Research Of Super-capacitor Bank Application In Grid Connected Converter

Super-capacitor, as a new energy-storage device, can provide very high power andvery long lifetime, with a good prospect for development. The introduction ofsuper-capacitor into grid connected converter can greatly improve the power quality ofrenewable energy such as wind or photovoltaic power generation system, enabling aneffective link with the grid. The super-capacitor bank is composed in series and/orparallel by a large number of units to meet the need of capacity and voltage level.However, the inconsistency of parameter may result in unbalanced voltage amongsuper-capacitors, with restriction on lifetime and reliability. Therefore, to balance thevoltage of the super-capacitor bank is of great importance. In this paper, the authors willresearch on the control method and voltage-balance scheme of the super-capacitor bank,based on the application of the direct-drive wind turbine.As one of the mainstream models for future wind power development, thedirect-drive wind turbine, with the full-power grid connected converter to isolategenerator and gird, has a great capability of Low Voltage Ride-Through (LVRT).However, there exist a lot of challenges to the full-power grid connected converter,especially under the asymmetric grid fault. In order to improve the capability ofasymmetric LVRT, a novel control method based on super-capacitor is proposed in thispaper, together with the modeling and capacity setting of the super-capacitor bank. Inthis method, super-capacitors are connected to the DC bus of the full-power gridconnected converter via a bi-directional DC converter, whose throughput power iscontrolled to limit the voltage rise and to reduce the2nd-order voltage ripple of the DCbus when in failure. Meanwhile, the grid voltage feed-forward approach is employed toreduce the negative sequence current component. Simulation results verify theeffectiveness of the proposed control method of asymmetric LVRT.A new power interface circuit is proposed in this paper, which has the ability tobalance the load voltage as well as power control. The topology and principle of thepower interface circuit, as well as the design of the corresponding controller, are alsodescribed with simulation verification.Furthermore, a voltage-balance scheme of the super-capacitor bank is proposed inthis paper and based on the power interface circuit, combining buck-boost module toimprove the voltage-balance speed and efficiency. Simulation results verify the effectiveness of the proposed voltage-balance scheme.