07-10-2014, 02:33 PM
Pilot Protection.ppt (Size: 1.22 MB / Downloads: 13) Pilot Protection of Transmission Lines
Outline
Transmission line introduction
Typical protection schemes
Need for pilot aided schemes
Various pilot schemes
Redundancy considerations
Desirable attributes of pilot relay
Introduction
Nonpilot protection using overcurrent and distance relays, contain a fundamental difficulty :
It is not possible to instantaneously clear a fault from both ends of a transmission line if the fault is near one end of the line.
Pilot protection is an adaptation of the principles of differential relaying that avoids the use of control cable between terminals.
The term ‘pilot’ refers to a communication channel between two or more ends of a transmission line to provide instantaneous clearing over 100% of the line.
This form of protection is also known as ‘teleprotection’.
Pilot Communications Channels
Distance-based pilot schemes traditionally utilize simple on/off communications between relays, but can also utilize peer-to-peer communications and GOOSE messaging over digital channels
Typical communications media include:
Pilot-wire (50Hz, 60Hz, AT)
Power line carrier
Microwave
Radio
Optic fiber (directly connected or multiplexed channels)
Directional Comparison Unblocking (DCUB)
Applied to Permissive Overreaching (POR) schemes to overcome the possibility of carrier signal attenuation or loss as a result of the fault
Unblocking provided in the receiver when signal is lost:
If signal is lost due to fault, at least one permissive RO functions will be picked up
Unblocking logic produces short-duration TRIP signal (150-300 ms). If RO function not picked up, channel lockout occurs until GUARD signal returns
Redundancy Considerations
Redundant protection systems increase dependability of the system:
Multiple sets of protection using same protection principle and multiple pilot channels overcome individual element failure, or
Multiple sets of protection using different protection principles and multiple channels protects against failure of one of the protection methods.
Security can be improved using “voting” schemes (i.e., 2-out-of-3), potentially at expense of dependability.
Redundancy of instrument transformers, battery systems, trip coil circuits, etc. also need to be considered.