18-03-2011, 03:24 PM
HIGH VOLTAGE TRANSIENT ANALYSIS.ppt (Size: 105 KB / Downloads: 141)
HIGH VOLTAGE TRANSIENT ANALYSIS
◊ SURGES ON TRANSMISSION LINE ◊
Due to a variety of reasons, such as a direct stroke of lightning on the line, or by indirect strokes, or by switching operations or by faults, high voltage surges are induced on the transmission line
The surge can be shown to travel along the overhead line at approximately the speed of light.
These waves, as they reach the end of the line or a junction of transmission lines, are partly reflected and partly transmitted
ENERGY STORED IN SURGE
The energy stored in a traveling wave is the sum of the energies stored in the voltage wave and in the current wave.
Energy = ½ c e2 + ½ l i2
But for a surge, e = Zo i, so that we have It is seen that half the energy of the surge is stored in the electrostatic field and half in the electromagnetic field.
REFLECTION OF TRAVELING WAVES AT A JUNCTION
When a traveling wave on a transmission line reaches a junction with another line, or a termination, then part of the incident wave is reflected back, and a part of it is transmitted beyond the junction or termination.
The incident wave, the reflected wave and the transmitted wave are formed in accordance with Kirchhoff's laws. They must also satisfy the differential equation of the line.
BEWLEY LATTICE DIAGRAM
This is a convenient diagram devised by Bewley, which shows at a glance the position and direction of motion of every incident, reflected, and transmitted wave on the system at every instant of time.
The diagram overcomes the difficulty of otherwise keeping track of the multiplicity of successive reflections at the various junctions.
PROPERTIES OF BEWLEY LATTICE DIAGRAM
All waves travel downhill, because time always increases.
The position of any wave at any time can be deduced directly from the diagram.
The total potential at any point, at any instant of time is the superposition of all the waves which have arrived at that point up until that instant of time, displaced in position from each other by intervals equal to the difference in their time of arrival.
The history of the wave is easily traced. It is possible to find where it came from and just what other waves went into its composition
Attenuation is included, so that the wave arriving at the far end of a line corresponds to the value entering multiplied by the attenuation factor of the line.