Main Tie Main, with two Generators

[brian john]

I have a site with a main tie main, two lines up of gear, 13.2kv steps down to 480/277of gear connected with 3000 amp busway through a tie CB in each switchboard. In addition each switchboard is fed from a generator (two generators) through a generator circuit breaker.

All mains have GFP and there are neutral CT?s on each neutral feeder (residual GFP). The service neutrals are grounded at the main and the generator neutrals are grounded at the generator. There are four points where the neutral is grounded. This is a solid neutral throughout the distribution system.

While I realize there are different ways to control the GFP through the 4 mains (two utility, two generator), it seems that a single point of connection from the distribution system neutral to the main system electrode would be a better way to operate this system. At present there is neutral current on the EGC?s and while this current is low, the site loads are minimal at present.

Lastly to through a curve into the mix they have a 4-pole ATS for emergency lighting, so if the generator neutral bonds were lifted the neutral in the 4-pole ATS would need to be reconfigured.

I would think at a minimum the generators should not have the neutral bonded to ground?

What about the services neutral to ground bonds?

Lastly I see no where that any portion of this distribution system was ever tested to verify the GFP or distribution CB?s are functional. In particular the GFP?s as we have had multiple issues with main tie mains, where the switchboard inter-connects are not properly terminated.

 

[jim dungar]

Sounds like a situation where a "Modified Differential Ground Fault" scheme should have been implemented.
Really simplistic explanation: in a MDGF, the neutral current sensor for each source is independent of the phase senors, this allows them to be used whether the current is flowing towards the source or away from it. Through the judicious use of contacts and interwiring, all of the neutral sensors are summed togther and then compared to the appropriate phase sensors. Commissioning a MDGF is definitely more complicated than simply pushing the GF relay 'test button'.

 

[brian john]

Jim, that was my thought, BUT, the systems I have tested in the past factory and field installation issues had issues and. I want to get the control drawings to verify how the GFP was designed.

What red flagged this is there are 4 UPS systems fed from this board with a 4 wire bypass they grounded the neutrals for each bypass. I figure o=if they did this maybe they messed up the complete grounding.

 

[jim dungar]

JI figure o=if they did this maybe they messed up the complete grounding.

Yep. :roll:

Designs like this seems to be, give GF protection a bad name.
GF is the most likely fault a system will ever see, why do people think it deserves the least 'implemented' protection scheme?