Question

1. Discuss the advantages and disadvantages of using the sphere gap as a high voltage measuring device.

Answer 1

Application for sphere gap as the voltage measurment device:

1)Voltage measurement that peak values of voltage may measurement from 2Kv to about 2500Kv by means of sphere.

2) Vertically with lower sphere grounded and horizantally with both sphere connected to the source voltage.

Procedure:

For dc and ac peak voltage measurement using sphere gap. (April 2014) A uniform field spark gap will always have a spark over voltage within a known tolerance under constant atmospheric conditions. Hence a spark gap can be used for measurement of the peak value of the voltage, if the gap distance is known. A spark over voltage of 30 kV (peak) at 1 cm spacing in air at 200C and 760 torr pressure occurs for a sphere gap or any uniform field gap. But experience has shown that these measurements are reliable only for certain gap configurations. Normally, only sphere gaps are used for voltage measurements.

Advantage:

1)They are used for voltage measurements.

2)They are suitable for all types of waveforms from d.c to impulse voltages of short times.

3)They are used for radio frequency a.c voltage peak measurements upto 1 MHz.

Disadvantage:

For very high voltages (1000 kV or more) problems arise due to large power dissipation, leakage currents and limitation of voltage stress per unit length, change in resistance due to temperature variations, etc. Hence, a resistance potential divider with an electrostatic voltmeter is sometimes better when high precision is needed. But potential dividers also suffer from the disadvantages stated above.

Both series resistance meters and potential dividers cause current drain from the source. Generating voltmeters are high impedance devices and do not load the source. They provide complete isolation from the source voltage (high voltage) as they are not directly connected to the high voltage terminal and hence are safer. Spark gaps such as sphere gaps are gas discharge devices and give an accurate measure of the peak voltage. These are quite simple and do not require any specialized construction. But the measurement is affected by the atmospheric conditions like temperature, humidity, etc. and by the vicinity of earthed objects, as the electric field in the gap is affected by the presence of earthed objects. But sphere gap measurement of voltages is independent of the waveform and frequency.