Generator Backfeed question

[bbartmasse r]

Hi all,

Installing around 20KW grid tied on a residence located within a compound of buildings. Compound is equipped with a hefty generator that supplies various transformers on the property, feeding numerous building.

What would be the transferring method here as to not back feed the generator?

thanks

 

[ggunn]

Hi all,

Installing around 20KW grid tied on a residence located within a compound of buildings. Compound is equipped with a hefty generator that supplies various transformers on the property, feeding numerous building.

What would be the transferring method here as to not back feed the generator?

thanks

Connect the PV to the line side of the transfer switch.

 

[bbartmasse r]

OK. So the array and equipment are on one building, the generator input is in another location. Does this mean we have to run the Inverter output all the way back to where the generator transfer switch is? Any way you can think of around this? Can we install something in the service panel with the PV?

thanks

 

[ggunn]

OK. So the array and equipment are on one building, the generator input is in another location. Does this mean we have to run the Inverter output all the way back to where the generator transfer switch is? Any way you can think of around this? Can we install something in the service panel with the PV?

thanks

I guess you could install some sort of contactors that would open when the grid goes down and something on the grid outside the ATS to send a signal to them. It doesn't seem easier than running the inverter output conductors out to the line side of the ATS, though.

 

[bbartmasse r]

In this case I think that would be easier. The ATS is manual, and located where the service enters the premise at 480V. The generator is also 480V.

From here it is distributed to all the buildings, each building has a step down transformer to 240/120. I am backfeeding the service at one of the buildings at 240/120

So there really is no way to run the power back there.

Can you suggest what a contact relay might look like in this case (part numbers perhaps?)

thanks

 

[ggunn]

In this case I think that would be easier. The ATS is manual, and located where the service enters the premise at 480V. The generator is also 480V.

From here it is distributed to all the buildings, each building has a step down transformer to 240/120. I am backfeeding the service at one of the buildings at 240/120

So there really is no way to run the power back there.

Can you suggest what a contact relay might look like in this case (part numbers perhaps?)

thanks

Actually, no. I was speaking theoretically; I have no idea how you would actually implement it. I have encountered a similar situation at a hospital, but in that case the generator was interconnected with several ATSs, each at the panels it was to back up in the event of an outage. We interconnected the 812kW of PV at the master MDP, which was on the utility side of all the ATSs.

However you do it, a grid tied PV inverter without batteries should never be connected to the output of a generator.

 

[pv_n00b]

Hi all,

Installing around 20KW grid tied on a residence located within a compound of buildings. Compound is equipped with a hefty generator that supplies various transformers on the property, feeding numerous building.

What would be the transferring method here as to not back feed the generator?

thanks

Last time I did one of these I used an AUX contact on the transfer switch to drop out a contactor on the PV system that was a good distance from the transfer switch. I put together a contactor and DC power supply from Phoenix Contact (www.phoenixcontact.com). Call them up and they can probably help you pick out the parts. If you don’t have an AUX contact in the transfer switch you will have to see if there is a generator run AUX contact in the generator you can hook into. If that is not available you are kind of out of luck. You might need to put in a better transfer switch with an AUX contact.

 

[cuba_pete]

In this case I think that would be easier. The ATS is manual...

I'm sorry...the ATS is manual?

 

[drcampbell]

... a grid tied PV inverter without batteries should never be connected to the output of a generator.

Isn't that precisely what "grid tie" means -- connecting the PV output to the output of many generators?

 

[ggunn]

I'm sorry...the ATS is manual?

I don't think that's possible since the "A" stands for "Automatic".

 

[ggunn]

Isn't that precisely what "grid tie" means -- connecting the PV output to the output of many generators?

No, it means what it says, i.e., tied to the grid. While it is true that the grid has many generators, it is extremely unlikely that a residential or commercial PV system is going to backfeed any of them. In a closed system when the current being drawn by the loads is less than the output of the PV system, that's very bad news for most generators unless the PV system also includes batteries to absorb the excess and the capacity to ramp down or shut off the PV when the batteries are fully charged.

 

[publicgood]

I don't think that's possible since the "A" stands for "Automatic".

It would simply be an MTS is his intent.

 

[electro7]

Yeah, you have to get a contactor or relay rated for the inverter amperage. Catch the utility side voltage to operate the contactor and run the inverter output through the contactor. So some way or another you need to get back to the utllity voltage but at least with a contactor you could run smaller gauge wire to operate the coil. Hopefully that makes sense.

 

[BillK-AZ]

In this case I think that would be easier. The ATS is manual, and located where the service enters the premise at 480V. The generator is also 480V.

From here it is distributed to all the buildings, each building has a step down transformer to 240/120. I am backfeeding the service at one of the buildings at 240/120

So there really is no way to run the power back there.

Can you suggest what a contact relay might look like in this case (part numbers perhaps?)

thanks

The system descriptions seem to indicate:

• Utility service is 277/480/3-ph
• Generator is 277/480/3-ph
• Transfer switch is 277/480/3-ph
• MDP is 277/480/3-ph
• Transformers on many loads and buildings
• One building has a single phase transformer 120/240 and a 20 KW PV system

The main problem is that the inverter may not detect that the grid has failed and the generator is feeding the system. In this case the inverter will end up backfeeding the generator, compounded by an imbalance. Not good for the generator.

You need a means to turn off the inverter whenever the generator is running and supplying the system. If the inverter does not have any inhibit input (some do), you will need a contactor (rated for the inverter output) on the inverter output. An automatic transfer switch can be used, they are listed for the current and voltage needed. In either case you need a means of determining that the remote generator is running and some sort of connection between the two areas. On possibility is a low power feed of a lower voltage derived from the main utility voltage (before the transfer switch, possibly by a 480/120V control transformer) to a control relay near the inverter transfer switch that can activate the transfer switch to open the inverter feed when the utility is not present. If a wired connection is not practical, a radio link can be used, but should be designed failsafe such that if the radio signal fails, the inverter is disconnected.

 

[ggunn]

The system descriptions seem to indicate:

• Utility service is 277/480/3-ph
• Generator is 277/480/3-ph
• Transfer switch is 277/480/3-ph
• MDP is 277/480/3-ph
• Transformers on many loads and buildings
• One building has a single phase transformer 120/240 and a 20 KW PV system

The main problem is that the inverter may not detect that the grid has failed and the generator is feeding the system. In this case the inverter will end up backfeeding the generator, compounded by an imbalance. Not good for the generator.

You need a means to turn off the inverter whenever the generator is running and supplying the system. If the inverter does not have any inhibit input (some do), you will need a contactor (rated for the inverter output) on the inverter output. An automatic transfer switch can be used, they are listed for the current and voltage needed. In either case you need a means of determining that the remote generator is running and some sort of connection between the two areas. On possibility is a low power feed of a lower voltage derived from the main utility voltage (before the transfer switch, possibly by a 480/120V control transformer) to a control relay near the inverter transfer switch that can activate the transfer switch to open the inverter feed when the utility is not present. If a wired connection is not practical, a radio link can be used, but should be designed failsafe such that if the radio signal fails, the inverter is disconnected.

For the most part I agree, but what you are talking about isn't a transfer switch, it's just a switch, i.e., a disconnecting means.

 

[pv_n00b]

Yeah, you have to get a contactor or relay rated for the inverter amperage. Catch the utility side voltage to operate the contactor and run the inverter output through the contactor. So some way or another you need to get back to the utllity voltage but at least with a contactor you could run smaller gauge wire to operate the coil. Hopefully that makes sense.

There is a danger in doing it this way. The generator might be running and supplying power to the system even with utility power available, so the PV system would not be dropped out. This often happens during generator testing and exercising. I think the best point to test is the position of the transfer switch. If it's in the generator position then drop out the PV. Doesn't matter if the generator is running or not or if the utility power is out.

 

[jaggedben]

There is a danger in doing it this way. The generator might be running and supplying power to the system even with utility power available, so the PV system would not be dropped out. This often happens during generator testing and exercising. I think the best point to test is the position of the transfer switch. If it's in the generator position then drop out the PV. Doesn't matter if the generator is running or not or if the utility power is out.

What about a normally-closed contact that opens when there is generator voltage? Or is that considered too risky since it will not fail gracefully?