Charging Currents in Transmission Lines

Any two conductors separated by an insulating medium constitutes a condenser or capacitor.In case of overhead transmission lines, two conductors form the two plates of the capacitor and the air between the conductors behaves as dielectric medium. Thus an overhead transmission line can be assumed to have capacitance between the conductors throughout the length of the line. The capacitance is uniformly distributed over the length of the line and may be considered as uniform series of condensers connected between the conductors.

When an alternating voltage is applied across the transmission line it draws the leading current even when supplying no load. This leading current will be in quadrature with the applied voltage and is termed as charging current. It must be noted that charging current is due to the capacitive effect between the conductors of the line and does not depend on the load. The strength of the charging currents depends on the voltage of transmission, the capacitance of the line and frequency of the ac supply.

If the capacitance of the overhead line is high, the line draws more charging currents which cancels out the lagging component of the load current (normally load is inductive in nature). Hence the resultant current flowing in the line is reduced. The reduction in the resultant current flowing through the transmission line for given load current results in:

• Reduction of the line losses and so increase of transmission efficiency.

• Reduction in the voltage drop in the system or improvement of the voltage regulation.

• Increased load capacity and improved power factor

Significance of Charging currents:

Capacitance effect (responsible for charging currents) of the short transmission lines are negligible. However they are significant in medium and long distance transmission lines.

In long distance transmission lines, during light loaded conditions receiving end voltage will be higher than sending end voltage. This is because of the charging currents and capacitive effect of the line.

Synchroscopes

Synchroscopes are electrodynamic instruments, which rely on the interaction of magnetic fields to rotate a pointer. In most types,there is no restoring spring torque for the magnetically produced torques to overcome therefore  pointer system is free to rotate continually. Synchroscopes have a damping vane to smooth out vibration of the moving system.

A polarized-vane synchroscope has a field winding with a phase-shifting network arranged to produce a rotating magnetic field. The field windings are connected to the incoming machine. A single phase polarizing winding is connected to the running system. It is mounted perpendicular to the field winding and produces a magnetic flux that passes through the moving vanes. The moving vanes turn a shaft that carries a pointer moving over a scale. If the frequency of the source connected to the polarizing winding is different from the source connected to the field winding, the pointer rotates continually at a speed proportional to the difference in system frequencies.

The scale is marked to show the direction of rotation corresponding to the incoming machine running faster than the running system. When the frequencies match, the moving vanes will rotate to a position corresponding to the phase difference between the two sources. The incoming machine can then be adjusted in speed  and than phase sequence is checked.

EM Wave Propogation

There are two main types of waves. Mechanical wave and Electromagnetic wave.
Mechanical waves propagate through a medium, and the substance of this medium is deformed. The deformation reverses itself owing to restoring forces resulting from its deformation. For example, sound waves propagate via air molecules colliding with their neighbors. When air molecules collide, they also bounce away from each other (a restoring force). This keeps the molecules from continuing to travel in the direction of the wave.

The second main type of wave, electromagnetic waves, do not require a medium. Instead, they consist of periodic oscillations of electrical and magnetic fields generated by charged particles, and can therefore travel through a vacuum. These types of waves vary in wavelength, and include radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. An electromagnetic wave (i.e., a light wave) is produced by accelerating electric charge. As the wave moves through the vacuum of empty space, it travels at a speed of c (3 x 108 m/s). This value is the speed of light in a vacuum. When the wave impinges upon a particle of matter, the energy is absorbed and sets electrons within the atoms into vibrational motion. If the frequency of the electromagnetic wave does not match the resonant frequency of vibration of the electron, then the energy is reemitted in the form of an electromagnetic wave. This new electromagnetic wave has the same frequency as the original wave and it too will travel at a speed of c through the empty space between atoms. The newly emitted light wave continues to move through the interatomic space until it impinges upon a neighboring particle. The energy is absorbed by this new particle and sets the electrons of its atoms into vibration motion. And once more, if there is no match between the frequency of the electromagnetic wave and the resonant frequency of the electron, the energy is reemitted in the form of a new electromagnetic wave. 

The cycle of absorption and reemission continues as the energy is transported from particle to particle through the bulk of a medium. Every photon (bundle of electromagnetic energy) travels between the interatomic void at a speed of c; yet time delay involved in the process of being absorbed and reemitted by the atoms of the material lowers the net speed of transport from one end of the medium to the other. Subsequently, the net speed of an electromagnetic wave in any medium is somewhat less than its speed in a vacuum - c (3 x 10^8 m/s).
How much the wave will delay will depend upon the optical density of material.
The optical density of a medium is not the same as its physical density. The physical density of a material refers to the mass/volume ratio. The optical density of a material relates to the tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave in the form of vibrating electrons before reemitting it as a new electromagnetic disturbance. The more optically dense that a material is, the slower that a wave will move through the material.