Dissimilar DG Sets with Common Output Bus

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greggs007

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I have 02 DG Sets of dissimilar makes, with each DG set having three-pole ACB and switched-neutrals (as DG output breakers) and connecting their outputs on a common TPN Bus-bar.

Due to some unwarranted conditions, the DGs paralleling and synchronization is not desired. I wish to run both the DGs as standalone DGs, in both the cases using the common output bus-bar to which multiple loads are connected. Under such scenario, I need to electrically and mechanically interlock both the DGs output ACBs. Since, the neutrals of both the DG sets are switched through dedicated relay, mechanically interlock is not possible for neutral line.

I wish to know, would the electrical interlocking only for the both neutral lines be a safer approach.
(a) If no, why?
(b) If yes, how?
(c) Is it essential to use 4-pole ACB for both DG set?

Appreciate, your responses.
 
greggs007 said:
I wish to know, would the electrical interlocking only for the both neutral lines be a safer approach.
(a) If no, why?
(b) If yes, how?
(c) Is it essential to use 4-pole ACB for both DG set?
Click to expand...

If you leave the neutral points of the two generators ungrounded, you don't have to switch the neutrals at all. The generators will derive their ground reference from the service grounding electrode system. You only need switched neutrals if the generators are providing backup to more than one wye connected system.
 
You did not give the system, voltage, or power. Following response is based on solidly grounded 480Y, say 500KW to 2MW. Although responses would be applicable to a 208V system as well.

adding to Joe's response:
In my experience (industrial - both self-generated and on-grid):
  • The CBs are not mechanically interlocked - just electrically interlocked.
  • As joe said:
    • remove the NG bond at the generators, bond at the common switchboard.
    • All are 3 pole CBs - neutral is not switched by the CB
  • At each gen install a neutral disconnect for maintenance. All I have seen use a standard 3pole disconnect. One pole is used in the gen run circuit so if the switch is inadvertantly opened the gen driver shuts down.
Mechanically interlocking and switching neutrals aren't generally considered safety issues - just design issues. Mechanical interlocks usually only show up on small generation (say <300KW) transfer switches. As for switched neutrals, charlieb has said he always orders 4 pole CBs - I never have had an application that needed one. Go figure???

ice
 
Thanks for your responses. Apologies for logging in late.

Would like to mail you the layout for better appreciation. I hope you are wouldn't mind.

Thanks
 
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