Minimum start voltage & operating voltage

[DomB]

My latest customer asked me a goodun:

He noticed from the SMA SB2500HF installation manual that its "startup voltage (adjustable)" was 220V but the "minimum voltage" was 175V.

His question was why not make the startup voltage as low as possible so the inverter kicks in earlier..

I promised I'd get back to him on it; any help much appreciated!

 

[dansk]

Because making the voltage lower means the inverter will stay on longer!

 

[DomB]

riight - you mean that it would therefore be wasting electricity? Wouldn't it be inverting the lower voltage DC power when it's on so be generating for longer?

 

[dansk]

Low input voltage doesnt mean it start up earlier - panels are at there highest voltage on a cold bright morning - and lowest voltage on a hot bright evening, so its wrong to assume inverters with a low voltage start sooner, its the opposite. I dont know the reason why they set the voltage at 220 and not 175, i can only assume the inverter is more effcient at 220 - its the same as saying this car can do 150 MPH, but crusing speed is 100MPH, everything has its sweet spot.

I welcome to be corrected if im mis-understanding.

Remember the cooler and brighter the day, the higher the voltage, the hotter it is the lower the voltage. Low volt inverters will stay on longer in the hottest weather, and high input means it will start on the coldest mornings.

 

[DomB]

Aye, & I understand the inverter's MPP tracker is 'moved around' by fluctuating the voltage it's working at - so that tallys with the efficiency being affected if you lower the starting voltage. Maybe lowering it effectively moves the bandwidth of the MPP tracker to the lower areas.

I'm with you that Voltage is affected by temperature whilst current is affected by insloation - have you ever Voc'd a panel that is totally covered over - must do that on my next install!

 

[Aurora_Service]

Folks

If I may add to the discussion;

Fundamentally the inverter will need enough power in the string(s) to stay connected to the grid. Without enough string(s) power, the reliability of inverter could be reduced (ie the grid connecting internal contactors could toggle on and off, hence reducing the lifetime of these components)


Therefore reduction of the activation voltage is not "a one size fits all" approach and needs to be applied in accordance with the output of manufacturers design software!

 

[SolarCity]

Thanks for the info, Aurora.

Although I'm a bit confused. Why would the internal contacts toggle on and off? Would they not just be on or off?

 

[Aurora_Service]

Hi BiggsSolar

I meant to add some dimension of time to my point! Assuming the inverter is connected to the grid and dosnt have the input power to inject current, it will wait for a given time before disconnecting (ie 60 secs as standard for Aurora, although adjustable). Given changing light conditions this could occur several times in a relatively small period.

Hope this helps

Regards all