wye to wye Step up/ step down transformer question

[joe.usip]

I am working on a 150 kw project where a wye to wye 480V primary 208V secondary transformer is needed. Today I received the unit, a 150 KVA autotransformer, only to find this on the name plate:



Did they sell me a step-up transformer? I know most of the time H refers to the higher voltage, on the name plate it is referring to H as the 208 side. It is an older refurbished model and I know sometimes manufacturers would use H to refer to the primary. Can I still use this in my application? Job details:

5 Solectria inverters ((3) 28TLs (2) 23Tls) 480Y output

to

200 amp load center

to

fused 200 amp disconnect

to

XFMR, 2 parallel output runs

to

fused 600 amp disco, 2 parallel runs

to

line side tap at 1600 amp 208Y switchgear

 

[SolarPro]

Looks like the right transformer to me. The PV system (via the inverter) is the source of the power. And this power source connects to the primary side of the transformer. In this case, the primary side of the transformer is the "low" voltage side and the secondary is the "high" voltage side.

Transformer naming/rating conventions can get confusing when dealing with a PV system. Typically, the utility is power source. Since the utility-supplied voltage is the high voltage in the system, the high voltage side of the transformer is typically also the primary side of transformer.

 

[GoldDigger]

Which is the inverter output voltage and which is the service voltage?
The primary of the transformer should ideally be on the service side to limit inrush current and provide proper tap operation.

 

[SolarPro]

Good point. I was actually misreading the application. A lot of central inverters tend to have a something like a 208 V native output, and it's not uncommon to step this up to 480 V. In this case, you need to step the 480 Vac inverter output down to 280 Vac.

My first thought is that if the inverter manufacturer's application engineers can't specify the right transformer for this application, I don't know who can. This is a super common application for them to have to deal with these days.

(It's occurred to me in the past that this application—stepping down the output of 3-phase string inverters—would make a good SolarPro article. Maybe in 2016...)

 

[GoldDigger]

OK. That nails it down. The transformer needs to be a step up transformer since it will originally be energized from the 208Y/120 side. And that side will be designated H and will have the tap points. It seems to me that you got the right transformer. Wye on the inverter side and 480V line to line.

 

[iceworm]

Which is the inverter output voltage and which is the service voltage?
The primary of the transformer should ideally be on the service side to limit inrush current and provide proper tap operation.

OK. That nails it down. The transformer needs to be a step up transformer since it will originally be energized from the 208Y/120 side. And that side will be designated H and will have the tap points. It seems to me that you got the right transformer. Wye on the inverter side and 480V line to line.

Educate me a bit. I don't know too much about autotransformers. The few I've seen don't look like they have a one winding wrapped over the other. I had thought that the LV winding was always the one closest to the wye point. The Higher voltage winding is just an extension of the LV winding. If so, then energizing the LV portion is the one closest to the core. And when energizing to the higher voltage - well that energizes both winding in series.

I'm not seeing that it matters which direction the xfm is energized - or which direction the powerflows.

ice

 

[GoldDigger]

I did not know that 30kV to 200kV transformers were usually wound as autotransformer.
In an isolation transformer one winding will usually be wound closest to the core, making the surge current asymmetrical.

 

[GoldDigger]

I did not know that 30kV to 200kV transformers were usually wound as autotransformer.
In an isolation transformer one winding will usually be wound closest to the core, making the surge current asymmetrical.

Continued..
Even in an autotransformer with that high a ratio, the HV connection energization will draw a higher percentage of its full load current than the LV connection.

 

[GoldDigger]

I did not know that 30kV to 200kV transformers were usually wound as autotransformer.
In an isolation transformer one winding will usually be wound closest to the core, making the surge current asymmetrical.

Wrong thread, sorry. But was the OP talking about an autotransformer?

 

[kwired]

Wrong thread, sorry. But was the OP talking about an autotransformer?

I hope there was a reply in the wrong thread, as OP was not talking about autotransformers, I think?

 

[iceworm]

I am working on a 150 kw project where a wye to wye 480V primary 208V secondary transformer is needed. Today I received the unit, a 150 KVA autotransformer, only to find this on the name plate: ....

I hope there was a reply in the wrong thread, as OP was not talking about autotransformers, I think?

OP says it is an autotransformer. What did I miss?

ice

 

[kwired]

OP says it is an autotransformer. What did I miss?

ice

OK, I missed that. Everything about it suggests it is a wye to wye transformer so I don't get why it is an autotransformer, other then maybe because the neutrals are tied together?

 

[iceworm]

... I don't get why it is an autotransformer, other then maybe because the neutrals are tied together?

Me neither. As well as not being an autotransforfer whiz, I'm also not a solar whiz.

GD -
I am still curious about the physics on why an autotransformer has a higher inrush (as in % FLA) on HV energization as opposed to LV energization.

 

[kwired]

Me neither. As well as not being an autotransforfer whiz, I'm also not a solar whiz.

GD -
I am still curious about the physics on why an autotransformer has a higher inrush (as in % FLA) on HV energization as opposed to LV energization.

All that differentiates an auto transformer from an isolation transformer is the primary and secondary windings are interconnected somehow on an auto transformer. Common example is a HID lighting transformer uses two input leads, but one of them is also physically connected to one of the output leads. NEC generally requires that common lead between them to be the grounded lead if it is a grounded system.

OP's transformer maybe has both primary and secondary neutral tied together? I guess that would make it an auto transformer. Most of the 480 x 208 transformers you find will have delta connection and isolation on the intended primary side. One could still have wye on both sides but to have true isolation you need to leave the neutral float on one side or the other - and preferably the primary side otherwise it brings up issues of where to ground the secondary side.

 

[iceworm]

All that differentiates an auto transformer from an isolation transformer is the primary and secondary windings are interconnected somehow on an auto transformer. Common example is a HID lighting transformer uses two input leads, but one of them is also physically connected to one of the output leads. NEC generally requires that common lead between them to be the grounded lead if it is a grounded system.

OP's transformer maybe has both primary and secondary neutral tied together? I guess that would make it an auto transformer. Most of the 480 x 208 transformers you find will have delta connection and isolation on the intended primary side. One could still have wye on both sides but to have true isolation you need to leave the neutral float on one side or the other - and preferably the primary side otherwise it brings up issues of where to ground the secondary side.

Kwired -
That does not make any sense at all to me. We are either not on the same page or I clearly don't understand - I'm not discounting either one.
(Following not including single phase buck boost)

All the autoxfm I have seen - that would be a total of three - look like the attached. All were 3ph, 480/208 used as stepdown 480/208Y - for office space in an industrial plant. Sizes were 150kva to 300kva. The 208 Y-point has to be grounded, which grounds the 480 Y-point.

Pull the covers and look inside. Here is what I think I saw:
Three leg core each with two coils.
The 208 coil is bigger wire, closes to core
The rest of the coil to make up the 480 winding, was smaller wire, wound over the 208 winding.

My current (although nebulous) understanding:
There is no separate 480V winding. When the 480 is energized, the current goes through the 208V winding series connected to the rest of the turns that make up the 480V coils.

There is only one winding (per phase). It changes wire sizes 43% down the coil.

So what am I missing?

ice

 

[kwired]

Kwired -
That does not make any sense at all to me. We are either not on the same page or I clearly don't understand - I'm not discounting either one.
(Following not including single phase buck boost)

All the autoxfm I have seen - that would be a total of three - look like the attached. All were 3ph, 480/208 used as stepdown 480/208Y - for office space in an industrial plant. Sizes were 150kva to 300kva. The 208 Y-point has to be grounded, which grounds the 480 Y-point.

Pull the covers and look inside. Here is what I think I saw:
Three leg core each with two coils.
The 208 coil is bigger wire, closes to core
The rest of the coil to make up the 480 winding, was smaller wire, wound over the 208 winding.

My current (although nebulous) understanding:
There is no separate 480V winding. When the 480 is energized, the current goes through the 208V winding series connected to the rest of the turns that make up the 480V coils.

There is only one winding (per phase). It changes wire sizes 43% down the coil.

So what am I missing?

ice

My only real experience is with single phase auto transformers, but I see how what you described can work.

Main difference from an NEC perspective is there is not a separately derived system here.

 

[iceworm]

... Main difference from an NEC perspective is there is not a separately derived system here.

Yeah, I got that - maybe.

Mostly I'm hoping GD has command of the physics on why an autotransformer has a higher inrush (as in % FLA) on HV energization as opposed to LV energization. Cause I'm lost on that.

ice

 

[joe.usip]

Good point. I was actually misreading the application. A lot of central inverters tend to have a something like a 208 V native output, and it's not uncommon to step this up to 480 V. In this case, you need to step the 480 Vac inverter output down to 280 Vac.

My first thought is that if the inverter manufacturer's application engineers can't specify the right transformer for this application, I don't know who can. This is a super common application for them to have to deal with these days.

(It's occurred to me in the past that this application—stepping down the output of 3-phase string inverters—would make a good SolarPro article. Maybe in 2016...)

Thanks for the help! I really do appreciate it, just got around to replying. My question now is grounding and bonding of this transformer. Would the same grounding and bonding rules for a delta-wye transformer apply to a wye-wye transformer?

 

[kwired]

Thanks for the help! I really do appreciate it, just got around to replying. My question now is grounding and bonding of this transformer. Would the same grounding and bonding rules for a delta-wye transformer apply to a wye-wye transformer?

Same rules apply to a single phase transformer, all that is different between each system is what point is grounded, bonding and grounding rules otherwise are the same.

 

[GoldDigger]

With a wye-wye, the line side star point can be connected. For a delta-wye with the delta on the inverter side, the line side star point should not be connected.
If a line side star point is not connected to a neutral it must also not be bonded/grounded.