SEF relay part of O/C , E/F relay REF 615 1

[bvc]

Hi everybody,
I have this query regarding Sensitive Earth fault protection. Normally in our installation, the feeder protection for 13.8 kv (distribution) is O/C , E/f ( 50/51,50N,51 N) provided in one relay and Sensitive earth fault (51G) provided in another relay. Presently my contractor has come up with an installation where it is suggested to combine all these functions into one relay i.e. ABB REF 615. My concern is as follows:
My phase CT is 600:1 and cable CT (for SEF is 50:1). Also the relay REF 615 has only one neutral coil for neutral current input. What is the implication of using the cable CT ( 50:1) for E/f protection of a distribution feeder ? Should I insist on separate relays to be provided for SEF and 50/51/51N/50N? I think the earth fault setting will be too low if I use the 50:1 Ct for 51N and 50N protection. Kindly advise.

 

[Kiribanda]

bvc,
50/1 ratio is appropriate for a high resistance grounded (HRG) system where the ground fault currents are limited.If it is a LRG network then 50/1 seems to be not enough.What is your max allowable ground fault current?In any case still you can use terminals 13/14 for your sensitive ground fault CT input and program 50/51N (residual)function for sensitive ground fault protection function(50/51G).

 

[bvc]

Thanks Mr.Kiribanda,

My transformer 132 kv /13.8 kV neutral is grounded through an NER (800 A/ 10 ohms). Max feeder load current is 300 A. Ph O/C Pick up is 400 A, highset of 2400 A. As per my company guidelines, SEF setting is having time delay of 5 sec. If there is a ground fault then before the SEF , phase o/c will trip. The fault level is as follows:
13.8 kV Single phase to Ground
Isym kA - 1.58
X/R - 0.1
Iasym kA - 1.58