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Low Capacitance measurement by High Voltage Schering Bridge


Schering bridge is widely used for capacitance and dissipation factor measurements. In fact Schering bridge is one the most important of an a.c. bridges. It is extensively used in the measurement of capacitance in general, and in particular in the measurement of the properties of insulators, capacitor bushings, insulating oil and other insulating materials. This bridge is particularly suitable for small capacitances, and is then usually supplied from a high frequency or a high voltage source. 

                            Fig. 1. High voltage Schering Bridge.

The special features of a high voltage Schering bridge shown in fig. 1. are explained below:

  1. The high voltage supply is obtained from a transformer usually at 50Hz. The detector, in this case, is a vibration galvanometer.
  2.  Arms ab and ad each contain only a capacitor and these capacitors are designed for high voltage work. The impedance of these two arms are very high in comparison with other arms, bc and dc. Thus the major potential drop will be in the arms ab and ad and very little voltage drop is there across that even if a voltage as high as 100kV is applied to the bridge, the voltage across arms bc and dc is a few volt above earth. 
  3.   It is necessary to prevent dangerous high voltages appearing across arms bc and ac in the case of breakdown of either of the high voltage capacitors. This is done by connecting a spark gap, across each of the arms bc and dc. 
  4.  The impedances of arms ab and ad are large and therefore the current drawn from the source is small and hence the power loss is quite small. But this small value of current also necessitates the use of a sensitive detector. 
  5. The fixed standard capacitor C2 has either air or compressed gas as dielectric. The dissipation factor of a dry and clean gas is sensibly zero, loss in the insulating supports cannot be avoided. This loss, however, can be prevented from influencing the measurements by  the use of a guard ring from which both electrodes of the capacitor are supported. With this arrangement the current through the high voltage supports passes direct to earth, and as the potential difference between low voltage electrodes has a negligible effect. For some general applications, mica capacitors are used and in such cases, the dissipation factor of the capacitor must be accurately known.

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