PV voltage rise affecting other loads?

[JoeNorm]

We're installing an array VERY far away from the service point and transformer. Voltage rise will be an issue. I was not part of the design and it's moving forward so there is not much to do about it at this point besides see what happens.

The voltage rise at the other end is one thing, but in this case we are tying into a sub-panel about a quarter of the way back to the transformer that has other loads such as a well pump. I am wondering how the inverter output may affect these other loads?

If the inverter has to put out 264VAC to make it back to the transformer that is what the well pump is going to see at that time too, correct? I am concerned we may damage other parts of the electrical system over time. Should this be a concern?

 

[Dennis Alwon]

If the motor is 240 then 264 v is 10% higher. That may be okay however if it is a 208 motor then you will have to drop the voltage down at the motor.

10% is really the max the motor should see so it may be worth dropping it to 240 or 208 for safety

 

[winnie]

.... but in this case we are tying into a sub-panel about a quarter of the way back to the transformer that has other loads such as a well pump. I am wondering how the inverter output may affect these other loads?

If the inverter has to put out 264VAC to make it back to the transformer that is what the well pump is going to see at that time too, correct?

If the inverter needs to put out voltage 10% higher than the service because of voltage drop along the feeder to the PV system, and the subpanel you are concerned about is 1/4 the way back to the transformer, then it will see a 7.5% voltage rise.

IMHO that is still enough to be concerned about.

A simple buck transformer won't help here because the voltage will be all over the place depending upon sun conditions and subpanel loading

Jon

 

[Dennis Alwon]

If the inverter needs to put out voltage 10% higher than the service because of voltage drop along the feeder to the PV system, and the subpanel you are concerned about is 1/4 the way back to the transformer, then it will see a 7.5% voltage rise.

IMHO that is still enough to be concerned about.

A simple buck transformer won't help here because the voltage will be all over the place depending upon sun conditions and subpanel loading

Jon

So what do you propose

 

[winnie]

So what do you propose

That would depend upon the loads being supplied, but I would try to select loads that could tolerate voltage variations.

For example if the subpanel were powering a pump, I might use a VFD to regulate the voltage to the pump, and confirm that the VFD can handle the maximum supply voltage safely.

If push came to shove there are voltage regulating transformers available, but they are expensive and modern electronic loads can tolerate wide voltage ranges.

Jon

 

[Dennis Alwon]

What would the vd be at the motor? And @winnie why is 7.5% voltage rise a concern. I thought a motor could handle +- 10%

 

[winnie]

What happens if the utility is 5% high on top of the voltage rise? I guess at some point the inverter shuts down....

-Jon

 

[electrofelon]

What happens if the utility is 5% high on top of the voltage rise? I guess at some point the inverter shuts down....

-Jon

That was my first concern, is whether the system will even operate. The inverter parameters can be modified to expand the voltage range, but that usually requires a special code from the manufacturer and they are reluctant to do it usually. One option would be to see what the voltage is at the service equipment and if high there too maybe the utility can be forced to help out. We had a 25kw system that pushed the voltage up quite high at the service equipment and the POCO replaced the transformer, presumably with one that was larger or had lower impedance so it had better voltage regulation. You are going to have "rise" on all parts of the system, even the POCO, so it's a matter of seeing what you can pick up and where.

 

[jaggedben]

A big load at the subpanel should actually help. If the solar was backfeeding the feeder between the sub and service, the load will reduce the current on the feeder, thus reducing the voltage rise, letting the inverters output at a lower voltage, and reducing the voltage at the sub. If the load is larger than the PV output in the given moment, then the voltage at the sub will be lower than at the service.

Note that while most utilities try to keep service voltage closer to nominal than 10% above, I believe equipment designers should be looking to the same ANSI standard that the inverters use. In other words, I think a 240V pump ought to be able to handle 264V if it meets the applicable product standards. Some due diligence might be in order to contact manufacturers and ask them to verify this. But I might be more concerned about small loads than the big well pump. Devices without regulated power supplies might fail. I suspect the most likely effect of this situation would be small electronic power supplies experiencing a shorter lifespan.

 

[electrofelon]

We're installing an array VERY far away from the service point and transformer. Voltage rise will be an issue. I was not part of the design and it's moving forward so there is not much to do about it at this point besides see what happens.

Out of curiosity, how long, how many KW and what size wire?

 

[JoeNorm]

Out of curiosity, how long, how many KW and what size wire?

17.3 KW
250 feet on 4/0 al
1000 feet on #2 al

 

[jaggedben]

At what voltage?

 

[electrofelon]

17.3 KW
250 feet on 4/0 al
1000 feet on #2 al

yeah that's pretty much not going to work (assuming we are talkin about 240 volt single phase).

 

[JoeNorm]

yeah that's pretty much not going to work (assuming we are talkin about 240 volt single phase).

240vac

and yes, I agree. But luckily it's not going to be my problem. I have made the designer/owner aware awhile ago. Project has moved forward anyway.

 

[electrofelon]

The only way I can think of to salvage this (short of redoing the wire run) is to use transformers on each end to bump it up to 600 volts. That will keep voltage drop to about 2.6% which is pretty good, although you probably would still want to have taps on the transformers as you will have voltage drop in the two transformers so it's all going to add up.

 

[winnie]

If you were to bump to higher voltage, you could match the inverter to the new system voltage. An autotransformer arrangement would also save on steel, copper, and losses.

The subpanel would require a transformer to get back to 120/240V

-Jon

 

[winnie]

17.3 KW
250 feet on 4/0 al
1000 feet on #2 al

Is the 250 feet of 4/0 between the solar panel and the subpanel with the pump?

Is there an expectation that the pump will be consuming the bulk of the output of the panel?

If the full 17.3 kW is consumed at the subpanel then the system doesn't have a problem at all. If the pump control does something clever like adjust rate to match available solar power, or there is energy storage at the pump location.....

-Jon

 

[jaggedben]

Is the 250 feet of 4/0 between the solar panel and the subpanel with the pump?

Is there an expectation that the pump will be consuming the bulk of the output of the panel?

If the full 17.3 kW is consumed at the subpanel then the system doesn't have a problem at all. If the pump control does something clever like adjust rate to match available solar power, or there is energy storage at the pump location.....

-Jon

Somehow I doubt there will be a problem when the pump is running, but that the solar system may not operate when the pump isn't running. The pump may also need to operate for 5 mins before the solar system reliably turns on in the middle of a sunny day. So it's a good question when the pump operates and for how long.

 

[JoeNorm]

I am unsure of the use of this particular well pump. I would not want to count on it to run in order for the inverter to work. But it is an interesting thought.

 

[winnie]

I agree that if the pump is a completely independent entity that it wouldn't help. Arguably it would make things worse as the voltage jumps all over the place.

I'm suggesting the possibility of the pump being controlled intentionally to minimize the problem.

Not even sure is this is practical to implement, but I could imagine this as reasonable for an irrigation system or the like.

Jon