Station design and layout

positioning the equipment cubicle. With this arrange­ ment however, diode monitoring and maintenance are more complicated.

14.4 Main connections

The standard 660 MW generator produces over 19 000 A at 23.5 kV, the earlier 500 MW generators produce 14 500 A at 23.5 kV and the proposed 900 MW will produce 24 750 A at 26 kV. This power has to be transmitted to the LV terminals of the generator transformer by the main connections.

The magnetic and phase-to-phase fault problems associated with transmitting such large currents led to the development of the phase isolated busbar (PIB) and this is now standard for all main connections.

In ai PIB system, each line connection consists of a current carrying high conductivity aluminium con­ ductor supported by post insulators inside a high con­

Chapter 2

ductivity aluminium sheath of fully-welded const ruc­ tion which is electrically continuous over its complete length, see Fig 2.43. The sheaths have insulated sections inserted at both the generator and the gen­ erator transformer ends, and at the ends of the tee-offs to the voltage transformers and unit transformers. Bonding the sheaths together at their ends ensures that the current induced in the sheath is in anti-phase to that in the main conductor. The sheaths are insulated from adjacent metalwork, bonded together al the generator end and earthed to the station earth bar. Hence the magnetic field from the conductor and th;it from the sheath cancel each other out both in normal working and under short-circuit conditions. There is little heating of surrounding or supporting steelwork nor is there electrodynamic strain between different phase connections.

Despite the large reduction in magnetic effects achieved by phase isolation, in order to further mini­ mise the effects of any induced currents, there must be

a gap of at least 300 mm between the main connections and any steelwork or pipework running parallel to them. When the run of steelwork is at right angles to the main connections the gap can be reduced to 150 mm. Closed loops of metalwork must be avoided, also the main connections must be insulated from all supporting steelwork.

Natural air cooling is practicable up to about 20 kA but a significant increase in current rating for the same conductor size is possible by forced air cooling.

Phase isolated busbars arrive on site in make-up sections which may be 10 m or even more in length. They are then supported in position and welded together. Being continuously welded they are naturally vermin and drip proof over their length so they only need sealing at their ends. Special provision is made at the generator to ensure that hydrogen, if it should leak from the generator, cannot collect in the main connec­ tions. similarly provision is made at transformer con­ nections to prevent the ingress of bushing oil. In order to prevent dust entering, and to guard against conden­ sation, which can cause a reduction in the dielectric strength of the insulatois and corrosion, the connec­ tions arc pressurised to approximately 12.5 mbar with dry conditioner! air.

'1 lie main conductors arc joined to the generator and other items of plant by means of flexible connections.

This prevents the transmission of vibration and conse­ quent work-hardening of the aluminium. Their route should be as straight as possible because bends can create hot spots. Expansion joints are required in long straight runs; however, these are usually designed so that thev do not particularly increase the sheath diameter.

The line and neutral connections are. in the majority of cases, brought out from the underside of the stator with the line connections close to the exciter end of the geherator and the neutral connections nearer to its centre line. The simplest arrangement of connections is when the unit is arranged transversely and they are then run straight out to the generator transformer passing between the generator supporting steelwork, through the turbine hall wall and over the unit trans­ former (see Fig 2.42). The voltage transformers are then also positioned between the generator supporting steelwork legs with their connections teed-off from the main connections passing overhead.

When the unit is arranged longitudinally this simple arrangement cannot be used. The longitudinal arrange­ ment creates additional bends and also sterilises part of the turbine basement area. Bringing the connections out of the top of the generator alleviates many of these problems but then creates one of its own because they are now susceptible to damage from loads being moved on the overhead cranes.

The LV side of the generator transformer is delta connected. The generator transformer now consists of three single phase units arranged to form a three-phase bank. This means that the delta must be formed exter­ nally. On early designs this was done using a delta box

Electrical plant layout

across the three transformers, but more recently the delta box has been made in the main connections just prior to their terminating point on the generator transformer (see Fig 2.44).

Also associated with main connections are the volt­ age transformers (VT), the neutral connections, the excitation busbar connections and the mounting of the current transformers.

The VT connections are tccd-off the main connec­ tions. The simplest position for the VTs is immediately below the main connections in the turbine basement.

The excitation connections interconnect the exciter, the rectifier cubicle and the field switch cubicle. The two cubicles are positioned side-by-side in the turbine basement, level with and slightly to the side of the exciter. Three-phase connections from the exciter are taken from above, the main busbars and at right angles to them, they then turn through 90° and drop vertically onto the rectifier. The busbars connections between the rectifier and the field switch run in the canopy. The connections from the field switch back to the generator brushgear take a similar route to those from the exciter (see Fig 2.45).

The neutral connections and the star bar are supplied with the main generator, but the neutral earthing transformer and resistor form part of the main connec­ tions. The neutral earthing transformer and its asso­ ciated resistance banks are supplied mounted on a frame as one unit which must be positioned as close to the star bar as is possible. This may well require it to be supported at some considerable height above the turbine hall basement. On the neutral connections immediately below the stator and above the star bar are mounted current transformers. There may be as many as seven of these on each phase. They are supported from the neutral housing which screens the whole of the neutral connections. In addition to the current trans­ formers on the neutral, there are others mounted imme­ diately before the unit and generator transformers.

14.5 Transformers

14.5.1 Generator transformers

The generator transformer is the largest transformer on a power station and connects the generator output to the grid. There is a generator transformer for each generating unit and it is rated according to the size of that unit. Like any other oil-filled transformer, the generator transformer should be located within an out­ door compound, protected by a high pressure water­ spray fire protection system and surrounded by a bund wall capable of containing not only all of the oil contained in a single phase transformer, but also the discharge from the fire protection system ovfer the whole bank of transformers on that raft following rain­ fall prior to the incident. For the generator transformer this could mean up to 320 000 litres, of oil and water

mixture. Also, as any other oil-filled transformer, the raft must drain into a special system equipped with an oil separator and interceptor, and there must be no possibility of puddles forming or being left after a spillage.

However, certain other considerations determine the location of the generator transformer (see Fig 2.46):

•It must be as close to the generator as possible so as to keep the main connections as short as possible.

•The generator transformer is one of the heaviest loads delivered to site so its location will seriously

affect the position of the site perimeter road along which it must be delivered and from which it must be manoeuvred into position.

• Its cooler bank needs an area of approximately nine times its own plan area to be free of major obstruc­ tions over 1.3 m high, although this area may include fences and roadways.

All single phase units of a similar rating are currently designed to be interchangeable. This means that all their interface dimensions shall be the same and there shall always be the same distance between the indivi­ dual phases forming the three-phase unit. This dimen­ sion is 4.9 m for the 800 MVA transformer and 5.1 m for the 1145 MVA transformer.

Whilst the oil Loimcclions to any tianslorinci i.mk are identical, the location of the conservator aiul coolct bank within the compound can be adjusted to suit any particular site.

On early designs of single-phase generator transfor­ mers, the LV delta was formed in an oil-filled delta box which spanned the three tanks, but this has now been superseded by making an air-cooled delta in the main connections just before they connect onto the trans­ former. Mounted on the main connections just prior to the delta are the protection current transformers. These transformers and the delta need to be supported in position and if the route of removal of the trans­ former unit is underneath this support then adequate clearance must be provided.

On the more recently-built power stations, the HV connections from the generator transformer have been made through SF6 (sodium hexafluoride) insulated isolators and earth switches into 400 kV cables which then run in concrete troughs out to the grid substation. This gives a much more compact arrangement than the air-insulated equipment and overhead connections pre­ viously used. It also improves access and site safety because there are no longer 400 kV overhead wires crossing the site perimeter road.

Each transformer has a cooling system comprising two oil pumps and four cooler fans. All of these are

supplied and controlled from the transformer marshal­ ling kiosk which is located just outside the bunded area. The marshalling kiosk also controls the on-load tap changer and marshalls all local cables for alarms, etc., originating on the transformer.

14.5.2 Station transformers

The second largest transformers on a power station are the station transformers. These step down the grid voltage to that of the highest plant auxiliaries which on modern power stations is always 11 kV. They are required for commissioning the first plant on a new station and supply loads not specifically associated with the generating unit such as lighting, cranes, CW pumps, etc.

The location of the station transformer within the power station is not as critical as that of the generator transformer, but it should be as close as possible to the buildings and the 11 kV switchboards that it is sup­ plying. Its compound must fulfil all the general condi­ tions applying to oil-filled transformers with respect to fire protection, drainage and bund walls (see Fig 2.46).

The HV connections to the station transformer may be overhead or underground depending on the site. The LV connections may be either 11 kV cables or phase isolated busbars depending on the relative loca­ tions of the station transformer and its associated 11 kV station switchgear. Phase isolated busbars are only favoured when the transformer and switchgear are close together because they are inflexible and need straight runs with a minimum of bends. When cables

are used, they usually leave the transformer compound via underground ducts or tunnels. These must be ade­ quately sealed to ensure that there is no possibility of oil or water escaping from the compound into the cableways.

14.5.3Unit transformers

The HV side of the unit transformer is teed straight off the main connections, so this transformer should be positioned very close to the generator transformer (see Fig 2.46). Its compound is equipped with high pressure water spray fire protection, bund walls, and drainage into an oil/water separator. Where two oil-filled trans­ formers occupy adjoining compounds, they arc sepa­ rated by a fire barrier wall.

The 11 kV connections from the unit transformer are by cables, two, three or four per phase depending on the rating. These leave the compound via ducts into a trench or tunnel. The ducts must be tightly sealed against the ingress of transformer oil. The star point of the LV is taken to earth via a liquid neutral earthing resistor (LNER) which limits the earth fault current. This LNER is also accommodated within the trans­ former compound.

The unit transformer is a heavy load so adequate haulage facilities must be provided to enable it to be placed in position.

The maximum permitted noise emission from the unit transformer is 30 dB at 400 m, and a noise enclosure is not normally required.

14.5.4Auxiliary transformers

Auxiliary transformers on power stations are many and vary greatly in size, rating, insulation, etc. On CEGB stations, the largest arc the 11 kV/3.3 kV auxiliary transformers which can' be 12.5 MVA, 10 MVA or 8 MVA. They are oil-filled and hence require all the safeguards of any oil-filled transformer, i.e.. outdoor location, high pressure waterspray fire protection, fire barrier walls, bund walls and drainage into an oil/water separator. Where possible they are positioned to suit the 3.3 kV switchgear.

Transformers of 3.3 kV/415 V are usually rated at 2 MVA, 1.6 MVA or 1 MVA as these values match the standard current ratings of 415 V switchgear. These transformers may be oil-filled, as are the majority of those on existing power stations. However, the require­ ment that oil-filled transformers be located out of doors