Two-Probe Setup with Improved Calibration for Online Impedance Measurement of Electrical Assets
The online impedance serves as one of the most crucial specification to evaluate the health status and efficiency of an electrical device or system. The inductive coupling technique is a preferred approach to measure the online impedance due to the ease of implementation of the circuit which has zero physical contact to the live electrical system. The existing inductive coupling method deployed to measure the online impedance of an electrical device under test (DUT) adopts two probes in total: an injecting inductive probe (IIP) and a receiving inductive probe (RIP). An open/short/load (OSL) calibration procedure is implemented to eradicate the ramifications of the probe-to-probe coupling, however, based on the assumption that the calibration criterions (shorted, open and 50Ω) are approximated to their theoretical values in a specified frequency range. Hence, any measurement with frequency outside the specified range (i.e. larger than 1 MHz) will not be accurate due to the frequency-dependent residual inductances and capacitances of the calibration model. To overcome the aforesaid limitation, this paper introduces an improved calibration procedure which is applicable for a wider frequency range which takes the frequency-dependent characteristics into consideration. With the two-probe measurement setup (TPMS), the adopted improved calibration procedure is introduced to eradicate the ramifications of the probe-to-probe coupling with the intention to refine the accuracy of the extracted online impedance.