10-04-2017, 09:29 PM
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.
Compensation:
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
Advantages:
independent of power line.
No necessity of coupling and trap equipment.
Signal level is not attenuated.
Facility for high speed relaying channel equipment.