High voltage AC resistive current measurements using a computer based digital watts technique

A computer-based technique is described which can accurately determine high-voltage AC resistive currents by obtaining the watts loss of the test specimen and dividing by the true r.m.s. (t.r.m.s.) value of the applied voltage Therefore, the normal concern for guarding against stray capacitive currents and for the exact repetition of test setup geometry on successive testing are not required for accurate measurements of AC resistive current. The watts loss calculation is based on the general formula for average watts so that the resistive current can be measured accurately with some harmonics of the fundamental present in both the test current and the supply voltage. A digital oscilloscope is used to acquire the test voltage and current so that special-purpose amplifiers are not required. The overall system accuracy is verified to less than ±1.21% of full-scale current. A microprocessor was used to compute the following parameters: average power, t.r.m.s. voltage, t.r.m.s. resistive current, t.r.m.s. capacitive current, total t.r.m.s. current, and the magnitude of calculated measurement error. The waveforms displayed are volt-ampere, voltage, and total current. An EHV aerial lift boom, a 115 kV station post insulator, and a metal-oxide surge arrester are evaluated using this technique to illustrate the measurement advantages