A untypical application, so Im not sure what equipment is out there to do it the best (cheapest).
There's basically 2 services, 120/240v single phase. The main house/shop service (200A - 2x100A subpanels) and a grid-tie solar array 10kw (60A). Same transformer/phase into separate disconnects side-by-side. What's wanted is to meter the main service and the solar, preferably in watts, and combine the two for a net value...the solar minus the main (or visa versa)...and if there is a significantly positive net (solar production greater than usage), then operate an optional load (via relay). The reason being, the energy is much cheaper for what is used directly from the solar, vs selling it and buying back.
Some sort of way to have 4 CTs (2 on each service) connect to some instrument that would figure out the net watts and close some contacts if there is say 3500watts extra, then open when it reaches 0 watts (looking at a 3000watt load). Any ideas??
Wait a minute! You are not using a net metering arraingement. You have one meter for the service feeding the house and shop, and a second meter for the inverter output. You are buying High (Utility) and selling Low (Solar). Correct?
You want to use the "cheaper" solar power when it less than the utility power and buy the utility power when there isn't enough solar.
Last edited by LarryC; 12/29/1102:43 PM. Reason: Epiphany and spelling mistakes.
There's one utility pole transformer, which goes to two utility meters (solar and main). But they are in SERIES, as in it goes first to the main meter (bi-directional), then after the main meter it branches to the solar meter, then both the main and solar run (separately) into the shop to two disconnects, then to respective panels. With the bidirectional meter, it is known what solar energy stayed on the property and what leaves the property. What stays on the property is bought back at a much lower rate as there is no delivery, etc.
I think Larry has the right idea, except they are actually selling the solar at a much higher rate (subsidized green power) than it costs for 'normal' power. But they are paid for 100% of the solar, then buy everything back at normal rates. Except what they use directly they buy back at much less than even normal rates (no transmission).
And yes, the utility meter is not a net meter, and maybe net is not the best term for what is wanted. Maybe summation? (combine/add the two). Ideally if we could meter it where the main bi-directional meter is, since it's that meter we want to duplicate (what is exiting the property), but the utility is not going to let us touch their stuff.
The optional load (water heating for example) comes in when there is excess solar production over what is being consumed. Load shed isn't quite right...it's the opposite, the load turns ON when it reaches the preset power.
As you are north of the border, one of the Canadian members may input if this is allowed in Canada.
Tou can purchase an electronic KWHr meter from a company like E-Mon D-Mon, and install that to record anything. It uses split core CTs so there are no lines to tap, cut, etc. One meter can record multiple loads (sub-panels) via additional sets of CTs. Square D and others also make similar units.
You could do what I think you want ('summation') with one meter and two sets of CTs.
E-mon looks interesting. A couple things I think Im seeing is most don't do real-time watts except the higher end 3-phase units. And only the class 3000 (3-phase) has load control. Something like that single-phase, real-time watts and load control would be needed.
This about multiple loads with additional sets of CTs....How is that? Some of those E-mons refer to paralleling sets of CTs...as in wiring more than one CT to one (pair) CT input on the meter?? I thought/was told that couldn't be done....It was one of my first thoughts...maybe we're wrong?