the star-bar, for the following purposes:

•Tariff metering.

•Efficiency testing.

•Unit instrumentation and turbine-generator automatic control input signals.

•Low forward power protection.

•Negative phase sequence and loss of excitation protection.

•Generator differential protection.

Within the generator neutral earthing module on the secondary side of the neutral earthing transformer for stator earth fault protection (see Fig 4.10).

At the unit transformer for

•Generator differential protection.

•Unit transformer differential protection.

On the 11V bushing of the generator transformer for generator differential protection.

The design of the CTs must be such that they do not reduce the electrical impulse-withstand level or the power frequency withstand level of the installation. The CTs positioned in the main connections busbar are of the 'slipover' type, mounted within a housing, for ease of erection. They include an earthed screen to shield the secondary winding from the electric field of the conductor, thereby allowing the secondary insulation level to be a nominal 2 kV. This assembly is held at earth potential by connecting the CT core-shield assembly to an independent earth cable at one point only, to prevent any circulating current paths. All secondary cabling should be glanded on an insulated glandplate, thereby maintaining the 3.6 kV insulation level of the enclosure. Adequate support and bracing of the CTs is required as a typical assembly is heavy; a neutral CT assembly may weigh 600 kg and a line CT assembly 350 kg. Sufficient ventilation must be provided to ensure that the heat produced in the windings does not cause unacceptable temperature rises. Any forces exerted on the CTs during fault conditions will be li mited to those attempting to centralise them around the neutral line of the enclosure. These are not significant if the CTs are mounted concentrically. There will be little, if any, axial force exerted on the CTs and this is easily contained by the mountings. The generator has an impulse-withstand level of 85 kV, consequently a similar figure can be allowed for the neutral-end equipment.

CTs should be clearly labelled with details of their duty and their orientation within the system must be identified to ensure the correct polarity of secondary signals.

7.10 Environmental conditions

For design purposes, the relative humidity should be taken as 100% and any equipment mounted out-of- doors should be completely weatherproof and capable of withstanding inclement weather conditions, including wind and snow loading, and solar heating. The entire installation should be drip proof, dust proof and vermin proof, with an enclosure rating to at least 11)45 of BS5490.

7.11 Portable earth access covers

The philosophy of portable earthing is discussed in Section 11 of this chapter. The mechanism by which

it is applied is covered here. The design of the access covers must allow reinstatement of the air conditioning system when portable earths are applied to prevent condensation and dampness forming within the main connections system during periods of prolonged outages. The access cover to the enclosure must allow good access to the conductor and earthing device within the enclosure, but be securely bolted and hinged when not in use, The earthing connection onto the conductor is made by a clamp and is applied using an insulated pole. A flexible cable then connects this via another clamp onto the main connections earth system. The access cover must also allow the application of a voltage testing device. CEGB safety rules require the cover to be lockable.

7.12 Viewing ports

Viewing ports, comprising clear glass or Perspex viewing windows, are provided in the enclosure at positions where there is a need to check the condition of flexible connectors and other equipment regularly. These also permit the use of infra-red heat measuring devices for checking the temperature of the connectors.

7.13 Connection of the conductor to plant

As detailed earlier in Section 7.5.2 of this chapter, braided flexible connectors are used to continue the conducting path to the connected plant. At the generator, a 'candelabra' assembly (see Fig 4.21) has been developed which forms a circular terminal arrangement fitted to the machine terminals. Braids then bridge the gap between the main connections conductor and the generator terminals forming, in effect, a short cylinder that assists equal current-sharing among the braids. Care must be taken in the design to ensure that there is adequate clearance from stator water cooling pipework.

Connections onto the generator transformer, which may be either a three-phase tank or three single-phase tanks, are by braided flexible connectors onto six terminal bushings. A typical arrangement in isolated phase busbar is shown in Fig 4.22.

To give a good current distribution, the connections onto the bushings should be arranged in as near circular configuration as possible, typically eight palms arranged in an octagonal formation (see Fig 4.23). A single palm connection would not give good current distribution and should be avoided. However, since the generator transformer connections carry phase current rather than line current, there is no need for the elaborate candelabra arrangement used at the generator. Connections to auxiliary transformers, e.g., the unit transformer, are simpler than the generator transformer since the load currents are much lower.

The temperature rise due to losses in the connections system must be reduced in areas where significant

heat may be conducted from plant, for example, at the generator and the generator transformer via its bushing. The specifications for these items permit higher temperatures than are permitted for generator main connections.

Access to terminals should be as easy as possible, with the removal of a minimum of enclosure components to gain access to the terminations.

7.14Joints in the conductor

Ahhoue.h apparently simple, joints have given serious Problems in the past, due to the unequal current-shar- ing in braids and poor pointing procedures. One cause sometimes aggravates the other and leads to the even-

tual failure of the joint, often with catastrophic results. A joint (which really consists of many joints in parallel) can be so badly burnt-out that the original cause of the fault is impossible to determine. Consequently, extensive experimental work on joints has been performed by the CEGB to establish the most suitable joint surface preparation assembly and fixing procedures.

There are several influencing factors to consider when making a joint:

•The material of the mating surfaces to be joined.

•The preparation of the mating surfaces.

•The bolt size.

•The bolt material.

•The size of spreading washer to give necessary clamping load.

•The necessary torque.

•The method of locking the nut.

The materials making up the joint are normally either aluminium or copper; the joints are therefore copper to copper, aluminium to aluminium or copper to aluminium, in order of increasing difficulty in making the joint. It is recommended in the CEGB that where surfaces are coated or plated this should be removed at the interface by linishing, so that one of these three joint interfaces is created. Without going into the science of jointing, the most satisfactory method will be described. The jointing surfaces are first cleaned

a wire brush, a separate wire brush being used for each material. A liberal coating of petroleum jelly is then applied to prevent further oxidation. High-tensile steel bolts, washers and nuts are then torqued-up and locked, the number of bolts depending on the size of the ferrule being bolted. Recommended sizes of these components are shown in Table 4.1.

TABLE 4,1

Recommended dimensions for bolts and washers used in jointing

7.15 On-load temperature measurement

As can be appreciated, temperature measurement of an IPB system is not easy, but it is necessary during commissioning to ensure that the design requirements have been met, and during operation to ensure that there has been no degradation of joints, flexible connectors, etc. During commissioning, there are various