Development of a Nonintrusive True-RMS AC Voltage Measurement Probe

The design and development of a new nonintrusive-type true-rms ac voltage measurement probe are presented in this paper. The measurement circuit of the capacitively coupled nonintrusive voltage sensor is designed to provide an output that is directly proportional to the rms value of unknown ac voltage $\text{V}_{\mathrm {X}}$ , which contains the power frequency (50 or 60 Hz) component along with the typical harmonic frequency components due to nonlinear loads. This is accomplished by a specially designed autobalancing signal conditioning circuit. The existing noncontact probes are designed for a set frequency and will not provide accurate rms value of a signal that has multiple frequency components. The output is independent of the coupling capacitance between the probe electrode and the insulated wire/cable under measurement. Thus, the sensor has low sensitivity to the variation due to the probe-to-wire capacitance, good linearity (0.95%), and accuracy (0.88%) over a range of voltages (0–1000 V) tested. Various parameters that affect the performance of the probe are analyzed, quantified, and discussed in this paper. A prototype of the probe along with the proposed autobalancing circuit has been designed and developed. The prototype has been tested in detail to evaluate the accuracy, linearity, and repeatability. The results obtained established the practicality of the proposed scheme.