Most transmission lines are protected by
directional distance relays. These may serve as backup protection to other
schemes in service, or these may be the sensing components in various forms of
differential protection.
Figure below shows the basic elements of a
PLC (power line carrier) system extensively used for protection of HV
transmission lines.
An HV transmission line is capable of
simultaneous functions of communications and electrical energy transmission.
PLC equipment consists of three distinct parts: terminal assemblies consisting
of transmitters, receivers, and protective relays; the coupling and tuning
equipment, which connects the terminals to selected points on the transmission
line; and the transmission line itself, which provides a suitable channel for
the transmission of carrier energy in PLC bands of frequencies between
terminals.
Coupling to lines is accomplished by means
of high-voltage capacitors, which provide a low-loss path to carrier signals
and block 60 Hz power frequency energy from the carrier equipment. Line traps
minimize the loss of carrier power into adjacent lines and prevent external
ground fault currents from short-circuiting the carrier signal of the unfaulted
line.
Carrier Frequencies.
Frequencies in the range of 30 kHz to 500
kHz have been employed for PLC relaying and other communication purposes. The
range is high enough to be isolated from the transmission line and the noise it
creates and yet not so high as to give rise to excessive attenuation.
There are two basic types of signals used
for teleprotection channels. Keyed carriers are sometimes referred to as AM,
amplitude modulation: It is normally off and intelligence is transmitted by
turning the carrier on and off.
This type of signal is normally used in
blocking-type relaying systems. The frequency may be in range from 29 kHz to 31
kHz, and the signal could be applied to a single sideband (SSB) PLC channel.
Frequency Shift Keyed Carrier.
This signal is always on, which provides a
means of continuously monitoring the channel. The frequency shift keyed (FSK)
carrier is less susceptible to noise and has a greater operating range. FSK
channels have been available with two-frequency operations, high and low shift
frequencies for additional security.
Blocking Schemes.
Transmission line faults are detected using
either high-speed phase comparison, in which the phase of the currents at the
two terminals are compared, or direction comparison relaying. The scheme
operates in a trip permissive mode.
A received signal is used to block tripping
of the protected line for external faults. The blocking scheme is biased toward
dependability because channel or remote relay failure will result in operation
of the local blocking relay.
Tripping Schemes.
A phase comparison tripping scheme channel
is keyed to the trip condition every half cycle during the fault. The scheme is
biased toward security so that a failure of the channels or relays would result
in non operation of the local relay for external and internal faults.
Directional distance relays can be used both for phase as well as ground fault
conditions.
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