Standard 33/0.4 kV Distribution Transformers

[musaUKPN]

Hi,

Our Electric utility (in IEC world - UK) is now providing customer connections at 33kV distribution voltage. Loads equal or above 5MVA threshold will be connected directly to 33kV rather traditional 11 or 6.6kV feeders.These bulk customers are metered at 33kV.

Most of the new developments include commercial (big offices/ retails or data centres) + domestic (block of flats or high rise residential towers) and as such LV supplies are required for the individual flats/ apartments. Some of these customers require total 8MVA - divided into 3.5MVA for distribution to domestic flats and bulk 4.5MVA for the commercial load.

Our standard design include 1MVA, 800kVA and 500kVA 11/0.4kV distribution transformers but we don't use any of 33/0.4kV transformers and as such there is no standard ratings available. We are interested to find few standard 33/0.4kV transformer ratings that are readily manufactured in Europe/ Uk to add them as our type tested standard equipment that customer or contractors can install on the sites that will be owned by our utility.

Can you please provide some guidance and information regarding the standard 33/0.4kV transformer ratings in the UK or Europe?

Kind regards,

musa UKPN

 

[cranky108]

In the US we do use as a common type of distribution transformer a 34.5 kV to 480 V. I know we do have stock (in our yard as spares) sizes of 2500 kVA.

So if you don't find anything there, they maybe available from the several US companies. But I don't know about the frequency differences.

I usually don't know about the distribution transformer we have, but because of a new customer, I have been trying to find out about what internal fuse these transformer have.

 

[prc]

In India we have standardised the following ratings at 33/0.433 kV
315,400,500,630,800,1000,1250,1600,2000,2500 &3000 kVA with impedance of 4 % (up to 630 kVA) 5 % ( for 800 &1000 kVA) and 6.5 % for 1.6 MVA &above.Europe also must be following the same ratings as these are as per IEC.

One trend coming all over is to reduce the number of voltage transformations in the transmission /distribution chain to get energy savings.In Malayesia they had 90 MVA 132/33 kv trf at substations to meet industrial loads and 30 MVA 132/11 kV transformer feeding distribution transformers meeting housing loads.In India we used 170 MVA 220/33 kV transformers( with over loading capability up to 190 MVA) in a big refinery to meet concentrated loads. This has become possible due to the availability of higher rated medium voltage circuit breakers.We are now planning similar rated units in another project to bring down energy directly from 400kV to 33 kV bypassing many layers of voltage step down.

 

[jkristinn]

I can confirm prc's suspicions, the ratings are indeed standardized by IEC 60076, and below 300 kVA you will find 160 kVA and 250 kVA.
You would be looking at HV side with Ur (highest voltage of equipment) = 36 kV.

For reference you could look at Efacec's online catalogue:

http://www.efacec.pt/PresentationLa...s/Catalogo oleo CEI - Mod TR 08 B 1007 A1.pdf

 

[Marmite]

Musa, the reason your standard ratings don't go above 1MVA is due to the security of supply planning standard P2/6 and the need to provide duplication of supplies above 1MVA demand. If you were supplying a block of apartments at LV from a single 3MVA transformer you would have a real headache restoring supplies in the event of a fault. Even connecting temporary generation would be problematic. If this is on the customer's side then fair enough, they take the risk, but as a distribution Company severely financially penalised for lost customer minutes I'm surprised that your Company will want to "own" this arrangement and the associated liability. What would be the incentive for going down that route?
Regards
Marmite

 

[cranky108]

We don't have a security of supply problem in the US, at least not here. But we do keep spare transformers to avoid outages of longer than a day.

I have also seen spare heated transformers in customer sites (we keep them energized to prevent water intrusion).

Some customers, data centers and chip manufacturers, and the like, prefer to pay extra for extra reliability.

Some customers have some remote sites and they accept a lower level of service to avoid the frount end charges to bring the service level up to our standards.

 

[musaUKPN]

Thanks all for the valuable comments.

@ Marmite: Thanks for your comments. We are aware of P-2/6 requirements. However, most of developers in London are reluctant to offer enough space for putting in 1MVA substations.These developers are ready to duplicate 3MVA transformers to get the required security.

I agree that 4x 1MVA substations will meet n-1 criteria for 3MVA high rise residential load. An alternative could be 2x 2/2.5MVA 33/0.4kV transformers and we are still satisfying P-2/6.

The above question to the forum has been raised to find the standard rated 33/0.4kV transformers readily available in the IEC market so that we can list a number of type approved standard 33/0.4kV transformers in our design standards (G-81). This is now necessary as ICP's and IDNO's are heavily contesting for new connection work

musa UKPN

 

[ScottyUK]

You need to look at fault levels on bigger transformers fed from further up the distribution network where fault energies are inherently high. Once you get above 2.5MVA then paralleling transformers becomes tricky because you are in danger of exceeding the capability of switchgear, and there's no provision in law to allow you to exceed the capability even though some operators do so for a short period. It's certainly an awkward position to defend as a designer or operator if something were to go wrong.


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If we learn from our mistakes I'm getting a great education!

 

[andybro]

I work for an industrial user in the UK rather than a distribution company and our standard transformer size is 1.5MVA. The main factor which determined this was the size and manageability of the LV connections rather than anything else. Is this approach common?

 

[musaUKPN]

@ Scotty: Thanks for the comments. It is usual practice at our DNO not to parallel the LV's of the two distribution transformers that feed the a particular customer's load. Under normal running conditions; the load is divided between the two transformers and bus section or LV links between the two transformers are only closed if one transformer is out/ faulty.

Here in London, we also have LV interconnected networks where a number of transformers (maximum rating is 1MVA) that are connected to different HV(11 or 6.6kV) cables of the same primary substation run in parallel. The maximum fault current allowed for these networks is 46kA @ LV.

The use of 33/0.4kV transformers would be limited to supplies which feed a single customer of a single property. Large developers are happy to pay and provide space for the duplicate the transformers to achieve higher security of supply.

However, we are still studying feasibility of this new design and any comments are welcome.

musa UKPN

 

[prc]

Can some one explain the P-2/6 criteria? Hope n-1 means, n-i units of transformers shall meet the total demand at any time.

 

[musaUKPN]

@ prc; yes your interpretation is right. P-2/6 is the security of supply criteria in UK. It gives the guidelines for the security of supply requirements of an existing or new system. Supplies are classified as maximum demands of A<=1MW; 1<B<12MW; 12<C<60MW; 60<D<300MW; 300<E<1500MW and F>1500MW. A,B,C,D,E,F are supply classes.

N-1 (a single fault) applies to all supply classes. This means that group demand of the particular supply class should be restored within specified time upon occurrence of a single fault in the network. The restoration time for class A (<=1MW) is the actual repair time.
N-2 (a single fault during an planned outage) applies to classes D and onwards.

You can find this documents from ENA website (link below:

http://www.ena-eng.org/ENA-Docs/EADocs.asp?WCI=SearchResults&DocSubjectID=109

musa UKPN