AC Transfer with multiple XW6048

JimAK

We have a customer with three XW6048 in parallel on a 120/240 1ph system. They are tied to a 40 kW generator. The AC transfer relay fried in one of the relays after about a year of operation. I am getting conflicting information fromo Schneider tech support regarding the situation. One said that, due to slight inconsistencies in the individual inverter transfer times, it is possible that one inverter could transfer slightly before the others, placing all of the load momentarily on the one set and overloading that set of contacts. And that we should be installing an external transfer switch in multiple inverter installations. The next tech said that no matter how many inverters are paralleled in the application, the pass-thru is limited to the 60A of one inverter, which makes no sense.

Can anyone shed any light on this for me?

thanks

MGar

Re: AC Transfer with multiple XW6048

I was told that the first inverter would load up to 60%? and then the next would turn on, this is a bit foggy as it was a while back.

Maybe adding two relay's to the first inverter as it could be as high as 12,000 watts for a moment.

inetdog

Re: AC Transfer with multiple XW6048

JimAK wrote: »

We have a customer with three XW6048 in parallel on a 120/240 1ph system. They are tied to a 40 kW generator. The AC transfer relay fried in one of the relays after about a year of operation. I am getting conflicting information fromo Schneider tech support regarding the situation. One said that, due to slight inconsistencies in the individual inverter transfer times, it is possible that one inverter could transfer slightly before the others, placing all of the load momentarily on the one set and overloading that set of contacts. And that we should be installing an external transfer switch in multiple inverter installations. The next tech said that no matter how many inverters are paralleled in the application, the pass-thru is limited to the 60A of one inverter, which makes no sense.

Can anyone shed any light on this for me?

thanks

The key point is that it is not safe to wire up the transfer switches of multiple inverters in parallel. There is too much chance that through mechanical or electrical or programming failure one of the switches will be in the opposite position from the others, effectively shorting the grid AC to the inverter output.
So although the several inverters will track each other on the amount of inverter power to produce, only one of them can have its transfer switch connected to the AC source.

RCinFLA

Re: AC Transfer with multiple XW6048

The tech is correct that there will be slightly different delays during a transfer. Normally, on transfer from a grid, things will work out okay but it is still rough on the contacts if there is heavy load >40 amps on the first relay to switch over.

With transfer to a generator things get a bit more rough. All inverters are sync'd up and matching the unloaded gen until first one switches over and begins to load the generator. The other two inverters, previously matching the unloaded gen, now transfer with not so well matched sync due to the gen changing its output voltage and possibly even frequency slightly as the gen output changes as the result of the suddenly applied load change on it. The inverters take several cycles to readjust their sync.

Putting switches and relay contacts in parallel do not mean you can directly add their ratings together to get 3x an individual relay rating. It is just bad practice to parallel DPDT or 3PDT switches or relays to achieve higher current handling capability.

The original SW series inverter used a 3 pole double throw relay, each contact rated at 30 amps. These three 30 amp paralleled contacts comprised the transfer switch for the inverter with a 60 amp rating (not 90 amp transfer). At least they were driven from the same relay coil so only the mechanical tolerances effected which of the three contacts hit first. Still poor practice but an inexpensive way to get a transfer switch for higher current.

You will never see a switch or relay manufacturer say you can get higher contact current ratings by paralleling poles of switching device together.

I don't agree with the statement of adding an external transfer switch unless there is way to disable the XW's internal transfer switches and have all inverters agree to sync and give the go ahead to activate the external transfer switch. I don't know of any such software control feature from the XW's.

inetdog

Re: AC Transfer with multiple XW6048

RCinFLA wrote: »

You will never see a switch or relay manufacturer say you can get higher contact current ratings by paralleling poles of switching device together.

But you can often put poles in series to get a higher voltage rating.

Dave Angelini

Re: AC Transfer with multiple XW6048

I would say that load management in this case would require the switching of whatever the huge loads (they are huge!) to be mostly done directly from the generator to the load. In other words do not use the XW transfer for some of the big AC loads. This will also allow the XW's to charge batteries and get that job done efficiently. What are the loads BTW?

JimAK wrote: »

We have a customer with three XW6048 in parallel on a 120/240 1ph system. They are tied to a 40 kW generator. The AC transfer relay fried in one of the relays after about a year of operation. I am getting conflicting information fromo Schneider tech support regarding the situation. One said that, due to slight inconsistencies in the individual inverter transfer times, it is possible that one inverter could transfer slightly before the others, placing all of the load momentarily on the one set and overloading that set of contacts. And that we should be installing an external transfer switch in multiple inverter installations. The next tech said that no matter how many inverters are paralleled in the application, the pass-thru is limited to the 60A of one inverter, which makes no sense.

Can anyone shed any light on this for me?

thanks