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Back-to-back converters for wind turbine systems (WTS) feature capacitors in the DC-link to maintain a stable DClink voltage and to decouple the generator from the grid. Electrolytic and film capacitors can be chosen to this purpose. Long-term field experience and recorded failure data reveal that capacitor failures are one of the main reasons for the downtime of WTS. The ripple current accelerates the wear-out of the capacitors, which is strongly dependent on the mission profile of the system. Therefore, it becomes important to estimate the useful lifetime of a capacitor as a function of the ripple current. In this paper, aging characteristics of electrolytic capacitors are described, and a power converter topology and its control strategy are designed to perform accelerated lifetime tests. The voltage and ripple current of the capacitor under test (CUT) can be controlled with low THD to correlate wear-out with ripple.

Liserre, "Impacts of rotor current control targets on DC-link capacitor lifetime in DFIG-based wind turbine during grid voltage unbalance," Abstract-In doubly-fed induction generator (DFIG)-based wind turbines, the DC-link capacitor of the back-to-back (B2B) converters is one of the components that tend to fail most often. The equivalent resistance of electrolytic capacitors is frequency dependent and presents higher values in the low-frequency range. Therefore, additional low-frequency ripple current components will stress the DC-link capacitor and reduce its lifetime. Under unbalanced grid voltage conditions, the conventional vector control strategy of the rotor side converter may be replaced by flexible multi-objective control strategies, leading to different low-frequency harmonics in DC-link capacitor current. Therefore, in this paper the differences between the different control targets with respect to overall RMS current, additional low-frequency ripple current and temperature are analyzed by means of simulations for a 2 MW DFIG wind turbine.

Abstract-Back-to-back converters for wind turbine systems feature capacitors in the dc-link to maintain a stable voltage and to decouple a generator from the electric grid. The electrolytic capacitors are typically chosen for their advantages; a higher energy density and a higher capacitance at lower costs. Long-term field experiences and recorded failure data revealed that the capacitors are one of the most frequent failure reasons for the wind turbine system. The current profile of the capacitors is highly responsible for this degradation, since it determines the dissipated power of the capacitor. This paper analyzes the actual current profile in the dc-link capacitor of a back-to-back converter for wind turbine application. A power converter is also designed to generate sinusoidal current at arbitrary frequency and arbitrary dc bias voltage for testing purposes. The experimental results confirm that the proposed power converter enables us to derive the correlation between the current frequency and the temperature variation of capacitor.Index Terms-Back-to-back converter, electrolytic capacitor, reliability, wind turbine, ripple current stress.

Abstract-Transformerless PV inverter topologies allow the PV inverter size and weight to be reduced while reaching a higher efficiency. The drawbacks of these topologies are that DC current components at the grid side must be cancelled by means of the employed controller and the number of power devices required to avoid currents through the parasitic capacitor of the PV generator increases.These topologies have been analysed in the literature but physical variations of power devices due to the manufacturing process have not been taken into account. This manuscript analyses the effect of such physical variations on the performance of three commonly employed transformerless topologies: H5, HERIC and Half-Bridge NPC. The impact of the parameter variations is compared regarding the efficiencies at different power levels, the European Efficiency and the DC current components. The results indicate that physical variations lead to different behaviors regarding the robustness of the analysed topologies. By means of statistical analyses these differences are obtained and exposed.

Abstract-Transformerless (TL) topologies are employed in 1φ PV inverter topologies due to their small size and low weight. Avoiding the grid side transformer requires the modulation technique and the basis topology to be accordingly changed in order to mitigate dc current components in the grid side and the leakage current to ground.This paper carries out a sensitivity analysis of selected TL topologies. This analysis investigates the impact of parameter variations of the employed semiconductor devices and detects the device which affects the most the overall efficiency. As a result, relevant info for engineers selecting the most suitable power devices for the implementation of a certain TL topology is provided.