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Equivalent power system impedance is an important electrical quantity from many points of view. Areas in which this parameter plays an important role include, in particular: Voltage stability analysis, power quality, or fault condition analysis. Power system impedance estimation in real operation conditions can be performed by one of the non-invasive methods described by different authors. This paper aims to investigate and compare seven different methods for power system impedance estimation based on voltage and current variations measurement. After a brief description of selected methods, these methods were applied for power system impedance estimation in the case of two simple simulation tests and then in the case of three real measured data. Voltage and current changes used for power system impedance estimation in real conditions were measured in high voltage (HV) and medium voltage (MV) substations feeding steel mill with the electric arc furnace (EAF) operation. As the results presented in this paper have shown, not all of the methods analyzed are suitable for determining the power system impedance based on the fast step changes of voltage and current that occur, for example, during an EAF operation. Indeed, some of the tested methods were originally designed to determine the power system impedance from changes in voltages and currents recorded at steady state.

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During electricity generation, transformation, distribution and consumption, distortions of voltage and current sinusoidal signals occur. Most consumers are non-linear, and a significant content of higher harmonics emerges when they are connected to the supply grid. The algorithms of modern electricity meters take into account most of higher harmonics, whilst with some fallibilities. However, when supply voltage and current have the same amplitude and a various harmonic composition, the readings of electricity meters can vary greatly. This article describes how the harmonic composition of supply voltage and current affects the readings of electricity metering devices. The analysed ratios do not depend on the amplitude of supply voltage.