GE L60 Instructions Manual page 673

CHAPTER 9: THEORY OF OPERATION OVERVIEW
tripping is blocked via the NOT input to AND4. This blocking of a trip output persists for the drop-out time setting of the
transient blocking timer after the AND3 output disappears as a result of FDH resetting or the trip integrator producing an
output.
The pickup time delay setting of the transient blocking timer must be longer than the expected time difference between
FDH pickup and a trip integrator output during an internal fault. This insures no delay in tripping in the event of an internal
fault, as well as prolonged blocking during the clearing of an external fault during which transient power reversals can
tend to cause false tripping.
9.1.7.4 Unblocking dual phase comparison
The unblocking dual phase comparison scheme is a combination of a blocking scheme with permission to operate. This
scheme can be used with the FSK carrier only, as it requires monitoring of the check channel status before the fault using
the Guard (Low) frequency and during the fault by detecting/not detecting switching between Guard and Trip (High)
frequencies. If both Trip and Guard frequencies disappear prior the fault Guard frequency (that is, during the fault), then the
relay produces a trip within the programmable trip window time (typically 150 ms) after the FDH detector operates.
Figure 9-17: Unblocking dual frequency phase comparison
9
The figure illustrates the state machine used for this scheme. The state machine is started when a fault is detected by FDL.
At this moment, the Guard frequency status (Rx1N) is checked. If the Guard frequency is absent, the scheme is locked up,
and 87PC is blocked until FDL resets.
If the Guard frequency is present upon fault occurrence, then a switch from Guard to Trip is expected during the next 15
ms. Once this occurs, the scheme is unblocked and regular dual phase-comparison (on both positive and negative halves
of the sinewave) takes place. If both Guard and Trip frequencies are not present when the 15 ms timer expires, then the
phase comparison scheme is allowed to operate during the trip window time after FDH picks up.
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 9-25