EE 4192 Laboratory Practice VIII

IMPULSE GENERATOR(High Voltage Laboratory)

Instructed by: Prof. J. R. Lucas

Group Members:Name : K.K. UyanahewaU. L. D. V. Deshapriya100093KIndex No : 100554CD. H. T. Dinumpura100101LGroup : 08J. M. S. M. Jayasekara100215NDate of Performance: 19 / 01 / 2015K. K. Uyanahewa100554CDate of Submission : 02 / 02 / 2015

OBSERVATIONS

Name: K.K. UyanahewaIndex No: 100554CGroup: 08Date: 19 / 01 / 2015

Average unintended triggered voltage with impulse generator sphere gaps

Sphere Gap(mm)Average Unintended Triggered Voltage(kV)

57.5 x 6 = 45.0

108.0 x 6 = 48.0

159.0 x 6 = 54.0

2022.0 x 6 = 132.0

2527.3 x 6 = 163.8

3024.7 x 6 = 148.2

3534.0 x 6 = 204.0

4034.0 x 6 = 204.0

Oscilloscope graphs for various matching units (5 mm sphere gap)

15

30

45

60

75

Oscilloscope graphs of unintended tiggered voltage waveforms for various impulse generator sphere gaps

5 mm gap

10 mm gap

15 mm gap

20 mm gap

25 mm gap

30 mm gap

35 mm gap

40 mm gap

Note: 2nd part of this practical couldnt be done.

RESULTS

1. Layout of the control panel of the impulse testing unit

2. The wavefront time, wavetail time, efficiency and stored energy at rated maximum voltager, r- Charging Resistance (20 k)Rs1 Internal Damping Resistance (15 )Rs2 External Damping Resistance (180 )ROWave Tail Resistance (2 k)C- Main Capacitance (0.25 F)C0 Wave Shaping Capacitance (3 nF)

Efficiency ()

Wave front time (tf)Defining wave front from 10% to 90% and considering only determines the wave front,

Wave tail time (tt) Defining wave tail time as the time to decay to 50% of peak and only considering

Stored Energy at Maximum Voltage

Maximum Voltage

Maximum Energy

3. Plot the variation of intended and unintended triggering voltages with gap setting and obtain the intended and unintended triggering zones.

Unintended Triggered Voltage vs. Sphere Gap

Sphere Gap(mm)Unintended Triggered Voltage(kV)

545.0

1048.0

1554.0

20132.0

25163.8

30148.2

35204.0

40204.0

3.

4. Plot the variation of the height of the second peak in the waveform distortion.

Gap is set to 5 mm and the resistance values are changed to matching unit in order to get the proper waveform from the oscilloscope. After analysing observed graphs, 30 was selected as best resistance value.

Ratio between the first peak and second peak vs. Matching ResistanceMatching Resistance ()Ratio between the first and second peak

15 2.24

30 1.12

45 1.36

60 1.15

75 1.17

4.

5. Plot the variation of 50% breakdown voltage of the sphere gap with gap distance.

50% Breakdown Voltage of the 125 mm Sphere Gap vs. Gap Distance

Gap Distance (mm)Positive (kV)Negative (kV)

0516.816.8

1031.731.7

1545.545.5

205959

247070

2675.575.5

3085.585.9

359897

40110108

45122119

50134129

55145138

60155146

65164154

70173161

75181168

80189174

90203185

100215195

DISCUSSION

1. Difference between the processes of this impulse generator and the standard Marx impulse generator circuit

The Nissin Impulse Generator is used for the practical and internal arrangement is as follows.

Marx Impulse Generator

Nissin Impulse Generator Standard Marx Impulse Generator

There is a trigger pulse circuit to create breakdown No Trigger pulse circuit

The amount of capacitors used are higher than Marx Impulse generator (doubled the amount)Has less number of capacitors

Allows the capacitor to be fully charged without occurring the breakdown. (controlled operation ) Allows variations in the amplitude of the impulses without a proper control mechanism. (uncontrolled operation )

2. Difference between any stated values, theoretical values and practically observed values during this experiments

TheoreticalPractical

Impulse Voltage300 kV280 kV

Wave front time2.4506 s2 s

Wave tail time61.9533 s28 s

Possible reasons for the deviations between stated, theoretical and practical values, Human errors done while doing practical. Calculation errors. Ionization of air during the practical would change the breakdown strength of air and result in air gap breakdowns before the theoretical value. Resistances of the connecting wires are neglected. Capacitor values and charging resistor values can be deviated from nameplate values. Theoretically impulse generator should produce 300kV, but practically due to the resistances that are in series during charging the capacitors are not all charged to the same voltage. Distances are controlled by a mechanical system and the system was not accurate since there was some play within the wheels. To have an accurate breaking voltage, the spheres must be cleaned and shiny. Because of the dust and the corrosion results can be changed from the actual values. 3. Significance of the layout of the control panel of the impulse generating equipment

Special interlocking system is used in high voltage laboratory. Control panel can only turn on if the gate of the testing area is closed. When the control panel is turned on, supply voltage is gradually increase by using rotating voltage regulator. Voltage and the charging current can be observed using the two dials in control panel. The Alarm button in the control panel is there for the safety purposes. When someone presses that button in an emergency situation it will emit an alarm sound to inform the people around the test area to be away from the danger. And also there is a power indicator bulb to show whether the impulse generator is at ON stage or OFF stage. It is useful to identify the status of the equipment. Current display is to show the maximum charging current. Circuit can be operated not to exceed the maximum charging current above 10mA. Safety of the people and equipment both can be achieved through this. Magnetic contactor should turn on before increasing the voltage by rotating the control dial.

4. Reasons for the deviation of output waveforms when different positions of the matching units used in the measurement

To observe the proper transient signal from the oscilloscope, there must be a perfect matching of the signals at its both sending and receiving ends. In this practical low voltage side of the voltage divider is connected to the oscilloscope through a delay cable. By changing the matching unit resistance, most appropriate value has to be selected for the resistance value at the receiving end to control the amount of reflection and transmission transient. That selected value must be equal with the resistance of the delay cable for the perfect matching. If we use different values of the matching unit, observed waveforms have different distortion. When complete matching is performed, the waveform distortion is the minimum. It can be identified by analyzing the output graph of the oscilloscope.

5. Important features of the oscilloscope that was used for the study of fast transient phenomena

Digital oscilloscope is used for the practical. The sampling frequency of the oscilloscope is much higher. Therefore, captured waveforms have high accuracy. To capture the high speed transient, oscilloscope must have a higher intensity. Since the speed is high, the intensity is lowered and hence a higher intensity is required. In these the beam should not come on till the transient comes in because if it is stationary, the spot of high intensity would fog the photograph before the transient comes on or if it is moving, the beam may have swept before the transient comes. Previously mentioned delay cable is used to delay the incoming transient waveform. So we can clearly capture the whole transient.

6. Comparison of sphere gap characteristic observed with the given value in the table.

Distances are controlled by a mechanical system and the system was not accurate since there was some play within the wheels. so actual gap distance may not be the distance shown in the linear scale. The reading may not be accurate due to the corrosion and dust in the equipment. Human errors done while doing practical is another cause. The breakdown voltages were observed by an observer. He might not be able to read the exact breakdown voltage as the breakdown occurs suddenly. Test was done in a sequence manner. Testing of one gap space might be effected by the previous gap space test as there might be a conductive path between two spheres for some time interval.

REFERENCE

High Voltage Engineering by Prof. J. R. Lucas