Traceability for AC Ripple Over DC Current

Direct current (dc) is experiencing a new renaissance. The consolidated high-voltage direct current (HVdc), together with new medium voltage direct current (MVdc), and low-voltage direct current (LVdc) are more and more present in research and development projects. One of the relevant topics, associated with the usage of dc, is the ac ripple that unavoidably accompanies the dc signals. The interest in the measurement of this quantity is linked with the determination of the performance of dc energy meters for billing purposes. The on-line power quality (PQ) analysis, the verification of the capabilities of the filtering systems, and the accurate determination of losses associated with the transmission, distribution, and conversion of the electric energy by dc systems require accurate determination of the ac ripple. National metrology institutes (NMIs) provide traceability only for pure ac or pure dc current signals. To guarantee a high-quality standard for such measurements, the article proposes a novel methodology, named ALFO, for the traceable calibration of measuring systems with signals composed by dc with superimposed ac ripple. The calibration setup is based on a decoupling transformer (DeT) which has been specifically designed by a numerical tool. A detailed analysis and quantification of the systematic errors affecting the measurement and of uncertainty budget have been performed. The setup can calibrate current measuring systems up to 100 A (dc) with ac ripple up to 1% of the dc in the frequency range 300 Hz-150 kHz. Preliminary evaluation of the standard uncertainty, performed at maximum 40 A dc and 1 A ac up to 250 mu A /A in the range of 300 Hz-150 kHz.