Add GEB var speed system power shedding capability for separate backup systems

[yzhou601]

Currently the variable speed power shedding control is only introduced for integrated backup systems. Separate backup systems in theory, have no way to receive the shedding signals, but considering that we introduced electric backup as "just in case" in BEopt/ResStock to avoid unmet loads, it's probably better to be handled the same as integrated backup than letting the backup run during the DR event.

From PR comment: #1325 (comment).

Some discussions as context:

[2/9 1:37 PM] Winkler, Jon
I see. So you mean an electric backup is sometimes added, even though one wasn't specified, to limit unmet loads? That makes sense but poses a challenge for modeling DR. The DR for VCHPs based on AHRI 1380, limits the compressor speed without adjusting the setpoint. So really, the load is being unmet. And then there is a maximum allowed offset to prevent the home from becoming too cold. Maybe in those cases it makes sense to treat the backup heat in the same way an integrated backup would be treated? I realize that's a bit idealistic but it's probably better than letting the backup run during a DR event. (Which would actually increase demand, not reduce it.)

[2/9 1:40 PM] Maguire, Jeff
Yeah that was my primary concern, you'd apply this control to try to limit power and you'd actually end up increasing it because the backup doesn't know about the signal you're sending. I'd be OK with not making this distinction if we think it's going to cause problems and/or be confusing to most people, but it is definitely idealizing what actually happens here.

[2/9 1:43 PM] Horowitz, Scott
I was also tempted to suggest we go that route. It may not be so simple to implement, but worth evaluating.

[2/9 2:23 PM] Zhou, Yueyue
Yes I vote, I just took a quick look and it didn't seem to be straightforward to implement, the separate backup system can be created before or after the EMS program being created, and also we need a way to pass the E+ backup system coil to the EMS method to actuate. I couldn't find some straightforward methodology off the top of my head. I feel that we may create an issue for this and probably address it later. So far I feel that we may be the only user for this research based feature

[2/9 2:51 PM] Horowitz, Scott
Yeah, you'd probably have create the EMS program after all the HVAC systems have been created instead of on the fly as you create each HVAC system.