Contactor needed for inverter output disconnect

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tallgirl

Senior Member
Location
Great White North
Occupation
Controls Systems firmware engineer
Greets,

I could wait until tomorrow morning and contact my sales rep at Graybar, but why when I can ask y'all!

I have a client who is having a very hard time finding a contactor for his solar power system. I had a conference call with him and his electrician today and the electrician insists he can't find a 50A two pole contactor that is normally-closed. I suggested he find a double throw contactor and ignore the normally-open terminals.

The application is as follows --

He has a twin stack of OutBack GVFX 3648 (grid-interactive inverters) that are connected to the main service on their inputs and the input of a critical loads panel on their output. There is a Cutler-Hammer ATS switch the grid out and a 14KW Kohler in when grid power fails. That still leaves the inputs of the inverters connected to the output of the generator. So the plan is to add a 50A contactor at the inverter inputs to keep it from trying to export power to the generator should the battery voltage rise above the "Sell RE Power" voltage.

For some odd reason (I dunno ...) the electrician would rather have the relay normally-closed.

Do any of you know of a part number for something like this, or do I have to go to Graybar in the morning and have me some free coffee?
 

tallgirl

Senior Member
Location
Great White North
Occupation
Controls Systems firmware engineer
Contactor needed for inverter input disconnect

Contactor needed for inverter input disconnect

Greets,

I could wait until tomorrow morning and contact my sales rep at Graybar, but why when I can ask y'all!

I have a client who is having a very hard time finding a contactor for his solar power system. I had a conference call with him and his electrician today and the electrician insists he can't find a 50A two pole contactor that is normally-closed. I suggested he find a double throw contactor and ignore the normally-open terminals.

The application is as follows --

He has a twin stack of OutBack GVFX 3648 (grid-interactive inverters) that are connected to the main service on their inputs and the input of a critical loads panel on their output. There is a Cutler-Hammer ATS switch the grid out and a 14KW Kohler in when grid power fails. That still leaves the inputs of the inverters connected to the output of the generator. So the plan is to add a 50A contactor at the inverter inputs to keep it from trying to export power to the generator should the battery voltage rise above the "Sell RE Power" voltage.

For some odd reason (I dunno ...) the electrician would rather have the relay normally-closed.

Do any of you know of a part number for something like this, or do I have to go to Graybar in the morning and have me some free coffee?
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
They are going to be hard pressed to find a 50 amp normally closed contactor, Is the grid tie on the load side of the transfer switch for some reason? Or do you also have a bank of batteries that pick up on utility fail? Not really understanding why the inverters would be on the load side of the transfer switch?
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
The inverters are suppose to disconnect upon loss of utility, so why would they go into "Sell RE power mode? They are not going to sense the generator, unless the inverters are on the load side of the transfer switch? Would not be a good design if they were.
 

hurk27

Senior Member
It would be an un-common relay, but this design sounds funky, why not just have a contactor tied to the ATS that will open when the load transfers to the generator?

Wouldn't having the inverter online help lower the fuel usage when on generator? I'm not much on PV but it would seem that having the inverter help carry some of the load while on generator would be a good thing.
 

tallgirl

Senior Member
Location
Great White North
Occupation
Controls Systems firmware engineer
There are a number of issues with synchronization as well as UL1741.

The issues is that the inverter must be completely isolated from the system whenever the generator is running. The inputs must be disconnected so the inverter doesn't try to "sell" power to the generator, and the outputs must be disconnected so the generator and inverter don't get out of sync.

The design is solid, and I'm pretty sure my sales dude at Graybar said they have something like that. I'm just impatient. And I hadn't stopped in to see you guys in a while :D
 

tallgirl

Senior Member
Location
Great White North
Occupation
Controls Systems firmware engineer
The inverters are suppose to disconnect upon loss of utility, so why would they go into "Sell RE power mode? They are not going to sense the generator, unless the inverters are on the load side of the transfer switch? Would not be a good design if they were.

A high-quality generator will trick the inverter into thinking it has the grid. Another client burned up a generator and his electrician was frantically looking for a solution to prevent the replacement from getting fried the same way. This approach seems to be pretty common -- use the grid to turn off the input, the use the generator to turn off the output.

And while that might not be a good design in your mind, that's pretty typical. The only oddity with this installation is that the inverters are "grid-interactive", where the usual approach is to use "standby" inverters then do something completely different to consume the solar power.
 

BillK-AZ

Senior Member
Location
Mesa Arizona
Can you put the ATS on the output of the inverter such that it selects between Inv and Gen?

Utility connects to input of the inverter all the time. If you want to disconnect the Util from the inverter input you can use a NO relay operated by an Aux switch on the ATS.

The basic wiring (if this comes through the same as I see it on a laptop):


[UTIL]--------[IN INV OUT]-------[in 1 ATS|
| |----->Panel
GEN out]--[in 2 ATS|

(add spaces if needed on each line so that 'ATS' lines up. For some reason the edit and post fonts differ.
 
Last edited:

tallgirl

Senior Member
Location
Great White North
Occupation
Controls Systems firmware engineer
No, because there are two different panels -- the critical loads panel that's connected to the inverter outputs and "everything else". "Everything else" gets transferred to the generator immediately when the grid drops. The critical loads panel gets transferred to the generator when the inverter shuts down due to low battery voltage.
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
So I take it, that the inverter IS on the load side of the transfer switch, the only correct way I see of fixing this is to have a second transfer, the inverter is feed from the utility side of the main transfer switch, Source one (utility) of the second transfer switch is feed by the inverter, while source two is feed by the load side of the first transfer switch. This way if source one fails (Inverter) source two (generator) safely and automatically picks up the critical load. This would isolate the two sources. Set the re-transfer to normal delay to 30 minutes or more so that if the batteries re-charge, and the inverter came back on line, it would not immediately transfer back and forth as the batteries discharge, as what will happen with a plain ol' contactor.
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
If you still want to go the contactor route, a Generac Guardian 100 amp single phase transferswitch would be a off the shelf and cheaper way to go, all you need is a 12 VDC powersource and a dry contact to control it. Asco probably makes them for Generac, so you may check them to. Another cheap route would be a 50 amp RV transfer switch, some of those have a non-powered fail close position on them.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I'm not clear on where the contactors would go. When you say "inverter input", do you mean on the DC side of the inverter? Makes a BIG difference, you cannot just use any old off-the-shelf contactor on DC, and 50A DC contactors at typical PV system peak voltages (usually 500V rated components) are hard to find.

If they are on the AC side, most of the IEC contactor mfrs sell a 4 pole contactor that is 2NO and 2NC power contacts, they have to use them for some specialized pole switching crane motor that's apparently common over there. I know for a fact that Siemens has them available, the 3RT15... series, but they only go to 40A inductive, 55A resistive max. I'm pretty sure ABB and Eaton (Moeller) have them too though.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I'm not clear on where the contactors would go. When you say "inverter input", do you mean on the DC side of the inverter?

No, she means on the AC side. The terminology is a little imprecise, but most Outback inverters (and some other brands) have both an AC connection for the grid and an AC connection for a downstream subpanel, aka the 'critical loads' panel mentioned above. Hence both an AC 'input' and 'output' on the inverter(s).
 

tallgirl

Senior Member
Location
Great White North
Occupation
Controls Systems firmware engineer
So I take it, that the inverter IS on the load side of the transfer switch, the only correct way I see of fixing this is to have a second transfer, the inverter is feed from the utility side of the main transfer switch, Source one (utility) of the second transfer switch is feed by the inverter, while source two is feed by the load side of the first transfer switch. This way if source one fails (Inverter) source two (generator) safely and automatically picks up the critical load. This would isolate the two sources. Set the re-transfer to normal delay to 30 minutes or more so that if the batteries re-charge, and the inverter came back on line, it would not immediately transfer back and forth as the batteries discharge, as what will happen with a plain ol' contactor.

That's not normally a problem as the cut-in and cut-out voltages are different. Typically the cut-in voltage is higher enough that the batteries have actually recharged enough applying a load won't cause the cut-out voltage to be reached.

The configuration is a bit weird because when the grid is dropped, and the generator picks up the rest of the load, the inverter MUST be disconnected from the generator. The reason is that if the inverters see the generator, it might just try to "sell" power to it and break it.

Then, when the inverter reaches low battery disconnect, the inverter outputs to the critical loads panel have to be switched over to generator power so that the inverter is completely disconnected from the AC system.
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
That's not normally a problem as the cut-in and cut-out voltages are different. Typically the cut-in voltage is higher enough that the batteries have actually recharged enough applying a load won't cause the cut-out voltage to be reached.

The configuration is a bit weird because when the grid is dropped, and the generator picks up the rest of the load, the inverter MUST be disconnected from the generator. The reason is that if the inverters see the generator, it might just try to "sell" power to it and break it.

Then, when the inverter reaches low battery disconnect, the inverter outputs to the critical loads panel have to be switched over to generator power so that the inverter is completely disconnected from the AC system.

Did you find the contactor you needed?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Educate me

Educate me

What's the difference between a contactor and a switch?
 

ACE1970

Member
Location
California
Try looking at grainger with my degree in electrical engineering I could get a two pole 50 amp contacter and two ice cube relays two switch back and forth from nc to normally open as needed for the system to work properly again I would have to open the inverter and solder a few wires
 

ACE1970

Member
Location
California
What's the difference between a contactor and a switch?
Automatic transfer switch or non automatic transfer switch has fuses inside normally for larger loads to maintain a power source for alarge load or sensitive equipment etc. A contacter is a normally open contact that needs to be manually or automatically closed using a start/stop station or on/off switch etc.
 
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