Is a neutral required on a transformer secondary?

[SunFish]

I am working on a project that is providing building level battery backup. The battery backup system utilizes a 480 V delta inverter that operates without a neutral. I was instructed by the manufacturer to provide a neutral forming transformer on the output of the inverter so the battery backup system can provide power to 277 L-N loads during backup mode. So I am currently planning for a 480 Delta (Inverter side) to 480/277Y (grid side) transformer.

My question, if there is a ground fault on the inverter side of the transformer, is this safe? How will a ground fault clear if there is no neutral for a N-G bond?

 

[ggunn]

I am working on a project that is providing building level battery backup. The battery backup system utilizes a 480 V delta inverter that operates without a neutral. I was instructed by the manufacturer to provide a neutral forming transformer on the output of the inverter so the battery backup system can provide power to 277 L-N loads during backup mode. So I am currently planning for a 480 Delta (Inverter side) to 480/277Y (grid side) transformer.

My question, if there is a ground fault on the inverter side of the transformer, is this safe? How will a ground fault clear if there is no neutral for a N-G bond?

What 480V inverter are you using that will run in off grid mode? What batteries are you using?

 

[synchro]

To make a neutral forming transformer, you would connect the line terminals on the wye side of a delta-wye transformer to the inverters and not connect anything to the delta side. The voltage rating of the delta side is not important because it's not connected.
The neutral on the wye side would be connected to the neutral of the system, which should already have a N-G bond.

Even when a net neutral load current is drawn from the system in backup mode, the delta windings will keep the L-N voltages balanced on the wye side. A current will circulate within the internal delta windings in order to make this happen, but that detail is not important to know for installation purposes.

 

[SunFish]

What 480V inverter are you using that will run in off grid mode? What batteries are you using?

Inverter is a Delta PCS 125 with Blue Ion 128 kWh LXHV. Support at Blue Ion recommended either adding a zig zag transformer to the delta side or an isolation resistance based GF relay on the delta side wired to a contactor that opens the wye side of the transformer when a GF condition is sensed.

 

[SunFish]

To make a neutral forming transformer, you would connect the line terminals on the wye side of a delta-wye transformer to the inverters and not connect anything to the delta side. The voltage rating of the delta side is not important because it's not connected.
The neutral on the wye side would be connected to the neutral of the system, which should already have a N-G bond.

Even when a net neutral load current is drawn from the system in backup mode, the delta windings will keep the L-N voltages balanced on the wye side. A current will circulate within the internal delta windings in order to make this happen, but that detail is not important to know for installation purposes.

Thanks, not sure I completely follow what you're saying yet... This is the diagram I was provided by the battery & inverter supplier showing the battery inverter connected to the delta side and the grid/loads on the wye side of the xfmr.



Are you saying we should connect the inverter L1/L2/L3 output to the wye side of the xfmr and also connect the grid/loads L1/L2/L3 + N to the same wye side of the xfmr? So nothing is connected to the delta side at all?

OR do we connect the grid/loads L1/L2/L3 to the delta side, connect the inverter output L1/L2/L3 to the wye side, and then connect the grid/loads neutral to the wye side?

 

[winnie]

Do you need isolation between the inverter and the loads?

The delta:wye transformer you described in post 1 provides a neutral on the isolated wye side, but as you note it doesn't provide a neutral on the delta side. The delta:wye transformer, connected as an isolation transformer, is suitable if the supply is already grounded or is intentionally ungrounded.

If the company is suggesting a 'neutral forming transformer', then this sounds like a non-isolating transformer which derives a neutral connected to its supply. An example of such a transformer is a 'grounding transformer' used to derive a neutral and ground a previously ungrounded delta system.

A big benefit of using such a transformer is that it need only be sized for the expected neutral loading, not for the full system load. A big potential disadvantage is that it grounds its supply; this is only a problem if the supply is already grounded or you want to keep it ungrounded.

There are two common types of neutral forming transformer: the zig-zag transformer which has six coils (2 coils per phase) arranged in a sort of bent leg wye arrangement with 2 phases on each transformer core leg. This zig-zag arrangement is needed to derive a low impedance neutral; an ordinary wye coil arrangement creates a high impedance neutral with poor voltage stability.

The second type of neutral forming transformer is as synchro describes: a common delta:wye transformer with nothing connected on the delta side. The source phases are connected to the wye line terminals and continue on to the load. The neutral of the wye provides the derived neutral. Circulating currents on the delta provide the necessary balance to give a low impedance neutral.

The datasheets for the battery and inverter make no mention of grounding. A battery can be grounded or floating; similarly the inverter might already be grounded or floating. The inverter datasheet states that a Dyn transformer is needed for 3P4W use, but doesn't specify if this should be an isolation transformer or used as a grounding transformer.

-Jon

 

[winnie]

Are you saying we should connect the inverter L1/L2/L3 output to the wye side of the xfmr and also connect the grid/loads L1/L2/L3 + N to the same wye side of the xfmr? So nothing is connected to the delta side at all?

This is what syncro is describing if the transformer is used to derive a neutral that is present both to the facility loads _and_ to the inverter. But based on the one line the manufacturer sent, I don't think this is what you want to do.

OR do we connect the grid/loads L1/L2/L3 to the delta side, connect the inverter output L1/L2/L3 to the wye side, and then connect the grid/loads neutral to the wye side?

This is what the diagram from the manufacturer says to do.

I assume that the battery system or inverter is providing the necessary protection for that part of the system, and that the isolation transformer is most appropriate. What you really don't want to do is connect a non-isolated neutral forming transformer to a system that already has its own grounding.

-Jon

 

[synchro]

Inverter is a Delta PCS 125 with Blue Ion 128 kWh LXHV. Support at Blue Ion recommended either adding a zig zag transformer to the delta side or an isolation resistance based GF relay on the delta side wired to a contactor that opens the wye side of the transformer when a GF condition is sensed.

Thanks, not sure I completely follow what you're saying yet... This is the diagram I was provided by the battery & inverter supplier showing the battery inverter connected to the delta side and the grid/loads on the wye side of the xfmr. ....

There appears to be some possibly conflicting, or at least confusing information here. As Jon mentioned, Delta Electronics includes the following instructions for the PCS125:
"If a transformer is required to be added between PCS and load, D type of transformer must be on PCS side."
Assuming this means delta transformer windings should be connected to the PCS125, the following from Jon is a reasonable justification for this requirement:

I assume that the battery system or inverter is providing the necessary protection for that part of the system, and that the isolation transformer is most appropriate. What you really don't want to do is connect a non-isolated neutral forming transformer to a system that already has its own grounding.

But if such isolation is needed, the recommendation by Blue Ion to add a zig-zag grounding transformer on the delta side would make no sense. I think consulting Delta Electronics directly about connecting their inverter could be wise.


If as Jon mentioned the battery/inverter system already provides sufficient protection, then the one-line looks good as long as the neutral from the generator and grid are disconnected when not providing power. That's because the delta-wye will be acting as a grounding transformer on the AC combiner bus.

By the way, Delta Electronics has a version PCS125HV that supplies 4-wire loads without a transformer. However, perhaps you will need a delta-wye anyway to act as a grounding transformer for the PV inverter output when in off grid mode. The PV connection in the 1-line says 4-wire, but it's likely the neutral is just used as a voltage reference.

 

[SunFish]

But if such isolation is needed, the recommendation by Blue Ion to add a zig-zag grounding transformer on the delta side would make no sense. I think consulting Delta Electronics directly about connecting their inverter could be wise.
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Thanks! The recommendation for the zig-zag transformer on the delta size is actually a recommendation from Delta Electronics that came through Blue Ion. I'll reach out to Delta directly to see if they have any additional information.