Reading material re grounding transformer for solar project? 1

[jimolson]

My church constructed one of the last PV solar projects in Indiana to receive net metering certification before the legislative door closed on December 31st, 2017. As the lone, gray-haired circuit board designer in this technology-averse congregation, I got tapped to work with the church's solar contractor. I'm a great single phase switch mode power supply designer but my knowledge of upstream AC power distribution is limited.

The church's project was a 33kW array with three phase 208V outputs from three string inverters.

At the 11th hour the local utility asked that our contractor install a grounding transformer and reactor between their 3 phase 120/208V pole drop and our solar inverters. Our contractor, who was not as technically adept as we had assumed at the time the purchase order was issued, asked why. The utility basically answered "do it...it's not our job to educate solar contractors on matters they are required to understand...".

Some political wrangling ensued, with the result that the church (and I) accepted on faith that a grounding transformer was technically necessary and that the utility's request for one was not a ploy to run out the net metering clock.

Can readers in this forum steer me to reading materials on the internet that talk about isolation requirements for PV arrays? Why is our neighborhood 208V 3 phase grid threatened by 11kw inverters injecting their power in delta configuration rather than the utility's preferred wye configuration? What is the role of the reactor used in conjunction with the isolation transformer?

The array is now up and running and we are harvesting 200kWH+ of energy each day. Thanks in advance for your advice on this topic.

 

[xnuke]

Try Effective Grounding for PV Plants, or PV and Inverter-based DER Ground Referencing.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jghrist]

The utility transformer is most likely connected grounded wye - grounded wye and provides a ground source when it is energized by the utility primary distribution system. Your PV system probably has an isolation transformer that does not provide a ground source if the system is disconnected from the utility system. With no ground source, your phase to neutral voltages will be unstable and there will be no ground source to trip breakers in the event of a ground fault. A grounding transformer will furnish this source.

 

[jfpe]

Whether the grounding bank is technically warranted doesn't matter. If the utility requires it you'll need to install it.

Speak to the inverter manufacturer. They should be able to give you grounding transformer design/sizing advice.

If the inverter manufacturer can't help you, the Xcel article xnuke posted has good sizing information. Your situation is covered in Example 3.

I see a lot of Hammond Power Systems customer grounding transformers. Try reaching out to them once you have your design numbers.

-JFPE

 

[jimolson]

Thanks for the feedback, guys.

In the end the solar contractor did interpose a 3 phase 45kVA Hammond isolation transformer containing a grounded wye secondary that faces the utility. I think there was also a reactor connected to the isolation transformer. These additional measures satisfied the local utility and we got our approval to turn the PV system on.

I just wanted to understand better the technical need for the isolation transformer. XNuke's reading suggestions allowed me to do that.

Hopefully solar inverter manufacturers can work with utilities to identify cheaper, lighter, and more energy efficient methods for assuring proper PV side grounding. The present solution in our system cost us $5k including installation, took four men to drag down the basement steps, and it turns 5% of our precious solar output to unneeded heat in the basement.

It is ironic that inverter manufactures tout extremely high conversion efficiencies but at the job site end users negate this achievement with lossy 60Hz iron core isolation transformers.

Jim Olson
Indianapolis, IN