Hi, folks!
Due to beaureaucratic bungling in my home state of Florida, I had to disconnect my SunTie from the grid when the public utility installed "smart meters" across the entire county. The smart meter considers all current passing thru it as energy consumed, whether it is drawn from the grid or supplied to the grid. So, I get billed. I want to modify my grid tied inverter to work normally up to the point where it just begins feeding power to the grid (a few mA, max). I want it to limit its output at that point. If there were an extra input to the PIC so that I could sense current magnitude and direction with a loop around the power line in my breaker box, the inverter would supply all the power being consumed by the household, but never supply any significant amount to the grid. Building the external hardware is no problem, getting useable info to the PIC is the unknown. thanks!

Do you have an official "grid tie" agreement with your utility ? They should have installed the proper meter, or you may have to have 2 meters installed. If you are not legal, bummer.

Hi, Mr. Guppy-
Florida requires that a written agreement be executed between the micro-generator (me) and the power utility. The agreement specifies that all equipment must be installed per code and inspected by the city building inspector. This requires a building permit. The building permit application stipulates that no work can begin until the permit is approved. I have an off-grid system which I installed in Feb 2006, after hurricane Wilma took 98% of the local power utility's customers off-line (according to local press reports) in October 2005. I was without commercial power for 3 weeks. At that time, Florida did not allow net metering. Since the PV panels from that system would be part of the proposed on-line system, technically, work has been started. One must understand that I am dealing with government employees and bureaucrats. They would likely never issue the permit if I disclosed the previous "construction". If they caught me with the previous "construction" after issuance of the building permit, I could face penalties. Or complete rejection of the system. Or a lot of extra expense. Or all the above. So, I'm screwed. Catch 22. So, I'm trying to use engineering to circumvent government bumbling. If I "jeep" a legal grid-tie inverter to the point where it supplies just enough power to cover the instantaneous household power consumption, I am not supplying power to the grid, not breaking any laws. I can do this by detecting the sense (direction) of current flow in the power line feeding my house and "throttling back" the inverter as soon as the sense changes from power consumption to power delivery to the utility. But I need to tell the PIC when this happens. Since all inverters have an output current sample so they can determine maximum rated power production and protect themselves from overload, I considered using that parameter to "talk" to the PIC. I could operate the inverter within its rated power production range by using a 3 kw inverter with 2 kw of PV panels. But then overcurrent protection may be jeopardized. So an additional input to the PIC would be a good solution. Yes? No? Thanks for any ideas.

You can get the permit, just make no mention of anything currently installed. Be prepared, there will almost certainly be aspects of your system that needs changes to be code compliant.

Having any inverter connected to the grid with sell ability requires the utility sign-off, whether is does its best not to sell doesn't matter

I have the impression your going in circles in your own mind, making calls and assumptions for something that is straight forward.

Get the permit and inspection, then get the required approval from the utility

As to modifying the Suntie, not a chance.

OK, Mr. Red Cobra (cool title, btw)--
I appreciate your sage advice. Sounds like a headache.
many thanks!

I went through this with my first system, built installed then had to back track when the utility found out about the system ( almost 10 years ago ) so I believe I understand where your coming from

Its completely doable, and to finish, you have to start, which means don't keep making mental reasons it can't be done or dwell on how hard it may be ... just start by talking with your building department and that will give you the to do list

Good luck!

That's interesting that you get billed of the energy you supply the grid... My home was installed electric kWh meter couple years back. It did not count kWh when it 'spun' backwards. (It didn't count backwards either...) Meter was made by Landis&Gyr. The model number I do not remember.

Just a month ago I noticed they had replaced my meter with a new Landis&Gyr one. It has now a sticker "DRG" in front of it which I believe means "Dual Read capable". There is an additional counter display that appear to be my "sold" energy. In addition they have placed a warning sticker to my meter housing saying that the meter is fed by on site power generation. I'm wondering if that's it or if they are going to ask me to do something else.

I have same principle that I do not plan to sell anything back to the grid. I have built a "load controller" using PIC microcontroller that controls a set of solid state relays, that turns on loads (Window Air Conditioners) based on the current the charge controller feeds the inverter (Outback charge controller feeding Outback grid tie inverter.) I measure the current with a simple hall sensor placed over the inverter power cable.

Hi, Matsovol-
That's kinda what I wanted to do. The state regulators write the rules in such a way that a lot of people (state-licensed solar installers, city and county inspectors, etc) get a piece of the pie if you build your grid tie system "legally". They have all kinds of good reasons why you should do it the "legal" way. What the "legal" way really does is make a solar grid tie installation prohibitively expensive, so that it will never pay for itself. This, in turn, makes the public utilities happy, so they continue to sell fossil-fueled electricity. I'm going to try another approach. Still not gonna put up with the inspection routine; there's no need for it. It's a waste of time and money.