Some zero export inverter questions

[Hedges]

Yes, I wanted a "fudge factor" to never accidentally back-feed. Mainly I just want to offset some of my HVAC usage, so that number could be higher if need be.
Is it a single speed or inverter?

With a single speed there is the trick of picking a grid tie inverter that is always smaller than the single power level. And then naughtily connect the inverter to the load side of the start contactor (or some other interlocked way to start). Microinverters or small grid tie string.

An old UL-1741 GT PV inverter behind the A/C compressor contactor AND any thermal overload switch is absolutely, positively, guaranteed to disconnect from the grid the same millisecond the compressor load is removed from the grid.

If possible, reduce the 5 minute qualification of grid in spec to a couple seconds.

You'll have to set max wattage of inverter so it never exceeds the lowest draw of compressor, which will vary with temperature.
A CT based zero-export would be better, ramp production up to match compressor draw, minus some margin for measurement error. But contactor instantly takes it off line, eliminating spurious exports.

Of course, this only works during air conditioning season. Maybe you can switch it over to heating circuit as well. (after temperature limit switch, actually on wires to element.)

 

[Texas-Mark]

Of course, this only works during air conditioning season. Maybe you can switch it over to heating circuit as well.

Yep, thought about that too.

 

[RCinFLA]

Most inverters with an export to grid limiting via external CT sensors on grid input lines hold a few amps of margin to prevent errors in CT measurements from allowing a little continuous export to grid. You need at least 3-5 amps of incoming current from grid before inverter will export to main electrical panel loads.

If AC coupling is enabled the inverter will not fully charge battery to keep a dump margin on batteries to take a momentary PV excess power output while freq shifting is setup, which can take up to a minute before PV GT inverters cut back their power. This is most likely to happen when a large AC load is switched off leaving excess AC coupled PV power that must be momentarily dumped to batteries. This is always the case even if grid is available to dump excess PV power because you never know when grid might drop out.

 

[zanydroid]

Most inverters with an export to grid limiting via external CT sensors on grid input lines hold a few amps of margin to prevent errors in CT measurements from allowing a little continuous export to grid.

What about in self consumption mode with export permission? I’d like to zero out as much as possible during peak, bc $$$&$&$

 

[zanydroid]

Ok, this is what confused me.

Doh. I was afraid I made that typo. I corrected it at least once

 

[Texas-Mark]

inverter behind the A/C compressor contactor AND any thermal overload switch is absolutely, positively, guaranteed to disconnect from the grid the same millisecond the compressor load is removed from the grid.

At first I was wondering why you mentioned the thermal overload switch, but then it made sense. However, I suppose it is possible it would not be guaranteed to be fool proof if the compressor failed somehow from an internal open winding (rare as that may be). There would still be grid voltage, but no load.

I was also thinking of using a current sensing relay on the HVAC power line (or even main line by the meter) and set it so it is only closed when current from the grid is high (higher than the grid-tie inverter could ever output). This method would require an inverter that can be switched off with contacts/switch vice dropping the grid voltage. The advantage is that it would work with cool or heat, and even other high demand items like a dryer.

https://www.amazon.com/Current-Sensing-Switch-Normally-Monitoring/dp/B0B566KGYT?th=1

I know this all may sound silly to some, but a lot of the projects I do are more about just trying to get something to work the way I want than anything else. Electricity is cheap here and solar projects have never be about saving money or trying to go off-grid.

<edit> After thinking about it some more, that last idea I had won't work. As soon as the grid-tie inverter starts outputting, the draw from the grid would decrease, and the current sensing relay would turn back off. This would result in an on/off cycle loop.

 

[kommando]

In the UK as well as zero export using CT or RS485 meter connected to the Inverter you can also divert using a Solar Diverter

Solar iBoost+ - UK's Favourite PV Immersion Controller

Solar iBoost+ is the UK's favourite PV immersion controller. Use the excess power generated by your solar PV to heat your hot water for FREE.

www.marlec.co.uk

It also uses a CT but monitors the input output 50 times a sec (UK main is 50 htz) and then diverts the surplus using a Triac to a load which is typically a hot water immersion heater.

Its independent of the Inverter and utilises the UK normal meters bucket of 3200 joules dead zone where no consumption is recorded as long as you stay within the bucket

As well as commercial version there are also self build options.

 

[RCinFLA]

An AC load dump is a good alternative to protect against PV excess power surge. Hybrid inverter freq shifting to reduce PV GT inverter power output is not fast enough to react to a large AC load disconnect leaving excess PV power to deal with.

Hybrid battery inverter idle consumption is same, to slightly greater.

Battery powered hybrid inverter must provide AC sync master (grid surrogate) for PV GT inverter and there will be some slight extra consumption if no actual grid is active for hybrid inverter to resist PV grid tied inverter AC testing for valid grid presence.

PV GT inverter will periodically attempt to wiggle its phasing to test for grid presence. If off grid with AC coupling, the hybrid inverter consumption will go up a very small amount periodically to resist the GT inverter grid testing.

 

[1201]

Question for @Texas-Mark if you live in the county, why not just interconnect? Are there high fees for connecting or are they making you jump through hoops?

I'm in the county too and it was extremely easy to interconnect

 

[Hedges]

At first I was wondering why you mentioned the thermal overload switch, but then it made sense. However, I suppose it is possible it would not be guaranteed to be fool proof if the compressor failed somehow from an internal open winding (rare as that may be). There would still be grid voltage, but no load.

True. But if you use a zero-export setup with CT, it will be approximately zero all the time. Connection behind the switch to to prevent momentary export when compressor switches off.

I've had a motor fail by not starting and overheating, not by going open circuit (induction motor; brush type of course goes open circuit when brushes worn out.)

<edit> After thinking about it some more, that last idea I had won't work. As soon as the grid-tie inverter starts outputting, the draw from the grid would decrease, and the current sensing relay would turn back off. This would result in an on/off cycle loop.

Right, CT by itself doesn't know if power is coming or going. Need power meter which also measures voltage.

 

[Texas-Mark]

Question for @Texas-Mark if you live in the county, why not just interconnect? Are there high fees for connecting or are they making you jump through hoops?

After I got the call about back-feeding, I looked at their web site to see how to get things "approved". I quit reading after about 15 pages.

 

[1201]

After I got the call about back-feeding, I looked at their web site to see how to get things "approved". I quit reading after about 15 pages.

wow! 15 pages?
mine had about that much and an application but it mainly boiled down to a single line diagram with the application.

if you dont mind sharing, who is your provider?

 

[Texas-Mark]

if you dont mind sharing, who is your provider?

It's been a while since I looked on theiir site, but it looks like they will do an install:

Solar by Medina EC

A renewable option for members.

www.medinaec.org

"The base price for the 5kW solar PV system the cooperative offers is $15,000."

Or if you want someone else to do the install or do it yourself:

Distributed Generation & Renewable Energy

www.medinaec.org

 

[1201]

It's been a while since I looked on theiir site, but it looks like they will do an install:

Solar by Medina EC

A renewable option for members.

www.medinaec.org

"The base price for the 5kW solar PV system the cooperative offers is $15,000."

Or if you want someone else to do the install or do it yourself:

Distributed Generation & Renewable Energy

www.medinaec.org

Find out from other pv owners in your area if they do all that.

Same with my co-op- they said they would come out and inspect, and they could require a stamped SLD, but they accepted my sld without a stamp and they never came out to inspect.

 

[Texas-Mark]

Find out from other pv owners in your area if they do all that.

I don't see any others around here with PV. When electric rates are at $.09 kWh, not much incentive. I do the solar I have more as a hobby than anything.

 

[1201]

I don't see any others around here with PV. When electric rates are at $.09 kWh, not much incentive. I do the solar I have more as a hobby than anything.

if its more of a hobby then I wouldnt consider zero export. I would consider
1. go grid tie. very simple. and your poco pays you back about 50% , which is very fair. even at .09/kwh you can still save money especially if you can diy for around .03/kwh long term
2. do a protected loads panel like everybody else and power that with an inverter and batteries. You can make the system as small or as large as you want then

 

[Texas-Mark]

if its more of a hobby then I wouldnt consider zero export. I would consider
1. go grid tie. very simple. and your poco pays you back about 50% , which is very fair. even at .09/kwh you can still save money especially if you can diy for around .03/kwh long term

Not really an option because I would have to change too much to meed their requirements.

2. do a protected loads panel like everybody else and power that with an inverter and batteries. You can make the system as small or as large as you want then

I said in one of my earlier posts that this was an option, but it is also not that easy to implement.

 

[1201]

Not really an option because I would have to change too much to meed their requirements.

What specifically? I looked at the application and it's as straightforward as can be

I said in one of my earlier posts that this was an option, but it is also not that easy to implement.

How so?

 

[zanydroid]

What specifically? I looked at the application and it's as straightforward as can be

I agree. This application (I reviewed both application and interconnect agreement) is simpler than the PG&E one I filled out with a modicum of forum help.

 

[Texas-Mark]

What specifically? I looked at the application and it's as straightforward as can be

Did you look at all of the compliance questions? Most of my stuff is out in a solar shed and most likely not in compliance with those codes. The automatic cutoff switches they want don't exist and would not be easy to implement. My meter is on a pole 40 feet away from everything. If I want to purchase an "approved" inverter, I might as well just pay them to do an install. So no, not going to happen.

As I said earlier, I started this project nearly 20 years ago so it is a mix of 3 different systems. It's been working fine all that time. And as I said, up until they installed the smart meters and started hawking their own systems, nobody cared what you did. The cost to do things they way they want is just not worth it.

How so?

My breaker box in the house is not in a good location to easily to branch off separate circuits.