Inductively coupled in-circuit impedance measurement based on two-port network characterization
Date of Issue2017
School of Electrical and Electronic Engineering
Electromagnetic Effects Research Laboratory
In-circuit impedance information is one of the parameters for condition monitoring of an electrical system. Besides condition monitoring, it also provides impedance information of a switching converter connected to high voltage power supply so that an optimal electromagnetic interference (EMI) filter can be designed for electromagnetic compatibility (EMC) compliance. Thus, an accurate in-circuit impedance measurement system without interrupting the normal operation of an electrical system under test is very useful for the applications mentioned earlier. Existing in-circuit impedance measurement approaches, such as transformer ratio-arm bridge, current sensing and the inductive coupling have been reviewed. The two-probe inductive coupling approach is found to be superior to other approaches, as the measurement setup does not have direct electrical contact to the electrical system being measured and it can be easily mounted on-site for any high-voltage electrical system under its normal condition without electrical safety hazard to the measurement personnel. However, the existing implementation of the two-probe inductive coupling approach has several shortcomings. To overcome these shortcomings, two-port network characterization technique is adopted so that the measurement system setup can be treated as cascaded two-port ABCD networks. With the accurate characterization of the inductive coupling probes, as well as the measurement setup, the setup calibration can be greatly simplified without interrupting the operation of the electrical system under test. In addition, it also enhances the measurement accuracy of the in-circuit impedance. The theory of the two-port network characterization is described in details in the thesis and it experimental validation is also covered comprehensively. Subsequently, the value of the proposed approach is demonstrated through two practical applications. Firstly, it is applied to condition monitoring of electrical systems. Using a DC powered motor and an AC powered transformer as two examples, the deviations of in-circuit impedance of the motor and transformer are detected by varying their loading conditions. Then, with the proposed approach, an impact analysis of conducted emission measurement without and with line impedance stabilization network (LISN) is carried out. It is shown that the EMI filter designed based on conducted emission measurement with LISN may not be adequate for the actual electronic product that is connected to the AC power mains without LISN. The impact of the conducted emission measurement without LISN will allow us to know the adequacy of EMI filter, especially for high power switching converters.
DRNTU::Engineering::Electrical and electronic engineering