INSTRUCTIOn MANUAL FOR BRUSHLESES. AC GENERATOR MODEL TWYFull description
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Graph of the Triggered Voltage Vs Sphere Gaps:
Triggered Voltage Voltage Vs Sphere Gap Distance 200
180
160
140
) 120 V k ( e g a t l o V100 d e r e g g i r T
80
60
40
20
0 0
0.5
1
1.5
2
2.5
Gap distance (cm)
3
3.5
4
4.5
CALCULATIONS
CS – Surge Surge Capacitance C0 – Shunt Shunt Capacitance R 1D Internal Damping Resistance 1D – Internal R 1 – Charging Charging Resistance R OUT Wave Tail Resistance OUT – Wave C 1 C S / 6
0.25 6
0.04 2 F
C O 0.00 3 F R1 6 × 15 + 18 0 27 0
ROUT OUT 2000 // 5000 1.4286 k
Efficiency
C 1 C 1 C O
0.042
0.933 93.3%
0.042 0.003
Wavefront time
Wave front is considered from 30% to 90% Tr 3.243 R OUT CO 3
DISCUSSION Difference between the practical values and theoretical values: Wave tail time
Produced impulse voltage
Theoretical value
= 44.57
Theoretical value
= 300 kV
Practical value
=
Practical value
= 280 kV
The difference between the theoretical values and practical values may be due to following reasons.
Human errors.
Calculation errors.
The ionization can be caused to change the breakdown strength of air while doing the practical.
The resistance of the connecting wires are neglected
There can be slight variations in the capacitor values and charging resistor values.
As a result of aging, temperature and usage the wave front and wave tail control resistances can be changed. Theoretically impulse generator should produce 300kV, but practically the capacitors are not all charged to the same voltage due du e to the resistances that are in series during c harging.
Use of impulse generator: In order that equipment designed to be used on high voltage lines, and others, be able to withstand surges caused in them during operation, it is necessary to test these equipment with voltages of the form likely to be met in service. Impulse test systems are used to generate impulse voltages simulating lightning strokes and switching surges.The Impulse Voltage Generator is the main part of an impulse voltage test system. It consists of capacitors, resistors, and spark gaps. Some of its applications are listed below.
Lightning testing on cables and insulators Material and dielectric testing Breaking of raw diamonds in mineralogy Bridge wire exploding Electron injection into nuclear reactors Electron accelerators Flash x-ray generation Pulsed electron generation Short duration luminous flash for ultra high speed photograph y Nuclear electromagnetic pulse generator Generation of axial plasma for injection purposes
Charging and Discharging Process of the Impulse Generator:
The impulse voltage generator consists of capacitors, resistors, and spark gaps.
The charging circuit consists of a high voltage step up transformer and a full bridge rectifier which is used to provide HVDC supply to the capacitors. The full wave bridge circuit has better voltage regulation characteristics than voltage doublers circuits and does not require any capacitors for voltage doubling. A stack of capacitors is simultaneously charged in a parallel configuration to a voltage "E" and then discharged in series with a voltage of "nE" where "n" in the number of capacitors chargedin order to obtain a higher impulse voltage. The discharge of capacitors occurs through a special preciselyspaced 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.
Operation of impulse generator: I.
Uncontrolled operation – Diode
Sphere gap
R a.c. supply
C
C R
e
h.v.
,.
In the uncontrolled operation, the break down voltage of the sphere gap is less than the peak value of the supply, so that it discharges as the voltage across the gap builds up above its breakdown value. Hence the capacitor most probably discharges through the impulse generator circuit while producing an impulse waveform. The impedance of the impulse generator circuit is much lower than that of the impulse generator charging circuit. Therefore the rectifier and other related components could be disregarded during the impulse. The capacitor would charge up once again and the process would be repetitive. Both the time of occurrence of the impulse or the exact magnitude are not controllable, as the breakdown of a sphere gap is not exactly a constant but statistical.
Impulse generator waveforms for uncontrolled operation
II.
Controlled operation –
Diode
Sphere gap
R
R
R 2
Pulse v
a.c. supply
C1
C2 R 1
e
h.v. transformer
In controllable method charging voltage is less than the spark over voltage.
The capacitor is permitted to reach the full charging voltage without the breaking down of the sphere gap, though the same basic circuit is used,.
The spark over voltage is arranged at a slightly higher value than the charging voltage. That is facilitated by a special arrangement of third sphere in between the 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. Hence
half the applied voltage V appears across each of the two auxiliary gaps.
Once the capacitor C1has charged up to the full value, a small pulse voltage v(about 20 %)is applied at the third electrode (also known as the trigger electrode).
This pulse raises the voltage across one of the auxiliary gaps to more than half the charging voltage (½V + 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 ionization present in the region would cause the main gap also to breakdown almost simultaneously and thus the impulse voltage would be applied.
Impulse generator waveforms for controlled operation
Control Panel: This is used to increase the input voltage to the impulse generator unil the breakdown occurs. There is a voltmeter and an ammeter to take the readings of input current and input voltage. Control Panel Impulse
enerat
Chargin g unit
V measuring potential device
Important Features of oscilloscopes:
Impulse waveforms have characteristics, which are changing vary rapidly, mostly in the order of micro seconds. So the sampling frequency of the oscilloscope should be high enough to capture the waveform with a greater accuracy.
It should also have fairly adequate storage capacity.
