Over voltage

[SRE]

Recently installed Sanyo 240's with Fronius 3.6tl but having problems with over voltage. DNO has monitored it but say it's going to take 3 months to assess it, meanwhile sun shines, inverter cuts out, customer not happy. The fronius has been re-set to 261v but still seems to be cutting out at 256v, but another installation in Feb down the road with same inverter isn't cutting out - Fronius problem or DNO?

I know the DNO are allowed a certain % of over voltage but of course we don't know how much it is because they aren't doing the assessment of the monitoring for 3 months. Any thoughts - the DNO has told the customer this is happening a lot.

 

[malcolmsanford]

Have you guys ever thought about Surge Protection Devices, designed initially for transient voltages for lightening, but we use then quite effectively over here as the voltage is not the most stable.

I always thought that perhaps you guys would get yourselves one to see if it worked on an instance liked this, and if it did you could advise the client to purchase their own

 

[BruceB]

Recently installed Sanyo 240's with Fronius 3.6tl but having problems with over voltage. DNO has monitored it but say it's going to take 3 months to assess it, meanwhile sun shines, inverter cuts out, customer not happy. The fronius has been re-set to 261v but still seems to be cutting out at 256v, but another installation in Feb down the road with same inverter isn't cutting out - Fronius problem or DNO?

I know the DNO are allowed a certain % of over voltage but of course we don't know how much it is because they aren't doing the assessment of the monitoring for 3 months. Any thoughts - the DNO has told the customer this is happening a lot.

The key thing from the DNO's perspective will be the voltage at the supply terminals of the installation. The ESQC Regulations specify 230V +10% -6%. So the maximum they are allowed to supply electricity to your customer is 253V, effectively at the meter. You or they might have to monitor the voltage to see whether it is sustained high voltage shutting down the inverter, or transients on the line which I think is what malcolmsanford is talking about. The DNO has no freedom to go above 253V by design - they would be in contravention of the ESQC Regs.

I do not know the Fronius inverters, but can you set it higher still? The G83 limit is 264V. And depending on inverter design the clearance time can be up to 5s.
Or can you run a thicker cable between inverter and CU to reduce voltage rise?

Regards
Bruce

 

[JD6400]

Hi SRE , why not just hire your own monitoring gear , even your locall wholesaler branch should be able to arrange it for you .
It will be around £100 or so to hire for a week or any where from £1200 to £8000 to buy , if you ask me the second option is best as you can put it out on prospective clients to just see what you are up against and then again when installed to show a true usage pattern change !
Hi Malcolm , We are meant to be doing such a test if the company that makes it ever gets back to us .
Hi Bruce , yes they are limited by the ESQC regs but are allowed there is a get out clause of 5% that allows them to do what they like in essence .

Yes you can up the limit on the inverter ( simple as pressing a couple of display buttons few times in a certain order ).

As for the larger cable i do not understand how this could be , surly this would aid voltage rise ?
I am honestly not trying to be smart with that last comment Bruce as i believe you to be a very good source of information , but have read several of your post where not just yourself but others on this forum that i equally respect the opinion of , have mentioned a larger sub main would help to keep the voltage down at the inverter .
I have held off posting this Query until now for fear of looking a Pratt but i need to know what am i missing here ?

 

[BruceB]

DPE,

I am not aware of any authority that allows the DNO to breach the ESQC Regs. Do you have a reference?

Whilst they may not be charged and convicted for a breach if the voltage goes above the limit in an unplanned way temporarily, my point is I am not aware of any authority that allows them to plan to continue breaching the limit if they know it is happening regularly. The ESQC Regs are the law and do not offer any flexibility.

The larger cable will help because the current from the inverter to the CU causes a voltage drop in the cable. Now the voltage reference is effectively the voltage at the supply terminals, so the voltage at the inverter will be higher than that at the supply terminals by whatever is the voltage drop (or rise if you want to look at it that way) is in the cable between the inverter and CU. Increase its cross-sectional area and the volage drop will reduce (standard Ohm's law stuff).

Regards
Bruce

 

[dansk]

DPE,

I am not aware of any authority that allows the DNO to breach the ESQC Regs. Do you have a reference?

Whilst they may not be charged and convicted for a breach if the voltage goes above the limit in an unplanned way temporarily, my point is I am not aware of any authority that allows them to plan to continue breaching the limit if they know it is happening regularly. The ESQC Regs are the law and do not offer any flexibility.

The larger cable will help because the current from the inverter to the CU causes a voltage drop in the cable. Now the voltage reference is effectively the voltage at the supply terminals, so the voltage at the inverter will be higher than that at the supply terminals by whatever is the voltage drop (or rise if you want to look at it that way) is in the cable between the inverter and CU. Increase its cross-sectional area and the volage drop will reduce (standard Ohm's law stuff).

Regards
Bruce

@Bruce - and therefore the voltage at the inverter would almost match the voltage at the supply terminals? is that right?

 

[BruceB]

Yes the thicker the cable the closer it will be. This is the 1% voltage drop limit the DTI guide talks about.

I did put together a picture to help explain in March. Post #12 in this thread:
http://www.electriciansforums.net/p...tall-first-g59-adivice-submains-advice-2.html

Regards
Bruce

 

[dansk]

Yeah im with it - all i was trying to get through my "ed" was how this benefits the inverter when there is overvoltage at the supply.

 

[JD6400]

Bruce , Thanks for the reply but i rater embarrassingly i just can not seem to get my head around this one !
I just can not see how the inverter will take its reference voltage from the service head side of the sub main and not from where it is connected to the sub main ?
Say just for a example that you have 240v at the service head and at the other end of a 100m sub main where the inverter is you have a 10 volt drop giving you 230v , how does the inverter then know to be + of the 240 figure ?
I feel like i am a 1st year apprentice with this one and need to know for piece of mind how after my years in the trade that this is so foreign to me !!!
Many Thanks
Dave

 

[BruceB]

What would be causing a 10V voltage drop at the inverter end of the sub-main? If there is other current using equipment there then you might have that situation, but for there to be a voltage drop then that current using equipment would have to be using all the inverter power and enough power from the mains to cause that voltage drop.

Probably better to consider this with only an inverter connected to the sub-main. When the inverter is first connected there will be no current flowing and the voltage at the inverter end of the sub-main will be the same as that at the service head end of the sub-main. When the inverter starts pushing current and power to the grid then the voltage at the inverter end has to rise by the I*R value to overcome the resistance R of the sub-main. Whether the voltage rises because of the current flowing or the current increases because of the rise in voltage is a moot point and much like the "what comes first the chicken or the egg" question. The inverter does not 'see' the voltage at the service head, it only 'sees' the voltage at its own terminals, but Ohms law gives you what it has to be in the steady state given that the DNO supply voltage is the constant. Of course that assumption is not completely true because there is also resistance in the cable between the service head and the DNO transformer, but it is good enough for a first approximation.

I find it helps to draw diagrams if I get confused over this sort of thing.

Regards
Bruce

 

[yellowvanman]

dpelectricalltd - but the voltage drop is the 'other way'. Making the assumption that the inverter is kicking out current, at the far end of the cable (at the head end) is 240V, so the voltage at the inverter terminals will be 250V (using the 10V drop you have mentioned) because the flow of current is from inverter to service head.

Obviously if you increase the size of cable the voltage drop could be reduced from 10V to 5V say, by doubling the CSA.

Well thats the way I see it! I can see some confusion at the time when the inverter is not outputting any current but in that state the voltage drop will be very small becuase the 'offload' consumed current of the inverter is very small (I assume).

 

[JD6400]

Hi again , the 10v drop was just a simplified example to work with .
And thanks for sticking with it !
I have always assumed that the inverter supply's at around + 2v and measures the true mains at point of connection to the individual inverter so if there is a volt drop on the sub it would work both ways , both on the true mains going to the inverter and from the inverter back to the main head .
That is why i have not been able to get my head around what you are saying and to be honest still can't .
I do appreciate the effort you are putting in to explaining this and would great full if you could persevere with me as i do not like to work blind after it has been pointed out to me .
Again Many Thanks
Dave

 

[yellowvanman]

Not sure if I understand correctly what you are saying.

If the inverter is on a sub main and the load on the sub main is greater than the power from the inverter then yes there will be a real voltage drop between the service head and the sub main because there is still power coming down the sub main from the grid, so to use your 10V example. In effect there is 230V at the sub mains, in which case as long as the inverter can create a voltage greater than 230V, the loads on the sub main will use the inverter power first. If as your saying the inverter generates a voltage @ measured +2V, in this case 232V, if the service head is at 240V then power from the inverter can't go up to the service head, it'll all be used 'locally'.

 

[JD6400]

Thanks for the reply yellowvanman , but still can not get it Bruce !
To be honest every one i have asked on my company can not either .
The way i see it is if the cable is under speck and you have a volt drop at the end where the inverter is connected of say 30 v then that inverter can only see 200 v and so will match its feed in at 202 v ( which would be under supply not over ) as that is all it can see and not the 230 v at the service head .
I am sorry to keep asking but feel i need to know how i could be so wrong ?

 

[yellowvanman]

Just think which way the current is flowing.

In your last post you say the voltage at the inverter is only 200V that means current if flowing from the head to the inverter sub main, in which case you are importing current from the grid, because there is something on the submain that is drawing all the current from the inverter and more!

More usually the current will be coming from the inverter to the head, so the voltage drop will be from the inverter to the head, in which case the voltage at the inverter will be higher than the head voltage.

Current always flows from a higher voltage to a lower voltage.