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Solar with Powerwall design question

normie

Member
Jun 8, 2021
6
0
home
Good day everyone, first time caller, long time listener.

I have been scouring the forum and it is quite obvious there are many extremely knowledgeable individuals posting here. With that said, I have some questions for the members here that for some strange reason (sarcasm) I can't get an answer from the people actually "designing" my system.

A few months ago (like six now) I placed an order for a 2 powerwall 8kW solar panel installation. Things progressed along very quickly to permitting and interconnect agreement with my local utility (PECO) here in PA.

PECO denied my system design an proposed some changes because the system as designed would violate Pennsylvania's Electrical Code for "Voltage Rise" I am pasting the particulars below.

Details of Rejection:

Proposed generation creates voltage rise exceeding PA Code maximum allowable limit. The application is conditionally approved pending solution acceptance and will be approved as revised if one of the following possible solutions is implemented:

1) Lowering proposed generation to 4.70 kW AC Output.

2) Lowering proposed generation to 5.50 kW AC Output AND upgrading service conductor to 1/0 AL triplex.

3) Lowering proposed generation to 5.70 kW AC Output AND all inverters within system operate at a constant 95% lagging/absorbing power factor (PF). Inverter also to maintain Q or Reactive power priority setting (as applicable). This option requires completion of an Inverter Addendum which supplements the PECO Interconnection Agreement.

4) Lowering proposed generation to 6.80 kW AC Output AND Upgrading service conductor to 1/0 AL triplex AND all inverters within system operate at a constant 95% lagging/absorbing power factor (PF).


We opted for the last/best option. However, the question I have is. Shouldn't Tesla be able to design the system with different inverters or some other appliances/software config to prevent the voltage rise? it seems like it would be rather simple in my opinion, but I am not an electrical engineer.

I have tried contacting my *third* Project Advisor VIA phone and email to little success, and of course they changed the original "whole house" backup with 2 powerwalls to a "most of house" backup, unless we added a third powerwall, so that redesign is taking forever.

Hopefully, we will be able to resolve at least the redesign quickly, as I am more concerned with power stability and reliability than I am with offsetting electric costs with solar. But in the meantime, maybe I can understand more about the Voltage Rise, and eliminating it while having more panels available.

Thanks!
 

Ampster

Active Member
Oct 5, 2012
1,780
477
Kenwood, California
Two issues here. Voltage rise caused by wire size and Power Factor correction. Wire size of the seevice conductor is not within Tesla's scope. I can't give you and explanation on PF correction.
 

normie

Member
Jun 8, 2021
6
0
home
Well, Wire size increased in option 4. So perhaps that is as big as can go?

As per the energy company req upgrading to (service conductor to 1/0 AL triplex) Get's my design up to 6.8kW in panels, but figuring something else is a factor here, as I see MUCH larger kW systems posted here on site, nearby my home and even for the same energy supplier at 110% of average power consumption, I am near 40% with an 8kW system.
 

BGbreeder

Member
Jun 19, 2020
292
178
Bay Area
Where are you located?

From the various limitations, it seems as if the original wiring in your house is small by modern standards.

It could also additionally be the case that your transformer is not up to the additional power. It happens. Many utilities will upgrade the transformer to take the added power, usually at a cost to you.

All the best,

BG
 

normie

Member
Jun 8, 2021
6
0
home
Where are you located?

From the various limitations, it seems as if the original wiring in your house is small by modern standards.

It could also additionally be the case that your transformer is not up to the additional power. It happens. Many utilities will upgrade the transformer to take the added power, usually at a cost to you.

All the best,

BG
NE PA, Philly area.

Our infrastructure is quite old, but they supposedly upgraded transformers a few years ago, additionally the power company's comments were that the design exceeded Pennsylvania's Electric Code Voltage Rise Limits, not my local limit. But am thinking that the line set upgrade will only achieve so much, and that perhaps the age of our local infrastructure can't handle more than the ~7kW limit set.
 

miimura

Well-Known Member
Aug 21, 2013
6,350
6,041
Los Altos, CA
200 amp service.
That is very odd. I assume Tesla would want to use a 7.6kW inverter for 8kW DC of panels. That is only a maximum back-feed of 32A on a 200A service. I have heard of utilities assuming the Powerwalls can export full bore into the grid, which they don't. I've never heard of this "voltage rise" issue on the forum before.
 

normie

Member
Jun 8, 2021
6
0
home
Yeah this seemed very sketch to me as well. Like how can I be at less than 40% generation of my usage and have an issue with what I am pushing back to the grid, potentially.
 

BGbreeder

Member
Jun 19, 2020
292
178
Bay Area
One way: not saying it is true in your locale, is that the neighborhood wiring can't handle all of the local solar back feed. Utilities routinely put in wiring and transformer services, knowing that customers will never draw that much, but it works in reverse, too. That said, I might expect it in say in an older neighborhood of Arizona, or California, where there is high rate of solar installation and old infrastructure. Your area, not so much.

Still, since you have a 200A meter, this seems very odd to me. One thing that seems especially odd is their focus on power factor specifications that suggest, to me, a lack of significant experience with solar inverters. It might be worth having a listening session with your utility to understand their concerns and the cause for their concerns. Contesting their assertions may not be without costs in the longer term (e.g. @holeydonut's experiences with a different utility.).

All the best,

BG
 

holeydonut

Supporting Member
Jun 27, 2020
1,833
1,210
East Bay NorCal
Well my issue was that PG&E originally granted me approval for my project and things were on track (the PG&E planners confirmed my service entry, gas riser, and blah blah).

Then they sent someone to my house to do the service disconnect and that person crapped all over my install citing some lame safety concerns with my gas riser (more like PG&E just hated Sunrun). From there, Sunrun was blocked by PG&E from building the design that their own team previously supported because of some 120% rule interpretation on my partial home backup.

PG&E didn't even give Sunrun options. They just told Sunrun to terminate my installation. So yeah, I don't think Normie is in the same boat yet since it seems like their PoCo is providing solutions instead of just providing problems.

FWIW, PG&E wouldn't let me do my install even if I had 1/0 AL PWC underground cable. I don't know what it costs you to do the upgrade to 1/0, but that seems like a good thing?

My home is 200A service, and PG&E required me to have at least 2/0 aluminum. Good news is PG&E sent a worker and he confirmed my underground entry actually 4/0 AL; so the real conductor size entering my home wasn't the issue at all for me. It was just whether I did a partial home backup vs whole home backup. Basically 2 batteries = nope. 3 batteries = ok.
 

wwhitney

Member
Nov 2, 2017
900
1,186
Berkeley, CA
PECO denied my system design an proposed some changes because the system as designed would violate Pennsylvania's Electrical Code for "Voltage Rise" I am pasting the particulars below.

In case it's not clear, voltage rise is the analogue for grid interactive inverters of voltage drop for loads. In either case it is due to the resistance (more properly impedance) of the wires (and other utility equipment, like the transformer), which means when you push current through them, you get a V = I R voltage change between the ends of the wire. And as the grid is very big, the voltage at the utility transformer is approximately fixed by the utility's configuration choices. So for a load, the voltage change shows up as a drop relative to the voltage at the transformer. While for a grid interactive inverter when it is pushing energy back to the grid, the voltage change shows up as a voltage rise at the inverter, again relative to the voltage at the transformer.

I think the issue your POCO raises is the following, at a technical level at least; I'm not sure that I have the regulatory details correct. I understand that the utility is required to provide service at your meter at a voltage within some allowable window around 240V, maybe something like 228V to 246V (not sure of the numbers, just an example of -5% to +2.5%), or a range of 18V. Now your 200A service likely only ever sees a maximum of 60A of current draw (maybe that's a little low, maybe it could spike to 100A), which the utility knows and uses when determining how it sizes its wires (in contrast, wires after the meter are sized per the NEC, whose load calculations are very conservative, generally resulting in bigger wires). Without PV, that's a 60A current window, 0A to 60A from the grid to you (again, 60A is a guess).

With those numbers, that would mean that the utility can get away with sizing its equipment (primarily the service conductors from transformer to meter, but also the transformer itself) to have an overall impedance of 0.3 ohms. Then if it sets the transformer no load voltage to the high end of the window, 246V, that complies when the current is 0A; and when the current is 60A, the voltage drop is 18V, and you'll see service at the low end of your voltage window, 228V in this example.

Now say you want to add PV with a 7.2 kW inverter. That enlarges the current window to -30A to 60A, 50% wider, which means the voltage window would be 50% wider. That won't comply with the regulations with the existing 0.3 ohm impedance service equipment. The utility will need to drop the impedance of its service to no more than 0.2 ohms (primarily, use bigger wire) in order to keep the voltage window only 18V wide. And it will also change the no load voltage to 240V, so that you have room for both voltage rise when exporting power, and voltage drop when net consuming.

Presumably the state utility regulations determines who has to pay for the cost of upgrading the power company's equipment/wires to allow for your PV installation.

Hope this background info helps.

Cheers, Wayne
 

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