How to create an artificial neutral from an step-up transformer YNd5 - 11/0,4kV - 1.250kVA - ucc=6%

[Fernando Lima]

Dear all,
I need your help and expertise under a big problem I have in hands right now.
This is about an 1.200kWp solar power plant we’re dealing for our customer in Funchal – Madeira.
Currently, our solar plant has an step-up power transformer (attached) feeding an Main Low Voltage Switchgear to which the 16 three-phase solar inverters are connected.
It turns out that the BT side (delta, 400V) of this transformer has no neutral, but the inverters need that reference connected to their PE terminal to place their phase-neutral input voltages between 230-277V.
My question is if you can help us with an auxiliary transformer (maybe zigzag or auto-transformer?) for inside mounting (with enclosure) to provide the neutral on the 400V side of our step-up power transformer.
These are 16x 75kVA inverters, model SHP75-10, which produce a total of 1,200kVA and which consume <3W each, as attached.
It should be noted that our 1250kVA step-up transformer also feeds building low voltage utilities around 20kVA (lighting, sockets, heating, etc).
Attachment for general overview of the installation.
REMARK:The "red dashed" connection under my SLD sketch doesn't work because our TSA auxiliary transformer is an insulation type so it's not same reference as our step-up transformer TPE.
Waiting for your quick reply with,
Many thanks and best regards,
FL

https://files.engineering.com/getfile.aspx?folder=ae67b740-03c0-4b87-9de5-c8f905cbabe6&file=SLD.JPEG

https://files.engineering.com/getfi...115-4ce2-b92e-4ceb407bc04a&file=4DT186358.pdf

https://files.engineering.com/getfi...97ccdd0b2a29&file=SHP75-10-DEN1744-V23web.pdf

https://files.engineering.com/getfi...3359e94&file=MV-Trafo-SB_SMC_STP-TI-en-12.pdf

 

[waross]

A zig-zag or an artificial neutral may be a special order.
That may not be an issue for the original installation but may cause a lengthy outage waiting for a special order spare should it ever fail.
My preference is three common "off the shelf" transformers connected in wye delta.
The wye side is connected to the line and the voltage rating must be suitable for 277 Volts or more.
The delta serves to stabilize the wye and the voltage is unimportant.
Sizing;
As I understand the load is 3 Watts times 16 units = 48 Watts.
It is unclear if this is the losses or the neutral load.
I would tend to go large and use three 100 VA transformers.
This sizing is for an ungrounded voltage reference only.
If the wye point will be grounded, then it is up to you to size the transformers to carry anticipated fault currents.
If this is used as an ungrounded voltage reference I suggest mounting the transformers in the same enclosure as the inverters.
If it will be grounded, then I would mount the transformers near the 400 Volt bus.

Bill
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"Why not the best?"
Jimmy Carter