MVA calculations 1

[Guzzy89]

I am currently working on sizing primary fuses for a three wire 34.5 kV distribution system. The connection is delta(HS) - grounded wye(LS). There are four single phase transformers, two connected in parallel - each with a 667 KVA rating, the other two are not in parallel and have a KVA rating of 1500 and 1670 respectively. How do I calculate the total KVA of this bank. I assume that I add the 667 KVA transformers together. Since the total of the two tranformers in parallel have a lower KVA rating than the other two, I would simply multiply this total(parallel transformers) by 3 to get the Bank total. For example ((667*2)*3) = 4002 MVA. Can anyone confirm this or let me know where I may be able to find some good reference material on this subject matter.

 

[tinfoil]

1) Unless the ohmic impedances of these three unlike tfmrs are exactly alike (not by %Z, as these are measured against transformer nameplate kVA, usually ONAN), there will be circulating currents in the delta side that will heat your bank, regardless of load, and require derating of the bank.

2) Once item 1 is addressed, I would rate the bank as 3 * smallest unit for capacity purposes.

3) You seem to be selecting primary fusing. You will have to address the issue that the fuse should not blow during transformer inrush. The inrush currents will vary depending on transformer %Z and individual kVA rating, and you will have to decide if they will be energized with or without preloading downstream.

4) If it were me, I'd ask why we're using such a hodgepodge of units rather than acquiring a better matched set.

 

[toto2002]

How is it possible to have single phase transformers with different ratings, working in paralel on the same buss !?

Can you sketch a one line?

From the info provided it seems that the problem is much bigger than sizing the fuse.

Other than that I totaly aggree with tinfoil, particularly with Item 4.

 

[Guzzy89]

I agree that it is a hodgepodge setup, however, it is an existing system that has worked for 15-20 years, don't ask me why it was done like this. I suspect that connecting the transfomers in parallel had to do with an emergency situation and what they had at the time to work with. As I stated originally, I am simply calculating primary fuse sizes and need to find the MVA of the entire bank. There are no bigger problems. Explanation - On the High side, there is a 34.5 kV three phase bus, R,W,B. The two transformers in parallel have the H1 terminals connected together and likewise for the H2 terminals. The H1 terminals connect to the R phase, where H2 terminals connect to the B phase. For the other two transformers, one of the transformers terminals, the H1, connects to the W phase, where the H2 connects to the R phase. For the Last transformer the H1 terminal connects to the B phase where the H2 terminal connects to the W phase. On the Low side, all X1 terminals are connected to ground. This is our neutral. For the Two transformers in parallel the X2 terminals connect to the same phase, where the other two transformers, terminals X2, connect to the last two phases respectively. I realize this is long , I will try to get a one line posted soon.

 

[rbulsara]

From strictly calculating total MVA, they are three single phase systems..simply add them up.

(2*667+ 1500+1670)/1000 = ________MVA.

This is your installed MVA, whether or not loads will be balance or there will be circulating currents is a different matter..

Obviouly maximum balanced three phase load you can have is
3*2*667 KVA. (base on the smallest bank)

 

[busbar]

 
Guzzy89, are intentions to use a single set of hiside fuses to protect all described transformers? Even with four 3ø sets if fuses, there will be a degree of ‘juggling’ required. In parallel sets, circulating-current quantites are a little weird, and may seem oddly incomprehensible at first take. {Er, “…vars to nowhere.”} If you have the luxury of 5/8% OLTCs, they can be trimmed to a some extent, but the saying, “…watch those eggs like a hawk!” particularly applies.

Short-term assemblages of surplus gear often have a lifetime well beyond originally anticipated, and after a time, the best of well-meaning shoestring economic efforts ‘catch fire’ and leave you as the bad guy during any outage.

References may be: IEEE transfomer-standards collection, IEEE red book and J&P transformer book.

 

[Guzzy89]

busbar,
My intentions are to use a single set to protect all described transformers. I am going to use - ((667*2)*3) for my bank MVA total. rbulsara has confirmed this as well. As for the tap changers, this DS feeds a residential area in the middle of nowhere and is slated to be re-built in the next year or two so I will stick to simple primary fusing. POINT TAKEN - "Short-term assemblages of surplus gear often have a lifetime well beyond originally anticipated, and after a time, the best of well-meaning shoestring economic efforts ‘catch fire’ and leave you as the bad guy during any outage."
Thanks

 

[busbar]

Thank you for the followup, Guzzy. I hate to see operations people get shoehorned between ancient gear and customers that seem to magically expect high service reliabiliry. My apologies for presupposing the circumstances behind the transformer application. [No doubt IEEE C57.109 through-fault capability will be given some review.]

Sounds like it's an installation that is slated for rebuild soon. Best to you and your efforts.