02-05-2012, 01:12 PM
Power System Components
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In order to develop effective protective relay settings and protective relaying
schemes, protection engineers need to develop an understanding of
design and operational characteristics of power systems and power system
components. They need to be able to identify normal operating conditions,
probable failure modes, and expected system response when postulated
failures occur. They also need to be aware of physical constraints that may
impact protective relay setpoints.
This chapter is intended to establish an understanding of power system
components—transmission lines, distribution lines, transformers, circuit
breakers, buses, reclosers, fuses, reactors, and generators from the perspective
of a protection engineer. Complete descriptions are not provided.
Discussions are limited to details that are relevant to protection engineers.
Transmission Lines
AC transmission lines, also called transmission feeders, are three-phase,
conductive connections, at preselected voltage levels, between substations,
switchyards, and generating stations. Transmission lines are used to
transmit large amounts of power across power systems. Important characteristics
are impedance, operating voltage, and ampacity. Transmissionline
steady-state loading is a function of many variables, including sending
end voltage, receiving end voltage, available generation, system load, and
current distribution among parallel current paths. Transient loading is a
function of the fault or abnormality that initiated the transient in addition
to the preceding factors. Both normal and emergency power-transfer
capability must be considered when setting transmission-line protective
devices.
Transmission lines are terminated at circuit breakers and connected to
lines in a network is a function of the transmission system design philosophy.
When higher voltage and lower voltage transmission lines are
built along the same right of way, lower voltage transmission lines may
be operated as radial lines to prevent lower voltage lines from becoming
overloaded when higher voltage lines are removed from service.