The assessment of the power quality could be a valuable tool to foster the efficiency of the railway systems. Power quality is a well-addressed topic in conventional AC 50/60 Hz power systems and many procedures, algorithms and measurement systems were presented in the international standards and widely discussed in the scientific literature. A less explored research field is the assessment of the power quality in the railway traction supply systems, in particular with reference to the DC and 16.7 Hz systems. The paper explores this theme, proposing an extension of the definitions and of the standard measurement procedures for some of main power quality indexes, well defined and widely used for conventional power systems, in order to be used also in all railway traction supply systems. The limits or difficulties of applicability are discussed with reference to measurements performed both on-board and in substation. The proposed procedures are applied to an experimental case of a real DC railway system with a large measurement campaign.

The energy efficiency of transportation is a crucial point for the rail and metro system today. The optimized recovery of the energy provided by the electrical braking can lead to savings of about 10% to 30%. Such figures can be reached by infrastructure measures which allow the recovery of the breaking energy that is not directly consumed by the rail system and dissipated in rheostat resistors. A methodology for the accurate estimate of such energy is valuable for a reliable evaluation of the cost–benefit ratio associated with the infrastructural investment. The energy can be estimated by measuring a braking current flowing in the rheostats. The varying duty-cycle associated with the high dynamic variation, from zero to thousands of amperes, makes the current measurement very challenging. Moreover, the digitization of such waveforms introduces systematic errors that affect the energy estimation. To overcome these issues, this paper proposes a technique to measure the power and energy dissipated by the rheostat of a DC operated train with high accuracy. By means of an accurate model of the electrical braking circuit (chopper and rheostat) and the frequency characterization of the current transducer, a correction coefficient as a function of the duty-cycle is estimated. The method is then applied to data recorded during a measurement campaign performed on-board a 1.5 kV train of Metro de Madrid during normal operation. Using the proposed technique, the estimation of the dissipated braking energy is improved by 20%.

The on-site assessment of the efficiency of a charging station is not a trivial process and is a topic of discussion for professionals. The efficiency of electric Vehicle Supply Equipment (EVSE), is an important parameter for both the user and the EVSE operator. This paper deals with a particular type of EVSE, using static wireless power transfer (WPT). This paper proposes a clear method to account for the parameters which can affect the correct determination of efficiency, such as in particular the accuracy of the meters and the effect of temperature. This work proposes a method to define the accuracy of the power and efficiency on-site assessment, and is aimed at clarifying that despite distorted waveforms at the charging stations, it is possible to reach a good accuracy in a wide temperature span (expanded uncertainty <0.5% between 5 °C and 40 °C). Analysis initiated from the measurement conditions and the actual waveforms recorded at two WPT EVSEs with differently rated power. This paper paves the way for the possibility of verifying class 0.5 m on-site, desirable for this type of application. The paper also clarifies that despite the evident presence of voltage and current ripple at the batteries, the weight of the ripple power on the total power is nevertheless lower than 0.1%. Finally, the paper highlights how, for the correct measurement of the ripple, it would be advisable to calibrate the instrumentation in DC and in AC, at a frequency double that of the working frequency of the EVSE coils.

This paper analyzes the impact of typical DC transient events occurring in railway grids on the frequency performance of instrument transformers (ITs) installed onboard trains and in AC substations for power quality (PQ) applications. PQ monitoring in railway systems is an issue of great interest because it plays a key role in the improvement of energy efficiency. The measurement chain for the PQ measurements, at 15 kV at 16.7 Hz and 25 kV at 50/60 Hz, commonly includes ITs to scale the voltage to levels fitting the input of the measurement units. Nevertheless, the behavior of ITs in the presence of PQ phenomena represents an open issue from a normative point of view, even for those installed in conventional AC power supply systems. In this context, the paper presents a possible definition of DC transient disturbances test waveforms, a measurement procedure, and a setup to assess the impact of these disturbances on the harmonic performances of ITs for railway systems. Preliminary experimental tests carried out on two commercial ITs under wide ranges of variation for the amplitude and the time duration of DC disturbances show that, in some cases, the error introduced in harmonic measurements can exceed 100%.

Direct current (DC) is experiencing a new renaissance. The consolidated HVDC, togheter with new MVDC and LVDC (Hight, Medium and Low Voltage Direct Current) 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 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 paper 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 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 μ A/A in the range of 300 Hz - 150 kHz.

The accurate simulation of the behavior of a supercapacitor (SC) and its control in an electronic system, cannot be achieved by a simple one-branch circuit. A correct simulation requires the use of more complex equivalent circuits, with at least two or three branches. These equivalent circuits guarantee a good reproduction of the device’s behavior. The effectiveness of an equivalent circuit is linked to the limits of the circuit parameter identification, which is commonly achieved by means of the voltage and current measurement of charge and self-discharge cycles. The uncertainty in the identification of these circuit parameters, is dependent on the accuracy of the measurement instrumentation and on the repeatability of the SC. A cycle for determining the parameters can extend over a few hours for larger SCs, also considering the time needed by the software algorithm for the parameter identification. Therefore, having a large set of cycles including the parameters determination is a time-consuming procedure. In this study, an efficient method for the repeatability and uncertainty assessment of the equivalent circuit parameters is proposed; this approach relies on a limited set of experimental data and on a single parameter identification process. The analysis presented in this paper highlights how the limited repeatability of the device is an important source of uncertainty for the identification of the equivalent circuit parameters, but it is not the main one for all parameters.