19-06-2012, 02:41 PM
POWER LINE CARRIER COMMUNICATION
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Introduction of PLCC
As electronics play a vital role in the industrial growth. Communication is also backbone of any power system. Communication between various generating and receiving stations is very essential for proper operation of power system. This is more so in the case of large interconnected system where a control load dispatch station has to coordinate the working of various unit to see that the system is maintained in the optimum working condition. Power line carrier communication has been found to be the most economical & reliable method of communication for the medium and long distance in the power network. For short distance the ordinary telephone phone system is used. Open wire or underground cable and in some cases VHF wireless communication is found to be more economical as they do not involve the use of costly high voltage coupling equipment. In PLCC the higher mechanical strength and insulation level of high voltage power line result in increased reliability of communication & lower attenuation over long distance.
PLCC is used for establishing speech/ data communication between two stations using a power line. PLCC is an approach to utilize the existing power lines for the transmission of information. In today’s world every house and building has properly installed electricity lines. By using the existing AC power lines as a medium to transfer the information, it becomes easy to connect the houses with a high speed network access point without installing new wirings.
This technology has been in wide use since 1950 and was mainly used by the grid stations to transmit information at high speed. Now days this technology is finding wide use in building as it avoids the need of extra wiring. The data collected from different sensors is transmitted on these power lines thereby also reducing the maintenance cost of the additional wiring. In some countries this technology is also used to provide Internet connection.
Communication System in PLCC
Power generation and transmission need a good &healthy communication system for proper functioning of the power system and to give uninterrupted supply to the consumers.
Power line carrier communication (PLCC) is mainly used for telecommunication, tele-protection and tele-monitoring between electrical substations through power lines at high voltages, such as 110 kV, 220 kV, 400 kV. In a PLCC system the communication is established through the power line. PLCC integrates the transmission of communication signal and 50/60 Hz power signal through the same electric power cable. The audio frequency is carried by a carrier frequency and the range of carrier frequency is from 50 kHz to 500 kHz.
Line Matching Unit (LMU)
LMU is a composite unit consisting of Drain Coil, Isolation transformer with Lightning Arrester on its both the sides, a Tuning Device and an earth switch. Tuning Device is the combination of R-L-C circuits which act as filter circuit. LMU is also known as Coupling Device. Together with coupling capacitor, LMU serves the purpose of connecting effectively the Audio/Radio frequency signals to either transmission line or PLC terminal and protection of the PLCC unit from the over voltages caused due to transients on power system. It is also used for matching the impedance of the power line and HF cable.
Lighting Arrestor
The lighting arrestor against high voltage surges caused by atmospheric effects or switching operations protects the wave traps .The nominal discharge current of this lighting arrester is selected to suit that of the substation lighting arrester behind the wave trap. The tuning elements however have been rated at 20to30%.
The lighting arrester used may be vacuum type arrester whose are over voltage lies below the rated voltage of the tuning capacitor, but about the voltage produced across the coil during a short circuit current surge. Therefore it protects the tuning capacitors against momentary over voltages caused by traveling waves. Sustained over voltages resulting from short circuit currents are not high enough to cause the lighting arrester to be over. Hence, sustained and consequent destruction of the arrester are avoided.
Coupling Equipment
The most important technical problem in a power line carrier is to devise methods and equipment to couple the low voltage and high frequency carrier set to the high voltage and low frequency power line.
During the annual stages of carrier operation on power lines, and aerial wire of more than 300 feet length, supported on power line structure, was used. On one end of it was connected the output from carrier equipment. While the aerial coupled the signals to power lines mainly due to the capacitance between them. The modern practice is to achieve the coupling by connecting a capacitor between the carrier terminal and high voltage line. The particular value of the capacitor is a compromise between two conflicting requirements.