Core and Shell units in the same bank

[sreiter]

Our company has an existing bank of (4) 1-phase shell-form transformers, 500kV - 345kV - 34.5kV, 120 MVA (OA), manufactured by Westinghouse in the 1970's. We need to replace two of the four units. We are reviewing bid proposals, and the current low-evaluted bidder proposes to supply core-form transformers with the same tap voltages, capacities, impedances, etc as the existing shell-form. Do you have any experience mixing shell and core form units in the same bank? Is there any reason this is not advisable? Any help will be greatly appreciated. Thank You, in advance....

 

[electricpete]

What do you mean by bank of 4? It should be be 3 single-phase transformers to make up a 3-phase transformer. Is there some parallel connection?

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[sreiter]

Sorry for the confusion....one of the transformers is a spare unit that can be connected to replace any of the other three. As such, it is nomally out of service. We expect the two new units will be placed in-service, along with one of the existing units. The other "good" existing unit will become the spare. So, the in-service bank can be either two shell-form and one core-form, or two core-form and one shell-form, if we purchase the new units as core-form.

 

[electricpete]

I have a hard time remembering what exactly is the difference between shell and core form for single-phase transformer.

At any rate it would seem that the externally measurable parameters are all that matter. Same voltage ratings, (ratios and taps(), same MVA ratings. The only other thing I can think that might be important is similar impedance.

I'm remembering there are some zero-sequence difference between shell and core form 3-phase transformers but not an issue for your single-phase transfomrers connected as three phase.

I may be way off base though. I hope you get some other comments.

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[mc5w]

Both the magnitude and X/R ratio ( resistance to capacitance ratio ) need to be identical. You also need to match magnetizing current magnitude and power factor just about exactly. If the transformer efficiencies are radically different you could have some problems. The idea is to not introduce negative sequence impedance, magnetizing current, and so forth. If you only have 1 delta tertiary winding some differences in transformer characteristic will circulate in the delta tertiary, but for this power level you do not want to introduce unequal phase shifts. I am assumiing that these are the usual wye wye delta transmission type transformers but I do not see how you are getting a 34.5 KV tertiary winding.

The newer transformers might have enough in the way of better efficiency that replacing 3 of the units might actually save you some money. You would then use 1 old unit as the in place spare and use the other 2 elsewhere.

I have seem some 3 wire wye primary 4 wire delta secondary distribution banks where 2 of the transformers were say 1/2 the size of the middle transformers. However, 120/240 volt 4-wire delta systems are not intended to be fully balanced systems from a load or source impedance standpoint.

 

[RRaghunath]

I think the effect of going in for core form transformer as a replacement for existing Shell form transformer would lie in the dimensions and the civil foundation requirements (and nothing electrical really).

Core form transformer is supposed to be taller and the other one broader. You may like to check out the GA. Afterall, for any replacenent, the identicality in dimensions is important, more so, when you want to mix and match the two types of single phase transformer units.

raghunath_n00@rediffmail.com

 

[mc5w]

But newer transformers are about 1/2 the size of what they were 30 and 40 years ago, partly due to better materials. Wound core transformers also utilize transformer steel better and have lower no load loss than the E&I lamination stacks as I stated in another post in this forum.

 

[dpc]

I agree with Pete, as long as the rating and impedance are equal, the differences between shell and core form will not be an issue for normal operation.

Physical dimension differences are likely no matter what you buy. They will obviously have to fit in the space available, but I'm sure you have assured yourself of that.