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13.8KV DISTRIBUITION CIRCUIT NEUTRAL CONNECTION QUESTION

13.8KV DISTRIBUITION CIRCUIT NEUTRAL CONNECTION QUESTION

13.8KV DISTRIBUITION CIRCUIT NEUTRAL CONNECTION QUESTION

(OP)
Gents,

I work for a small town electric utility that has 4 distribution circuits at 13.8kv GRDY.  Each circuit has at least one tie point where it can be tied with another circuit.  These tie points only connect or disconnect the phase conductors, the neutral runs right through these points. The substation breakers and system reclosers monitor phase and ground currents, and react to fault conditions as programed.  My question is: how accurate is the ground current readings at these points when the neutral is interconnected at so many points throughout the circuits?

thanks,
Chris  

RE: 13.8KV DISTRIBUITION CIRCUIT NEUTRAL CONNECTION QUESTION

Any neutral current has to return from whence it came - the substation transformer.  The path it takes may be hard to state with certainty, but eventually it ends up back at the transformer.  

David Castor
www.cvoes.com

RE: 13.8KV DISTRIBUITION CIRCUIT NEUTRAL CONNECTION QUESTION

Usually, what is monitored is the residual neutral current (sum of CT phases either calculated or measured from the CT secondary neutral) rather than the ground current.  Some of the residual neutral current will return in the metallic neutral and some will return in the ground.
 

RE: 13.8KV DISTRIBUITION CIRCUIT NEUTRAL CONNECTION QUESTION

(OP)
jghrist,

After further research, I believe you are correct.  The neutal and ground currents at the recloser control (SEL 351) are calculated, as there is no CT physically measuring either paths.  

Thanks
Chris   

RE: 13.8KV DISTRIBUITION CIRCUIT NEUTRAL CONNECTION QUESTION

Use the ground/neutral readings with caution when they're calculated from the phase currents. I've seen problems in the past on heavily unbalanced 3 phase ckts. One cold Thanksgiving a few years back when everyone was cooking turkeys, watching football, etc. a line recloser on the ckt would trip and reclose on ground current about every 15 mins. This was due to A phase having most of the single phase transformers causing a huge unbalance on the ckt beyond the recloser. This was not normally a problem because EVERYONE didn't have their oven on at exactly the same time on most days. The unbalance was never seen back at the substation because B and C phase was heavily loaded up to the recloser. We rebalanced the ckt that Fri.

RE: 13.8KV DISTRIBUITION CIRCUIT NEUTRAL CONNECTION QUESTION

COURVILLE, how are these distribution neutrals grounded in the field? Common practice is to ground the distribution neutrals at each pole. Or at least at each distribution transformer. If this is the case, a certain amount of the neutral (unbalance) current will return to the substation through the ground and return to the transformer neutral through the station ground grid.

As this is a single point, there is no easy way to allocate a measured transformer neutral current to each circuit. And a CT on each circuit neutral would be 'fooled' by the portion of the unbalance returning through the ground in parallel with the neutral.

In the final analysis, ground current trip levels have to be set with some margin above the greatest anticipated unbalance current to prevent nuisance tripping. Keep in mind such factors as normal unbalance (as pointed out by Jebb) as well as unbalances that arise from single phase fuses blowing.  

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