EE1402(UNIT-IV)

UNIT-IV
1. What are the factors that affect the spark over voltage of sphere gap?
• Nearby earthed objects
• Atmospheric conditions and humidity
• Irradiation
• Polarity and rise time of voltage waveforms
2. State the measurement techniques that is suitable for measuring A.C. high frequency
Voltages and impulse.
• Potential divider with CRO (resistive (or) capacitive dividers).
• Peak voltmeters.
• Sphere gaps.
3. What are the methods that are used to measure D.C. voltages?
• Series resistance micro ammeter.
• Resistance potential divider.
• Generating voltmeter.
• Sphere and other spark gaps.
4. What are the drawbacks in the series resistance with micro ammeter method?
• Power dissipation and source loading.
• Temperature effects and long time stability.
• Voltage dependence or resistive elements.
• Sensitivity to mechanical stresses.
5. State the advantages and limitations of generating voltmeters?
Advantages:
• No source loading by the meter.
• No direct connection to high voltage electrodes.
• Scale is linear and extension of range is easy.
• Convenient instrument.
Disadvantages:
• They require calibration
• Careful construction is needed and it is cumbersome instrument requiring an auxiliary drive.
• Disturbance in position and mounting of the electrodes makes the calibration invalid.


6. State the principle of operation of electrostatic voltmeters.
When one electrode is free to move, the force on the plate can be measured by controlling it by a spring or balancing it with a counter weight. The force is proportional to the square of the applied voltage, the measurement can be made for D.C or A.C.
F= abs (½ V2 δC/δS)
7. What are the elements that mainly constitute errors in potential dividers for impulse voltage measurement ?
• Residual inductance in the elements.
• Stray capacitance occurring.
i. Between elements.
ii. From sections and terminals of the elements to ground.
iii. From the high voltage lead to the elements or sections.
• The impedance error.
i. Connecting leads between the divider and the test objects
ii. Ground return leads and extraneous current in ground leads.
• Parasitic oscillations due to lead and cable inductances and capacitance of high voltage terminal ground.


8. State the measurement techniques that is suitable for measuring A.C. high frequency
Voltages and impulse
• Potential divider with CRO (resistive (or) capacitive dividers).
• Peak voltmeters.
• Sphere gaps.


9. Why capacitance voltage divider preferred for high voltage ac measurement?
It is used to eliminate the errors due to the hamonics with the help of an Electro static voltmeter(ESV) or high impedance voltmeter.