cable size selection

[infinity]

I'm just looking at a cable for a large install 50KW. With a 120M run,

When doing the calcs in using 50mm swa underground with IB of 80A
And getting a volt drop of 8.3V which is above the 1% allowed,

What do you do in these circumstances? Surely I can't install a 240mm cable that's ridiculous!

 

[jason121]

How about 2 x 120mm cables. We have 2 x 150mm to drag in the ground never managed the 1 % volt drop. Its kinda trade off money v volt drop. But if you end up with inverters tripping due to over voltage could become costly laying more cables. Its your call in the design

 

[davesparks]

Pick the smallest cable size which satisfies all of the requirements.
If the smallest cable which satisfies the volt drop requirement is 240mm then install 240mm.
But there a few other cable sizes between 50 and 240 you could check first. If 50mm comes close then 70mm might be ok.
Also worth doing the calcs for parallel cables to see what you get.

 

[Worcester]

You could always put a G59 relay at the far end of the cable and set the inverters to say G83/2 ....

 

[SolarCity]

I'd check the incoming voltage as it currently is.

Risky, as voltages can rise.

 

[infinity]

Yes I will check the voltage,
I suppose there has to be some compromise between the cable size and volt drop, I think 8.2V will be too much but would think closer to 5V would be more acceptable, what would the maximum voltage tested for at the origin be , anything under 240V? Then surely the over voltage relay in the inverter should never cause a problem?

 

[pvman]

Cant you change the location of the inverter distribution board

Matt

 

[wozz]

is using measured voltage acceptable when designing a circuit, as opposed to using the nominal voltage? if that what you are getting at?

 

[Gavin A]

Yes I will check the voltage,
I suppose there has to be some compromise between the cable size and volt drop, I think 8.2V will be too much but would think closer to 5V would be more acceptable, what would the maximum voltage tested for at the origin be , anything under 240V? Then surely the over voltage relay in the inverter should never cause a problem?

worth getting some voltage monitoring kit for this sort of situation, stick that on for a couple of weeks, work out what the max voltage is, make a bit of allowance for different times of year, then use that to give you the volt drop that should be ok without the inverter tripping out.

or as Worcester says, stick a G59 relay at the grid connection point and adjust the voltage higher at the inverter, as long as it's a dedicated circuit for the inverters only, and clearly marked as such.

 

[Gavin A]

is using measured voltage acceptable when designing a circuit, as opposed to using the nominal voltage? if that what you are getting at?

for embeded generation - yes, if what you're aiming for is a guide to what the actual voltage range is so as to avoid the inverters tripping out at the top of their allowable voltage range. YOu'd need to be getting data on the actual voltage range as opposed to using a single voltage reading.

If you're not going to do this then you'll have to stick with the 1% guidance figure, but then even that can result in inverters tripping out regularly in some situations.

 

[jason121]

[or as Worcester says, stick a G59 relay at the grid connection point and adjust the voltage higher at the inverter, as long as it's a dedicated circuit for the inverters only, and clearly marked as such.[/QUOTE]

Now add the cost of the relay/panel and test engineer to your quote

 

[wozz]

for embeded generation - yes, if what you're aiming for is a guide to what the actual voltage range is so as to avoid the inverters tripping out at the top of their allowable voltage range. YOu'd need to be getting data on the actual voltage range as opposed to using a single voltage reading.

If you're not going to do this then you'll have to stick with the 1% guidance figure, but then even that can result in inverters tripping out regularly in some situations.

Thanks for reply.

So how does that stand if you base calcs off recorded voltage that may be lower than nominal, then next week the DNO changes their transformer and the voltage changed but still within their allowable range and your equipment stops working? Could this situation arise? and where would you go from there? or am i barking up the wrong tree?

 

[infinity]

I like the idea of the G59 relays, but not sure how much extra this would cost with the relays and test engineer, it may out way the extra sized cable?

Measuring the voltage for a week or so is also a good idea!

At least we know if its high then the 1% may be the only way.

 

[jason121]

Can someone explain to me how by moving the relay this would work

thanks

Jason

 

[Gavin A]

Can someone explain to me how by moving the relay this would work

thanks

Jason

it would mean that the reference voltage for the G59 settings would be taken at the grid connection point rather than 120m away, so it would protect the voltage going into the grid rather than the voltage at the inverters 120m from the grid.

ie the inverters could then be set to run 10V higher than G59 limits without any problems (or however high you wanted them really), as long as it was a dedicated circuit clearly marked to prevent some muppet coming along and wiring some sockets into the inverter end in the future.

The cost / benefit ratio of doing this may or may not work, depends on the alternative cable costs, but it's an option to consider.