17 Unit transformers

 Unit transformers

The unit transformer is teed-off from the main connections of the generator to the generator transformer. 

It is energised only when the generator is in service, except where a generator switch scheme is used as described above, and supplies loads which are essential to the operation of the unit.

The design criteria to be met by the unit transformer are as follows:

The HV voltage is relatively low, being equal to the generator output voltage, that is usually between 11 and 23.5 kV.

The LV voltage is usually 11 kV nominal, although on some CCGT stations 6.6 kV is used to supply the unit auxiliaries.

Impedance must be such as to enable it to be paralleled with the station transformer at 11 kV (or 6.6 kV, as appropriate) without exceeding the permissible fault level – usually this will be about 15 per cent.

Since the HV voltage is maintained within 65 per cent of nominal by the action of the generator AVR, on-load tap changing is not needed. 

This also enables a design fl ux density of 1.7 Tesla to be used as for the generator transformer.

As in the case of the generator transformer, operating load factor is high, so that load losses and no-load losses should both be as low as is economically practicable. 

(Except in some nuclear stations, where two fully rated unit transformers are provided per unit for system security purposes.)

Paralleling of unit and station transformers during changeover of station and unit supplies can result in a large circulating current between station and unit switchboards. 

This generally adds to the unit transformer load current, and subtracts from that of the station transformer.

The unit transformer must therefore be capable of withstanding the resultant

short-time overload.

General design features

The above design criteria result in a transformer which will probably have a

fairly close voltage ratio, say 23.5/11.8 kV in the case of a unit transformer associated with a large 660 MW generator. 

The LV (11.8 kV) winding must have a neutral to provide for an earth on the unit auxiliaries system, so the connections will probably be delta/star. 

The open-circuit voltage ratio of 23.5/11.8 kV is equivalent to 23.5/11 kV at full-load 0.8 power factor. 

Circuit taps on the HV winding of 67.5 per cent in six steps of 2.5 per cent

will probably be provided to enable fi ne trimming of the system to be carried

out during commissioning. 

For the reasons explained in Section 4.6, these are nowadays often varied by means of links under the oil rather than using an circuit switch which was the previous practice.

Unit transformers on CCGT stations, if used, will often have quite low ratings, perhaps no more than a few MVA, since these employ few unit auxiliaries.

These may well be very similar to large distribution transformers. 

However on coal fi red or nuclear stations, the need to provide supplies for electric boiler feed pumps, circulating water pumps and/or gas circulators plus many other lower rated auxiliaries, means that ratings of from 20 to 50 MVA are common.

Such a relatively large rating and modest voltage can lead to some design and manufacturing problems. 

Both HV and LV currents are relatively high, so that windings have a small number of turns of fairly large cross-section conductor. 

The large cross-section means that stray loss will be high, probably necessitating the use of continuously transposed conductor for HV and LV windings. 

The number of HV turns will be relatively few, so that it will be difficult to build in the necessary strength to resist outward bursting forces under short-circuit . 

In order to improve the bursting strength it is desirable to employ a disc winding but if a disc winding is used there will be a very small number of large-section turns per section which will not make this an easy winding to produce. 

These manufacturing difficulties will probably make a unit transformer of this type as costly as one having a similar rating but higher HV voltage, and the level of QA appropriate during manufacture will be greater than that normally associated with other types of transformers of similar voltage class.

The changeover of unit and station supplies normally only requires that

these transformers be paralleled for a few seconds. 

This is long enough for the operator to be sure that one circuit breaker has closed before the other is opened. 

During this time, however, a circulating current can fl ow which is dependent on the combined phase shift through the unit, generator and station transformers, plus any phase shift through interbus transformers, if generator and station transformers are not connected to the same section of the transmission system. 

This can result in the unit transformer seeing a current equivalent to up to two and a half times full load. 

Should the operator take longer than expected to carry out this switching, the unit transformer windings will rapidly overheat. 

Such a delay is regarded as a fault occurrence, which will only take place fairly infrequently, if at all. 

It is considered that parallel operation for a time as long as 2 minutes is more likely to occur than a short circuit of the transformer and so the limiting temperature is set lower than the temperature permitted on short-circuit. 

The latter is set at 250ºC by EN 60076

and so the CEGB considered it appropriate that a fi gure of 180ºC should not be exceeded after a period of 2 minutes parallel operation.