GE T35 Instruction Manual page 533

CHAPTER 9: THEORY OF OPERATION SATURATION DETECTOR
restraining signal develops rapidly. Once one or more CTs saturate, the differential current increases. The restraining signal,
however, yields by at least a few milliseconds. During internal faults, both the differential and restraining currents develop
simultaneously. This creates characteristic patterns for the differential - restraining trajectory, as shown.
Figure 9-3: CT saturation detection internal and external fault patterns
The CT saturation condition is declared by the saturation detector when the magnitude of the restraining signal becomes
larger than the higher breakpoint ( ) and at the same time the differential current is below the first slope (
HIGH BPNT LOW
). The said condition is of a transient nature and requires a seal-in. A special logic in the form of a "state machine" is
SLOPE
used for this purpose as depicted in the following figure on saturation detector state machine.
As the phasor estimator introduces a delay into the measurement process, the aforementioned saturation test fails to
detect CT saturation occurring very fast. In order to cope with very fast CT saturation, another condition is checked that
uses relations between the signals at the waveform level. The basic principle is similar to that described. Additionally, the
sample-based stage of the saturation detector uses the time derivative of the restraining signal (di/dt) to better trace the
saturation pattern shown in the following figure.
The saturation detector is capable of detecting saturation occurring in approximately 3 ms from fault inception. The
saturation detector, although having no dedicated settings, uses the main differential characteristic for proper operation.
This must be kept in mind when setting the characteristic as its parameters must retain their original meaning.
The operation of the saturation detector is available as the FlexLogic operand
XFMR PCNT DIFF SAT A/B/C.
9
T35 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL 9-3