Bus Differential Protection for Load Side Connected PV

310 BLAZE IT

Senior Member
I am very familiar with section 705 so I want to steer this post away from reviewing those code requirements. Perhaps this discussion goes beyond code since code only gives requirements for the rating of busbars in 'Panelboards'.

Since many utilities require cold sequence metering, where a connection on the supply side of the service disconnect is prohibited due to net metering requirements, I am looking for a way to connect on the load side in a way that ensures the bus is protected. Reducing the main breaker to a smaller size, connecting on the opposite end of the busbar, and increasing the bus rating beyond the main breaker rating are all not options here.

Has anyone run into this and determined the best way of adding differential protection? I am thinking that I will need CTs after the meter and on the PV breaker and will need to use an SEL relay protection with an RTAC unit to temporarily curtail the PV Intervters. I was thinking of using an SEL 587 but I don't see any relay specific for this application so I'm not too confident with this application.

Anyone have experience with this?

Thanks,
 

wwhitney

Senior Member
Reducing the main breaker to a smaller size, connecting on the opposite end of the busbar, and increasing the bus rating beyond the main breaker rating are all not options here.
I can't speak to your question, but I wanted to mention there is one more standard option:

If the service disconnect/OCPD is in its own enclosure (or can be put in own), and it feeds a panel that can take a main breaker, then you can simply intercept that feeder using (2014) 705.12(D)(2)(1).

Cheers, Wayne
 

ggunn

PE (Electrical), NABCEP certified
I can't speak to your question, but I wanted to mention there is one more standard option:

If the service disconnect/OCPD is in its own enclosure (or can be put in own), and it feeds a panel that can take a main breaker, then you can simply intercept that feeder using (2014) 705.12(D)(2)(1).

Cheers, Wayne
And of course you have to install that main breaker.
 

pv_n00b

Senior Member
A few ideas.
  1. It's not a requirement for cold sequence metering that the disconnect ahead of the meter is the service disconnect (NEC 230.82(3)). It can be a meter disconnect and the service disconnect comes after the meter. It's worth checking what you actually might have. If the service disconnect is after the meter then you can still do a supply side interconnection.
  2. Interconnections to SWBDs on the load side of the main disconnect can be done under engineering supervision without making any other changes. This is implied in the NEC up to 2017 and specifically spelled out in 2020.
  3. If it's possible, put another OCPD downstream of the PV interconnection to protect the bus.
  4. The interconnection can be done with relay protection but that is not in the current code so you are back to doing it under engineering supervision. Power control has been added to the 2020 NEC in 705.13.
 

310 BLAZE IT

Senior Member
I figured it out. I am going to use a 751 relay and parallel the CTs on the main solar and tie using partial buss differential. This creates an ideal source that can be coordinated w downstream loads. Easypower actually now has this capability to model and they have a webinar on it coming up. This is a much lower cost solution than anything else. Pseg requires cold sequence metering on all secondary services with solar new construction (and all 480v new construction) causing this scenario.

Pure power illudes to this solution on their website but it's their secret sauce so not explicit in their writeup

Sent from my SM-G965U using Tapatalk
 

pv_n00b

Senior Member
That is a solution, measure the current from the utility and from the PV system being delivered to the load bus and use a relay to make sure it does not go above the bus rating by controlling the PV system output. Of course, the only way this would happen would be if someone overloaded the distribution section in the first place. Without the PV system supplying power, this would just trip the main OCPD. So if the distribution load exceeds the bus rating reducing the PV output will increase the current from the utility until the main OCPD trips. Fun times then.

The only problem with this solution is that the NEC does not explicitly allow it as an option. The NEC only allow OCPD protection. This solution was added as an option in the 2020 NEC because it solves issues with AC coupled energy storage. But in 2017 and before an AJH can choose not to accept it.
 
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