This practical course for the beginner level is a useful and fun tool for learning Romanian. It is intended for people who want to easily learn and communicate in a relatively short time in …Full description
Full description
EC501 PRACTICAL WORK 4.pdf
difference between lightining impulse 7 Switching impulse
% Wave front time (Tf ) Wave front is considered from 30% to 90%
Tr
3.243 R 1 C 2
3.243 0.933 270 (0.003 10 6 )
Wave tail time (Tt) Tt
0.693 R out C 1
0.693 1428 .6 (0.042 10
6
)
0.933
Stored Energy at Maximum Voltage 1. Maximum Voltage E ma x V ma x
C 1 C 1 C 2
300
0.042 0.042
0.003
2. Maximum Energy
2007.6 J
Peak inverse voltage of the diode
Peak inverse voltage of the diode
= 30√ kV = 51.96kV
Discussion 1. Charging and discharging processes of impulse generator Impulse generators basically consist of an array of capacitors (accompanied by resistors and spark gaps) which are charged in a parallelconfiguration to a voltage "E" and then discharged in series with a voltage of "nE" where "n" in the number of capacitors charged.
The initial charging of the capacitors is done via a controlled current source by the use of a high voltage step up transformer and a full bridge rectifier. These capacitors once charged would be discharged in a series configuration through special precisely-spaced spark gap switches for each capacitor. The breakdown of the controlling sphere gap occurs first and it initiates the triggering of the other sphere gap. By changing the gap distance between the controlling spheres, it is possible to change the magnitude of the breakdown voltage. Selection of the capacitors will determine the peak current and rate of current rise during the discharge cycle. Not only the value of capacity and voltage is selected, but the discharge loop inductance and peak current handling is also considered. The operation of the impulse generator could be explained in two ways according to the breakdown of sphere gaps.
Uncontrolled operation In the uncontrolled operation, the break down voltage of the sphere gap is less than the peak value of the supply, so that it effectively closes when the voltage across the gap builds up above its breakdown value. The capacitorwould then discharge through the impulse generator circuit producing an impulse waveform. The impedance ofthe impulse generator charging circuit is much higher than that of the impulse generator circuit so that during theimpulse the rectifier and other related components can be disregarded.
Controlled operation In the controlled mode of operation, the same basic circuit is used, but the capacitor is allowed to reach the full charging voltage without the sphere gap breaking down. The spark over voltage is set at slightly higher than thecharging voltage. In this case, at the sphere gap a special arrangement is used, such as a third sphere betweenthe other two, to be able to initiate breakdown of the gap. The potential across the main gap is divided into two by means of 2 equal resistors R, each of about 100 MΩ. By this means, half the applied voltage V appears across each of the two
auxiliary gaps.
Once the capacitor C1 has charged up to the full value, a small pulse voltage v is applied (about 20 %) at the third electrode (also known as the trigger electrode). This pulse raises the voltage across one of the auxiliarygaps to more than half the charging voltage (½ V + v) so that it would be just sufficient to breakdown the gap.As this auxiliary gap breaks down, the full voltage would be applied across the remaining auxiliary gap causing it also to breakdown. Once both auxiliary gaps have broken down, the ionisation present in the region would cause the main gap also to breakdown almost simultaneously and thus the impulse voltage would be applied. 2. Diagram of the charging unit and details of its operation
This consists of a high voltage step-up transformer with a rectifier assembly connected to form a full wave bridge circuit. So it does not require any capacitors for voltage doubling. The circuit uses compensated selenium rectifiers which are selected to withstand high current surges, voltage reversals and flashovers. Protective resistors are used to limit the current to a safer limit of rectifiers in case of a short circuit. 3. Layout of the control panel of the impulse generator The control panel for the impulse generator mainly consists of a dial and ammeter and voltmeter interfaces. The input voltage to impulse generator until the point of breaking down is controlled by the dial on the control panel. By slowly turning the dial clockwise the applied input voltage to the impulse generator could be gradually increased. For input voltages and input currents, readings are taken from the voltmeter and ammeter. The buildup of voltage at the sphere gap could be observed in the voltmeter and at the point of breakdown a sudden drop of voltage could also be observed.
4. Important features of oscilloscopes used for the study of fast transient phenomena in the work on high voltage and on spark breakdown in small gaps
Ability to capture wave forms which appear in substantially short time period. This feature is useful in measuring impulse voltages as the occurrence of an impulse takes place in microsecond range, it is impossible to measure the voltage values using ordinary measuring equipment.
The sampling frequency of the oscilloscope could be adjusted to be high enough to capture the waveform with a greater accuracy.
Oscilloscopes have the capability of saving the waveforms to an external storage device for further analysis and later reference.
5. Layout of the potential divider used in the laboratory and various types of potential dividers Potential dividers are important when the oscilloscope is used to monitor the high voltage transients as it is essential to reduce the impulse voltage. Potential dividers can be found as capacitive or resistive.
Resistive potential divider In this method, a high resistance potential divider is connected across the high-voltage winding, and a definite fraction of the total voltage is measured by means of a low voltage voltmeter. The ratio of the divider is determined by the sensitivity of the oscilloscope.
Capacitive potential divider In this method, two capacitances C 1and C2 are used in series, the electrostatic voltmeter being connected across the lower capacitor and connected to the oscilloscope.
