Three phase transformer question

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ggunn

PE (Electrical), NABCEP certified
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When connecting a 480V inverter to a 208V service through an isolation transformer, there's no need for a neutral on the 208 side, is there? 480/277V wye to 208V delta should do the job. Separately derived on the 480 side, N to GND bond, new ground rod bonded to service GND.
 
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ggunn

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ron said:
Is the 208V loads on the secondary 3 wire or 4 wire?
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I'm not sure what you are asking. This is for connecting a 480V PV (solar) inverter to a 208V service. There are no loads on the 208V conductors between the transformer and the MDP.
 
ron

ron

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ggunn said:
I'm not sure what you are asking. This is for connecting a 480V PV (solar) inverter to a 208V service. There are no loads on the 208V conductors between the transformer and the MDP.
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I'm curious if the loads downstream of the service are 3W or 4W connected. I'm taking for granted that it is solidly grounded service and not resistance or ungrounded.

I only suspect that a delta source connected to a 4W system with 4W loads (such as L-N), make act weird. I do not have facts, just surmising.
 
Smart $

Smart $

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208V delta is preferred... and there nowhere to even connect a neutral. But if it is 208/120V wye configured on the utility side, do not connect a neutral.
 
electrofelon

electrofelon

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Smart $ said:
208V delta is preferred... and there nowhere to even connect a neutral. But if it is 208/120V wye configured on the utility side, do not connect a neutral.
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I agree. I admit I have not yet installed an sds for a pv system, but my understanding is that there is no need to deviate from a typical delta->wye sds just because pv is involved. Utilities typically use wye wye for their serving padmounts, but that is for other reasons and in my experience they don't do anything different if pv is in the picture.
 
dkidd

dkidd

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ggunn said:
When connecting a 480V inverter to a 208V service through an isolation transformer, there's no need for a neutral on the 208 side, is there? 480/277V wye to 208V delta should do the job. Separately derived on the 480 side, N to GND bond, new ground rod bonded to service GND.
Click to expand...

The 208 volt side is the SDS. A neutral would be required for 120 volt loads.
 
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GoldDigger

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dkidd said:
The 208 volt side is the SDS. A neutral would be required for 120 volt loads.
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The 208V side is the source of the magnetizing current and sets the frequency and voltage on the inverter side. For most practical purposes it is the primary. And it should not have a neutral connection even if it were a wye.
We are talking about a grid interactive inverter, not a standalone power generator.

Sent from my XT1585 using Tapatalk
 
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ggunn

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Consulting Electrical Engineer - Photovoltaic Systems
dkidd said:
The 208 volt side is the SDS and needs a grounded neutral.
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Yes, the 208 service needs a neutral for its 120V loads but a transformer that connects a 480V inverter to a 208V service does not need a neutral. You need not reference the 208 conductors to ground at the transformer because they are already so referenced at the service.

Both sides are separately derived. The 208V side is separately derived from the 480V side and vice versa. If the inverter needs a neutral, then the 480V side of the transformer is a 480/277V wye with the center grounded; hence, the neutral. There is no connection between the 480/277V neutral and the 208/120V neutral because, yes, they are separately derived from each other.
 
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dkidd

dkidd

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http://apps.geindustrial.com/publibrary/checkout/Transformer2?TNR=White Papers|Transformer2|generic


When the secondary (wye) of a delta-wye transformer is energized instead of the primary (delta), then the wye side of the transformer is not a separately derived service. As such, the neutral should not be connected to building ground nor should it be bonded to the transformer enclosure. The delta side of the transformer becomes the output, which is the separately derived system. The output delta “B” phase should be tied to ground unless the facility distribution system utilizes a different grounding scheme.
 
electrofelon

electrofelon

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dkidd said:
http://apps.geindustrial.com/publibrary/checkout/Transformer2?TNR=White Papers|Transformer2|generic


When the secondary (wye) of a delta-wye transformer is energized instead of the primary (delta), then the wye side of the transformer is not a separately derived service. As such, the neutral should not be connected to building ground nor should it be bonded to the transformer enclosure. The delta side of the transformer becomes the output, which is the separately derived system. The output delta “B” phase should be tied to ground unless the facility distribution system utilizes a different grounding scheme.
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The problem with your thinking is that you are treating the interective inverter like a utility service. It doesn't work that way. In terms of nec seperate derived systems, the inverter is just like any other widget you connect to the secondary side - it doesn't make the utility side an SDS. Check out some of the definitions of sds and service. (Granted there is a little inconsistency as, IIRC, there is not an allowace for the utility to receive energy).
 
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GoldDigger

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jaggedben said:
oldsparky52 said:
If the PoCo drops power, does the inverter automatically disconnect from the electrical service?
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Yes.
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Depends on what you mean by disconnect.
It stops trying to source power into the AC connection, which probably also involves shutting down the DC front end, at least partially.
But most if not all will continue to draw standby power from the array while monitoring the voltages on the AC terminals to see whether they can start up again.
 
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