24-07-2014, 12:42 PM
POWER LINE CARRIER COMMUNICATION
INTRODUCTION TO R.S.E.B
Rajasthan state electricity board stated working from 1st july 1957. This is the body of big organization and is to function under provision electricity act , like public limited companies. The board does not have article and memorandum of association.
In order to carry out its function, it has rules and regulations and has made other necessary administrative arrangement, after acting of RSEB six dimension along with 64 offices and about 300 employees were transferred to its control by the State Government.
The main aim of RSEB is to supply electricity to entire Rajasthan State in the most economical way. There is no possibility of staking or electricity so the target of board is to distribute the energy in new area as possible. The board has to carry the business on profit without losses.
After an efficient starting as from last many years RSEB is not earning profits . So far for betterment privatization of RSEB has been done recently. It has been divided in 5 main parts they are-:
BASIC PRINCIPLE OF PLCC
• In PLCC the higher mechanical strength and insulation level of voltage power lines results in increased reliability of communication and lower attenuation over long distance.
• Since telephone communication system cannot be directly connected to the high voltage lines, suitably designed coupling device have to be employed.
• Coupling devices consists of high voltage capacitors in conjunction with suitable line matching units (LMU’s) for line impedance matching to that of the co-axial cable connecting the unit to the plc transmit-receive equipment.
• Carrier currents used for communication have to be prevented from entering the power equipment used in GSS as this would results in complete loss of power system.
• To prevent loss of communication signals, wave traps or line traps are employed. This consists of suitably designed choke coils connected in series with the line, which offer negligible impedance to RF carrier currents.
• Wave traps also usually have one or more suitably designed capacitors connected parallel with choke coils so as to resonate at carrier frequencies and thus offer even higher impedance to the flow of RF currents.
• The RF is prevented from entering the stations bus (yard) and the power frequency is blocked of coupling capacitor.
Coupling capacitor
A modern coupling capacitor consists of a stack of flat wound elements of pure cellulose paper and aluminum foil held between insulating rods under optimum pressure to minimize capacitance changes with temperature and time. The interconnections are designed to obtain highest possible surge withstand capacity, highest cut-off frequency and lowest series resistance at carrier frequencies.
Phase to Ground coupling
The wave traps and coupling capacitors are all connected to one conductor of the power line. The remaining two conductors, though not directly connected to the line, carry a portion of the returning carrier current because of the capacitance between the conductors. Because these two conductors do not have wave traps, a portion of the carrier energy is lost. Also radiation losses are high as earth forms a part of the circuit, and the noise pick-up is correspondingly higher.
This type of coupling is more economical than the other types of coupling as it use only half number of wave traps and coupling capacitors used with the other types of coupling.
Inter-line or Inter-circuit coupling
This is the same as phase-to-phase coupling but with the difference that the two conductors used for communication belong to two different power circuits carried on common towers. This type of coupling is not employed where the two circuits are carried on two separate sets of towers as it then behaves more like a double phase-to-ground coupling and is found to be impractical. This is because the two widely separated conductors do not constitute an efficient transmission line but tend to behave more like two antennas, with all the consequent disadvantages
Lightning arrester
A lightning arrester (in Europe: surge arrester) is a device used on electrical power systems and telecommunications systems to protect the insulation and conductors of the system from the damaging effects of lightning. The typical lightning arrester has a high-voltage terminal and a ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrestor, in most cases to earth.
In telegraphy and telephony, a lightning arrestor is placed where wires enter a structure, preventing damage to electronic instruments within and ensuring the safety of individuals near them. Smaller versions of lightning arresters, also called surge protectors, are devices that are connected between each electrical conductor in power and communications systems and the Earth. These prevent the flow of the normal power or signal currents to ground, but provide a path over which high-voltage lightning current flows, bypassing the connected equipment. Their purpose is to limit the rise in voltage when a communications or power line is struck by lightning or is near to a lightning strike.
If protection fails or is absent, lightning that strikes the electrical system introduces thousands of kilovolts that may damage the transmission lines, and can also cause severe damage to transformers and other electrical or electronic devices. Lightning-produced extreme voltage spikes in incoming power lines can damage electrical home appliances.
Tuning Capacitors
These are used with high voltage, high stability mica capacitors with no losses. For lower voltage class of tuning units (with impulse test voltage rating up to 40 KV)Polystyrene capacitors are used by some manufactures. For higher voltage class of tuning units with impulse test voltage rating up to 150 KV, capacitors with mineral oils impregnated paper dielectric are used which rare similar in construction to coupling capacitors all types are mounted in epoxy resin. Single frequency traps have a single and double frequency traps and double tuned parallel resonant circuits.
All the elements belonging to the tuning circuits are usually mounted in a common housing, which can be resolved and substituted with another similar tuning device to resonate trap to a different frequency.
Circuit description
The 3-phase AC input is applied through the 3 poles 2 way switch(RS-1) and fuse f-18 to f-20 to the float input contractor (CON-1).
Resistance R-3, capacitor C-2 and also resistance R-2, capacitor C-1 are incorporated to remove the instabilitf likes like hunting. Operation amplifier IC-2 liner amplifier the mv drop across shunt. the ratio of R-14/R-15 determine the gain of the amplifier and RV-2 on sub assembly sets the charging current. When charging current increases the mv drop across pin no. 2 & 3 of IC-2 will increased. This voltage is applied to the base of TR-4 through R-11. Transistor TR-4 will be the base current of TR-3 and TR-3 will increase the voltage from D-2 will control the voltage correcting operational amplifier IC-1. This will result in decrease in DC output voltages to keep the battery current at set level, which can be adjusted by potentiometer RV-2.
Operation of charger
The float or boost charger can be switches ‘ON’ by means of selector switch RS-1. Thus at a time only one charger either float or boost can be operated.
When the charger is operated in float mode the battery is on float charge and all the VDD’s are bypassed through the contacts of DC contractor. This enables complete voltage appearing on the load. In case of mains fall also the entire battery voltage is available on load through contacts of DC contractor. When the charger is operated on boost mode, the contacts of DC contractor opens.
Load voltage can be adjusted by VDD switch RS-8 as per the requirement main switch RS-9 have been provided to isolate the charger from load and battery. When the selector switch RS-9 is in charger mode then it will supplying load as well as trickle charger. The batteries in float and boost charger, the battery mode when the switch RS-9 is in main mode, then the load will be supplied by the battery and the charger is totally isolated from battery for charger main purpose.
MODES OF OPERATION
The equipment is suitable for connecting to a telephone exchange and furthermore, a 4-wire remote/emergency call station can be created by operating it in parallel with the built in service telephone equipment. The transmission facilities for tele-operation working (telemetry, tele-control & protection signals) use separate output circuits according to their classification.
When 4 KHz equipment carries simultaneously the speech and tele operation signals, they are transmitted in frequency multiplex and accordingly the audio frequency band is divided into parts the part of the band is used for speech and the lower part for tele-operation signals.
CONCLUSION
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. PLCC has been found to be the most economical and reliable method of communication for is medium and long distance in a power network.
Telephone communication system cannot be directly connected to the high voltage power lines therefore we have to suitably design the coupling devices. These usually consist of high voltage capacitor and line matching unit. For matching the line impedance to that the impedance of the coaxial cable connecting the PLCC equipment. PLCC Equipments are used for point-to-point communication over high voltage power lines. PLCC equipments are used send/receive speech/data/teleprotection signals by using HF carrier signals ranging from 50KHz.
When compared with ordinary lines the power lines have appreciably higher mechanical strength. They would normally remain unaffected under the conditions, which might seriously damage telephone lines
PLCC works on rectified AC or main. When supply goes off we use of a device for proper functioning of PLCC called BATTERY CHARGER. It provides DC to the panel by battery of 48V. In this type 24 batteries are connected in series and individually per battery has approx. 2v capacities
The battery can be charged by using the two rotatory switches provided on front panel for coarse and fine control and that charging current can be read by ammeter A-3 provided on the front panel. The operator must ensure that the rotatory switches are in minimum position before switching on the boost charger.