Fuse Protection of Power Transformers

In my experience fuses we(a)re common up to 1MVA on DNO networks.  Ring Main Units generally had a trip bar arrangement to disconnect all three phases on the first fuse operation.  Overhead network tended to use either oil filled or drop out fuses but generally for smaller (pole mounted) transformers.

Few if any transformers fed through fuses either above 11kV  or 1MVA on GB networks.


VTs for protection / metering also tend to be fuse protected but these are small VA rating.


Hope this helps.

It is not my area of expertise, except where I happen to have come across it in the course of other works, I cannot comment on 60 MVA - that to me is a very large genset and more than a typical 11k/400 transformer, or even more than the supply end to an 11kV line feeding a few . However, in the UK the loads on the secondary of an HV LV  substation are normally limited with fuses, though there may also be breakers and  these may be triggered by a variety of things as well, and can be interlocked to kill the primary side if a secondary side fault develops.

There may be   expulsion fuses   somewhere in the primary feeders, such things are sometimes seen on the pole tops of 11kV (and maybe 33kV)  rural lines. However, at the feeder end of the HV lines protection is far more sophisticated, and will include current transformers to operate breakers with 'inverse time' characteristics, and also to trip fast on HV earth faults. On meandering rural networks there may be trips that include automatic or remote control re-connectors as well, to give a degree of restart with only the faulty branch permanently disconnected after 3 goes or whatever.

Realise it is not just small transformers that have thermal trips built in -  the  Buchholz Relay  fulfils this purpose, and turns off a transformer if the oil pressurre build perhaps if it  is boiling for any reason. In the crudest form these may just short the HV, and rely on something further back to do the disconnection, but this is old hat, and more often would be linked to a proper HV breaker.


Industrial sites with private transformers that take in HV directly, at least those that are relatively modern,  generally see something similar to that described by OMS in the middle posts in this thread from a few years ago. older thread about 11kV overload and fault systems . Even so, a long way further back is generally something simple that will melt, vapourise or perhaps just blow up, but by design,  if the clever stuff does not behave itself.

60MVA would be small genset?  Typically 60MVA is what steps 132kv down to 33kv and 11kv in many newer installations. 5-20MVA typically takes 33 down to 11kv. 600MVA for 400-132kv autos. Though they are autos so technically the core is something like half that relative to full galvanic isolation.

Bear in mind that the whole UK demand is perhaps 60 gigawatts on on a very busy day, about a kilowatt per person - given how the load is spread out around the country, there are not that many places where concentrating half a gigawatt of transformer makes much sense -  a whole power station is measure in single figures of gigawatts and many are  quite a bit less than that (list of all UK power stations)  -  by comparison a genset is a small thing wheeled out to prop up a region when the power is lost and for some reason cannot be restored quickly.

There is a tendency to call the stuff above 33kV 'transmission' rather than 'distribution' which takes place at at 33k/11k/6k or 230/400v. In some more spread out places like the Scottish Highlands, you will find 33kV coming down to 230/400 directly, but an 11kV tier is far more common.

Transformers of the kind you describe are likely at the ends  of long transmission runs, and are serious national infrastructure projects.  There are a few hundred 132 and 400kV transformers in the whole of the UK  mainly at the tap points on our   High power UK transmission lines

By comparison, the 11kV to 400V star delta distribution transformers are numbered in the tens of thousands per each DNO region.

Hi I'm not sure but I thought that at grid stations they would have 2 or more 30/60/90 MVA Txs dropping 400 Kv down to 132kv  is this right? I think the only place you would find 600MVA Txs is in a power station  one thing I know for sure the hum from these huge Txs permeates every part of the station

I can't argue with any of the above, so I'll make a subtly different point. Yes, you can provide basic overcurrent protection with fuses, although you might struggle to discriminate with protection further downstream, but for something this valuable (and with long lead times to get a replacement), I suggest that you need something more. 

a) you should fit earth fault protection

b) as others have mentioned in passing, if you're waiting for a fault to develop to the point where it'll melt a fuse, you've waited too long. therefore, you definitely want a Buchholz trip. for anything more than the commodity sub-MVA DTC transformers, it's generally worth fitting biased differential protection, which doesn't cost much in the scheme

All this leads me to having a proper CB with DC relays


hope this helps

Dave Miller FIET