Research on the Effect of Ramp Waveform with Ripple on Electric Energy Measurement in Subway DC Power Supply

During the start and stop of the subway, the current and voltage ramp waveforms with ripples frequently appear on the bus of DC power supply. These waveforms are different from the common DC power supply operating waveforms, so it is necessary to investigate their effect on DC energy measurement. The normalized coefficient method is used to derive the expressions of the three main energy measurement algorithms including the mean algorithm, the RMS algorithm, and the instantaneous integration algorithm. The expressions contain the waveform ramp and ripple parameters. The theoretical errors of different energy measurement algorithms are calculated with the ramp and ripple parameters as variables, and the corresponding measurement error graph is obtained. Then, the typical parameters are used for the subway traction power supply system modelling, and the subway DC power current and voltage simulation ramp waveforms with ripples are obtained. Finally, according to the simulation waveforms, the error results of different energy measurement algorithms are obtained, and these results are consistent with the theoretical errors. The results show that the measurement accuracy of the mean algorithm is high when the ripple is small, but the measurement error of RMS algorithm is relatively larger. The normalized ripple amplitude has a greater impact on the energy measurement error than the normalized slope. The instantaneous integration algorithm is suitable for the energy measurement of the DC power supplied subway. This provides a reference for the selection of the measurement algorithm of the DC energy meter used in the subway.