Measurement and analysis of zero sequence current levels during normal operation

In the Netherlands, power grids and railway systems are located in close proximity over considerable lengths. Zero-sequence currents in the transmission grid may have an influence on neighbouring systems through inductive coupling. Not much is known about these currents during normal operation: Literature on the topic is scarce and modelling is complicated for various reasons.Several measurements were analysed to obtain insight in 150 kV and 380 kV branches. Both PMU measurements and measurements using the secondary wiring of current transformers were used. The results are presented in this paper. The paper also presents a measurement uncertainty analysis. on the reconstruction of zero-sequence currents for both measurement types, indicating the influence of the measurement principles.The considered measurements show several types of correlation between positive-sequence and zero-sequence currents.The 150 kV measurements show constant zero-sequence currents that do not vary with increasing positive-sequence currents.Three of the 380 kV branches show an increase of zero-sequence currents as positive-sequence currents increase.All measured zero-sequence currents that have been observed are below the values used in the interference studies