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RECTIFIER TRANSFORMERS
The requirement for DC power supplies is nowadays relatively uncommon.
The advent of inexpensive and rugged thyristor drives has meant that AC three-phase motors can now be used for traction as well as for all types of hoists, winders, rolling mills and the like which might hitherto have relied on DC derived Ward Leonard supplies. Telephone exchanges now operate with solid-state digital controls drawing currents of only a few amps so that large batteries of Planté cells are no longer necessary. The exceptions are certain process plants, aluminum smelters, electrolytic gas production plants, electro plating plants and uninterruptable power supplies (UPS) systems and there are still some locations, such as power stations, both fossil fuelled and nuclear, where the provision of a large battery represents the ultimate guarantee of sup ply security for essential plant such as turbine barring gear, lubricating and jacking oil supplies, or post-trip reactor cooling systems. Most power stations will also have batteries for control and operation of HV switchgear.
For large plants such as aluminum smelters, rectifier transformer ratings may be as large as 20-60 MVA, taking a supply at 33 kV and providing a low-voltage output to the rectifiers at between 500 and 1000 V. LV currents may therefore be as high as 10-30 kA and the LV conductors will need to have a substantial cross-section. This usually means that in order to bring out the large cross-section LV leads, the LV winding must be made the outer winding rather than occupying its usual position next to the core and it will probably consist of a number of parallel disc-wound sections arranged axially with their ends connected directly to vertical copper busbar risers.
For power station battery supplies, the rectifier transformers will be very much smaller and of more conventional construction with the LV winding next to the core. They will probably be supplied at 3.3 kV or 415 V with output voltages of 220 or 110 V.
Regardless of their rating, the feature which singles out rectifier transformers for special attention is the problem of harmonic currents created by the thyristor rectifiers and fed back into the supply system. The problem has two aspects: one, the additional heating which these produce within the transformer and the other the waveform distortion which is created on the supply network.
In the case of the first of these, the important requirement is that they should be taken into consideration when designing the cooling for the transformer and also when carrying out any temperature rise test. Ideally the temperature rise test should be carried out with the transformer coupled to its rectifier, although this might not be practicable in the case of the larger rectifier transformers.
IEC 60136 Semiconductor converters, Part 1-3 Transformers and reactors deals specifically with the issue of harmonics and the implications of these in relation to works testing.
FIG. 41 'Six-phase' rectifier transformer and rectifier transformer bank
of delta/star and interstar/star transformers
FIG. 42 Core and windings for 5590 kW, 130 V DC, 43 kA rectifier transformer.
This unit is connected with 2 x 6 phase double star valve windings and has
a center yoke (Areva T&D)
It is the second aspect, however, which can be the most serious, particularly for the very large rectifier loads, and especially if the loads from a number of parallel rectifiers are all drawing harmonics in phase with each other. Many rectifier transformers employ a 'six-phase' delta/star/star connection arrangement as shown in FIG. 41(a) and this of itself helps to reduce harmonic distortion by elimination of even harmonics. However, an improved arrangement can be obtained by doubling the number of supply transformers and providing half of these with an interconnected star secondary winding (FIG. 41(b)). This has the object of displacing half the rectifier load, and its associated harmonic cur rents, by 30º so as to reduce the resultant magnitude of any given harmonic cur rent drawn from the supply. FIG. 42 shows a large rectifier transformer with two sets of star-connected secondaries installed on a common core to provide double six-phase outputs. Although control of the harmonics generated by most medium sized rectifiers is unlikely to represent a major problem for the system, the problem is nevertheless an increasing one due to the very large growth in the use of thyristor drives and there is considerable literature on the subject, for example, Energy Networks Association Engineering Recommendation G5/4 [3] and in the technical press [4].
In addition to the heating produced by harmonics, certain connection arrangements of rectifiers fed from polyphase transformers can lead to the transformers being subjected to a DC component of current in their secondary windings. This is another reason why, if possible, any temperature rise test should be carried out in conjunction with the associated rectifier.
Many smaller rectifier transformers operating from 3.3 kV and below are of the dry type, class C, variety so that they can be installed indoors in a cubicle adjacent to the rectifier. Smaller units may well be installed within the same cubicle as the thyristor equipment, in which case, in order to avoid the generation of too much heat within the cubicle, a transformer of the lower temperature rise insulation class F could be used with its temperature rise limits specified as for class E materials.