100A Ground Fault Protection for 100A, 200V MPPT Controller

MStar1

We have been looking for a 100A GFP device for the Morningstar GenStar MPPT controller.
The highest current rating I could find is 100A, 50A GFD breakers from Midnite and Outback.
Does anyone know if two of these can be used in parallel with two subarrays or if there is a 100A GFP device for 100A > 200V?

BB.

I have seen circuit breaker products that (as I recall) can be "stacked" in series--take two 150 VDC rated breakers (mechanically linked trip) and place them in series for a single 300 VDC rated breaker.

So if you can get (for example) 2x 100 VDC rated 100 amp breakers and place them in series (with the 1 amp "pilot" ground fault sense breaker), you should be able to get the Listing Agency to accept that configuration.

If you work with a breaker manufacturer and UL/NRTL Listing agency, you might be able to get them to "List" placing two (or more) breakers in parallel (again, mechanically linked trip).

HOWEVER for parallel connected breakers, I would be looking at the series resistance of each breaker and the breaker "combination". There needs to be a "guarantee" that 50% (+/- for two breakers in parallel) of the current flows through each breaker so that each is measuring the accurate trip current (normally some sort of "ballast" resistance to spit current flow in parallel DC circuits). Each breaker should be rated for its maximum interrupt current (x,000 Amperes each)... So the interrupt should work OK for a solar array with less than 100 Amp Isc rating.

IF you have, for example, 2x 50 Amp sub arrays, then you should be able to use the standard 1 amp "sense" breaker to mechanically trip the two (or more) parallel breakers that each have a 50 Amp (example) breaker (or, for example 2x 50 Amp/100 VDC Max voltage rated) breakers in series per 50 Amp array leg.

Note: Personally, I have major safety concerns with the "sense" GFP fuse/breaker current sense system. This appears to be a method to try and shoehorn "Listed" breakers into an in appropriate system to create this GFP function.

I even wrote up a little white paper that basically a fuse/breaker in a safety ground connection is never acceptable. A single ground fault in the positive solar array will trip the sense fuse/breaker and then cause the (for example negative grounded DC system) to have the formerly "hot" positive to ground, and the negative (formerly grounded) lead(s) to be taken to negative battery voltage or negative array voltage (depending on where the ground fault occurs). 

https://iaeimagazine.org/2008/january2008/ground-fault-protection-for-pv-systems/ (ground fault trips safety ground fuse/breaker)

https://www.morningstarcorp.com/wp-content/uploads/datasheet-ground-fault-protection-device-en.pdf (MorningStar's ground sense, ground stays connected)

Links to way too long discussions about breaking safety grounds for DC GFP devices/systems (the eventual answer was I was talking about some other European code or something--as I recall and did not address the lifting of safety grounds in a system):

https://forum.solar-electric.com/discussion/comment/272194

If you had two pole disconnects on (for example) the solar array +/- ... That would "safely" disconnect the solar array voltage/energy source from the DC system... However, that assumes that the GF occurs at/on the solar array... What happens if the ground fault occurs (for example) on the "battery bus" side of the system... The solar array is disconnected, but the DC battery bus is still energized, + is taken to near zero volts (via ground fault), but the - (negative) terminal is now energized to negative battery bus voltage. Obvious system issues ("converted" DC bus from negative ground battery bus to positive ground battery bus) and safety issues.

I am a little confused about your requirements... Why 100 amp rated breakers? The GenStar is rated for 100 Amp DC output, but if you run the Array at 120 Vmp nominal, the maximum PV Array current would be 50 amps or less... I see that would "limit" the maximum charging current specifications for lower voltage arrays...

Personally, I would suggest looking a different solution. Last I looked (it has been decades since I have cracked open a NEC book), but as I recall, the GFP just requires the equipment to shut down, not to physically break (for example) the PV Array connection (one or two pole?).

A possible solution would be for you to talk with the MorningStar engineer(s) about their GFP system. For example, take the DC Fault Current sensor and use the to connect to the GenStar's "new? inhibit" input. The remote GF detector connects to the GenStar and shuts it down. As far as I know, that is the only code requirement--And it keeps the safety ground(s) connected (no detection fuse/breaker).

Your thoughts?

Sincerely,
-Bill

MStar1

Thankyou Bill,

Using the max PV current = 1.25 X Isc a 50A PV breaker would have a max Isc = 40A and 100A PV breaker would have a max Isc = 80A.
I did some array sizing with 60 cell and and 72 cell modules and found the following max array sizing with 150V and 200V max voltages.

GFP-150V (100A)     5200W - 7300W        100-125% max output power

GFP-300V (50A)     3,800W - 5,100W      75-90% max output power

80A is actually good enough if it isn't oversized too much. 

GFP-300V (80A)    5,600W - 8,000W    115-140% max output power

This really limits the size of the array that can be used especially with the GFP-300V (50A).

I was thinking that if the 300V 50A GFD breakers had a .5A trip it might be okay, but just noticed they have 1A fuses so you would need a GF current of ~ 2A for the first one to trip.

I was looking at a Midnite Epanel with two MPPT controllers and they have a GFP for each controller. I don't know how that would work since the PV negative (common) conductors from each controller will be connected to ground through both GFD trip breakers.
I believe this is why they started making the 4-pole gf breakers like the Outback PNL-GFDI-80Q PV ground-fault detector interrupter, 80A 150VDC four pole panel mount.


Maybe it would be easy to get this listed for 2-pole 300V by wiring the two breakers on the left in series and the two breakers on the left in series. This way it could be used for two GenStar's for 80A.

Or alternatively, a safer solution as you indicated, use it with a single array as a 2-pole 300V, 80A GF Breaker and break both + and -.

I really apprechiate your insight into this Bill.

- Doug