Maximum transformer rating/circuit breakers in parallel configuration

[Adam1980]

Dear all,

all manufacturers has a certain upper limit or maximum rating for the circuit breakers that they manufacturem, i.e. feeder rating. For example some manufacturer would have for 33 kV rated breakers a maximum current rating of 2500 A. In other words if I am to serve a load from a transfomrer with 33 kV at the LV side the maximum rating of the transfomer should be sqrt(3) x 2500 x 33 = 145 MVA.

If i am to use a larger transfomer lets say 200 MVA i would need to install two breakers in parallel since there is no one breaker at the 33 kV level that can take around 3500 A. In this case i came to know that this configuration is problametic and it is recommended to avoid due to possible issues with synchronized openning and so on.

I would like to benefit form your experience concerning this topic and whether this is, i.e. parallerl breakers, an industry normal practice and what are the possible problems and limitations of such a configuration.

Thank you.
Adam

 

[m3ntosan]

200 MVA is a big beast for a distribution transformer, most of them are 100 MVA 400/33 kV, I've seen a few 150 MVA with 2500A breakers. Never heard of paralleling breakers, is it possible to get a transformer with two LV windings 200/100/100 MVA to get around the issue? Alternatively you should be able to get a very expensive generator breaker to do the job

May you grow up to be righteous, may you grow up to be true...

 

[Adam1980]

Hello thanks for the reply.
actually it isnt for distribution but kind of generation. These configuratinos are mainly used in offshore wind farms as i believe using many transformers would take more space than having a big transformers. Splitting the secondary winding to two is also an options which is used sometims. But i saw a lot of configuratinos with parallel breakers at the secondary and was wondering what is the operative limitations or problems with that.

 

[m3ntosan]

Hi

I got a bit confused with your example above and I thought you're talking about distribution, feeding demand.

I looked at two offshore windfarms just now and one of them has 240 MVA transformer offshore 132/33 kV with 2 breakers in parallel, Siemens 8DA10 2500 A and the other one 150/33 kV 180 MVA transformer with two 2500 A breakers in parallel.

With the restriction being the current continuous rating, 1 breaker can cope with the fault break easily (most of the 33 kV breakers are rated for 31.5 kA) on a platform behind a big transformer and next to nothing fault infeed from the mills. Why do you think you would need synchronised opening?

May you grow up to be righteous, may you grow up to be true...

 

[collies99]

Yes they do make very large generator breakers up to 16ka continuous rating.
BC Hydro have 460 mva, 16 kv generators.

ABB can parallel some of their breakers to operate in parallel so you can get about 6ka continuous.

Three winding xformer sounds optimum for 2 collector rings as suggested.

 

[veritas]

Be aware of ANY differences in impedance between the two breakers in parallel. Contrary to the belief that the current will split 50/50 I have seen a case of two identical short cables carrying vastly different currents. Difference was due to a very small difference in impedance between them.

For example, suppose impedance of breaker path between the points of paralleling is 0.1ohms. A small resistance increase at a clamp in one of the circuits of say 0.1ohm will have one circuit seeing 66% of the total current, with the balance in the other.

 

[Adam1980]

so if i am not considering using a generator breaker, the parallel 2 x 2500 A breaker configuration is valid and shouldnt cause any operational issues with all switching operations? Energization, fault clearance, load rejection,...
Thanks!

 

[collies99]

Juat remember to perform timing tests with all 6 poles timed simultaneously. There should be less than a few milli-second difference between poles for both closing and tripping.