Theory Of Operation; Saturation Detector; Ct Saturation Detection - GE M60 Instruction Manual

9 THEORY OF OPERATION

9.1 SATURATION DETECTOR

9 THEORY OF OPERATION 9.1SATURATION DETECTOR

9.1.1 CT SATURATION DETECTION

The saturation detector of the M60 takes advantage of the fact that any CT operates correctly for a short period of time
even under very large primary currents that would subsequently cause a very deep saturation. As a result of that, in the
case of an external fault, the differential current stays very low during the initial period of linear operation of the CTs while
the restraining signal develops rapidly. Once one or more CTs saturate, the differential current will increase. 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 depicted below.
OPERATE
BLOCK
restraining
836728A1.CDR
Figure 9–1: CT SATURATION DETECTION: INTERNAL & 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 (HIGH BPNT) and at the same time the differential current is below the first slope (LOW
SLOPE). The said condition is of a transient nature and requires a seal-in. A special logic in the form of a "state machine" is
used for this purpose as depicted in the following figure.
As the phasor estimator introduces a delay into the measurement process, the aforementioned saturation test would fail 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 above. Addition-
ally, 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. It is worth
emphasizing that 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 STATOR DIFF SAT A/B/C.
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GE Multilin M60 Motor Protection System 9-1