Phase Selection - GE C60 Instruction Manual

CHAPTER 8: THEORY OF OPERATION SINGLE-POLE TRIPPING
8.2.1.3 SLG fault evolving into LLG
When an AG fault occurs, the events unfold initially as in the previous example. If the fault evolves quickly, the phase
selector changes its initial assessment from AG to ABG fault and when the trip request is placed either by the zone 1 or the
POTT scheme, a three-pole trip is initiated. When this is the case, all three A, B, and operands are asserted. The
TRIP PHASE C
command is passed to the breaker control element and results in a three-pole trip. At the same time, the recloser is
initiated as per settings of the trip output. As the operand is asserted (not the operand), the open
TRIP 3-POLE TRIP 1-POLE
pole is not activated. Because the in progress is asserted, the phase selector is blocked as well.
AR RIP
If the fault evolves slowly, the sequence is different. The relay trips phase A as in the previous example. The phase selector
resets, the open pole detector is activated and forces the zone 1 and zone 2 AG, AB, CA and negative-sequence
overcurrent elements to reset. If the zone 1 BG element picks up, or the zone 2 BG element picks up resulting in operation
of the POTT scheme, no trip command is issued until the is asserted. This happens 1.25 cycles after the
AR FORCE 3-P TRIP
first trip. If at this time or any time later a request for trip is placed (due to an evolving fault), a three-pole trip is initiated. The
operand is de-asserted by the operand, resetting the open pole detector. Shortly all three-poles are
TRIP 1-POLE TRIP 3-POLE
opened.
When the dead time expires, the recloser signals the breaker control to close the breaker. At this time, all the protection
elements are operational, as the open pole detector is not blocking any elements. If the line-side VTs are used, the line
pickup element is armed as well. If there is a fault on the line, these elements pick up the fault and issue the next request
for trip. This request results in a three-pole trip as the is still asserted.
AR FORCE 3-P TRIP
The response of the system from this point is as described earlier for the second trip, except that the recloser goes to
lockout upon the next initiation (depending on the number of shots programmed).

8.2.2 Phase selection

The C60 uses phase relations between current symmetrical components for phase selection. First, the algorithm validates
if there is enough zero-sequence, positive-sequence, and negative-sequence currents for reliable analysis. The
comparison is adaptive; that is, the magnitudes of the three symmetrical components used mutually as restraints confirm
if a given component is large enough to be used for phase selection. Once the current magnitudes are validated, the
algorithm analyzes phase relations between the negative-sequence and positive-sequence currents and negative-
sequence and zero-sequence currents (when applicable), as shown in the figure.
Figure 8-4: Phase selection principle (ABC phase rotation)
8
Due to dual comparisons, the algorithm is very secure. For increased accuracy and to facilitate operation in weak systems,
the pre-fault components are removed from the analyzed currents. The algorithm is very fast and ensures proper phase
selection before any of the correctly set protection elements operates.
C60 BREAKER PROTECTION SYSTEM – INSTRUCTION MANUAL 8-7