Feeder Dedicated To A Transformer; Characteristics; Phase Timed O/C Pickup; Phase Instantaneous - GE 735 Instruction Manual

A.2 FEEDER DEDICATED TO A TRANSFORMER
A.2 FEEDER DEDICATED TO A TRANSFORMER
A

Feeder Dedicated to a Transformer:

From short circuit study:
• Maximum load current =
• Maximum transformer inrush =
• 3Φ fault in high voltage cable = 1760 A
• Ground fault in high voltage cable = 2100 A
• 3Φ fault in 600 V cable = 20600 A
The phase time overcurrent pickup is approximately equal to 1.2 to 1.5 times the maximum current, for example: 1.3 × 138
A = 179.4 A. Thus,
• Phase Pickup dial setting =
From time coordination curves established in the coordination study, with a minimum coordination between devices of 0.3
seconds (normally between 0.2 and 0.5 seconds), determine curve shape and time multiplier.
• Phase Curve Shape dial setting = VERY INVERSE
• Phase Time Multiplier dial setting = 3
It is desirable to operate this protection for a fault on the 600V bus.
• Fault current at 13.8 kV =
This current is less than the maximum possible transformer inrush current of 1255 A. Thus this value cannot be used.
Instead set instantaneous pickup at 120% of inrush level.
Instantaneous pickup current level = 1.2 × 1255 = 1506 A
Phase Instantaneous dial setting = 1506 A / 150 A ≈ 10
A-2
3300 kVA
--------------------------------- - = 138 A
×
3 13.8 kV
2500 kVA
×
--------------------------------- - 12 = 1255 A
×
3 13.8 kV
179.4 A
×
-------------------- - 100 = 120%
150 A
×
20600 A 600 A
-------------------------------------------- - = 895 A = 895 Amps
13800 A
735/737 Feeder Protection Relay
APPENDIX A

A.2.1 CHARACTERISTICS

A.2.2 PHASE TIMED O/C PICKUP
A.2.3 PHASE INSTANTANEOUS
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