Low-Cost System of Direct Measurement of Dissipation Factor for High-Voltage Electrical Machine

Schering bridge network can be used to measure the loss angle of an insulating material of electrical machines and its winding capacitance. But, this type of bridge measurement technique suffers from error due to the effect of stray capacitances in both between the bridge output nodal points and also between bridge output nodal points and the ground. These errors can be minimized by using the Wagner earth technique. The limitation of this method is the repetition of bridge balance and Wagner earth balance that is required in every observation. In this paper, a modified Schering bridge network has been developed using operational-amplifier (op-amp) to minimize the stray capacitances. A potentiometer has been used to adjust the sensitivity of the bridge. The experiment has been performed on a 3.3-kV, 130-kW, three-phase squirrel cage induction motor in a process plant. An additional electronic measurement circuit has been incorporated to measure the dissipation factor (tanδ) directly in terms of voltage ratio. For a particular value of the bridge sensitivity factor, the change of dissipation factor with the variation of voltage at line frequency has been observed and found satisfactory.