Diagnostic measurement based on bridge out-of-balance signals

Considerable use is made of HV bridge measurements in the assessment of rotating plant stator insulation. An alternative technique displays the charge transfer oscillograms due to discharging which is derived from the imbalance signal of a capacitance bridge. It is shown that the imbalance signals of both capacitance and Schering Bridges can be used to calculate capacitance and loss tangent values. This procedure is quicker and more accurate than conventional balancing. Comparisons are also made with the information provided by a charge transfer oscillogram. It is shown that the same values for total energy loss should be found by both techniques and that a different loss energy is only given from the oscillogram if the capacitance bridge is rebalanced by adjustment of its resistive arm. Low and variable frequency measurements using imbalance technique are described and it is shown that insulation properties can be conveniently measured down to 0.01 Hz at high and low voltage. An automatic computerized system for capacitance and loss tangent measurements over voltage and frequency ranges is described and some sample results are given. Other potential applications of imbalance techniques, such as measurements during overpotential tests and testing with controlled stress patterns, are discussed