Solar Transformer 1

[KS20]

I am using a Transformer in solar project where my inverter will give output of 600V, I have to step up this voltage to 12.47kV system which will be connected to utility. The existing infrastructure of utility is 12.47kV delta.
I am thinking to use YNy0 transformer but not sure. Can someone explain me importance of neutral on HV and LV side of transformer when it is connected as step up specifically in solar plant.
Thanks in Advance
KS20

 

[davidbeach]

You'll get a much more useful answer if you ask the utility what they want. Different utilities will answer that question differently as will all sorts of people on the internet, but there's only one answer that actually matters.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Kiribanda]

Since your site is a generating station, to continue to satisfy the "Effectively Grounded" 12.47kV Utility,
you have to have a WYE grounded on the 12.47kV Utility side. Low side connection (whether DELTA or WYE)
depends on your PV design.

 

[bacon4life]

As a utility engineer, I keep hoping there will be clear guideline developed that I could reference.

Kiribanda- A grounded-wye/delta transformer will attempt to "fix" any voltage imbalance on the 12.47 kV circuit, which can result in excessive circulating currents in the transformer.

 

[waross]

"fix" any voltage imbalance on the 12.47 kV circuit, which can result in excessive circulating currents in the transformer.

Those unbalances may not be readily apparent.
Voltage regulator banks on long distribution circuits correct the line to neutral or ground voltages.
This often results in unbalanced line to line voltages and unequal phase angles.
The circulating currents are related to the percent impedance of the transformer bank, so a relatively minor primary unbalance may result in excess circulating current.
With a low impedance transformer it won't take much unbalance to overload the delta winding.
There are other unwanted effects.
Consider a distribution circuit protected by fused disconnects possible miles away fro the wye:delta transformer.
If a line,en opens one fused cutout, both sides of the cutout will still be at line voltage due to back-feed from the wye:delta.
Best to avoid wye:delta transformers.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[KS20]

What will happen if I use Grounded Y on 12.47 Kv side and not use Grounded Y on LV side. i.e YNy0?
I was reading C57.159-2016 and it suggest that grounded connection should not be used on LV side of transformer connected to inverter in solar plant. Is there any specific reason for this?

 

[prc]

Usually, Dy11 transformers are preferred for this duty. The secondary Y is not grounded to avoid circulating current through the inverter. So most of the inverter makers insist on y connection and not yn.
Please see the attached which I wrote some time back.

https://www.linkedin.com/pulse/62-transformers-photovoltaic-solar-projects-part-1-p-ramachandran

https://www.linkedin.com/pulse/63-transformers-photovoltaic-solar-projects-part-2-p-ramachandran

 

[jfpe]

Like Kiribanda said, the LV winding choice depends on the PV system design. See what is required by the inverter manufacturer. With central inverter installations, we see mostly delta LV windings. With string inverter installations, we see mostly grounded wye LV winidngs.

If you use a grounded-wye/delta transformer, you will want a neutral grounding reactor to limit zero sequence current.

-JFPE.

 

[waross]

If you use a grounded-wye/delta transformer, you will want a neutral grounding reactor to limit zero sequence current.

There are a several good reasons to avoid a grounded-wye/delta transformer.
A common issue (not that grounded-wye/delta transformers are that common) is the effect of voltage regulators on long distribution circuits.
On a long distribution circuit, particularly in rural areas, Unequal single phase loading will lead to unbalanced voltages.
The unbalanced phase to neutral voltages will cause unbalanced phase to phase voltages and uneven phase angles.
If the distribution circuit has voltage regulators, the voltage regulator will balance the line to neutral or ground voltages at the expense of unbalanced line to line voltages and uneven phase angles.
The end result of unbalanced primary voltages at the grounded wye/delta transformer will be circulating current in the delta and current in the neutral grounding reactor.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[b_gibb]

I'll build on a couple of points that have been mentioned.

My First step would be to the utility, as they may dictate what is required and then you are over halfway there. The other half is what the inverters require. Depending on the manufacturer, you may get lucky in talking to an engineer on staff there.

Anecdotally, I've seen two projects like this (both from a distance). One was a 25kV delta (rural feed) to a 480V delta but then a zig-zag on the LV to create a neutral with a resistor. The idea of the resistively grounded LV was to limit under-voltage on the HV side, I believe. I don't think anybody was able to prove this in simulations, however. This project had many people new to distributed energy resources.

The other was 13.8kV to 600V and it was a simple delta-wye grounded. The inverter manual indicated the LV system could be solidly grounded.

There is the IEEE 1547 standard that may be helpful.


ben
bengibb.ca
powerdesignerpro.com

 

[iop95]

Most solar inverters use 3-level topology (or variations as in SMA or Refu).
This topology require to use neutral.