Replacing E-Rated Fused Switch with a HV bkr feeding a xfmr, is Ground Fault Protection Not Reqd? 2

[bdn2004]

There is an existing overhead utility line that feeds a 12.47kV-480V, Delta-Wye 2500 kVA transformer that has E-rated fuses at the primary of the transformer. For reliability they are changing this feeder out and running underground cable directly from the Utility's switchgear to a new 12.47kV outdoor metal clad switchgear. The new switchgear has been specified with a vacuum breaker and relay with (3)CTs, one to go around each of the phase conductors. This will replace the E-rated fuses.

I would have thought the CTs would be connected in a residual way so to give the 51N function. Is this common to do it this way ?

 

[davidbeach]

Any decent overcurrent relay can calculate the residual value, no need to wire it in separately.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[bdn2004]

That's interesting... so why do I usually see it wired like this ?

 

[xnuke]

That image seems to predate microprocessor-based relaying where all phase currents go to the relay and the relay calculates the residual. That appears to be for electromechanical relaying.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[RRaghunath]

SEL 787 relay includes overcurrent as well as earth fault protection. Relay will be able to compute internally the residual current as well as includes input to measure.
So, overcurrent as well as earth fault protection can be configured for the transformer with this relay.

 

[stevenal]

I believe with SEL you will need to invoke 50/51G for calculated residual. 50/51N is directly measured from an input you don't appear to be using.