dc surge protection

paulsamuel1984 · Jun 28, 2011

What are peoples experience on adding the additional class II surge protection to transformerless inverters? Particuarly the sunny tripower range?

Are people finding that this is becoming a regular requirement for their installs?

I have seen a guideline regarding surge protection being required at the array as well as the inverter if there is more than 10m between the array and the inverter. What are peoples opinion of this? If I have 12 strings, this is getting very expensive. Even worse if class I is required!!

Again, any help would be greatly appreciated.

malcolmsanford · Jun 28, 2011

I always thought that you designed the invertor on your voltage open circuit x 1.25 and size the invertor accordingly. So your invertor would never suffer from DC voltage surges as it was designed to take the maximum DC that an array can ever produce, so the use of a voltage supressor seems strange.
Is it to somehow counteract the possiblilty of fault currents from the DC back into an AC side if you don't have a transformer and therefore seperation. If that was the case then you would need to somehow get an earth up to your DC side I would imagine.

paulsamuel1984 · Jun 28, 2011

I design all DC components based on this (Voc x 1.15 and Isc x 1.25). Regarding the inverter, I design this based on the best power ratio, and also the voltage it can be exposed to at low temperatures (-10 degree C). As it is a transformerless inverter, I would bond the frames and take one connection down to earth. However I have been advised that additional class II surge protection may be required at both inverter and array.

Please see comment below from Furse:

"You mention 1.25 x Isc, which is a known safety factor for cabling, but this should not be confused with the surge current rating which may be much higher, but as a 'transient' is active for a much shorter duration; measured in microseconds.
Overvoltage is certainly your main threat, as this may cause flashover and damage within the system."

Any thoughts?

malcolmsanford · Jun 28, 2011

Well I have great respect for Furse, who sell these SPD, lol is that a coincidence.

I'm not sure where these surge currents are coming from, are they referring to fualt current ? Does an array have the capability of producing the surge currents in production. I would have thought if you had 10 panels and at maximum harvest potential they produce 2.4Kw, and you size an invertor to that with the 1,25 safety factor, can these 10 panels produce 3kw plus so give these "transient" values.

They surely though must be talking about fault currents.