In-situ impedances measurement and influence of 2 k - 150 kHz disturbances on measurement of electrical parameters

Abstract

With the growth of the number of electric appliances which suffer from electromagnetic inference (EMI) in the frequency range 2 k – 150 kHz, the setting up of researches in this field has become more and more popular. Measuring impedances accurately in this frequency range is of great assistance for better understanding of the circuit as well as for EMI filter designs. Thus, this thesis aims to develop a safe, flexible and accurate in-situ measurement approach to measure the source and load impedances at 2 kHz – 150 kHz and then apply it to investigate the influences of the variable impedances on measurements of electrical parameters when experiencing disturbances. In this thesis, the current probe being one of the most important measurement devices is discussed first. To investigate the transfer impedance of the current probes used in the measurements a spectrum analyzer, a vector network analyzer (VNA) and a LCR meter are applied respectively. It is found that the input impedance on the wire side influences the bulk current injection (BCI) probe but not the normal current monitor probe in the transfer impedance measurements at discussed frequency range. The electric models are illustrated. Secondly, an in-situ measurement method – the three-probe method which is verified to be applicable at discussed frequency - is proposed. The measurement accuracy is guaranteed through a proper Fast Fourier Transform (FFT) process by collecting both amplitude and phase information of the measurement. In addition, the three-probe method was compared with its predecessor – the two-probe method which was proposed in 2004 to measure noise source impedance at 150 k – 30 MHz. Then the impedances measurement results of different sources and loads are illustrated by using this three-probe method. At last, by using a variable inductance/capacitance box to simulate different source conditions, it comes out that the loads which property will turn out to be capacitive at certain frequency (2 k – 150 kHz) are sensitive for source impedance variations. Their voltages and currents all are influenced significantly with the resonance between source and load impedances. Fortunately, with further investigation, this effect seems not to interfere with the reading of the smart meter under the test.