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