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??

I guess I am confused. How many service drops are there? How many meters? Where does the solar tie in?

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.

Greg, you are not the only one confused! I read this three times. I think LarryC got it!

I hope the OP returns and provides a little more info.

Now, where does the 'optional load' come in? Load shed?

Why not just a POCO net meter arrangement?

Edit: I sent the OP a PM requesting additional info.

Sorry I guess it wasn't clear...I'll try more.

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.

OK, I'll throw this out....

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.

Or, I may not understand your intent fully.

Google 'E-Mon' & check out the info/details.

Lets see if I have this right. Prices are just WAGs (Wild *ss Guess)

You sell solar power @ .10/KWHr
You buy grid power @ .15/KWHr
You can buy local solar power for .12/KWHr

You want to buy the .12 power when there is enough of an excess to power up your optional load. Correct?

What you are asking for is how to determine automatically when the property is producing excess power. Correct?

Sounds like you need the equivalent of a reverse power relay to tell you when the power is flowing out of the property.

Do you own the PV & Inverter setup? Are you allowed to modify the setup?

If I read this right, it sounds like they are selling you your own solar power.

Sweet

I doubt they will help you stop that gravy train.

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?

Close... the actual figures are about:
sell solar power for 60cents kwh
buy grid power for 20cents
buy local power for 6 cents (generation portion of grid power)
And yes don't think too hard, it's a 'game', but hey that's the way it is.

So correct, they want to buy the 6cent power when there is enough available, preferably automatically so they don't have to watch the meter and flip a switch when there is excess...and if something varies (i.e. a cloud goes over the solar or the stove is turned on) they aren't buying the 20cent power for something optional.

So what you need is a directional power sensing device. AKA a Reverse Power Relay which is normally used with paralleled generators setups. It trips when a generator tries to become a motor.

It's relatives that own the setup. Modify? Depends what you mean...maintenance or something like sticking on a meter would be fine. Adding/expanding/changing the solar array would require approval/inspection.

Reverse power relay: Something like what we're after but I think they only monitor one circuit? Here we need to combine two circuits. That seems to be the tricky part - combining/summing them. I can't go on the utility pole where the power lines connect together...else I could just put it there beside the bi-directional meter.

The relays normally come in two formats, from what I can see. Single phase for single phase and balanced three phase applications, and three phase for unbalanced three phase applications.

Apparently you need a CT, a voltage connection, and the actually relay. If your power setup is like a conventional house / small commercial installation, all you would have to do is install a suitable insulated split core CT on a phase conductor at the weatherhead.

OR

Mount the CT on one of the phases immediately before or after the bidirectional meter. How does the Bidirectional meter feed the two disconnects on the shop? Is there a gutter that houses the splices for the two shop mounted disconnects? If there is space, install the CT in the gutter or splice box.

Mount the relay assembly in a suitable enclosure and feed it a circuit from the breaker panel.

Obviously you need the shorting switch for the CT secondary leads and appropriate protection for everything else.

Back to the 'E-Mon' types of meters....
Yes, you can install multiple sets of CTs back to one (1) meter. Research time at the website, or call their customer service engineer guys, they used to be happy to provide any help that I required.

It actually sounds like a great idea, but I don’t think that type of system exists as you described. Probably because it wouldn’t benefit the electric utility companies directly. I’m wondering if you did manage to make your own system like that if the POCO would even let you interconnect it to their grid system without their approval.

If it on the owner's property, would the PoCo even have any say in it? Assuming the owner's electrician does not have to break any PoCo seals to perform the work safely. As it stands right now, the owner can manually turn their optional load on and off manually. All they want to do is automate the process. All they are doing is replicating the PoCo energy meters' information and using it to control loads.

sprdave,

Questions about the physical installation.

1) Where is the bi-directional meter mounted? On the power company owned pole or on the property owner's structure.

2) How and where do the wiring between the two shop drops (solar and PoCo) tie together?

3) Can you get legal physical access to one or both of the phase conductors either before the bi-directional meter or after the bi-meter but before the splice that leads to the two service drops? Not to make any electrical connections, but rather to install split core current transformer(s).

Reasoning:

All the customer really wants to do is automatically turn on a load when he is a net exporter of PV generated power. If the distribution wiring is on the property owner's property, I believe he can do whatever he wants AS LONG AS IT IS ACCEPTABLE to the authorities having jurisdiction. Assuming he is not stealing power, causing actual or potential damage to the utility's equipment, trespassing or any other criminal activity, I do not see an issue.

What he wants to do is duplicate the information that the power company gets in real time. He is not asking for the power company to provide it to him, using their equipment. He is looking to be allowed access to safely install non-intrusive monitoring equipment.

DISCLAIMER

I am not a Canadian resident, lawyer, or even a licensed electrician. My advice is worth exactly what you paid for it, absolutely nothing.

I don't know, I'm not sure how things like that are handled up in Canada. I’m not trying to quash the creative juices, because I really do think it’s good idea, but I am just assuming that unless he can somehow utilize an existing approved net metering setup for this purpose, then he will either have to otherwise be able to interconnect with the utility meter directly or install CT’s ahead of the POCO meter and they do tend to get touchy about things like that, at least around here. If somehow the inspector ends up getting involved, unless the assembly has some sort of NRTL listing, I can see him possibly having issues with it. I don’t think that should stop the OP from trying though.

It exists and has been connected and working for a couple years now. As someone said, they have just been manually guessing...sunny day run the dryer or hot tub, etc.

As far as setup...Im not good at describing but here goes.
High voltage from road to pole transformer/pole (utility owned). Secondary of transformer has like 3 feet of wire then connects to the 2 "services", the "load" going underground to the disconnect in the shop, the "generation" to a meter on the pole and to its disconnect in the shop. The CT for the bidirectional meter is on that 3 feet beside the transformer, the CT wires run down the pole to the meter. So I would have to go up the pole within about a foot of the transformer with my CTs to get a net reading that way. Not happening.

I came across a watt transducer called Wattnode http://www.ccontrolsys.com
Was thinking of using one of these with a pair of CTs by each disconnect to get a pulse output (equivalent to watts).
Then I would need a instrument that can use those two pulses, combine them and give a relay output based on my settings.

Any ideas?
Im looking at a programmable relay BB Electronics TECO SG2 http://www.bb-elec.com/product_multi_family.asp?MultiFamilyId=39&Trail=158&TrailType=Main
Not sure if it suites...probably fancier than needed anyway.

All right, you probably cannot get access to PoCo metering CTs.

Is there an monitoring output of the inverter that will provide a output KW signal in realtime? If there is, half of your work is already done.


WAIT A MINUTE !!!

How does the customer hook up a load to the inverter output before the generator meter?

Does the generation meter have some sort of one way clutch that prevents it from spinning backwards? I hope so or else you are buying power at 60 cents per KWHr when the inverter can't support all of the additional load.

Does the inverter service count as a separate service? Does it share a neutral and earth with the main service?
How do you power any loads off of the inverter panel?

Something sounds wonky about this setup.

Im not doing very well describing this...I'll try my hand at a diagram

Pole Transformer
|
|
PoCo CT ---(CT wire)--> bi-directional meter
|
|(top of pole)
|---(underground)-> load disconnect ---> subpanels
|
|
Generation meter
|
|(underground)
solar disconnect
|
|
Solar array

both disconnects are in the shop. As you can see the generation meter measures all solar production, the bi-directional measures what leaves from the solar and what is brought in from utility. The difference between the solar generation and what leaves is what is used on-site at cheap.

I'm going back to confused again (or still)

Try posting this image of the layout:

http://s10.postimage.org/vvmnel37d/layout_diagram.jpg


upload images

I believe you are overthinking this. Perhaps this model will make more sense.

Every KWHr leaving the inverter meter costs you 6 cents. Every KWHr leaving the property earns you 66 cents. Every KWHr entering the property costs you 20 cents.

Property is a net exporter of energy? You earn 60 cents/KWHr. Property is a net user of electricity? It costs you 20 cents/KWHr. Every KWHr you generate and use on site costs you 6 cent/KWHr.

Assuming KWHr meters are read once a month, there is no need to read the meter in real time. If you use 10 KWHr during a 24 hr period and generate 10 KWHr during the same 24 hr period, your net cost is 60 cents. It doesn't matter when you use the energy.

If you hook your load up to the inverter panel upstream of the inverter meter, AND you consume more energy than the inverter can put out. Your energy cost will be either be 14 cents a KWHr if the inverter meter runs backwards, or 20 cents a KWHr if the inverter meter does not run backwards. If you consume less energy than what the inverter puts out, your energy costs will be 0 cents per KWHr.

Sorry, I'm confused...Doesn't make sense, seems really complicated.

This is it:

1) Every KWHr leaving the generation(inverter) meter earns 60cents. 100% of the energy the inverter outputs. That's a given regardless of what the load is doing and whether property is exporting or importing.

2) Every KWHr that leaves the generation meter, but does NOT pass through the bi-directional meter (not exported), and used on site in real-time costs 6cents.

3) Every KWHr that is imported (consumed) costs 20cents.

This is not like net ACCUMULATIVE metering. It is bi-directional. If 10kwhr is exported that day and 10kwhr imported that night, the meter knows 10kwhr exported and 10kwhr imported, and charges for all 10kwhr imported at 20cents. Basically we don't want the energy reaching the bi-directional meter.

Example (24hr time period):
Inverter produces 60kwhr, then 60kwhr is recorded on generation meter.

40kwhr is exported, 30kwhr is imported. 40kwhr export, 30kwhr import is recorded on bi-directional.

Customer is paid $36.00 (60cents x 60kwhr)
Customer is charged $6.00 (20cents x 30kwhr imported) plus $1.20 (6cents x 20kwhr used real-time[60kwhr generated - 40kwhr exported]). Total charged $7.20

Does that help at all??

I'm sooo confused!! My head is hurting.
Seems that we do this with a line tap (or backfed CB) and a net POCO meter. With a solar production meter on the inverter output.

Lol...Ya it's like that except it's not a net meter but a bi-directional meter. I wish I could delete "net" from the subject, that might be what threw off some people. I meant net/surplus real-time KW, not POCO net Kwhr meter.

If it's any consolation, only the few first ones got hooked up this way. They don't allow this anymore.

"They don't allow this anymore."

To that I say....thanks!!

Yes, yes it does.

What exact make and model inverter is installed? Does it have or can you add a real time KVA output monitor.

Basic work around would be adding a current monitoring relay to one leg of the inverter output and have it trip when the current exceeds 12.5 A(3 KW) and opens up when the current reaches some lower limit you decide.

These relays are often used on belt driven fans to determine remotely if the fan has a mechanical failure or the belt broke.

This setup will allow the load to only operate when the inverter is putting out current.

Would it make sense to approach the power company and have them change it to a true net metering setup? No more complicated billing methods and make it simpler for everyone?

They are two 5kw Aurora inverters. You can plug in a laptop and/or get monitoring for them, but expensive.

The problem with that or a current relay (and main issue here) is that only goes by the generation. It does not factor the consumption. The inverter could be putting out 3kw, but they could already be consuming 4kw and thus not want to run the extra load.



NO actually. The usual method is now parallel. That means that they buy 100% of the power they consume at normal rates (20cents in the example), no 6cent power if they use it direct.
Net metering is even worse. That means no 60cents for the generation. It would just replace 20cent power.

Thanks for your thinking...eventually something might materialize

I was thinking about it, almost would be tempting to re-route the solar/generation wiring through the load/consumption splitter, so the solar and load wires could be together and put a large CT over them all...Then the CT or current relay would measure the "net" current...

Except that would be a lot of work and they wouldn't like that....

But thinking that, would it be feasible to put individual CTs over each wire (one on each phase of the generation and the consumption) and run the CT secondaries through something like a current relay, to measure the "net" of the CT secondaries??

That doesn't give watts, but might be a compromise.

I think that would work.

1) Wire the same ratio CTs from the same phase in opposing series so that the output equals Generation minus Consumption.
2) Swap the polarity of the Red leg from the Black leg so that the outputs from each leg are now in phase with each other.
3) Run a single wire from each of the leg CT secondaries thru the Current monitoring relay. That is a total of two wires thru the current monitoring relay sensor coil.
4) Summation of (Generation - Load)red phase + (Generation - Load)black phase. When the result is greater than zero, then the property is producing a net excess of current.

ASSUMPTIONS

1) You don't need to know actual KW values. You just want to know when the property is exporting excess energy.

2) The current monitoring relay IS polarity sensitive AND it can tolerate prolonged "reverse" currents.

3) Reverse current is defined as the current it will see when consumption is greater than generation.

My point was simply that if the system with the control configuration setup that you are looking for existed, you wouldn’t need to be trying to make it work the way you want yourself. Don't give up on it though.