GE M60, Instruction Manual

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CHAPTER 5: SETTINGS GROUPED ELEMENTS
M60 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL 5-227
5
Figure 5-124: Stator differential logic
5.7.4.1 Saturation detection
External faults near generators typically result in very large time constants of DC components in the fault currents. Also,
when energizing a step-up transformer, the inrush current being limited only by the machine impedance can be significant
and can last for a very long time. In order to provide additional security against maloperations during these events, the
M60 incorporates saturation detection logic. When saturation is detected, the element makes an additional check on the
angle between the neutral and output current. If this angle indicates an internal fault, then tripping is permitted.
The saturation detector is implemented as a state machine (see figure). "NORMAL" is the initial state of the machine. When
in "NORMAL" state, the saturation flag is not set (SAT = 0). The algorithm calculates the saturation condition, SC. If SC = 1
while the state machine is "NORMAL," the saturation detector goes into the "EXTERNAL FAULT" state and sets the
saturation flag (SAT = 1). The algorithm returns to the "NORMAL" state if the differential current is below the first slope, SL,
for more than 200 ms. When in the "EXTERNAL FAULT" state, the algorithm goes into the "EXTERNAL FAULT & CT
SATURATION" state if the differential flag is set (DIF = 1). When in the "EXTERNAL FAULT & CT SATURATION" state, the
algorithm keeps the saturation flag set (SAT = 1). The state machine returns to the "EXTERNAL FAULT" state if the differential
flag is reset (DIF = 0) for 100 ms.