• Energy Research

After a fast photovoltaic (PV) expansion in the past decade supported by many governments in Europe, in this postincentive era, one of the most significant open issues in the PV sector is to find appropriate inspection methods to evaluate real PV plant performance and failures. In this context, PV modules are surely the key components affecting the overall system performance; therefore, there is a main concern about the occurrence of any kind of failure in PV modules. This paper aims to propose a novel concept for monitoring PV plants by using light unmanned aerial vehicles (UAVs) or systems (UASs) during their operation and maintenance. The main objectives of this study are to explore and evaluate the use of different UAV technologies and to propose a reliable, cost-effective, and time-saving method for the inspection of PV plants. In this research, different UAVs were employed to inspect a PV array field. For this purpose, some thermal imaging cameras and a visual camera were chosen as monitoring tools to suitably scan PV modules. The first results show that the procedure of utilizing UAV was effective in the detection of different failures of PV modules. Moreover, such a process was much faster and cost effective than traditional methods.

The present paper deals with a characterization of supply voltage variations on induction motors when transient faults occur on distribution feeders or busbars. The main parameters that can qualify the voltage perturbation are presented and discussed both on the theoretical point of view and with example of application. The test network chosen shows the effectiveness of the index that represents the quality of the motor supply voltage when network faults occur.

Photovoltaic (PV) systems are well known for their simplicity of design, environmental friendliness, and low maintenance. Among the PV technologies, the behaviour of bifacial PV modules was studied in this research. Measurements of the I-V curves were carried out in the SolarTechLAB test facility at the Department of Energy of Politecnico di Milano, Italy, to detect the bifacial PV module behaviour, mainly in terms of power performance. In particular, I-V and power-voltage curves were measured at different tilt angles to consider several irradiance and cell temperature levels with both sides uncovered as well as with the back side covered. This last configuration was tested to evaluate the contribution of the rear face in the overall photoelectric conversion process. The comparison between the bifacial and monofacial operations highlighted that the power at the maximum power point of the bifacial operation can increase up to 13%. At the same time, leaving the rear face free allows for reducing the bifacial cell temperature up to about 6°C.

A common problem in wind power plants involves fixed-speed wind turbines. In fact, being equipped with a squirrel-cage induction generator (SCIG), they tend to drain a relevant amount of reactive power from the grid, potentially causing voltage drops and possible voltage instability. To improve SCIG power quality and transient stability, this paper investigates a new control strategy for pitch angle control based on proportional-integral (PI) controller and a fuzzy logic controller (FLC), considering both normal and fault ride-through (FRT) schemes. In the literature, often, the mechanical torque output is assumed constant for a specific wind speed. This might not be accurate, because the mechanical torque-speed typical of a wind turbine depends also on the power coefficient or pitch angle. In this paper, an analytic model of transient stability is proposed using the equivalent circuit of the SCIG and using the concepts of stable and unstable electrical-mechanical equilibrium. The method has been evaluated by comparing the results obtained by the analytic method with the dynamic simulation. The results show that the proposed hybrid controller is effective at smoothing the output power and complying with FRT requirements for SCIG in the power system.

In the present paper two new methods for the evaluation of the internal impedance of a rail are proposed. These methods are based on FEM and they consider different steel's magnetic behavior models. The first one is based on the normal magnetization curve and the equivalent sinusoid concept. In the second model, the hysteresis loops have been considered introducing the evaluation of complex magnetic permeability function. Rail's internal per unit length parameters are calculated in function of current and frequency. The results from FEM models are compared with the available measurements at the frequency of 50 Hz, and they are extended to other frequencies.

Energy systems around the world are undergoing substantial changes, with an increasing penetration of Renewable Energy Sources. For this reason, the availability of a pool of suitable forecasting models specific for the needed time horizon and task is becoming crucial in the grid operation. In addition, nowcasting techniques aiming at provideing the power forecast for the immediate future, are more often investigated due to the spread of micro-grids and the need of facing changing electrical market environments. In this paper a novel comprehensive methodology aiming at computing the PV power forecast on different time horizons and resolutions is introduced. Moving from the 24-hours ahead prediction provided by the Physical Hybrid Artificial Neural Network (PHANN), a technique to refine the power forecast for the following 3 hours with an hourly granularity is analyzed, leveraging on newer information available during the operations. Moreover, in order to provide the power forecast for the following 30 minutes on a minutely basis, an innovative modification of a statistical technique is proposed, the robust persistence. The proposed comprehensive approach allowed to greatly reduce the overall error committed when compared with the benchmark models. Finally, the proposed methodology is validated and tested on a freely available database consisting on different parameters recorded at both the meteorological and photovoltaic test facility at SolarTechLAB, Politecnico di Milano, Milan.

Photovoltaics, among renewable energy sources (RES), has become more popular [...]

The paper deals with the study and analysis of the influence of power quality perturbations on the dynamic stability of synchronous machines supplying isolated power networks. The synchronizing power coefficient is considered. The theoretical stability study is extended to the non-sinusoidal and/or unbalanced systems using the Park transformation methodology. An example relevant to the electrical power system on board of a military ship is reported in order to highlight the application of the proposed method of stability analysis.