Transmission Line Protection Principles

Factors Influencing line Protection

The protection system selected should provide redundancy to limit the impact of device failure, and backup protection to ensure dependability.

The high level factors influencing line protection include:

• The criticality of the line (in terms of load transfer and system stability),
• Fault clearing time requirements for system stability,
• Line length,
• The system feeding the line,
• The configuration of the line (the number of terminals, the physical construction of the line, the presence of parallel lines),
• The line loading,
• The types of communications available, and
• Failure modes of various protection equipment.

The more detailed factors for transmission line protection directly address dependability and security for a specific application.

Reclosing may be applied to keep the line in service for temporary faults, such as lightning strikes. The maximum load current level will impact the sensitivity of protection functions, and may require adjustment to protection functions settings during certain operating circumstances. Single-pole tripping applications impact the performance requirements of distance elements, differential elements, and communications schemes.

The physical construction of the transmission line is also a factor in protection system application.

The type of conductor, the size of conductor, and spacing of conductors determines the impedance of the line, and the physical response to short circuit conditions, as well as line charging current. In addition, the number of line terminals determines load and fault current flow, which must be accounted for by the protection system.

Parallel lines also impact relaying, as mutual coupling influences the ground current measured by protective relays.

The presence of tapped transformers on a line, or reactive compensation devices such as series capacitor banks or shunt reactors, also influences the choice of protection system, and the actual protection device settings.