Multilevel converters have good potential in high power and high voltage applications due to their advantages of reduced voltage or current stress on power devices. In recent years, hybrid multilevel converter (HMC) have attracted increasing attention since less equipment is required. In this paper, the topologies and evolutions of HMCs are presented, where five topology derivation ways are given by using basic cells in series-parallel/parallel-series. Some general topologies or structures that are used to generate higher levels are also deducted. Then many existing HMCs can be derived, and new topologies of the HMC might be inspired. The capabilities of neutral point and FC voltage balancing control are investigated. The performance of the selected FC-based HMCs is analyzed. Finally, the verifications of operation principle and control strategies for the derived HMCs are carried out.

Nowadays, most of the doubly fed induction generators (DFIGs) are equipped with rotor crowbar for the requirement of low voltage ride through (LVRT). The crowbar resistance is an important parameter, and it is selected taking rotor overcurrent and dc link overvoltage limits into consideration. However, the impact of grid impedance on the LVRT performance of DFIG and crowbar resistance is not adequately researched. This paper proposed an improved method to analyze the LVRT performance of DFIG system with rotor crowbar taking the influence of the grid impedance to fill this gap. The impedance of the present grid would be decreased in the future due to the installation of more wind farms to the grid. As a consequence, the performance of DFIG under LVRT with rotor crowbar is degraded. So the design rules of the crowbar resistance need to be reconstructed. Additionally, the analytical expression of the crowbar resistance is derived considering the grid impedance. The effectiveness of the proposed method is validated through theoretical analysis and MATLAB simulations.INDEX TERMS DFIG, crowbar, grid impedance, wind energy, low voltage ride through.

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The advantages, such as the mature control method, less volume of the converter and generator, make the doubly fed induction generator system prevalent in the wind power industry. Nevertheless, the doubly fed induction generator is more susceptible to grid faults and disturbances. The instantaneous high voltage fault may occur due to the excessive local reactive power after the doubly fed induction generator system achieving low voltage ride through, and then the wind turbine might be disconnected again. Hence, not only the low voltage ride through but also high voltage ride through capability should be required for the doubly fed induction generator system to meet the grid code requirements. A comprehensive review of the state of the art low voltage ride through and high voltage ride through technologies for the doubly fed induction generator system is presented. Firstly, different types of common low voltage ride through and high voltage ride through techniques are classified according to their features, i.e. auxiliary hardware, linear or nonlinear control strategies etc. The pros and cons of different low voltage ride through and high voltage ride through techniques are given. Furthermore, the latest developments of low voltage ride through and high voltage ride through technologies are introduced. Finally, the future trends of both the low voltage ride through and high voltage ride through technologies are discussed. 1 INTRODUCTION Wind energy has become one of the most extensively disseminated new energy sources owing to the clean and efficient characteristics. The wind power industry has developed rapidly in China since the beginning of the 21st century. The installed capacity of grid-connected wind power will be expected to reach 210 GW by the end of 2020 [1]. Wind energy development and utilisation have started to develop in a decentralised manner, especially at the end of the power grid. Due to the long distance of the transmission line, the impedance of the power grid seen from the wind turbines increases, and the short circuit ratio (SCR) becomes small, which is the main characteristic of the weak grid. As the proportion of grid-connected wind power is increased and scattered into the weak grid, operational characteristics of the power grid, such as weak inertia, low SCR, This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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