The Directional Comparison Blocking (DCB) scheme is the most popular pilot relaying scheme, implemented to protect high voltage power lines. This scheme is more dependable than a permissive transfer trip scheme because it trips the breaker even when there is no carrier signal from the remote end pilot relay. Let’s dive into details.
Equipment Needed for Directional Comparison Blocking Scheme
- Inside the control building:
- Distance relay like the Schweitzer 421.
- Pilot relays like Pulsar TC-10B or Ametek’s UPLC-II. They generate carrier frequency. Depending on the type of relay, the carrier frequency generated is either ON-OFF (amplitude modulation) or Frequency Shift Keying (frequency modulation). The UPLC-II offers both in one package.
- Hybrids – either balanced (resistive or reactive) or unbalanced a.k.a. skewed. Hybrids are required to multiplex multiple signals (Tx/Rx) from the carrier equipment onto a single coax wire heading out to the tuner in the yard.
- Out in the yard:
- Line tuner. It’s a bandpass filter. It couples carrier frequency from the pilot relay with the power frequency.
- Capacitor Coupled Voltage Transformer. This device provides a path for the carrier frequency, onto the transmission line.
Wavetrap. It’s a bandstop filter. It limits the carrier signals to the intended line section.
Implementation of Directional Comparison Blocking Scheme
Let’s examine the DCB scheme using the figure below and its scenarios.
Fault @ F1 on T-line:
This fault is internal to the circuit breakers CB1 and CB2. These breakers are the closest to the fault and tripping them will isolate the fault. In this scenario, CB1 and CB2 will be tripped without any delay by SEL-421.
Fault @ F2 on T-line:
The fault is external to CB1 and CB2 but internal to CB3 and its remote end breaker(s). Since zone 2 protective element on the CB1 relay can trip on it, it should be blocked from doing so. The intent is to trip the breakers local to the fault rather than taking out a larger portion of the system.
Thus, while CB3 takes measures to isolate the fault, CB2’s zone 3 element (which looks backwards) detects this fault as external and keys a “block from tripping” signal to CB1. It should be clear now that the SEL-421 at CB1 uses zone 1 and zone 2 to detect faults and that zone 2 is a time delayed logic that acts as a backup to CB3 when it fails to operate.
Fault @ Bus1
CB’s 1, 4, and 5 should trip to isolate the fault on Bus 1. A bus differential relay does this job via a 86 lockout device. Contact from this LOR relay is also used to engage the 85 device – to key the block signal to remote-end (to stop CB2 from tripping on
The distance protection using
Advantage of Directional Comparison Blocking Scheme
Dependable scheme because in the event the communication channel is disrupted, the relays do not sit idle – waiting on a trip signal. The relay still trips for Zone 2 fault after the timer expires. To check the integrity of the communication channel, a test signal a.k.a. checkback is transmitted 3 or 4 times a day.