Three Winding 132/11/11kV transformers

[imriep]

I am to propose the installation of a double 60MVA transformer primary substation with three windings i.e 132/11/11kV.
We have very little experiance of this arrangement and I am getting some reluctance from my company .
What are the problems I should look out for?

 

[patrick7]

could you please go into more detailed explanation?
thx

 

[imriep]

We have a 132/11kV double 30MVA (N+1) transformer distribution primary substation that is now overloaded. It supplies 11kV to a local town and a small amount of rural network.
The most economical solution is to replace the existing transformers with units that have two 11kV 30 MVA secondary windings and construct an additional 11kV switchboard.
Each of the transformers will supply the two 11kV bus-bars and will run with a closed bus section. There will be a single voltage regulator and earthing transformer on each transformer.
Although this arrangement may be common in other parts it is unusual for us.
So this is why I need to find the problems and hopefully the solutions.

 

[jghrist]

Some concerns:
1. Fault level - this probably double from the existing system. Is the existing switchgear and distribution equipment rated for the higher fault level?
2. Cost - Assuming that the total load will be 60 MVA, you could get N+1 with one additional 30 MVA single secondary transformer.
3. Circuit congestion - Getting twice the load out of a single substation will require double the number of distribution circuits unless you increase the load on each circuit. Increasing the load on each circuit will reduce reliability.
4. A single earthing transformer might expose you to loss of an earth source if you had to split the buses. Why not use grounded wye secondaries and avoid separate earthing transformers.

You might be better off with a second substation. This will allow shortening the distance to loads and reduction of distribution system losses. With some automated distribution switching, you might be able to keep your N+1 criteria with a second substation that has only one 30 MVA transformer.

 

[powerjunx]

I agree to jghrist.

BUt i think you better refer further to the Standard Installation Procedure.

 

[imriep]

Sorry I also missed out the fact that we are limited to space available within the existing substation without the prospect of getting additional land as it is "land locked".
We would not get an additional transformer into the space. We will just about get the additional 11kV switchouse building. Our N+1 opperate in parrallel as a closes 11kV bus-bar so an additional transformer of the same reactance as the existing two would exceed the fault level capability of the 11kV circuit breakers.
Simple isn't it.
By the way this in the UK!

 

[stevenal]

Why 2 30MVA windings rather than a single 60MVA?

 

[ScottyUK]

I think I see where this is going. Is this what you are describing?

[ol]
[li]You have an existing dual feed 11kV board with a bus coupler between the two half boards.[/li]
[li]Each half board is fed from its own 132/11kV transformer.[/li]
[li]Each existing transformer is large enough to carry the full load in the event of an outage on the other transformer.[/li]
[li]You propose to install a pair of new transformers to replace the existing transformers.[/li]
[li]Each new transformer will have 2x 30MVA 11kV secondaries.[/li]
[li]Each half board in the existing substation will be fed from one winding of the new transformers, giving redundancy.[/li]
[li]A new substation similar in configuration to the existing one will be constructed.[/li]
[li]The two half boards in the new substation will be fed from the remaining winding on each transformer.[/li]
[li]You are proposing this because you have inadequate space for a pair of new transformers to feed the new substation.[/li]
[/ol]

If any of the above is incorrect, please help us understand what you are proposing.

----------------------------------

I don't suffer from insanity. I enjoy it...

 

[imriep]

Yes, I think what you have described is what i am proposing.
I am finding some reluctance for this arrangement but I am unable to see any major issues that can't be resolved.
Anyone else know of any problems?

 

[jghrist]

I had misunderstood the setup. If the two dual feed switchboards are not connected, then this will not increase the fault level from the existing arrangement. I was involved (engineering construction support) for a similar substation in Saudi Arabia in 1982. We had two 2x50 MVA transformers (115-13.8 kV) connected to two dual switchboards. The only problem was getting all of the circuits out of the station (32 circuits if I remember correctly). We had a basement under the switchgear with the circuits running in tray to ductbank entrances. It was very congested, but it worked. Two smaller substations would have been better.

 

[prc]

Proposed three winding transformers are possible and are regularly used in many countries -mainly to limit the SC level to suit the secondary breaker( station start up transformers in thermal power stations) or to take power from two generators using single generator transformer.
Care must be taken in specifying the impedances between windings as manufacturer has many options which will not suit your service requirements.In your case, the following % impedances may be ideal.H-L1 = H-L2 = 10%on 30 MVA base
L1-L2=20 %on 30 MVA base H-L1+L2=11.5 %on 60 MVA.Then buy the transformer from an experienced source as poorly designed LV windings are liable for failure from Sc forces in service.In India such transformers are in service up to 130 MVA and voltage level 400 kV.