Industrial Power Transformers -- Special features of transformers for particular purposes [part 2]

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OTHER POWER STATION TRANSFORMERS

Station transformers

The station transformer generally supplies the power station auxiliary system for starting up the boiler/turbine generator unit or gas turbine/generator and for supplying those loads which are not specifically associated with the generating unit, for example, lighting supplies, cranes, workshops and other services.

In addition, in order to provide a diversity of supplies to certain plant, the station switchboard is used as a source of supply for certain large drives which are provided on a multiple basis for each unit, for example, the gas circulators of a nuclear reactor and the circulating water pumps for the main condensers of a steam-turbine generator. A minimum of two station transformers will normally be provided in order to provide diversity of supplies with all units shut down. In a four-unit station each transformer will probably have the capability of starting up two units simultaneously whilst also supplying a proportion of the power station load. The station transformer will usually, therefore, have a larger rating than that of the unit transformer. The station transformer is usually the first major connection to be made with the transmission system for a power station under construction, providing supplies for the commissioning of the plant.

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

• In the UK the HV connection is usually from the 132 networks, however it is possible to use the 275 or 400 kV systems.

• The LV is almost invariably 11 kV nominal on modern main generating stations.

• Impedance must be such that it can be paralleled with the unit transformer at 11 kV to allow changeover from station to unit supplies and vice versa with out loss of continuity and without exceeding the permissible fault level for the unit and station switchgear -- this usually means that it is about 15 percent.

• An on-load tapchanger is required to maintain 11 kV system volts constant as load is varied and as grid voltage varies.

• Operating load factor is low, that is, for much of its life the station transformer will run at half-load, or less. Load losses can therefore be relatively high, but fixed losses should be as low as possible.

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FIG. 6 Arrangements for deriving auxiliary supplies employing a generator switch

400 kV substation; Generator transformer 15/400 kV; Unit transformer 15/6.6 kV; Unit & station auxiliary supplies; Generator switch 15 kV; Gas turbine generator; Generator 400 kV circuit breaker

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General design features

The station transformer is almost invariably star/star connected, since both HV and LV windings must provide a neutral for connection to ground. For a four-unit fossil fuelled or nuclear station its rating will be of the order of 50 to 60 MVA. If supplied from the 275 or 400 kV system, this represents a rather small rating for either voltage class so that special care must be taken in the design of the HV winding. This will have a large number of turns having a relatively small wire size. A disc winding having a large number of turns per section must be used which will require particular attention to the distribution of impulse volt age. An interleaved winding arrangement will almost certainly be necessary.

At CCGT stations where the station auxiliaries loading is considerably less than the values mentioned above, so that, even at 132 kV, the design of a 'conventional' station transformer would be very difficult and certainly uneconomic, it is common to employ a generator switch scheme as shown in FIG. 6. With this type of arrangement a switch or circuit breaker is provided on the LV side of the generator transformer so that the generator can be disconnected when not operating. A tapping off the LV side of the generator transformer can then be used to provide station supplies and, when the unit is in operation, this will also double as a unit supply. Generator circuit breakers and even generator switches are costly and a full discussion of the merits and demerits of the generator switch scheme is beyond the scope of a volume dealing with transformers, but at least a transformer and its associated connections is saved, and a transformer which would be fairly difficult technically at that.

FIG. 7 Connection of gas turbine neutral in parallel with station transformer neutral

Until the late 1970s, a star/star-connected station transformer would automatically have been provided with a delta-connected tertiary for the elimination of third harmonic. However, as auxiliary systems and the transformers feeding them became larger, fault levels increased and it became clear these could be effectively reduced and third harmonic remain at acceptable levels if a three-limb transformer without a tertiary winding was used. The thinking behind its omission from the station transformers for the CEGB's Littlebrook 'D' power station designed in the 1970s is described in Section 2.

If the tertiary is omitted, zero-sequence impedance will be greatly increased and it is necessary to be sure that, in the event of an 11 kV system line-to ground fault, there will be sufficient fault current to enable the protection to operate. Works testing of the Littlebrook station transformer showed that the actual value of zero-sequence impedance was low enough to meet the auxiliary system protection requirements. It was also necessary to ensure that the absence of a tertiary would not give rise to excessive third-harmonic currents circulating in the system neutral. Such currents flow whenever the system has more than one neutral grounded, as, for example in the system shown in FIG. 7 where an auxiliary gas-turbine generator with its neutral grounded is operated in parallel with the station transformer supply, thus setting up a complete loop for circulating currents. The impedance of this loop to third-harmonic currents can be increased by connecting a third-harmonic suppresser in series with the gas-turbine ground connection. This is an iron-cored reactor whose design flux density is carefully chosen to be fully saturated at 50 Hz, thus having a low impedance at normal supply frequency, whereas at 150 Hz it operates below the knee point and, being unsaturated, has a high impedance, effectively equal to the magnetizing reactance. In order to ensure that protection problems are not encountered when deciding to omit the tertiary from a star/star transformer it is good practice to specify that the zero-sequence impedance should fall within a band from, say, 0.9-6 times the positive-sequence value.

Mention has been made above of the use of an on-load tapchanger on the station transformer as a means of compensating for grid voltage variation and for regulation within the transformer itself. This has an important bearing on the design of the station transformer.

In order to ensure that the 11 kV station board voltage remains at an adequate value under full-load conditions, the open-circuit ratio of the station transformer is usually selected to give a low voltage somewhat above nominal. A figure of 11.8 kV is typical.

Under normal operating conditions the UK transmission system voltage may be permitted to rise to a level 10 percent above nominal. On the 400 kV system this condition is deemed to persist for no longer than 15 minutes. For the 132 and 275 kV systems, the condition may exist continuously.

Should the station transformer HV volts rise above nominal, the operator may tap up on the tapchanger, that is increase the number of turns in the HV winding. If the HV were to fall, he would operate the tapchanger in the opposite direction, which would reduce the HV turns: both these operations maintain the flux density constant.

The operator can also use the tapchanger to boost the LV system voltage, either to compensate for regulation or because a safe margin is required, say, to start an electric boiler feed pump. The tapchanger would increase the volts/ turn and this would thus increase the flux density. The use of the on-load tap changer in this way to control the LV system voltage is discussed more fully in Section 4.6.

The station transformer will probably have been provided with a tapping range of +/-10 percent to match the possible supply voltage variation. On the limit, it is possible for a voltage which is 10 percent high to be applied to the -10 percent tapping. This is an overvoltage factor of 22 percent and would result in an increase in flux density of this amount. To avoid saturation, it is desirable that the operating flux density should never exceed about 1.9 Tesla; this results in a specified nominal flux density of 1.55 Tesla at nominal volts for all station transformers, a value considerably lower than that specified for other transformers, for example, a generator transformer as discussed above.

Unit transformers

The unit transformer is teed-off from the main connections of the generator to the generator transformer. It is energized 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 volt age, 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 percent.

• Since the HV voltage is maintained within +/- 5 percent of nominal by the action of the generator AVR, on-load tapchanging is not needed. This also enables a design flux 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 (see FIG. 8 and below). 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.


FIG. 8 Paralleling of station and unit transformers

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 a ground 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. Off circuit taps on the HV winding of _7.5 percent in six steps of 2.5 percent will probably be provided to enable fine 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 off 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 fired 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 (see Section 4.7). 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 pro duce. 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 flow 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 (FIG. 8). 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 figure of 180ºC should not be exceeded after a period of 2 minutes parallel operation.

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