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Pilot Relaying Schemes
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Pilot Relaying Schemes

¢ Pilot relaying schemes employ a communication channel in conjunction with protective relays to detect a fault in the transmission lines.

¢ High speed determination of fault locations.

¢ 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

¢ The protective relays used at each end include a set of phase and ground tripping devices.

¢ The relays operate only for faults in the direction of the protected line.

¢ The relays used are usually of impedance with directional type or mho type.

¢ Applicable to schemes using blocking pilot.

¢ Not suitable for microwave channels.

Phase Comparison Schemes

¢ In these schemes phase angle of the current entering the terminal is compared with the phase angle of the current leaving the other terminal.

¢ If these two currents are in phase, there is no fault.

¢ If the two currents are essentially 180 0 out of phase then there is a fault in the line.

¢ The carrier channel used is a blocking pilot.

Transferred Tripping Schemes

¢ These schemes utilize frequency shift audio tone signals over either wire pilot or microwave channels.

¢ Under normal conditions continuous guard frequency is transmitted.

¢ 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

¢ The high loop impedance and the relatively low shunt capacitive impedance make the discriminating factor low.


¢ A shunt connected reactor across the terminals at each end.

¢ Use of directional relays.

¢ 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


¢ independent of power line.

¢ No necessity of coupling and trap equipment.

¢ Signal level is not attenuated.

¢ Facility for high speed relaying channel equipment.

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