IEEE 1547.7

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msheets

PE Electrical
Location
Salt Lake City, UT, USA
Occupation
Electrical Engineer
We completed a small commercial PV system last month, but the POCO has not yet approved the net metering agreement, and just yesterday told us that due to the requirements of IEEE Standard 1547.7, we will have to install a transformer grounding bank before we can start generating. For those unfamiliar with 1547, the relevant portion essentially states that if the generating facility is connected to a 3ph 4W system, and the total generating peak power is more than 10% of the minimum load on the circuit, then the generating facility has to be effectively grounded. Solectria wrote an interesting paper on the subject: http://solectria.com//site/assets/files/1484/solectria_effective_grounding_for_pv_plants.pdf. Quite a bit of PV has gone up in the area lately, and the POCO just realized that they are at or near that 10% limit.

The prospect of adding the transformer grounding bank doesn't bother me too much, but due to the lead time of some of the components, we may have to put off generating another two months. I can't imagine our client is too happy about that. With that in mind, I'd like to know whether this applies to this project. The transformer feeding the building is 12470Y-Ground / 208Y. I can't think of any reason why this transformer should not be solidly grounded. If that is the case, is it safe to assume that the "effectively grounded" portion of the 1547 requirement is satisfied, and we therefore do not need the transformer grounding bank? Am I missing anything?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Probably not.
Possibly the grounding transformers would be needed for a pure delta generating plant, where corner grounding was not acceptable?
I also get the impression that POCO is not clear on a concept somewhere. But I might be missing just what the definition of "effective grounding" is.
 

msheets

PE Electrical
Location
Salt Lake City, UT, USA
Occupation
Electrical Engineer
the definition of "effective grounding".

Not sure if this really helps, but from the paper I linked: The IEEE green book defines ?effective grounding? as the impedance ratios between zero sequence impedance and positive sequence reactance:
0<X0/X1<3, 0<R0/X1<1.

I believe (don't quote me on this) that a solidly grounded system is also considered to be effectively grounded.
 

big john

Senior Member
Location
Portland, ME
It doesn't get much more effectively grounded than a Y:Y with the neutral points earthed and connected to a primary MGN.

A grounding transformer wouldn't do anything to improve on that. That's for a delta secondary.
 

msheets

PE Electrical
Location
Salt Lake City, UT, USA
Occupation
Electrical Engineer
For my own curiosity, how big was the system?

(I'm reading the whitepaper. Probably won't have any advice to offer though.)

69kW DC. The transformer serving the building is 75kVA.

It doesn't get much more effectively grounded than a Y:Y with the neutral points earthed and connected to a primary MGN.

A grounding transformer wouldn't do anything to improve on that. That's for a delta secondary.

That was my thought. I just need to convince the utility of that so we can finally flip the switch on this thing. Unfortunately, this is the first I've come across the issue, and I'm not certain I understand it well enough to discuss it effectively.
 

msheets

PE Electrical
Location
Salt Lake City, UT, USA
Occupation
Electrical Engineer
What, specifically is their objection? That grounding-transformers are necessary in all applications?

That appears to be what they are saying. I have spoken with the Customer Generation Engineer who informed us that the ground bank is necessary, but I'm not sure he fully understands it, either. I told him that my interpretation of the white paper that he provided (linked above) was that it was unnecessary since this is a solidly grounded system. He told me he would discuss that point with his colleagues and get back to me.
 
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