GE 845, Instruction Manual

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4–144 845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
PROTECTION CHAPTER 4: SETPOINTS
For example, let the CT 3000:5 on winding 2 be the worst one which would saturate first
during external faults, and let the maximum external fault current be 25kA, with an X/R
ratio of 15. The CT voltage kneepoint from the saturation characteristic is 800 V, with
winding resistance of 0.5 Ohms, Burden resistance of 2.19 Ohms, and Burden reactance of
0.01 Ohms.
Logging this data into the CT sat tool, produces the following CT saturation waveform:
The waveform shows that during this external bolted fault with maximum fault current of
25 kA, the CT (3000:5) will saturate severely, and produce only a ¼ of a cycle saturation
free time. Break 2 must be set to a through fault current so that the CT (3000:5) produces at
least ½ cycle saturation free time. The solution is to change the fault current to 15000 kA.
The waveform now shows a bigger part of the first cycle before saturation.