Pilot Relaying Schemes
Â¢ Pilot relaying schemes employ a communication channel in conjunction with protective relays to detect a fault in the transmission lines.
Â¢ High speed determination of fault locations.
Â¢ There are basically two types of pilot relaying schemes.
1. Blocking pilot protective scheme
2. Tripping pilot scheme
Types of channels
There are basically 3 types of channels used in pilot relaying.
1. Wire pilot- Twisted pair of copper wires used in Telephone cables.
2. Carrier current pilot- The pilot signal is coupled directly to the power line to be protected. (freq- 500-700 kHz)
3. Microwave pilot- a radio channel of very high frequency (450 to 10,000 MHz).
Directional Comparison Schemes
Â¢ The protective relays used at each end include a set of phase and ground tripping devices.
Â¢ The relays operate only for faults in the direction of the protected line.
Â¢ The relays used are usually of impedance with directional type or mho type.
Â¢ Applicable to schemes using blocking pilot.
Â¢ Not suitable for microwave channels.
Phase Comparison Schemes
Â¢ In these schemes phase angle of the current entering the terminal is compared with the phase angle of the current leaving the other terminal.
Â¢ If these two currents are in phase, there is no fault.
Â¢ If the two currents are essentially 180 0 out of phase then there is a fault in the line.
Â¢ The carrier channel used is a blocking pilot.
Transferred Tripping Schemes
Â¢ These schemes utilize frequency shift audio tone signals over either wire pilot or microwave channels.
Â¢ Under normal conditions continuous guard frequency is transmitted.
Â¢ The contacts of the trip and guard relays are in each receiver are arranged so that the combined action of dropped out guard relay and a picked up trip relay constitutes the reception of transferred trip signal.
Problems With Telephone Pilots
Â¢ The high loop impedance and the relatively low shunt capacitive impedance make the discriminating factor low.
Â¢ A shunt connected reactor across the terminals at each end.
Â¢ Use of directional relays.
Â¢ By using replica impedances for the pilot loop under external fault conditions.
Application Of Microwave Channels For Protective Relaying
1.Transformer and C.B. Failure Protection
2. Transmission line protection
Â¢ independent of power line.
Â¢ No necessity of coupling and trap equipment.
Â¢ Signal level is not attenuated.
Â¢ Facility for high speed relaying channel equipment.