Sizing Medium Voltage Transformer

[Saturn_Europa]

I work at the Doctors Without Borders office in Freetown Sierra Leone. The office is served by a 3 phase, 230/400 volts. There is a 50 kva back up generator.

There are low voltage and medium voltage power lines directly in front of our office. the low voltage lines start from a pad mounted transformer about 400 meters away. There are many taps on the low voltage line, some are illegal and poorly done. We only have power about 50% of the time and often run on generator for days on end.

The medium voltage power lines come directly from a turkish "power ship" that is moored in freetown harbor. the medium voltage is very consistent and reliable.

We have asked the poco to supply us with our own medium voltage transformer. they are trying to sell us a 100 kva transformer for around $30,000. The 50 kva transformer is around half as much.

On the hottest day of the year the office pulls around 33 amps across the three legs for around 23 kva. No future growth is predicted. Peak loads do not exceed 30 kva.

I have zero experience with medium voltage transformers. What is the correct size for this transformer. Is there an engineering rule of thumb to quadruple the predicted size of the transformer?

All loads are single phase. Most of the load comes from mini-split air conditioners.

 

[retirede]

There is no such rule of thumb AFAIK. Here, it is not unusual for POCO transformers to be loaded to the max, maybe more.

I think in your case the 50 kva is more than adequate.

Disclosure: Sizing MV transformers is not directly in my area of expertise. I only comment based what I’ve heard and read it, plus basic engineering facts.

 

[Saturn_Europa]

There is no such rule of thumb AFAIK. Here, it is not unusual for POCO transformers to be loaded to the max, maybe more.

I think in your case the 50 kva is more than adequate.

Disclosure: Sizing MV transformers is not directly in my area of expertise. I only comment based what I’ve heard and read it, plus basic engineering facts.

Thank you!!

 

[Julius Right]

If the transformer is 50 kVA 4%uk and if you have a 10 kW compressor starting D.O.L.[direct on line], then the voltage drop on transformer will be 8.75%.

It depends on how many times per hour the compressor starts and how sensitive the connected devices are, to disturb unpleasantly.

 

[retirede]

If the transformer is 50 kVA 4%uk and if you have a 10 kW compressor starting D.O.L.[direct on line], then the voltage drop on transformer will be 8.75%.

It depends on how many times per hour the compressor starts and how sensitive the connected devices are, to disturb unpleasantly.

He has a bunch of mini splits. I don’t think your scenario applies.

 

[bwat]

As others have said, there isn't such a rule of thumb for the type of loads it sounds like you have. The only time people do almost that (xfmr KVA = 3x motor HP) is when there is a large motor starting across the line. So what's the largest load (or simultaneous loads) that will start at one time?

It sounds like you don't have a large single load, so there's no reason to go with 100kVA.

In general, there aren't vast differences in sizing a MV vs LV transformer.

 

[Hv&Lv]

We have asked the poco to supply us with our own medium voltage transformer. they are trying to sell us a 100 kva transformer for around $30,000. The 50 kva transformer is around half as much.

Wow.
that’s ridiculously high IMO.
I’m basing my opinion on what I can buy these things for here...

 

[bwat]

Wow.
that’s ridiculously high IMO.
I’m basing my opinion on what I can buy these things for here...

I was thinking the same. Even my company, who is not a POCO, got a recent quote from a POCO for pad mount at around $14,000 for a 1,000 kVA transformer. MV dist. to 480V. That's 1/2 the cost for 10x more.

You and others probably can get that for even less.

To the OP, any chance the cost is also including them upgrading cabling?