The offline capacitance and dissipation factor bridge test is an industry accepted standard test to assess the health condition of ground-wall insulation of industrial motors. However, so far, it was not possible to conduct this test online on large motors due to the great difficulty in online measurement of very small motor insulation leakage currents (few mA) in the presence of large motor line currents (hundreds of amps). The line currents would produce enormous noise to mask the tiny magnetic field produced by the insulation leakage currents. A recently invented novel sensor, High Sensitivity Differential Current Transformer (HSCT), is presented in this paper that enabled online measurement of stator insulation leakage current of large industrial motor for the first time in the world. Thus HSCT also enabled to conduct the industry accepted capacitance and dissipation factor test on large industrial motors for the first time in the industry. Such an online test adds a new dimension to state-of-the-art online condition monitoring and greatly benefits the motor users to continuously assess the insulation condition since no motor outage is required.
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