Voltage Rise Question in PV System.

[JasonC]

First thank you for your help ahead of time. I am versed in Theory and calculations but I am not an EE that is part of the reason I am here.

I have taken on a problem site of ours that continues to go off line due to the inverter(s) output voltage rising and causing it to go out of limits (264V). I know some of the fault is ours due to not being conservative enough in the VD%, however I would like to try and figure out if the utility is also compounding the issue. As stated I inherited this issue, I did not spec it.

Knowns: When Fault has occurred Grid voltage at array with zero load / zero production was 248.2V
Circuit resistance according to calculation is .103 ohms
Single Phase 120-240 service.
When only one inverter is exporting power the voltage rises from 248.2 up to 258.8 at 8800va (10.6 volt rise)
When the second tries to export power the rise goes beyond listing requirements and one will drop off.

My calculations should put the rise at 3.6 volts before grid impedance. This is telling me we have somewhere in the magnitude of 3 times the impedance in the system.

The overall length of the AC Circuit is aprox 460' (150' with #2, 160' with 4/0 Alu, & 150' 350kcMil) it is connected to the service through a supply side tap according to 705.12 (A), the service conductors going up to the weather head are new and is 4/0 Alu triplex. The triplex coming from the utility is old and does appear to be in not the best of condition. Then the transformer supplying the house(s) is a new 25KvA that powers a 120/240 LV network for a few houses.

I can upsize some of our conductors without changing the conduit, up to 39% fill. A little frustration in the pull but beats changing EMT. The underground 4/0 Alu I don't want to change until last if it can be avoided. I have asked the utility a few times if they know the transformer impedance, I have yet to get a response.

Will the voltage rise in a linear fashion or does it follow a curve that would require more information. Beside the utility and the system are there other factors that i should be looking into that maybe exacerbating the issue. I would like to resolve the issue in a timely manner, but also in an efficient manner.

 

[GoldDigger]

If the utility does not have voltage regulators (e.g. tap changers) on the circuit the voltage rise should be linear with produced current.

Sent from my XT1585 using Tapatalk

 

[beanland]

Utility Transformer

Utility Transformer

A typical 25kVA transformer will have 2-3% impedance. About 0.05 ohms. There will be negligible impedance from the rest of the utility system.

How does the voltage vary with load? If you add 8800VA of load, does the voltage drop 10.6V?

 

[ggunn]

First thank you for your help ahead of time. I am versed in Theory and calculations but I am not an EE that is part of the reason I am here.

I have taken on a problem site of ours that continues to go off line due to the inverter(s) output voltage rising and causing it to go out of limits (264V). I know some of the fault is ours due to not being conservative enough in the VD%, however I would like to try and figure out if the utility is also compounding the issue. As stated I inherited this issue, I did not spec it.

Knowns: When Fault has occurred Grid voltage at array with zero load / zero production was 248.2V
Circuit resistance according to calculation is .103 ohms
Single Phase 120-240 service.
When only one inverter is exporting power the voltage rises from 248.2 up to 258.8 at 8800va (10.6 volt rise)
When the second tries to export power the rise goes beyond listing requirements and one will drop off.

My calculations should put the rise at 3.6 volts before grid impedance. This is telling me we have somewhere in the magnitude of 3 times the impedance in the system.

The overall length of the AC Circuit is aprox 460' (150' with #2, 160' with 4/0 Alu, & 150' 350kcMil) it is connected to the service through a supply side tap according to 705.12 (A), the service conductors going up to the weather head are new and is 4/0 Alu triplex. The triplex coming from the utility is old and does appear to be in not the best of condition. Then the transformer supplying the house(s) is a new 25KvA that powers a 120/240 LV network for a few houses.

I can upsize some of our conductors without changing the conduit, up to 39% fill. A little frustration in the pull but beats changing EMT. The underground 4/0 Alu I don't want to change until last if it can be avoided. I have asked the utility a few times if they know the transformer impedance, I have yet to get a response.

Will the voltage rise in a linear fashion or does it follow a curve that would require more information. Beside the utility and the system are there other factors that i should be looking into that maybe exacerbating the issue. I would like to resolve the issue in a timely manner, but also in an efficient manner.

I have encountered similar situations where the utility could not/would not lower the voltage. That, and once the installers moved an inverter ~100' farther away from the service without telling me they were going to do it so I could rerun Vd calcs. We handled them with buck-boost transformers.

 

[GeorgeB]

With your measured vs calculated differing by about 3:1, I'd do 2 things. First, verify (as beanland suggested) how much drop there is with a similar applied load. A home dryer or water heater are similar, and together likely exceed the 8800 kVA of your system.

I'm going to assume the load doesn't create a similar drop. Second, I'd try to measure the drop at your junctions between series conductors. Either thermal or electrical would be useful.

My bet is on one of your splices.