Inductance Capacitance
2010
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Inductance Capacitance
Modes and Issues in Wireless Power Transfer
Wireless media is effectively being utilized in communication networks. Mankind is taking great strides forward in multiplying the data rate for both voice and data oriented networks. Wireless, therefore, has become the mainstream channel to send and receive the information signals. Compared to message or information signals, electric power signals have received very little attention as far as wireless medium is concerned. Although the concept of wireless power transmission is not new, only recently researchers have developed several techniques of moving electricity over long distances through wireless channel.
Let me briefly discuss few methods of transmitting electricity through wireless. Antenna is the tool used in communication networks to send or receive message signal over wireless channel. The size of the antenna is determined by the wavelength of the transmitting signals, larger the wavelength larger will be the size of the antenna. Since the 50 or 60 Hertz AC electric power signals have a very large wavelength, the possibility of wireless electric power distribution through antenna is ruled out.
Next is the method called "Inductive Coupling", this essentially is the principle on which transformers work. The principle says—any current carrying conductor, say A, creates circular magnetic field around it, if we bend the wire into a coil the magnetic field will be enhanced and more loops the coil makes the greater the field will be, if we place another conductor, say B, in this generated field, conductor A will induce a current in conductor B—the induced current can now be used to charge various electronics devices. This simple principle is being used in various other electric gadgets like electric toothbrushes and Splashpower recharging mat. But, wait a minute; can this method be used to generate current in devices located at quite a distance? Yes, a strong magnetic field could induce current from farther away but the process would be very inefficient since the magnetic fields spread in all directions.
A more attractive and radically innovative solution was proposed by researchers at MIT in 2006. They used the concept of resonance to transmit wireless power. Resonant frequency is the objects natural frequency in which it tends to oscillate when disturbed. It's easy to vibrate objects at their resonant frequency and difficult at others. Researchers at MIT demonstrated that if a conductor can tune itself to the frequency of magnetic field generated by the current carrying conduction coil, induction can take place. But for this to happen both current inducing coil and current induced coil need to have same inductance and capacitance—the two parameters on which depends the frequency of resonance. These two parameters can be made equal for transmitting and receiving coils by accurately matching their physical contours. Researchers called it "non-radiative energy transfer" since it require stationary fields around the coils and not the fields which radiate in all directions. Although such kind of setup can be used to recharge devices within a range of few meters, it will take more research to transfer power over large distances using this method.
About the Author
Mr. Sami ur Rehman is an electrical engineering graduate of Pakistan's premier engineerng university: National University of Sciences and Technology. After his graduation he was hired to serve as Analog Desing Engineer in the field of CMOS Microelectronics technology. He aims to take up his Masters studies abroad this year.
For a longtransmission linewhen line capacitance dominates over inductance we have to supply leading VAwhy?
This happens typically in a hydro power plant situated in remote place where generators are under excited. Why ?
First, if the transmission is open-wire, overhead construction, the series inductance always dominates over shunt capacitance. The only situation where shunt capacitance is greater than series inductance is if the transmission is in underground cable.
This occurs as a consequence of the physics of the transmission line, and in particular, is a consequence of the phase-to-phase spacing. As the phase-to-phase spacing increases, series inductance increases, and shunt capacitance decreases. Obviously, the phase-to-phase spacing of an open wire line is much greater than the phase-to-phase spacing of a three-phase cable.
Second, this situation can happen with any kind of generation. The source does not have to be a hydroelectric plant.
To make that point, the system that is most notorious for having more shunt capacitance than series inductance is the system surrounding the city of Riyadh in Saudi Arabia. Its an exclusively cable system. And there are no hydroelectric plants in the Kingdom of Saudi Arabia.
When you say that generation is 'underexcited', what you are really saying is that they are operating in the mode in which they are absorbing reactive VA from the system. That is, the generators are underexcited because the system has more shunt capacitance than series inductance, and in order to regulate voltage, the excess reactive VA must be extracted from the system. If that did not happen, the system voltage would rise to intolerable levels.
Another way to extract excess reactive VA from a system with excess shunt capacitance is to install shunt reactors at the transmission level.
sec 15-3 tophat communication, not inductance, not RF, not hv, not capacitive either?
