pole mounted transformer identification -3 phase, 2 phase???

[Gavin A]

Can anyone assist with identifying what this transformer is?

The customer has a 3 phase supply cable run up their drive, but apparently the conductors have bee paired up and is now only single phase supply - they think the transformer was swapped a few years ago after a major fault, and that things changed then.

They thought the transformer was single phase as well, as it only has the 2 HV cables coming in to it, however it has 3 x LV connections coming out of the transformer on the back, which is making me think it could well be a split phase supply from the transformer, with 2 phase plus neutral on the LV side, and the neutral on HV side being sent back via the L1 / L2 conductors as in delta formation 3 phase, or similar. I suppose they could be live, neutral and earth on single phase, but I'd have thought they'd have paired up neutral and earth before the transformer, and usually they'd have spiked the earth connection at the pole IME, and no sign of that.

Obviously I'll be checking with the DNO, but just wanted a quick sanity check on this as it make a lot of difference as to what pv capacity is likely to be feasible

ps the transformer is supplying 2 farms on either side of the road, so I'm thinking they might have just split it to one phase per farm when the transformer was renewed.

answers on a postcard to the usual address please..... ta

 

[Marvo]

Difficult to say without seeing how the 3 LV secondary wires are connected below the photo. Is there a local rod at the pole? Do you have a picture of the connections a meter below the transformer?

 

[Gavin A]

It was in a field I didn't have access to unfortunately, not sure if there's enough detail in this one

 

[Gavin A]

I couldn't see any evidence of an earth cable running down to a rod, but was looking from a good 10-15m away and could only see 2/3 of the pole, so could have been something on the other side.

Stupidly I didn't get chance to look at the meter / check the earthing arrangements in the farm either, as they distracted me with a long conversation about heat pumps during which I worked out they were about to make a massively expensive mistake because the UFH company had used a design flow temp of 55 degrees for the UFH... then they had to go out.

 

[Marvo]

I'd suggest without further info you'd be better asking the DNO what type of 11kV transformer is installed. It could be split phase and it could be 3 phase but it's impossible to know for sure.

 

[Gavin A]

I'd suggest without further info you'd be better asking the DNO what type of 11kV transformer is installed. It could be split phase and it could be 3 phase but it's impossible to know for sure.

but is it possible to say that it's not single phase? (or unlikely to be)

as that's my main query, as the customer was under the impression the transformer itself was single phase, but that didn't seem right to me when there are the 3 outputs from it on the LV side.

 

[Gavin A]

I will be asking the DNO, but that's going to take 2 weeks, and I'm trying to work out how much time to put in to design options for larger arrays in the meantime.

 

[Marvo]

It could be a 180 degree split phase output with a centre tapped neutral and two phases.

 

[BruceB]

I think Marvo's suggestion of split phase is the most likely from what I can see in the photos. I hope there are no other more standard transformers locally as the earth reference and currents will be unusual if that is a centre tapped neutral referenced to earth. I am no expert in identifying them though. There are some well informed people in the IET forum if you post there.

 

[Gavin A]

It could be a 180 degree split phase output with a centre tapped neutral and two phases.

ah right, that makes more sense, knew I'd seen 2 phase LV supplies from 2 wire HV systems, I'd just misunderstood how it was done.

Think that sounds about right then, and would make some sense as the transformer is feeding only 2 properties.

 

[Gavin A]

I think Marvo's suggestion of split phase is the most likely from what I can see in the photos. I hope there are no other more standard transformers locally as the earth reference and currents will be unusual if that is a centre tapped neutral referenced to earth. I am no expert in identifying them though. There are some well informed people in the IET forum if you post there.

nowt for a few hundred meters.

 

[Gavin A]

it was indeed split phase, thanks for the assistance folk.

for reference they would allow 6kW export on single phase or 15kW per phase / 30kW total if distributed across both phases if we brought the split phase up to the site.

Unfortunately the existing 3 phase supply cable obviously isn't rated for split phase voltage difference, so it'd be a new cable which is too expensive, so spending the extra on batteries etc instead for self consumption on site, but it shows what a difference a full balanced output makes to the transformer's capacity to handle export.

 

[Marvo]

Thanks for the follow up, it's always nice to know the outcome. The higher phase to phase voltage does present design challenges on many systems. I know in my country they're phasing this system out...no pun intended .

 

[BruceB]

Gavin, Why can't the existing 3-phase supply cable take split phase voltage difference? For example Batt's SWA is rated at 600/1000V and ordinary PVC twin+E or triple+E is rated at 300/500V.

 

[Gavin A]

Gavin, Why can't the existing 3-phase supply cable take split phase voltage difference? For example Batt's SWA is rated at 600/1000V and ordinary PVC twin+E or triple+E is rated at 300/500V.

because the DNO says it can't.

If it were running at say 253V L-N that'd be 506V between the 2 phases L1-L2, which I'm presuming must be higher than the rating for the cable, but I am only guessing on that really.