UG-H

[Ervin R Dilly]

I have 2 inverters connected to the same feed going into the main breaker box.1- 4.2from solar and the other is a 5.0from anouther company.My problem is when I start the 5.0 unit it starts and then goes into default (stop) and states UG-H.Is the problem that I have both units on 1 line.Voltage reading will be at 256 and then jumps to 262,which is the top limit.Using 12 guage to the garage junction box,from there it is 6 ga.to the main box.Any ideas as what needs to be done? Thanks

[Rob Beckers]

Ervin, what's the regular voltage (without backfeeding) from the utility at your place? If this is on the high side you can ask them if they can move your feed to a lower-voltage tab on the transformer (some transformers have tabs, some don't, so no guarantee this is possible).

You'd need to measure what the voltage is doing with/without backfeeding at the panel, to see if the problem is caused by excessive resistance on your side of the wiring, or the utility's. In the latter case you're probably up-the-creek (without a paddle)...

Many inverters have settings that can be changed to allow a wider/higher voltage before throwing an overvoltage error. That said, it may not be a great idea to have 260+ Volt (130+ for your outlets) much of the time. That's pretty high and not all appliances will take kindly to that long-term.

-Rob-

[Ervin R Dilly]

rob,thanks for getting back with me.Did a volt check with a very nice sunny,cool day and this is what I get.At the house with the grid on127V,with grid off 126V so it may be that I am getting to much to start with.Will try the electric company and talk to them,doubt if they will do or can do anything.Probably mad because my bill is not high enough.Im leaving the 4.2 alone as this is working good,but what if I took the 5.0 and try to cut down on the incoming voltage.Just a thought. Ervin

[Rob Beckers]

So, with the PV switched off you have 126 - 127 Volt? That's 252V coming in (split-phase), and that's too high. The utility should lower this, and get it closer to 240V.

I'm not sure what you mean by "what if I took the 5.0 and try to cut down on the incoming voltage". You can't lower the incoming voltage (unless the utility does it for you).

-RoB-

[Ervin R Dilly]

rob,got your 2 emails and sorry I didnt post them on this site.Went to the electric company this morning and they will come by tomorow to check the V at the meter.Also stated that the only control they have is at the terminal station,and they cant/wont change that as they run high for the people that are furthur away.And they also stated that they try to keep it at the 126V for this reason.Also said out of the 36 other private homes that they service that I am the only 1 to say anything about this.Go figure,got to be a trouble maker somewhere.Will try to keep you posted.One ?,is it posible for 2 differnt inverters not get along?

[Rob Beckers]

The inverters are getting along fine, that's not it at all. The inverters have a maximum voltage in them, at which they switch off. All grid-tie inverters have this, it is a UL1741/CSA22.2 requirement. To feed power back into the grid the voltage has to be higher than the grid's voltage; how much depends on the impedance (resistance) of the grid, it will get worse (higher voltage) the further down the line you are.

The standard requires that inverters switch off above 110% of 'rated' voltage (ie. 240V normally), within 1 second. There are additional requirements; if the voltage goes up to 120% of rated the inverter has to disconnect within 0.16 seconds and so on. The actual voltage that a given brand/type inverter will switch off at depends on the inverter, they are all different, and they are all below the norm. For Power-One string inverters they will go up to 108%, and if that lasts more than 0.9s the inverter will switch off. For 240V that means the inverter goes off at 259 Volt.

The reason nobody complains to the utility is that they don't know. Other than light bulbs burning out a little faster there's not much to tell that the voltage is high (besides measuring it). It's by and large only people with UL/CSA compliant inverters that will notice, like you.

For Power-One it is possible to change the 'grid' voltage that it expects. By making it a little higher the inverter won't switch off quite so soon. For that you'll have to give me a call, when you're standing next to the inverter (613-686-4618 x301), so I can talk you through it. For other inverters this may be possible too, but I do not have the procedure for it, for those you'll have to contact the manufacturer.

-RoB-

[Ervin R Dilly]

Rob.I think that I have a handle on it .Talked to the company and they stated to take the earth ground off of the inverter and leave only the power from the service panel connected.Did this yesterday and it has worked yesterday and today without shutting down.Never have had a problem with the 4.2.They seem to think that the earth ground and the power grnd is to much.Left my solar panels grounded as these need protected.It seems to work so will try this for a while.I am getting a difference of about 6 V between the 4.2 and the 5.0,the 4.2 being the lower 1,which is good.Thanks

[Ervin R Dilly]

Ok Rob,I thought I had this under control but the ugly head showed up again.If I get up around 400W per input the V goes up to 263 and the 5.0 shuts down.Here is somthing I was wondering if this could cause it.I have 12/3 wire running from my 2 inverters to the box that the electric company put in,from there it is a 6G alum wire.Is it possible that the (25ft) of 12/3 wire cannot handle the power and it is resisting it.Just a thought,cant have this all summer.By the way the 4.2 and 5.0 sit side by side and both feed into the same line,by way of independent breakers.The 4.2 is always reading 3 to 4 V lower than the 5.0.The company says it should hadle 265V and there is no adj.Thanks

[Rob Beckers]

Not sure 12 AWG is even legal for 37.5 Amp (9,000 Watt)...
The short length of it, 25ft, won't cause much of a voltage drop, a few Volt maximum. But for that current 10 AWG is marginal, and 8 AWG is the usual (we normally use 6 AWG when combining two 5kW inverters). Anyway, apart from legality I doubt that's causing you're problem.

Different voltage readings between inverters are normal: The inverters are not very accurate in measuring voltage, a few Volt is well within their margin of error. You need to measure the voltage with a decent multimeter to find out what the actual value is. How long is that 6 AWG aluminum cable? That's pretty small wire for feeding a house!

Not sure what "taking the ground off" means, but you must ground all the metal parts/housings, such as the inverters. Not doing so means you risk electrocution.

My earlier message still stands. The inverters are simply switching off due to overvoltage IMO. So the transformer that drops the voltage from the pole for your house doesn't have tabs that the electrical company can change?

-RoB-

[Ervin R Dilly]

Thanks for the quik reply.Dont take this wrong but it is funny that I have to get all my info from a canuke,just kidding,apprecite it.The 6Ga wire is about 30 Ft long to the main box.I thought that the 6 would be heavy enough.Do you think that if I went to 6 leading into the electric box it would help?Dont have a transformer at my location,it is on the corner about a half block away.They told me to take the earth grnd off of the inverter and to leave the panel grnd hooked to the inverter.Will try later to get a good reading from a volt meter at the inverter.