preventing PV Inverter Backfeed into Grid in NYC

Either of these should work in your applications. Both will handle either 1-phase or 3-phase installations.

https://www.byramlabs.com/product_info.php/products_id/8475

https://www.hoytmeter.com/products/products_details.php?pro_id=789

You stated you've installed PV's for a number of years. Is there an inverter that can variably limit the amount of power it produces? The reason I ask is the scheme shown in your drawing would completely disconnect your PV system. The 32 would have to be reset somehow; timed, manual or otherwise. If there was someway you could limit the PV power production not to exceed what was used by the load, it would be close to ideal for your set-up.

Lendog,

You are right. Using active feedback control is the ideal solution. That's possible but not without some custom programming in the inverter and perhaps some custom hardware. Unfortunately, the size of this project does not warrant that much extra cost.

I suppose it depends on the size of the PV system compared to the load it would feed. If the PV system is small in comparison, then the likelihood the power it supplies exceeding the size of the load would be minimal and the 32 arrangement in your diagram would work just fine.

Looking over the solution I suggested, I originally envisioned a PV system capable of exceeding and/or assuming the whole load. For a system of that size, the suggested approach may be 'salable', otherwise it is just overkill.

Good luck on your installation. It looks like the links previously posted have all the info you need.

It strikes me that the problem is with ConEd. They should have an alternative metering system which separately measures the forward power (what you are consuming from the line) and the reverse power (what you are producing into their line). They are required by Federal rules to purchase any excess power you produce at their "avoided cost" rate. By prohibiting "net metering" they are keeping you from selling them power at the full retail rate (which is usually higher than the avoided cost rate). This requirement is OK and legal, but they should have the alternative I mentioned above.

Hopefully, they will not try to have your PV systems sell their entire output to ConEd and them sell back your entire consumption, because this would increase their profits and decrease the payback your system can obtain.

In other words, is a reverse power relay the proper approach to the problem?

--JMM

I'm with JMM on this one. In this day and age of global warming and the necessity to reduce out dependence on fossil fuels they should be accepting surplus capacity and distributing it on the grid.

In Australia Origin energy not only accept surplus capacity but pay considerably more for it than it costs you to buy it from them. Mind you, they also manufacture the solar panels and inverter/control system so it's in their best interest to sell more systems. I also think that they are using it as a carbon offset for their coal fired power stations. Either way, the end user ends up better off.

Seems to me that ConEd should be asked why they do not accept surplus capacity and if they don't give you a valid technical reason get the customer to put pressure on them to change.

If it woks in Australia why wouldn't it work in the USA?

New York has a Net Metering law that the governor will be signing soon, if he hasn't already. All the utilities in the state will be required to develop interconnection rules for Net Metering. Some net metering is allowed right now but ConEd claims they cannot do it for technical reasons. New York City is served by a Grid Network where multiple transformers feed the loads in parallel. They claim that their network protection equipment will see the backfeeding current and sense that as a fault causing a trip. I think that in most cases, any current going back from a properly-sized solar array will not be large enough to cause this to happen.

G'day gals, guys & fellow gurus,

H'm,

• They claim that their network protection equipment will see the backfeeding current and sense that as a fault causing a trip.

I've got to have a bit of a think about this. It could be true but it feels a little bit of a dodgy excuse. However, I could very well be wrong so if anybody would like to correct me with a technical explanation it would be appreciated.

• I think that in most cases, any current going back from a properly-sized solar array will not be large enough to cause this to happen.

I agree with you on this as we would normally only be talking in the single digit kW range but as in the previous paragraph I could be wrong.

As a former employee of another utility, you are overlooking the SAFETY issue for utility employees who may be attempting to repair the system. Electricity flows both ways, and will transform up to transmission voltages as well as down to service voltages. The fact that the relay opens the circuit with no automatic closure (requiring reset manually) indicates that the utility rate is not the issue.

As a licensed contractor you can call 1-800-752-6633 and speak to the ConED energy services department. They will be glad to fill you in on the reasons for the relay, and it's proper connection. I really doubt that free electricity input to the system would be refused under normal circumstances, what do you think?

Hi CJMcGill

• As a former employee of another utility, you are overlooking the SAFETY issue for utility employees who may be attempting to repair the system. Electricity flows both ways, and will transform up to transmission voltages as well as down to service voltages.

That's a very valid point and I must admit something I have not given much attention to until now. Nonetheless, given that in numerous jurisdictions around the world it is possible to feed surplus capacity from solar cells and other distributed generation systems back on to the grid there must be some way of isolating such systems that allows work on the final distribution stage of the grid.

In the US, all inverters connected to the grid are required to have "anti-islanding" to prevent supplying power to the local grid during a blackout (or maintenance). The inverters will shutdown immediately upon sensing a drop in the grid voltage. (UL 1741,IEEE 1547) Many jurisdictions require an external AC disconnect switch for utility usage although some have dropped that requirement recognizing the inherent safety built into the inverter.

Hi wjwlitespeed,

Thanks for the reference. We more than likely have a very similar or identical standard in Australia. Although, trying to read standards from other parts of the world requires considerable skill in translating local terminology and abbreviations.

CR4, being an international forum is for ever having problems with local interpretations of terminology. I made up the term CRIMINAL to describe it, but you will have to follow the link to find out what it means

Not to pile on, but as stated and drawn, your diagram would prevent any power from flowing into the PV system, not into the grid. To perform the function stated, the sensing CT would be on the line to the panel from the grid not on the line from the PV to the panel.

As drawn, the relay would be protecting the PV array, and since this type of relay was designed to protect the generation source, could your assumption about why the relay is required be altered with more detailed input from the Energy Services folks at ConEd?

wjwlitespeed, did you successfully found reverse power relay and interconnected to ConEd grid? I am also looking for reverse power relay in order to interconnect a 25 kW solar PV system to PSEG grid network. All I can find is RPR with max contact current 5 Amps. But I need one that can handle 60 Amps. Could anyone give us sample product model numbers?