Dual position transformer

[ogbuehi]

I am working on a project to expand the electrical distribution from a power plant to add a feeder for some loads. I went to the existing electrical distribution to determine configurations and voltages. What I observed on the nameplate of the transformer was something I haven't come across before. It was labeled as a dual position transformer. According to what I can discern from the dataplate, it can be "switched" to receive 2400V or 7200V delta high side OR 4160V/2400V or 12470V/7200V wye high side. No matter the configuration on the high side, the transformer is designed to output 480V/277V wye.

The power plant feeding all these transformers is outputting 415V/240V wye stepped up to what appears to be 11kV delta (I tried talking with the power plant contractors but none of them were engineers nor could they speak very good english, so all I could rely on was observation of the metering and a 1-line). All loads at this location are designed to receive standard european voltage of 415V/240V.

When I measured the output of these "dual position" transformers, I read a voltage of 380V L-L. What I am trying to figure out is how the above transformer is outputting that voltage while receiving 11kV delta?

The transformers are 2MVA but they aren't heavily loaded (probably 30% or less). They've been operating like this in very high temps (110 degrees F average daytime WBGT for 8-9 months out of the year) for 10+ years with no failures. None of the maintainers of these transformers has ever thought to measure the voltage received at the high side of the transformer. I've requested they do that to confirm what voltage is present on the high side of the transformer. The feeder lengths (2km - 4km at best) for the 11kV from the power plant shouldn't be causing anything more than a negligible voltage drop (cables are 300mm^2/600MCM).

Some theories I was tossing around:
1) I know for sure that the power plant operates at 50Hz but the transformers are 60Hz rated. I know that this can affect impedance and operating temps but I'm not sure how it would affect the voltage.

2) Maybe the transformer was being fed a "wye" voltage but the neutral wasn't being connected to the transformer. But I'm not sure what happens if you don't connect the neutral to the high side of a wye-delta transformer.

Any help is greatly appreciated.

 

[LionelHutz]

To convert a 60Hz transformer to 60Hz you use a 5/6th ratio on the voltage and power rating.

It might be 11kV / 12.47kV * 480VAC = 423VAC then operating on a tap to reduce the output voltage 10% or it might really be operating on 10kV.

 

[ogbuehi]

That would mean that the transformer is expecting a wye voltage on the high side. But there are only 3 cables on the input side of the transformer. Does that mean that not connecting the neutral to the high/wye side of the transformer has no affect?

 

[prc]

This is a transformer which can take primary voltages of 2.4,7.2,4.16,12.47 KV (L-L voltage).From the rating plate and change over switch position,find out the voltage ratio in which it is in. Then check the tap changer position. It looks like connection is at 12.47 KV,tap changer is at +10 % voltage tap.Transformer is given only L_L voltages, star or delta.

 

[tinfoil]

A 60 Hz-designed transformer operated at 50 Hz will require 60/50 or 120% flux to transform the same energy, so you have to 'de-rate' the unit accordingly to avoid overheating.

If you connect 11000V delta to the primary of a xmfr expecting 12500 L-L ('floating Wye'), then you can expect to get 11000/12500 or 88% of the expected output voltage.

So 11000/12500 * 480 = 422V L-L on the secondary side.

You measured 380V, so perhaps the supply voltage is lower than the nominal 11000V, or there are off-load tapchanger adjustments being done here to drop the voltage more.

There's nothing to prevent the use of a Wye-wye transformer with the primary star point floating, but I am afraid to ask what your protection arrangements must be like.

 

[ogbuehi]

I was afraid to ask as well. The load has protection but based on what I'm seeing, that means the cables from the high side of the XFMR back to the switchgear at the power plant has none. I'm having the contractor get the configuration of the transformer. We can't measure the voltage at the transformer because the connections are sealed boots.