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

Hypothetically if a large wind farm had a grid connection, with the point of connection being 400kV would there be any issues using a 400/33kV transformer to connect the wind farm to the grid? 

I read on a paper published by the Indian government that a 400kV tx with low MVA rating is not desirable. Is there any technical issue or purely for maintenance and standardisation of assets?

No, and I’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’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… it’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.

Ps I believe, but cannot substantiate, that electrified rail supplies often come from higher voltage networks than you’d expect from the demand in MVA because (in the UK at least) they’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’t take it.

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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Accommodating Mike, thank you very much.

Thats really helpful, thank you. 

Thinking about your wind farm question, it all depends how far you are from the load. A wind farm miles off the coast will need a surprisingly long, and therefore high voltage line, even if its power generated is small compared to the normal MVA associated with  a line of that voltage. Something similar applies to solar farms in the desert - there may be no massive area or load served immediately surrounding the generation, as there are no houses or factories in the sea or desert (!), but the reach needed still drives you to use the higher voltages.

In which case to step up to go the distance and then to step down again once near the load, to a voltage whose reach is a better  match to the available power and service area may well be most sensible. It is not the normal 'grid planning' situation, but is probably becoming more common.

Mike.

As Mike says, in the UK at least, 400kV is the supergrid - if you like the equivalent of motorways. Whereas 33kV is more like a country lane.

Generally you don't see many country lanes connecting directly with motorways - it just adds too much complexity to the motorway system.

Likewise a 400kV system will serve a large area and so need to be very reliable - adding lots of little connections would mean adding a lot of complexity to the 400kV system - many more points of failure, much harder to disconnect for maintenance. In general it would be better to group smaller loads together (probably via some intermediate voltage) and minimise the number of connections direct to the 400kV system.

   - Andy.

400 to 33Kv is a heck of a drop I've not heard of it being done before so I guess its not a common thing. I remember being surprised when I first heard they some times drop from 132 to 11Kv  but only if the loads over 100KVa. Does anyone know when that first  started? I think Hastings 132Kv grid station has it it also used to have a power station but that's long gone.