Corona Measurement and Interpretation

This discussion of the performance of the more common types of corona measurement circuits has emphasized certain points. They may be summarized as follows: 1. The coulomb charge in individual corona discharges and the number and phase of these individual discharges are considered the significant quantities in corona measurement. 2. The voltage step pulse produced by a single discharge varies directly as the pulse coulomb charge and inversely as the test specimen capacitance and the capacitance in parallel with it. This voltage is then divided between the detector input shunt capacitance and the series-coupling capacitance. 3. The observed shape of the pulse following the very rapid step change in voltage is determined by the inductance or resistance shunting the detector input. These produce, in the case of a resistance, a nonoscillatory exponentially decaying voltage, and in the case of an inductor an oscillatory decaying voltage. 4. A wide-band amplifier is needed to indicate a major fraction of the initial pulse height if the time constant of the exponential decaying is short. This is particularly true of the nonoscillatory pulse produced by a resistance input, where conventional narrow-band amplifiers have an output crest voltage which is only a small fraction of the input pulse crest. A much greater sensitivity may be achieved with a fairly narrow bandwidth amplifier tuned to the natural oscillation frequency of the input circuit having an inductance. A high Q in this input circuit is preferred to give maximum sensitivity. 5.