In losses must be low and the insulation

                In any electrical power
generation, transmission and utilization system, insulation quality of
electrical apparatus is to be very high. However, that same insulation can
unnecessarily be the cause behind equipment failure resulting in costly
replacement or repairs and downtime of the plant. To avoid this situation,
testing of the capacitance and dissipation factor periodically is very much
necessary, for detecting moisture, contamination or deterioration of
insulation. If the high voltage is to be insulated within a small space, a
compressed gas, solid, liquid or compound insulation is required (e.g.
transformers, motors, generators, cables etc.). In all cases, the insulation
must be designed so that, its breakdown strength is high enough to withstand
occasional surges, which may be several times of the working voltage. The dielectric
losses must be low and the insulation resistance high in order to prevent
thermal breakdown.

For design a high voltage electrical machine, such as transformer,
motor, generator, capacitor, cables etc. the accurate measurement of loss angle
1, 15, 16, 17, 20, 21 at high excitation voltage is important. The loss angle
and dielectric constant of a material can be measured by accurate measurement
of the capacitance of a material capacitor. Normally, this capacitance is very
small value and comparable with the stray capacitance. So, the capacitance of a
capacitor is required to measure accurately. The different bridge measurement
technique 2, 3, 5, 10, 11, 14, 16, 17, 18, 19 for the measurement of
capacitance has been reported. Schering bridge network is one of the most
sensitive bridge measurement techniques for measuring the loss tangent and
capacitance at power frequency and high excitation. Therefore, the Schering
Bridge 16 is normally used to measure the dielectric loss and the capacitance
between the motor windings and ground of the high voltage squirrel cage motor
and found the effect of increase of high voltage on dielectric loss in the
motor. These investigations have been performed by comparing it with a standard
capacitor having negligible loss over a wide range of voltage. The error due to
the effect of stray capacitance has been minimized by using Wagner-Earth
technique 2, 17, 19, where the bridge components and the lead wire in low
voltage side has been screened from the high voltage side with a repeated bridge
balance in each observation. for the measurement of loss angle of a high
voltage transformer 24  with a modified
operational amplifier based Schering bridge network can effectively reduced the
stray capacitance negligible.

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In the present paper, the
conventional Schering bridge network has been modified using two high gain
operational amplifiers. The non-inverting terminal of the operational amplifier
is connected to the ground. The inverting terminals, which are connected to the
output nodal points of the bridge network, are both virtually at the same
potential with respect to the ground. Hence, the effect of the stray
capacitance between the output lead wires and the ground may be assumed to be
minimal without the use of Wagner- Earth technique. The equation of the bridge
balanced condition of this modified Schering bridge network is also same with
that of the traditional Schering bridge network. In addition to that, an electronic
measurement circuit has been connected with the modified Schering Bridge
network to achieve the direct measurement of tand in terms of voltage ratio. Moreover, this
technique also gives an additional bridge sensitivity factor adjustment by a
linear potentiometer.


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