• Energy Research

Compared with energy storage equipment based on supercapacitors, flywheels, or lithium batteries, inverters have obvious advantages in installation space, cost, reliability, and service lifetime. For this reason, regenerative inverters are increasingly installed in the substations of urban rail transit systems. They have controllable operating characteristics, but their impact on the energy saving and railway operation cost has not been fully studied. In this paper, based on the typical application scheme of regenerative inverters, the operating characteristics of the inverter including power limitation, start working voltage, and virtual internal resistance are introduced at first. A simplified power system model containing regenerative inverters and trains has been built. The impact of operating characteristics on the amount of regenerative braking energy and its distribution is analyzed. The inverter operating characteristics are optimized by a cost function considering total energy consumption, brake shoes wear, and inverter expense. A case study based on a multiple-train scenario is presented to validate the cost-saving performance. The work is expected to provide some guides for the design and optimization of the traction power supply system with inverting substations.

Due to environmental concerns, governments around the world are taking measures to decarbonise railway transport by replacing diesel traction systems with cleaner alternatives. While the electrification of railway systems is spreading rapidly, it is unlikely that all routes will be electrified as the volume of passengers will not justify the high infrastructure costs. Therefore, it is expected that, for several lines, a combination of hydrogen and electric traction will be used, with the latter partly provided by fixed infrastructure and partly by batteries. Railway traction drives will then need to change to accommodate these new types of power supply. A detailed review of the available traction motors and drives is provided with this review, given application to the new hybrid-electric systems. In particular, permanent magnet synchronous motors with multiphase windings are evaluated in comparison with traditional three-phase machines. Additionally, low and medium-voltage multisource power converters have been reviewed, taking into account the introduction of wide band-gap semiconductor devices.

A wide literature review of recent advance on monitoring, diagnosis, and power forecasting for photovoltaic systems is presented in this paper. Research contributions are classified into the following five macroareas: (i) electrical methods, covering monitoring/diagnosis techniques based on the direct measurement of electrical parameters, carried out, respectively, at array level, single string level, and single panel level with special consideration to data transmission methods; (ii) data analysis based on artificial intelligence; (iii) power forecasting, intended as the ability to evaluate the producible power of solar systems, with emphasis on temporal horizons of specific applications; (iv) thermal analysis, mostly with reference to thermal images captured by means of unmanned aerial vehicles; (v) power converter reliability especially focused on residual lifetime estimation. The literature survey has been limited, with some exceptions, to papers published during the last five years to focus mainly on recent developments.

In the recent years, there has been a considerable increment of diffused generators connected to distribution grids by means of power converters. These converters operate frequently below their rated power due to the fluctuation of the power generated by the renewable sources. In this scenario, power electronics interfaces could be also used as active harmonic compensators for other converters or distorting loads. Typically, these converters do not have information on other loads or on the level of grid current distortion. For this reason, this paper presents a new control algorithm for the grid harmonics compensation that relies only on the measurement of the voltage at the point of connection of the power converter. The reference of the compensating current is calculated from the harmonic content of the voltage in a reference frame synchronous with the grid voltage. Simulation results carried out on a sample distribution network with linear and nonlinear loads show that the proposed method does not require the knowledge of the harmonic current either of the grid or of the load and it is suitable when the distorting loads are unknown.

The paper aims to contribute to the use of electric double layer capacitor (EDLC) sets for boosting voltages of contact lines in urban and suburban railway traction systems. Different electrical configurations of contact lines are considered and investigated. For each of them, proper mathematical models are suggested to evaluate the electrical performances of the contact lines. They give rise, also, to sample design procedures for the sizing of the most appropriate energy storage systems, to be distributed along the lines, for boosting line voltages and avoiding undesired voltage drops. A numerical example based on the “Cumana” suburban Naples railway network is presented to give an idea of the weights and sizes of electric double layer capacitors needed to boost the voltage of a sample contact line. In particular, three different EDLC systems, for a overall installed energy of 9.6 kWh, have been placed nearby the stations presenting the highest voltage drops during the most representative situation of trains’ service. The new voltage drop is equal to 32% of that obtained in absence of EDLCs.

The battery pack is at the heart of electric vehicles, and lithium-ion cells are preferred because of their high power density, long life, high energy density, and viability for usage in relatively high and low temperatures. Lithium-ion batteries are negatively affected by overvoltage, undervoltage, thermal runaway, and cell voltage imbalance. The minimisation of cell imbalance is particularly important because it causes uneven power dissipation by each cell and, hence, temperature distribution that adversely impacts the battery lifetime. Several papers in the literature proposed advanced cell-balancing techniques to increase the effectiveness of basic cell-balancing approaches, reduce power losses, and reduce the number of components in balancing circuits. The new developments and optimisations over the last few years have been particularly intense due to the increased interest in battery technologies for several end-use applications. This paper reviews and discusses recent cell-balancing techniques or methods, covering their operating principles and the optimised utilisation of electrical components.

The ever-increasing development of wind power plants has raised awareness that an appropriate condition monitoring system is required to achieve high reliability of wind turbines. In order to develop an efficient, accurate and reliable condition monitoring system, the operations of wind turbines need to be fully understood. This article focuses on the online condition monitoring of electrical, mechanical and structural components of a wind turbine to diminish downtime due to maintenance. Failure mechanisms of the most vulnerable parts of wind turbines and their root causes are discussed. State-of-the-art condition monitoring methods of the different parts of wind turbine such as generators, power converters, DC-links, bearings, gearboxes, brake systems and tower structure are reviewed. This article addresses the existing problems in some areas of condition monitoring systems and provides a novel method to overcome these problems. In this article, a comparison between existing condition monitoring techniques is carried out and recommendations on appropriate methods are provided. In the analysis of the technical literature, it is noted that the effect of wind speed variation is not considered for traditional condition monitoring schemes.

The majority of electrical failures in wind turbines occur in the generator‐side semiconductor devices. This is due to temperature swings affecting the layers of insulated‐gate bipolar transistors in different ways; these effects are aggravated by the variation of wind speed. The implementation of accurate on‐line condition monitoring mainly relies on on‐line tracking of the temperature variation of components. The maximum temperature stress is observed at the junction terminal of the devices, which cannot be easily measured. Additionally, it is difficult to track the exact dynamic of temperature due to the slow response of sensors. Thus, several methods have been presented in the technical literature to estimate the junction temperature of the semiconductor devices. Each method has merits and disadvantages in terms of accuracy and complexity, as failure mechanisms have their own effects on the variation of junction temperature and some or all of the electrical parameters. Therefore, detection algorithms have to determine the root cause of the temperature variation. This study comparatively reviews the condition monitoring methods presented so far and gives directions on the future steps that should be addressed by research in this area.