Transformers

[MUTHU]

Sirs,

Seasonal greetings.

In a water supply pumping station, we propose to install 3 sets of Vertical turbine pumpsets out of which two will be effective and one will be stand bye. The prime movers are 3oo Kw(400 HP) capacity, 3.3 kv squirel cage motors. The starters will be FCMA ( Flux Compensated Magnified Amplitude) Soft Starters. For power supply we are proposing 2 Nos of 11Kv/3.3 Kv transformers of 800 KVA capacity each (one will act as effective and one stand bye.)

I want to get the following points to be clarified.

i) Is the capacity of the transformer (i.e. 800 KVA) is sufficient. ( The pumpsets will be started one after the other and in no case more than one pump will be started at the same instant. But after starting the pumps one after the other, two pumpsets will be operated continuously. Here the maximum current drawal will be at the time when the second pump is started while the firtst pump is running.)Kindly let me know whether the trasformer capacity is safe and sufficient for the above

ii) If the transformer capacity is considered to be inadequate, which of the following option is preferrable?

a) Connecting both the transformers and operating in parallel

OR

Installing transformer of higher capacity?

If so what will be the approprate capacity?

Kindly furnish reply.

 

 

With kind regards

 

 

[epsngr]

It will be nice to get people with experience to respond on this one cos I am moving to a job where i will be required to do similar things.

 

Anyway my thoughts:

 

Transformer xteristics = 800KVA, 3.3kv should supply 140A current (each).

 

Turbine load = 300KW, assume pf of 0.91 then max demand = 330KVA with max current = 58A each. Total current for both will be 116A which is well below what the transformer can supply under normal conditions. But starting current will be about 2.2 ~ 2.5 x the normal current for soft starts. So we anticipate an inirush current of 127A.

 

If turbine one is running and turbine 2 kicks in, we should see increased demand required = 127 + 58 = 185A. This is 32% higher than the transformer capacity.

 

Now what I don't know is if like motors transformers are design for such short duration increases. Again I cannot say if parallel is the answer, obviously you can resolve the challenge if you are able to parallel the xformers but there are issues around parallel transformers i.e. same impedance ratios, and secondary voltage. Like I said, i'm not very knowledgeable in this area.

 

I would actually install a higher capacity transformer (capacity > 1100KVA), I were in your place as an easy option - again thats if you have not alredy purchased xformers.

 

comments from the experts please!

[jraef]

The problem you are going to have in getting help from a worldwide forum like this is, nobody else outside of India is going to have much exposure to FCMA starters. There is only one manufacturer in the world, they are in India. I have posted my concerns for their promotion in here in the past, and one of my issues is exactly this. As far as I know, you have only one source for help on FCMA starters, and that is the one manufacturer.

 

If you had a more traditional electronic soft starter, I would estimate that your transformer was sufficient to start one motor and maybe the second after #1 was running, but it will depend on how loaded the first motor is when you do it. This is based solely on my experience. My observations are that for a solid state soft starter, your transformer kVA needs to be at least 1.5x the HP to work without causing an unacceptable voltage drop. You have 2x, so if you subtract the running load from that and it is only 200kVA, then you might get away with it. But that is completely subjective to start with, because what is an unacceptable voltage drop to one user is fine to another. What you really need to do is a study of all of the available parameters fed into a software package that does Transient Motor Starting analysis, such as SKM or ETAP. But again, the unfortunate thing is that those packages will not likely have any re-loaded data on FCMA starting; it's just too rare.

 

Good luck.

[marke]

Hello MUTHU

 

One transformer will certainly be sufficient to run two pumps.

 

In regard to the starting condition, the worst case is the situation where one pump is running and the second is started.

 

The transformer has an impedance and the pump will require a starting current.

The start current is certainly going to be several times the run current of the pump, so the transformer will be overloaded at start. This overload will be short term and should not damage the transformer. The major issue will be the voltage drop and this is a function of the transformer impedance and the start current.

If we assume that the transformer has an impedance of 5% and the start current is 400%, then the voltage drop due to starting one motor will be 20%. As one motor will already be running, the voltage drop will be 25%. This would probably be OK for most situations, but at 25% voltage drop, the pump that is running will slow a little and the current will rise. The actual effect on that pump is dependent on the motor characteristics. The torque will drop by the the voltage drop squared and in some cases, this ay be sufficient to cause that motor begin to stall and that would prove to be a problem.

 

The actual figures are dependent on the transformer impedance and the start current of the motor. You need to determine those to get a better indication of the results.

 

Best regards,