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.
-Power system protection blog
Electrical transmission lines are designed to hold up in adverse weather conditions. Transmission line hardware and electrical transmission in india is manufactured by expert enginners.
ReplyDelete