Seeking help interpreting power company note on "excessive voltage-rise"

[brycenesbitt]

I'm seeking help understanding the electrical concern identified by a large West Coast power company:

"Transformer's thermal rating of 35 kVA (summer coastal region); however, due to the almost 300 ft. of secondary conductor to the interconnection customer, this may result in excessive voltage-rise. Therefore, it is proposed to remove and replace the existing transformer, and up to 200 ft. of secondary conductor, to mitigate this. Estimating to confirm the mitigation(s) needed during the implementation phase. The transformer and secondary conductor replacements are considered Distribution Upgrades... updates will take 6 months to 12 months.... Any facilities (transformer or secondary cables) that are found to be dedicated to the customer and that would need to be replaced would be at the cost responsibility of the customer ."
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The early review of the project showed no problem:
the supposedly final signoff prior to interconnection generated the above.
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The project added a 57.1 kW nameplate microinverter solar generating facility to an existing building with two 100A meter mains.
The same poco transformer feeds six other properties at least, on two poles between lot lines.
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I'm trying to understand how adding solar generation impacts the voltage on the transformer.
The southwire drop calculator shows:

Engineering Information 4/0 Aluminum
0.1100 Ohms Resistance (Ohms per 1000 feet)
0.0410 Ohms Reactance (Ohms per 1000 feet)
0.9 Power Factor
2.92% Voltage Drop
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I see https://mcelectrical.com.au/solar-voltage-rise-explained/
But am looking to understand exactly which voltage the poco is worried about above.

 

[Fred B]

Generally utility company equipment is not under the requirements of the NEC but rather the NESC. But a large scale solar system probably has little difference from a small residential system and concerns only become larger on the Large scale system. Voltage rise or over voltage condition needs to be addressed when adding the secondary simultaneous power generation as it could result in an overload of the equipment. Likely NESC would have some criteria similar to NEC articles 690 and 705 but not familiar with NESC codes.

 

[Hv&Lv]

I have a couple of questions if you don’t mind..

You added 57kw to a 35 kW transformer?
How many other solar installs are around this area or on that transformer?
I’m not sure of the layout either.
Even without doing any calculations we would not allow the 57kWto be connected when the transformer is 37.5kW

Think of voltage rise as “solar saturation”.
One inverter pushes 2V above grid. A neighbor now sees the “grid” at +2V so their inverter pushes another +2V until you get close to the trip point of the transformer fuse or the inverter kickoff point.

 

[jaggedben]

In California the magic number to stay under is 30kw if you want to avoid big complications. Beyond 30kW it is a different, more extensive type of review and the customer may be on the hook for utility upgrades, like they told you.

I would say with backfeeding more than 200 amps on an existing 200A service you should have known there might be issues. (You are upgrading the service, right?) Now that they've told you the transformer is 35kW it's a good bet that the total customer load on all six properties rarely, if ever, exceeds the output of your proposed solar. Which means the solar is going to be backfeeding the transformer and primary conductors many hours of the typical day. The voltage rise will continue to whichever node takes current from both utility and DER at that moment. Or, as HV&LV said, there may be other DERs on the grid affecting the voltage dynamically. So to partially answer the question, they are likely not merely concerned with voltage drop on the service drop or lateral to the service where you're connecting.

FWIW I've had customers have to wait for upgrades with as little as 14kW on a shared transformer (the 14kW being their system and one neighbor's) although I think that was probably a transformer at/near its end of life already.

 

[brycenesbitt]

I have a couple of questions if you don’t mind..
You added 57kw to a 35 kW transformer?
How many other solar installs are around this area or on that transformer?
I’m not sure of the layout either.
Even without doing any calculations we would not allow the 57kWto be connected when the transformer is 37.5kW

It's a high solar penetration urban area of mixed single family homes many with ADU structures that themselves have solar.

And a big correction: it's 50 panels at 450W, so 22.5kW nameplate
For some reason 57kW appeared on a document I referenced when posting last night: a sleepy mistake.
It's definitely 22.5kW and thus 94 amps.

The solar contractor and utility company did all the permitting and calculations. It was not until Friday that we even were told the transformer size at the street: previously it had just been a green light project moving forward. No field measurements were done by the POCO, and no other details (such as existing local grid voltage) are available. I could measure (or even graph) the voltage at the connection point myself however, but have not done so.

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It's virtual net energy metering, so all solar generation is its own meter and could be fed back to the grid anywhere, not necessarily at the existing connection point.

 

[brycenesbitt]

Think of voltage rise as “solar saturation”.
One inverter pushes 2V above grid. A neighbor now sees the “grid” at +2V so their inverter pushes another +2V until you get close to the trip point of the transformer fuse or the inverter kickoff point.

Where is the typical "breaking point" in this type of situation....

The inverters themselves reaching maximum voltage and giving up is inefficient of course: would your POCO allow the inverters to be tuned to match the wire/transformer capacity?

 

[wwhitney]

This is the way in which I think about the physics of this issue from the POCO's point of view:

Let's say the POCO is required to supply a voltage at the customer's meter of 240V +/- 5% = 228V - 252V. That's a 24V voltage window. Suppose further than the load on the transformer and POCO secondary conductors will vary from 0A to 300A, a current window of 300A. Then the POCO can comply with the voltage limits at the meter as long as the transformer and the secondary conductors to the meter have a joint impedance of no more than 24V / 300A = 0.08 ohms.

Now a DER customer comes along and wants to be able to export 100A. The transformer and secondary conductors will now see a current window from -100A to 300A, or 400A in size. That means for the POCO to keep the voltage at the meter within their window, the transformer and secondary conductor impedance needs to be no more than 24V / (400A) = 0.06 ohms.

So if the existing transformer and secondary conductors have an impedance between 0.06 ohms and 0.08 ohms, adding the DER creates a compliance issue for the POCO. They need to upgrade the transformer and/or secondary conductors to reduce their joint impedance to meet their mandate of keeping the voltage at the meter within the regulatory window.

Cheers, Wayne

 

[Hv&Lv]

This is the way in which I think about the physics of this issue from the POCO's point of view:

Let's say the POCO is required to supply a voltage at the customer's meter of 240V +/- 5% = 228V - 252V. That's a 24V voltage window. Suppose further than the load on the transformer and POCO secondary conductors will vary from 0A to 300A, a current window of 300A. Then the POCO can comply with the voltage limits at the meter as long as the transformer and the secondary conductors to the meter have a joint impedance of no more than 24V / 300A = 0.08 ohms.

Now a DER customer comes along and wants to be able to export 100A. The transformer and secondary conductors will now see a current window from -100A to 300A, or 400A in size. That means for the POCO to keep the voltage at the meter within their window, the transformer and secondary conductor impedance needs to be no more than 24V / (400A) = 0.06 ohms.

So if the existing transformer and secondary conductors have an impedance between 0.06 ohms and 0.08 ohms, adding the DER creates a compliance issue for the POCO. They need to upgrade the transformer and/or secondary conductors to reduce their joint impedance to meet their mandate of keeping the voltage at the meter within the regulatory window.

Cheers, Wayne

One of the issues we have is DER toward the end of the regulated line before tge secind set of regulators.
Some of the older regulator controls don’t play well with reverse power, the solar saturation gets to a point the voltage is so high toward the end it feeds toward the regulators catching all the neighbors needs as it goes. But at a higher voltage.

We have had days where there was no load 79 degree day yet it was very sunny.
The combined DER on our substation backfeeds through the transformer, back on the transmission line, and towards another substation. This combined DER on this station also backfed to a 125kV delivery point. We were almost at the point of having to cut off some solar

 

[Hv&Lv]

Where is the typical "breaking point" in this type of situation....

The inverters themselves reaching maximum voltage and giving up is inefficient of course: would your POCO allow the inverters to be tuned to match the wire/transformer capacity?

How would you do that?

 

[ggunn]

How would you do that?

One way would be to limit the size of the array.

 

[jaggedben]

How would you do that?

Newer inverters can limit the amount of power export using Power Control Systems. On site load would allow higher (potentially up to full) solar output at times. This is far from ideal for a VNEM site though.

 

[Hv&Lv]

Newer inverters can limit the amount of power export using Power Control Systems. On site load would allow higher (potentially up to full) solar output at times. This is far from ideal for a VNEM site though.

Yes, a lot I see have adjustable export limits.
I meant how was he planning on doing his application specifically and if the POCO would accept that

 

[brycenesbitt]

How would you do that?

As @jaggedben said : Some models of inverter have software configurable thresholds.
They will hard cut if the voltage creeps too high, but can be set to ramp back production before reaching that threshold thus hopefully staying online more consistently.
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I have NO IDEA how such thresholds could be cooperatively managed between the POCO and the generator, at least not for modest residential scale systems. However if one had a site that has the problem, using the software thresholds could resolve the issue temporarily while awaiting a physical upgrade.
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For this particular site I'd rather just hook the system up if allowed to do so, and see how bad the issue issue is. Sometimes the calculated values are more conservative than necessary. AFIK, the highest voltage to the grid will be at the point of generation, so no other properties are at risk from pushing to exactly 240V +5%.
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I've read in passing of some new solar communication requirements related to grid management: does anyone know if this particular overvoltage issue is covered or even on the regulator's radar?

 

[jaggedben]

It sounds like you're talking about 'smart inverter' requirements, UL1741 SA and or SB, etc. Hopefully their engineers are already accounting for those features being enabled because that's required. But I don't know that those features are necessarily necessarily sufficient to avoid upgrades.

Those features are totally on the CPUCs radar and have been for over a decade.

Smart Inverter Working Group

www.cpuc.ca.gov

 

[jaggedben]

As @jaggedben said : ...
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...AFIK, the highest voltage to the grid will be at the point of generation, so no other properties are at risk from pushing to exactly 240V +5%.
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I don't know what PG&E is obligated to try to maintain but the inverters typically cut out at +10% (264V), not +5%.

 

[Walter Melville]

I don't know what PG&E is obligated to try to maintain but the inverters typically cut out at +10% (264V), not +5%.

By Rule 2 .C. 1(a) For a 120/240 volt service, PG&E's maximum allowable voltage 252 Volts with 2.5% voltage difference between phases. However, there are exceptions to the voltage rule big enough to drive a line truck through. I suggest you read Rule 2 yourself.