400/33kV Transformer

Question

The transformers of 400/33kV, 160MVA & 125MVA rating should be avoided as practice of using such low MVA rating transformers are not desirable at 400kV level . It is desirable that Transformers of above ratings are only procured by utilities to have standard ratings across the country. Can anyone explain why this is?

mapj1 · Accepted Answer

Not totally related but if you look at the UK national grid map then 
 A) there are not that many 400kV lines, and 
 B) except in high population centres or near to power stations (London, Southampton etc) The 400kV transformer stations (blue dots) are more or less uniformly spread. 
 The spacing distance is voltage dependent. The load areas can then be estimated as similar nos of square miles around each one. The red dots - 270kv lines and transformers are closer, spacing reduced almost, but not quite, proportional to voltage, and the area served (and therefore power handling needed) changes more or less as square of voltage. 
 An 11 or 33kV line will have proportionally smaller reach again, there are far more of them than fit on a UK scale map, and they each carry less power. DNOs publish their own maps for these. 
 As a very noddy rule of thumb the line voltage is at or near the maximum economic line length in metres. 400v going 400 metres is probably becoming worth investing in transformers and stepping up for all but very small loads. 
 Very few 11kV lines are much more than 11km long, 33kV lines 30km long etc. It can always be done with oversized cables of course, but the losses and rising costs to install mean it is getting uneconomic not to consider the higher voltage. 
 Mike 
 
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mapj1 · Answer

Mainly standardization of parts to allow quick repairs and maintenance procedures, and the relative costs and reach of lines of different voltage 
 It mainly a matter of network design rather than technical. There is no point in running a 400KV line a short distance to power half a dozen light bulbs ;-) so 400KV stations are expensive and far apart, and serve very large areas. So given that large service area, small transformers make no sense. 
 There is a similar effect at smaller scale in the LV side of things. In England we usually come down to LV from 11kV rather than 33kV, and if we have 33kV it steps down to 11kV first, though in some less densely populated areas, and in Scotland, the longer reach of a 33kV line is helpful. The fault level at 33kV is higher, the switch gear is different... 
 Mike

Jam · Answer

No, and I&rsquo;m aware of proposals along those lines. There would be details to consider of course; not least as Mike says the 400kV substation cost is substantial (with timelines to boot) aand if the project is big enough to justify the cost, it&rsquo;s likely to also be pushing the practical limits of the 33kV side and some intermediate voltage level may be needed due to limits on switchgear ratings etc&hellip; it&rsquo;s often more attractive to go considerably further to an existing 33kV PoC. 
 Oh and 400kV substations are rather large compounds, which come with the associated wider impacts and planning risks.

Jam · Answer

Ps I believe, but cannot substantiate, that electrified rail supplies often come from higher voltage networks than you&rsquo;d expect from the demand in MVA because (in the UK at least) they&rsquo;re single phase and electrically noisy and they need a strong network to minimise disturbance to other users. 
 So while it mIght run at 25kV SP, the supply might be either 132kV or straight down from 400kV if the DNO can&rsquo;t take it.

400/33kV Transformer

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