To Fuse or Not To Fuse 2

[JBUDA54]

Hello Everyone,

I am currently charged with decommissioning a 15kV ckt bkr that is in series with another 15kV breaker. The ckt bkr to be decommissioned is mainly a disconnecting means. The XFMR has a neutral grounding resistor with a set of CT's that are connected to a 51G and an 86 relay that interfaces with the breaker that the owner wants to decommission. The owner would like to pull a new feeder from the upstream 15kV breaker that has the relaying to a disconnect switch on the primary of a 7500kVA 13.8kV-2400V Cast Coil XFMR. My question is does the new disconnect need to be fused? The upstream breaker is interfaced with a 50/51 Relay that has a 600/5 CT connected to it. I believe I am bound to NEC Table 450.3(a) for the Maximum Rating of Overcurrent Protection for Transformers over 600V. My XFMR has a % Imp of 9.9 so therefore I have a supervised location with a XFMR % imp of more than 6 not more than 10% the chart leads me to 400% of the XFMR FLA. I calculate the primary FLA to be 7500kVA/13.8kV*SQRT(3)=313.77A therefore the maximum size breaker is 400% * 313.77 = 1255.10A. The upstream breaker is a 1200A breaker that is relayed with a 600/5 CT into a 50/51. Does this mean I don't need fuses? It appears I need to pull the 51G/86 relay signal to the new breaker as well. Any advice with this situation is greatly appreciated. Thanks in advance.

 

[tlink0824]

Use breaker & relaying only, IF you have compliant secondary overcurrent protection with T450.3(A) note 2.

Note this transformer, at 5MVA, is recommended by IEEE Buff Book to have 87T/86T differential protection and 63FP/63X rapid pressure rise protection, also.

 

[davidbeach]

tlink0824, not much use for 63 relays on a cast coil transformer, best to save those relays for oil transformers.