This paper presents several lesser-known aspects of the difficulties involved in analysing energy flows in complex systems such as DC railways, especially when energy recovery systems are present. In particular, it shows that quantifying energy savings based solely on local measurements taken on the recovery system is misleading and leads to significant overestimation. To address this issue, this paper proposes a system-level methodology for assessing energy flows across an entire DC railway network. The approach is based on a distributed and time-synchronised measurement architecture, which allows for the simultaneous acquisition of voltage and current at all substations along the line. The proposed system was deployed in a measurement campaign conducted across an entire Madrid Metro line where a reversible substation was installed. To facilitate data analysis, a dedicated software tool has been developed that allows the visualisation of all power flows of the entire system. The obtained experimental results revealed several unexpected and complex energy behaviours, including long-distance energy transfers between remote substations and night-time circulating currents. A daily energy analysis carried out over two weeks across the entire line confirmed that the energy savings inferred from local measurements can be overestimated.

Fiber-optic current sensors (FOCSs) have significant potential as traceable References in metrological applications, yet their widespread adoption remains limited. Laboratory calibration and comparison are essential for establishing traceability and reliability under real-world conditions. This article presents the operating principle of self-developed FOCSs and evaluates key performance metrics. It reports the first comparison among National Metrology Institutes (NMIs) for calibration and performance assessment of FOCSs, covering stability, accuracy, linearity, temperature dependence, and frequency response. The results demonstrate that self-developed FOCSs achieve an accuracy of 0.1%, linearity of 0.03% up to 4 kA at room temperature, and an error variation of less than 0.2% across the temperature range -10 degrees C to 60 degrees C. The wideband (WB) FOCS exhibits a bandwidth exceeding 120 kHz. Furthermore, FOCSs were deployed for current measurements in a medium-voltage substation and compared against Hall-effect current sensors (HECSs) under operational conditions. Findings confirm that FOCSs meet performance requirements in practical environments, offering excellent repeatability and traceability for on-site calibration and highlighting their unique value in metrological applications.

Smart Electrified Traction Systems (SETS) ma: play a pivotal role towards the decarbonization of the transpor sector and the integration of renewable energy sources. For thi reason, the paper proposes an up-to-date state of the art revier of the main configurations, operational schemes and flexibl assets forming these networks. Also, the intrinsic flexibility 0 these tractions systems calls for the optimal management of th assets in order to minimize the operational costs. This pape also reviews the main formulations of customized optimal powe flow problems, which incorporate the intrinsic features of thes networks. The critical analysis focuses on the reformulation techniques or algorithms adopted to overcome non-linear an non-convex constraints, typical of optimal power flow problems

The aim of the study here presented is to evaluate how temperature variations affect the metrological performance of inductive voltage transformers (VTs) in harmonic measurements. The analysis specifically targets the harmonics up to 1 kHz, where measurements using VTs can be challenging due to their nonlinear behavior. In fact, recent scientific literature has evidenced that when a VT operates under distorted conditions, the harmonics at its secondary side are not simply a scaled version of those injected. Instead, they are a combination of the injected harmonics and the spurious ones generated by the iron core due to its B-H curve. This phenomenon has been investigated in laboratory under controlled conditions, and the specific techniques have been proposed to compensate for it. However, no studies have analyzed the impact of temperature variation on this phenomenon and, consequently, on the introduced errors. This article addresses this open issue through experimental activities on two different VTs using a high-accuracy reference setup. Results show that, for different temperatures, the measured VT harmonic errors can differ up to an order of magnitude, indicating that this influence quantity cannot be neglected.

The urgent need to decarbonize our cities places the transport sector—responsible for a significant share of greenhouse gas emissions—at the center of global sustainability efforts. Achieving this goal requires not only the electrification of public transport systems but also ensuring their efficiency and effectiveness to encourage widespread adoption over private vehicles. A promising strategy to modernize such systems is the implementation of a Digital Twin (DT), a virtual counterpart of the infrastructure that integrates real-time field data from strategically deployed sensors. This work focuses on trambased urban transport systems, where one of the most critical parameters for DT applications is the traction current absorbed by each vehicle. However, the deployment of dedicated onboard current sensors is often constrained by economic and timerelated limitations. To address this, we propose a transitional solution: a data-driven model capable of estimating tram current consumption using only GPS-based vehicle position data. Specifically, this paper presents a hybrid neural network architecture combining Long Short-Term Memory (LSTM) and Gated Recurrent Unit (GRU) models. The network processes speed and acceleration profiles derived from open-access GPS data, eliminating the need for onboard instrumentation. The model was trained and validated with data from a targeted field measurement campaign, achieving a Mean Absolute Error (MAE) of 61.3 A and a Root Mean Square Error (RMSE) of 95.6 A. Given that the maximum absorbed current exceeds 900 A, these error values indicate that the model's predictive performance is within an acceptable range. This methodology provides a robust, low-cost solution for enabling early-stage DT development in Urban Traction Electrification Systems (UTES), supporting simulation and planning in the ongoing digital transformation of public transport.

The frequency characterization of instrument transformers is a topic of ongoing discussion, and the identification of a standardized methodology to perform it is still an open issue. Among the possible approaches discussed in international standards and literature, the use of pulse signals is particularly attractive due to the rapidity of execution and their compatibility with existing standard tests already performed on voltage transformers. In this context, this paper investigates the pros and the issue of using impulsive voltages as test waveforms for the MV voltage transformers wideband characterization. The study is conducted from both the theoretical and experimental point of view by performing preliminary tests on two commercial sensors.

Fiber-optic current sensor (FOCS) technology offers intrinsic galvanic isolation, a wide linearity range, immunity to electromagnetic interference, and non-invasive measurement, making it a strong candidate for applications in on-site calibration. However, its use as a traceable reference in metrological applications remains limited so far. Laboratory calibration and cross-institutional comparisons are essential steps toward establishing the reliability and traceability of FOCS in real-world conditions. This paper provides an overview of the FOCS operating principle and evaluates the key performance of a new FOCS sensor tested in two National Metrology Institutes (NMIs), including accuracy, stability, linearity, and temperature dependence. It furthermore describes the on-site measurement campaign using the FOCS for efficiency measurement in a medium-voltage substation. The test results reveal that FOCS can meet the performance requirements under practical conditions and demonstrate excellent repeatability and traceability for on-site calibration in operational environments: the linearity is better than $0.1 \%$ over the 2.4 kA current range, with temperature effects less than $0.2 \%$ from -10 to 60 degrees Celsius. This work contributes to understanding the metrological characteristics of FOCS and supports their broader adoption in on-site calibration.

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 determination of the parameters 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 article 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.

This paper presents the study and development of a high-performance voltage transducer for the accurate monitoring and control of traction voltage in tramway systems. The proposed transducer offers a more effective alternative to the Hall-effect sensors currently adopted, which suffer from some drawbacks such as limited frequency bandwidth and unsuitability for outdoor applications. The research activity is carried out in different steps. Initially, the typical characteristics of the voltage signals to be monitored, such as amplitude dynamics and spectral content, are analyzed. Attention is then focused on the environmental conditions in which the monitoring voltage transducer must operate. These preliminary analyses allowed for the definition of the key design constraints for the sensor under study. The article describes the implementation of the voltage transducer, with particular focus on the transducer typology, components selection, and insulation techniques, tailored to the identified design constraints. Preliminary results from laboratory experimental tests are presented and discussed, demonstrating the voltage transducer's high performance, with amplitude linearity within tens of ppm from 50 V to 1 kV and a frequency response error lower than 0.15% up to 2 kHz.

Reducing transport emissions through electrification and improving energy efficiency is a widely studied and crucial topic. Although electric railways are inherently efficient, traditional DC systems often waste significant regenerated braking energy due to unidirectional power flow of supply substations. Solutions like reversible substations (RSS) offer a remedy by re-injecting this energy into the main AC grid. However, there is a lack of standard methodologies to evaluate their efficiency and experimental procedures to accurately quantify the energy savings associated with their use, for example by measuring improvements in “line receptivity”, in real systems. This paper presents the first-ever synchronized, distributed measurement campaign conducted underground across an entire Madrid metro line, which operates with a RSS. The paper provides some preliminary data on real-world energy flows, highlighting the importance of accurate and synchronized measurements to understand the role of energy-saving technologies such as RSS in the operation of real metropolitan systems.

The determination of the equivalent parameters of a supercapacitor requires the galvanostatic measurement of a charge and self-discharge cycle. If a more general determination is needed than that on the single cycle, it is necessary to take into account the repeatability of the charging and self-discharging cycles. A cycle can also extend over a few hours for larger supercapacitors. Therefore, having a large set of cycles including the time required for parameters determination is definitely a time-consuming procedure. In this work, an efficient method for the repeatability assessment of the equivalent circuit parameters is proposed; this approach relies on a limited set of experimental curves and on a single parameter identification process.

High-Frequency Distortion (HFD) in both Low Voltage (LV) and Medium Voltage (MV) grids is gaining growing interest from the scientific and technical community due to its increasing occurrence and the issues they can cause. Furthermore, the phenomenon of HFD is expected to rise as it primarily stems from newly installed devices essential for achieving decentralized generation from renewable sources. To monitor HFD in MV grids, the use of Instrument Transformers (ITs) is essential to scale down voltages and currents to levels compatible with the input stages of Power Quality (PQ) instruments. In this respect, the recently released edition 2 of the IEC 61869-1 standard extends the IT accuracy class concept up to 500 kHz. However, within the IEC 61869 standard family, guidelines for testing ITs only exist at power frequency, lacking information on the procedure and setup for assessing the frequency behaviour of ITs. This paper proposes a flexible architecture for generating realistic currents with superimposed HFD. It involves the use of two current sources, one devoted to the generation of a fundamental tone at rated amplitude (theoretically up thousands of ampere) and frequency (DC or AC 50/60 Hz) and one for the superimposition of tones at reduced amplitude and frequencies in the HFD range. Through preliminary tests, the applicability of this proposed architecture has been experimentally validated and it is presented in this paper.

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.

The metrological characterization of DC static meters in normal operating conditions poses several challenges, both from a methodological and technological point of view. This paper presents the preliminary results of an inter-laboratory comparison between METAS and INRIM. The obtained results have two objectives: to prove the stability of the reference systems, and to assess the possible effects of AC disturbances in DC power measurements.

Low-voltage DC microgrids are a popular solution for a direct integration of more and more renewable energy sources. In this scenario, it is reasonable to expect that the power signal will not consist of pure DC components but will be affected by disturbances and power quality (PQ) events coming from the AC bulk grid as well as from switching power supplies or time-varying loads. To the state of the art, the normative framework does not provide a rigorous approach towards DCPQ analysis. We lack a clear and unambiguous definition of some quantities (e.g., DC power) or measurement methods (e.g., DC ripple). In order to solve this problem, the recent project DC grids has developed two reference systems for DCPQ at METAS and VSL laboratories. In this paper, we compare the systems' performance by means of a measurement campaigu on the same transfer standard. The results confirm the consistency of the two reference systems and provide some insightful inputs for the standardization of these tests in the normative framework.

At first glance, when tasked with designing a measurement system for a DC electrical network, engineers often underestimate the complexity of the operation. It may be considered a simple task of monitoring constant electrical quantities, which are therefore easily estimable. The common bias is to consider that in a DC microgrid, both the voltage applied and the current absorbed are continuous quantities. At most, these might experience slow variations or step changes corresponding to load insertions. A deeper analysis should also consider the ripple, which is always present on both voltage and current. However, it's worthwhile noting that the ripple is usually characterized by a small amplitude compared to the DC quantity. This article emphasizes that reality quite differs from common assumptions. The measurement campaign provides data from an actual DC microgrid, revealing that AC current components of relevant amplitude can circulate within a DC network even during normal system operations. These findings challenge traditional expectations and offer valuable insights for grid designers. By understanding the behavior of these AC components, designers can enhance the reliability and efficiency of DC microgrids, ultimately leading to more robust and resilient power systems.

The dataset refers to the measurement and simulations of the supply system and rolling stock of line 10 B of Metro de Madrid. Simulations have been performed by changing the position of the reversible substation and computing the current flowing in the braking rheostat of the simulated rolling stock. The data refer to the paper "Energy Efficiency Improvement with Reversible Substations for Electrified Transportation Systems" published in "The Open Transportation Journal".

This paper explores the calibration of Instrument Transformers (ITs) used in power grids metering. It highlights the lack of information in in-force standards regarding the non-idealities of the current and voltage generated for ITs calibration. Existing literature has proposed innovative calibration systems based on commercial ADC acquisition boards or digital multimeters. However, no studies have analyzed the impact of undesired non-idealities of the test signal on the measurement of power frequency ratio and phase errors. The paper aims to investigate and quantify the influence of Total Harmonic Distortion and frequency variation of the input voltage and current on IT accuracy assessment. The study is carried out through numerical simulations generating various THD levels and frequency variations and evaluating ratio and phase error through common algorithms. The simulation results show that especially for industrial laboratories in which uncertainty targets are more relaxed, THD levels higher than 2% can be tolerated by choosing proper measurement algorithms.

More and more often overall energy efficiency of an electrified transportation system appears as target of new constructions, sometimes with incomplete definitions of performance indexes to assess that target has been reached. A worked out example is based on the introduction of reversible substations. Reversible substations are a method to improve energy efficiency, whose application in an existing system can be progressive and does not require large traffic intensity to be effective. Their effectiveness depends on some system parameters (nominal catenary voltage in particular). A simulation model fed with experimental data from a line section of Metro de Madrid is used to demonstrate the operation and optimization of reversible substations. Background The ambitious reduction of CO2 greenhouse gas emission within 2050 declared by the European Commission also involves transportation systems. In this context an optimum recovery of the electric braking energy produced by railway vehicles is more and more relevant. Several strategies to completely recover the braking energy are being developed and applied. A methodology that allows the accurate determination of the amount of recovered energy in real operating conditions becomes then a valuable tool. Objective Reversible substations are one of the braking energy recovery methods that are widely applicable. Their effect is analyzed in a system perspective, considering them integrated in the transportation system with its dynamics and various operating points. The effectiveness and consequences of the operation of one or more reversible substations are evaluated by identifying relevant system conditions and scenarios. Methods An electric network model is provided, fed by measured timetable and traction current. Simulation results are analyzed and compared with some experimental results: simulation configurations will be selected to match those of the available experimental data. Results The selected substation no-load output voltage level has a significant effect on efficiency and performance of reversible substations. A reduction of 50 V, from 1700 V to 1650 V produces a decrease of the energy dissipated by the braking rheostat on-board trains of about 13%. The voltage increase caused during braking phases is kept under control better for track positions close to reversible substations: tests show that line voltage increase is 2.5% of nominal value with a reversible substation, and only 0.5% when it is operated. Trimming the thresholds that trigger the operation of the on-board braking chopper (intermingling regenerative and dissipative braking) has a dramatic effect on regenerability: a reduction of 50 V causes an improvement of 19%. Conclusion The paper presents a methodology that merges measurements and circuital model to investigate on the energy saving provided by reversible substations supplying railway system. The methodology has been applied to a real case. Preliminary results regarding the impact of supply voltage level, reversible module position and threshold levels of the braking chopper control system on the dissipated braking energy and power quality has been carried out.

In recent years, the adoption of DC technology in power systems is attracting renewed interest from the technical-scientific side not only in traditional high voltage applications (HVDC) which allow more efficient transmission of large quantities of energy over large distances but also in low voltage DC networks (LVDC), which allow for more efficient and sustainable integration of renewable energy sources and battery storage systems. An obstacle to its widespread diffusion is the lack of traceable reference systems able to evaluate the performance of energy meters for billing purposes in operating conditions characterized by DC with AC components (up to hundreds of kHz). The present work proposes a methodology to calibrate current measuring systems experiencing DC with AC ripple providing the metrological traceability for this combined signal. The developed and tested archetype, here presented, proves the feasibility of the methodology.