Measurement of High Test Voltages : Sphere gap; Construction

Sphere Gap

This is one of the oldest technique adopted for the measurement of all the types  (dc =, ac ~ and impulse ) high voltages of either  polarity.  It remained the most widely used method for decades. The field between two identical spheres is a classical example of  "weakly nonuniform"  field. The breakdown characterstic of such a gap is linear for the gap distances not greater than the radius of the spheres. Measurement voltage is made as a function of minimum distance at which it can flash over or sparkover. The breakdown voltage of the gap does not depend upon the duration of application of voltage and also not upon its variation with time.

Construction of the Sphere Gap

A sphere gap consists of two adjacent metal spheres of equal diameters whose separation gap distance is variable. The ability to respond to peak values of impulse voltages, if the time to peak is not too short ( 1-3 µsec), is governed by a short statistical time lag or the delay  in time required for the availabiilty of an electron to initiate an electron avalanche and hence the breakdown.

Fig 31.1  Vertical sphere gap. 1. Insulating support. 2. Sphere shank. 3. Opening gear, showing maximum dimensions. 4. High-voltage connection with series resistor. 5. Stress distributor, showing maximum dimensions. P. Sparking point of .v. sphere. A. Height of P above ground plane. B. Radius of space free from external structures. X. Item 4 not to pass throuhg this plane within a distance B from P. Note: This figure is drwan to scale for a 100-cm sphere  for gap spacing equal to their radius.

Within the limited gap distance between the spheres, a weakly nonuniform field exists, hence no PB or corona appear before the complete breakdown or flashover. For a given dimension of the diameter/radius of the spheres and a gap distance between them, the magnitude of the voltage at which the breakdown occured is read from the standard calibrated tables available for different types of voltages and for both their polarities. The inaccuracy of measurement of voltage by this method is about 3%.

Fig 31.2   Sphere gap in a HV Laboratory

The accuracy to measurement of voltage with sphere gap is considerably affected by earthed objects around the gap since they form stray capacitance.

Correction Factor

Since in general the actual air density during measurements differ from the reference conditions, the breakdown voltage of  the air gap is given as

Ub = kd Ub0

where Ub0 corresponds to the table values and kd is a correction factor related to air density.

The actual relative air density (RAD) is given in general terms by

where

p0 =  air pressure for standard conditions p   = air pressure at the test conditions t0   = 20 0C t  =  temperature in 0C at the test conditions.

The correction factor kd, given in table below for increasing values of RAD are slightly non linearly. However, it can be seen from the table that the correction factor kd is almost equal to RAD,δ.

Table 31.1   Air density correction factor

Relative air density RAD, δ Correction factor kd 0.70 0.72 0.75 0.77 0.80 0.82 0.85 0.86 0.90 0.91 0.95 0.95 1.00 1.00 1.05 1.05 1.10 1.09 1.15 1.13

Disadvantages:

For the measurement, breakdown in the gap has to take place. Continuous measurement of the voltage is not possible. This method is not very accurate.