• Energy Research

Summary Wind power plant operators are often faced with extra charges when their power production does not match the forecasted power. Because the accuracy of wind power forecasts is limited, the use of energy storage systems is an attractive alternative even when large-scale aggregation of wind power is considered. In this paper, the economic feasibility of lithium-ion batteries for balancing the wind power forecast error is analysed. In order to perform a reliable assessment, an ageing model of lithium-ion battery was developed considering both cycling and calendar life. The economic analysis considers two different energy management strategies for the storage systems and it is performed for the Danish market. Analyses have shown that the price of the Li-ion BESS needs to decrease by 6.7 times in order to obtain a positive net present value considering the present prices on the Danish energy market. Moreover, it was found that for total elimination of the wind power forecast error, it is required to have a 25-MWh Li-ion battery energy storage system for the considered 2 MW WT. Copyright © 2015 John Wiley & Sons, Ltd.

AbstractThis article proposes a fault identification method, based on the complementary analysis of the light and dark current–voltage (I–V) characteristics of the photovoltaic (PV) module, to distinguish between four important degradation modes that lead to power loss in PV modules: (i) degradation of the electrical circuit of the PV module (cell interconnect breaks; corrosion of the junction box, module cables, and connectors); (ii) mechanical damage to the solar cells (cell microcracks and fractures); (iii) potential‐induced degradation (PID) sustained by the module; and (iv) optical losses affecting the module (soiling, shading, and discoloration). The premise of the proposed method is that different degradation modes affect the light and dark I–V characteristics of the PV module in different ways, leaving distinct signatures. This work focuses on identifying and correlating these specific signatures present in the light and dark I–V measurements to specific degradation modes; a number of new dark I–V diagnostic parameters are proposed to quantify these signatures. The experimental results show that these dark I–V diagnostic parameters, complemented by light I–V performance and series‐resistance measurements, can accurately detect and identify the four degradation modes discussed. Copyright © 2015 John Wiley & Sons, Ltd.

Peer Reviewed

The classical control techniques for regulating the active and reactive power delivery in doubly fed induction generators (DFIG), for wind power applications, are normally based on voltage oriented control (VOC) strategies. Among these algorithms, those that work in a synchronous reference frame, attached to the magnetic flux vector, became very popular. In spite of the good behaviour of such algorithms their performance depends highly on an accurate detection of the stator flux position, something that can be critical under unbalanced or distorted grid voltage conditions. This paper presents a new VOC strategy able to control the operation of a DFIG in the αβ reference frame, with no need of flux position estimation, something that conducts to a more simple and robust algorithm. In order to evaluate the advantages of this new control proposal, namely VOC-RRF, their performance will be compared with the response obtained with a classical VOC algorithm by means of PSCAD/EMTDC® simulation models. Peer Reviewed

The modular multilevel converter is capable to reach high-voltage levels with high flexibility, high reliability, and high power quality as it became the standard solution for high-power high-voltage applications that operate with fixed frequency. However, in machine-drive applications, the modular multilevel converter shows critical problems since an extremely high submodule-capacitor voltage ripple occurs in the machine start-up and at low-speed operation, which can damage the converter. Recently, a new converter solution named modular multilevel series converter was proposed as a promising alternative for high-power machine-drive applications since it presented many important structural and operational advantages in relation to the modular multilevel converter such as the reduced number of submodule capacitors and the low submodule-capacitor voltage ripple at low frequencies. Even though the modular multilevel series converter presented a reduced number of capacitors, the size of these capacitors was not analyzed. This paper presents a detailed comparison analysis of the performance of the modular multilevel converter and the modular multilevel series converter at variable-frequency operation, which is based on the proposed analytical description of the submodule-capacitor voltage ripple in such topologies. This analysis concludes that the new modular multilevel series converter can be designed with smaller capacitors in comparison to the modular multilevel converter if these converters are used to drive electrical machines that operate within a range of low-frequency values. In other words, the modular multilevel series converter experiences extremely low submodule-capacitor voltage ripple at very low frequencies, which means that this converter solution presents high performance in the electrical machine start-up and at low-speed operation.

The increasing penetration of generation power plants based on renewable energies in the electrical networks has boosted the number of systems connected to the grid. In this scenario the microgrid, providing advanced functionalities to improve the stability and operation of the network, have become very popular. In this paper a control strategy for the microgrid management is presented. The proposed system improves the performance of microgrids and its interaction with the main network, or another microgrid, under grid voltage transients. This technique gives rise to a simple and robust control scheme to ensure the appropriate microgrid disconnection, and further resynchronization and reconnection, from the main grid. In the following, the control algorithm for the intelligent connection agent will be shown, and the grid synchronization system, as well as the voltage and current control loops will be detailed. Finally, results obtained using the MATLAB/Simulink & PSIM platform will be presented and discussed. Peer Reviewed