POTT scheme works using the following logic:
High-Speed Tripping Logic = Fault in Zone 2 AND permission-to-trip signal from remote end relay
Consider the following scenario when the fault occurs on T-line.
- A fault on t-line shows up in Zone 2 elements of each relay at each end. Both relays key permissive signal to each other. The relays go ahead and trip their respective breaker.
- Zone 1 elements are typically not used however some utilities may use them.
- For double-circuit line installations it is possible for current from one line to backfeed into other. To prevent mis-trip from this current reversal problem, zone 3 elements can be enabled to block the keying.
Consider the following scenario when the fault occurs outside the zone
Line relaying for Breaker B does not see fault since the Zone 2 elements are not looking behind. This means
- Permissive signal is NOT keyed to breaker A
- Permissive signal is sent to Breaker B
- Based on logic mentioned at the beginning of the article, line relays at breaker A and breaker B will not initiate trip.
- For this fault location, the bus differential relay would pickup and trip breakers B and C.
As mentioned above, POTT scheme relies on receiving a “permission to trip” signal from the remote end relay and the local relay to initiate breaker trip. The signal is keyed using a Frequency-Shift Keying (FSK) transmitter/receiver (see Ametek TCF-10B).
Under normal conditions, the relay transmits a “guard” frequency. This frequency prevents the relay from operating.
Under abnormal conditions, the relay switches to a higher or a lower frequency to transmit a “trip” signal. When relays on both ends of the line shift to the “trip” frequency, only then are the breakers tripped. The guard/trip frequencies are maintained and specified by the utility such that no two lines in the system use the same frequency.
Usually POTT schemes are not implemented using the power line carrier. A fault on the T-line may short out the trip signal and therefore inhibit relay operation. In this scenario, multi-phase coupling of the carrier signal provides the necessary redundancy for the secure operation of this scheme.
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