Designed a project installing a generator to provide backup power for some Telco equipment. I have seen in the past that a lot of times the emergency system is not tested under load since the short transfer interruption causes problems for the loads. I specified a closed transition ATS to eliminate this issue. Now I see where this ATS is being provided with a closed transition, switched neutral. This would seem to indicate I have to consider the gen-set as a separately derived source and provide a bond between the genset neutral and the ground rod system I had installed and taken to the genset. Okay..... but then, if the closed transition during testing occurs will I be creating the potential for transient currents and/or voltages? During a real outage there is no problem, but when testing will this cause trouble? Thanks for your time.
Switched Neutral in ATS
- Thread starter cleveland
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dbuckley
Senior Member
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- Canterbury, New Zealand
By "closed transition" do you mean "make before break"? If so (and I don't mean to teach my grandmom to suck eggs here) you will be paralleling the utility for a bit, and thus the transfers need to be in sync, which requires a standby genset that can sync and all the appropriate control equipment. Most standby generators are not intended for paralleling.
Get this wrong and transfers with utility power present will be destructive.
There is also the "islanding" problem to consider. If the normal operating sequence is utility fail, genset start, then ATS transfer, then as the ATS transfers it will momentarily backfeed power to the utility. As well as being illegal (which I'm certain of but have no idea under which bit of regulation it is disallowed) it's dangerous, as the apparently dead downed utility lines could be wrapped around a car and its victims or whatever. Also if there is a hard short upstream then you'll be feeding a short during the transfer, and this could pop the genset OCPD, which in my opinion would require fitting for precisely this reason.
I cannot see a benefit to a switched neutral in your case; indeed for a telecomms installation I'd see it as a distinct negative, as it is likely the apparent ground potential will shift during transfer.
Doesn't this telecom plant have a UPS downstream of the genset? Or a battery based -48v system that can tolerate the normal interruption of a standard ATS?
Get this wrong and transfers with utility power present will be destructive.
There is also the "islanding" problem to consider. If the normal operating sequence is utility fail, genset start, then ATS transfer, then as the ATS transfers it will momentarily backfeed power to the utility. As well as being illegal (which I'm certain of but have no idea under which bit of regulation it is disallowed) it's dangerous, as the apparently dead downed utility lines could be wrapped around a car and its victims or whatever. Also if there is a hard short upstream then you'll be feeding a short during the transfer, and this could pop the genset OCPD, which in my opinion would require fitting for precisely this reason.
I cannot see a benefit to a switched neutral in your case; indeed for a telecomms installation I'd see it as a distinct negative, as it is likely the apparent ground potential will shift during transfer.
Doesn't this telecom plant have a UPS downstream of the genset? Or a battery based -48v system that can tolerate the normal interruption of a standard ATS?
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- Location
- Illinois
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- retired electrician
dbuckley,
Don
That is all done by the transfer switch. If the generartor and the utility are not in sync, they will be as some point in the near future. The ATS just looks at the two voltages and closes the switch when they are within 5 degrees or so of each other. Yes the generartor and the utility are in parallel for a few cycles with this system.
That does not happen on power loss. In that case the normal contact is opened before the emergency contact is closed. They are only paralleled on return to utilty power and with some units when you go from utility to generator power for testing.
Don
As I get it now from the ATS provider, the neutral is an overlapping style. This should prevent any transients. Don is correct on the make before break aspect. We checked with the local utility company and they had no problem as long as the time was something in cycles. We didn't check with the generator manufacturer but they were selling their own ATS's which could be closed transistion style.
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