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Neutral required on 480 VAC system

Neutral required on 480 VAC system

(OP)
Hi all,

We are currently installing a 1 MW Battery Energy Storage System which the inverters consists of 3 phase+ Neutral 480 VAC. This system is to be connected on a 3 phase 480 VAC system which doesn't use Neutral. The supplier thought that the Neutral of their system could be connected to the ground of the power plant. However this is not a normal practice. The main Transformer of the switchgear at the power plant us a WYE system where the neutral is connected through a High resistance grounding equipment and afterwards to earth. Could this problem be resolved with an Delta- WYE isolation transformer? Attached is a sketch of the system mentioned above.



In advance many thanks for your cooperation and support.

Jairo

RE: Neutral required on 480 VAC system

Two options seem to present themselves:
1. The inverter manufacturer provides instructions how to connect the inverter without a connection to the neutral. You then follow those instructions.
2. You provide a suitable isolation transformer, could be delta-wye or wye-delta-wye, that provides you with a solidly grounded neutral.

You can't connect an identified neutral terminal to an impedance grounded neutral.

RE: Neutral required on 480 VAC system

(OP)
Hi davidbeach,

Thanks for the prompt reply. The inverter provider already indicated that the neutral needs to be connected and is used as voltage reference. However, they stated to work on another solution awaiting their response.
Would a zig-zag type transformer or an earthing transformer be a reasonable solution? I believe this will require additional disconnect equipment, cabling, etc. leading to a probable project delay.

Kind regards,

Jairo

RE: Neutral required on 480 VAC system

Earthing transformer is a possibility but fraught with problems. If you now solidly ground that existing 480V system (might as well remove the resistor instead) what happens? I certainly can't say. Isolation transformer allows the existing stuff to remain is it presently is.

RE: Neutral required on 480 VAC system

You can probably connect the inverter neutral directly to the neutral of the main transformer.
That will give you a valid voltage reference and preserve the high resistance grounding scheme.
This highlights the difference between a ground connection and a neutral connection. The terms are often used almost interchangeably.
In this case, there is a high resistance between the neutral terminal and the ground grid. It makes a difference.
A possible advantage of this solution is that ground faults fed by the inverter will also be limited by the NGR.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Neutral required on 480 VAC system

(OP)
Thanks all for the info. Connecting the inverter neutral directly to the transforme WYE neutral might work. However, this might require connection to the transformer to be modified since it consists completely of a bus bar system.

Lets say that an isolation transformer is a solution. What should be considered to size this Transformer?

RE: Neutral required on 480 VAC system

Wouldn't the inverter then experience L-G voltage during ground faults on the neutral? How would that affect the inverter?

RE: Neutral required on 480 VAC system

With a connection to the transformer neutral, the inverter will see line to neutral voltage.
If there is a ground fault on the neutral, the inverter will still see line to neutral voltage.
The inverter wants to see line to neutral voltage, that is the issue.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Neutral required on 480 VAC system

Are you convinced 100% that the inverter requires a neutral? Just asking, I have installed many inverters on large flow-batteries and have never required a neutral. Mind you I spec'd the inverters (to fit our power systems).

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: Neutral required on 480 VAC system

(OP)
Hi GroovyGuy.

Thanks for your information. Indeed, the inverter/ BESS supplier was well aware of the electrical conditions in the power plant where 3 phase 480 VAC is not used with a neutral connection anywhere. Still they assumed that there’s a neutral connection in the system that could be used.
Still we remain with a challenge since the supplier stated that they require a neutral connection ("as voltage reference"). We are currently stressing out that the supplier should provide the main purpose of the neutral and if it could still work without a neutral. As I can understand the inverter supplier doesn't want to share much information about their "proprietary" equipment.

However, we as a client still want to recommend a probable solution to the matter. Any information/ ideas is highly appreciated.

Regards,

Jairo

RE: Neutral required on 480 VAC system

Hi Jairo.
I think you have 3 options, none of which will be easy to swallow;
1) Convert your 480V 3P3W system into a 3P4W system, or
2) Install a isolation xfmr rated 480V-480/277V, one per inverter, or
3) Replace you inverter OEM with one that can read (& comply) with specs.
Obviously there are pros & cons with each of the above solutions.
Solution 2 is likely the best O/A solution. Any chance y0u can convince the inverter OEM to pay for this?
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: Neutral required on 480 VAC system

waross, I don't think they way I wrote that conveyed what I meant. What I meant to say is it seems like the neutral terminal on the inverter would see full L-G voltage for a single phase to ground fault. The neutral terminal on the inverter may not be insulated to handle that if it was designed with the expectation that it would be installed on a solidly grounded system. Operationally it might be a problem for the inverter as well.

RE: Neutral required on 480 VAC system

Sorry for the misunderstanding. Mostly my fault.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Neutral required on 480 VAC system

Per NEC, a high impedance grounded system can not serve Line to Neutral loads.

RE: Neutral required on 480 VAC system

Is an inverter a load?

RE: Neutral required on 480 VAC system

Having had similar discussions myself with inverter manufacturers regarding earthing of systems and the inverter arrangements, I'd suggest GroovyGuy's option 3 bears some relevance.

As far as I'm aware, there are a number of configurations of inverter around, some float the system as there's no isolation between the DC bus and the AC output, and those types don't want a grounded system, and there are others that effectively run single phase inverters with a common neutral, and all hell breaks loose when the neutral floats. There are also the considerations on what voltage the battery runs at, which also has a bearing on the isolation between the AC and DC stages, as well as the inverter topology to handle it. Some of these systems are VFD derivatives, some aren't.

There is also the possibility that the system could use VTs for voltage measurement, but doesn't, and the system is configured to monitor the phase - neutral voltage, but doesn't actually need a neutral. Whether that is the case is really something for the supplier, and if they can't tell you...

Getting to the technical people who can advise on these sorts of things can be quite difficult, particularly when the package is supplied by the battery people rather than the inverter people.

EDMS Australia

RE: Neutral required on 480 VAC system

(OP)
Hi wroggent,

The inverter itself is being part of the complete Battery Energy Storage (BES)system, thus charging (a load) and discharging (source of power). The total capacity of the system consists of ~2x 500 kW connected with two separate breakers on the same bus with a tie in between.

Regards,

jairo

RE: Neutral required on 480 VAC system

Back to the confusion between a neutral and a ground:
Does the inverter want a neutral? OR
Does the inverter want a ground? OR
Does the inverter want a grounded neutral?
If the inverter wants a neutral only, (not grounded), run a conductor from the inverter to the transformer wye point.
If the inverter wants a GROUNDED NEUTRAL you need either an isolating transformer or a different inverter.
Looking at two 500 KVA transformers it may be cheaper to send the inverter back and purchase a suitable inverter.
As for generating a neutral with a zig-zag transformer, three transformers connected in wye delta will generate a neutral.
The three transformer solution is probably cheaper and more available that a zig-zag transformer.
The problem with generating a neutral and grounding it is that the system then becomes a grounded system. You will have bypassed or shunted the NGR.
While only a few VA of capacity may be required for the neutral reference, if the neutral point of the transformers or zig-zag transformer is grounded, you can expect rapid failure of the small transformers.
If an ungrounded neutral will serve, ask the supplier to advise as to the current expected on the neutral. Use three appropriately sized lighting transformers in wye delta.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Neutral required on 480 VAC system

(OP)
Hi waross,

Thanks for the information. As can be seen on the sketch above. The inverter may need only a neutral and the Ground is on a separate connection bus. Till now it is not clear what/ how the neutral is used in the inverter. That is something that is currently amplifying the challenge.
As mentioned, connecting the neutral on the transformer wye point may be a plausible solution, however all the connection on the transformer is a bus bar system which is rather complicated.
Is there any violations connecting the neutral to the transformer wye point? In the current scenario, from there it is routed to the high resistance grounding.

Jairo

RE: Neutral required on 480 VAC system

I've seen something similar with a battery charger supplied from a high-resistance earthing system. The charger used the phase-to-neutral voltage to establish a timing reference for switching. In our case, the system designer worked with the charger manufacturer and ended up installing three small resistors, one connected to each phase, in a star configuration, and then connected the star point to the charger neutral connection. I forget whether the star point was also connected to earth. This solved the problem.

Cheers,
mgtrp

RE: Neutral required on 480 VAC system

Some codes require the neutral to be solidly grounded when there are any line to neutral loads served.
This is a grey area as the inverter is not really a load.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Neutral required on 480 VAC system

I would argue that an inverter is a load since it consumes electrical power. Yes it provides power too but only after consuming it (not a power generator). There is probably some room to argue otherwise. If it is a 'load', then a high impedance grounded system can't serve any L-N loads. This is per the NEC.

RE: Neutral required on 480 VAC system

Howdy Ya'll,
I think it goes beyond saying; if you have a NGR, then you cannot have any L-G loads. There that's taken care of.
I'd also be surprised of the inverter OEM would be terrible pleased with a NGR at all. I suspect that they would be happier with a solidly grounded system. These guys don't get out much.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: Neutral required on 480 VAC system

I'm pretty sure that in NEC land any connection to the inverter's N terminal would be a code violation on an NGR system and not connecting could also be a code violation if the installation instructions say to connect the neutral and don't address an NGR system. The isolation transformer or the removal of the NGR seem to be the two alternatives.

RE: Neutral required on 480 VAC system

1) NEC Violation
The specific section that applies is NEC 250.36. This article says high-impedance grounded neutral systems cannot have any line to neutral loads, and I would well argue that this inverter would be considered a line to neutral load (think of it as a negative impedance load). Another concern is if the neutral has a ground reference internally, this could be construed as a second ground reference where NEC says the neutral can be grounded only through the impedance.

2) Using Isolation Xfmr to Reduce Harmonics
2a) Three phase inverters tend to carry a lot of triplen (3rd, 9th, 15th...) harmonics over the neutral path, and in your 1200A application, I might expect in the 10s of amps of triplen harmonics flowing in the system neutral. An isolation xfmr would keep the triplen harmonics out of the system and also reduce other harmonics in the inverter output. Ideally, you would use specialized xfmrs for inverter applications that are derated for harmonic loading and might have heavier insulation.
2b) It is mentioned that there are 2x500kVA inverters, so this tells me you should use 2 independent isolation xfmrs. Ideally, each inverter would be on its own isolation xfmr. This allows you get into custom xfmrs with custom phase shifts (phase shifts other than +/- 30 degrees are available) for each xfmr. This allows very improved harmonic reduction. If for instance the 7th harmonic is shifted +15 degrees on one xfmr, and +30 degrees on the other, the harmonics cancel out and the 7th harmonic is not seen on the output. (I am making up this example; I do not know the correct angular differences to create the best harmonic cancellation.) The theory of the right phase shift and isolation xfmr design is something that engineers for your supplier should be able to assist with.

RE: Neutral required on 480 VAC system

Howdy Jensen,
I suspect that the inverter(s) in question here have active front-ends, therefore harmonics should not be an issue.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

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