| Renewable energy power generation has the characteristics of volatility,intermittent and unpredictability,the high penetration of which brings great challenges to the stability and power quality of the power system.The integration of the energy storage system can smooth power fluctuations,adjust the frequency of the grid and improve the quality of power,which is one of the key technologies in the renewable energy grid.Noting the development trend of large-scale penetration of energy storage systems in the power grid,a novel hybrid energy storage system(HESS)based on modular multilevel converter(MMC)is proposed,which integrates both super capacitor(SC)and battery.Different from other topologies,batteries and SCs are allocated respectively at the sub-module(SM)and the DC side.The energy storage system which has the advantages of both high power-density and high energydensity,is suitable for the energy storage occasions of both high-voltage and large-capacity.Moreover,it can be directly connected to the grid without any transformer.Firstly,the topological structure and basic working principle of MMC-HESS are presented in this article.And the mathematical model of the MMC-HESS is built to analyze the relationship between internal circulation and energy flow,which provides a theoretical basis for the control strategy.The power transfer relationship between super capacitor,battery and power grid is analyzed.Then four different working modes of HESS are discussed.Secondly,the control strategy in the converter layer for the system is proposed,including AC side control strategy,super capacitor power control strategy and state of charge(SOC)equalization strategy.The AC side power is controlled by the current decoupling method.The super capacitor power is controlled by the DC side current.The SOC of the battery between phases and the SOC between the upper and lower arms are respectively controlled by the DC circulating current and the AC circulating current.And the nearest level modulation(NLM)is applied to balance the SM SOC in the arm.In addition,the feasibility of the proposed strategy is verified by PSCAD/EMTDC simulations in the results.Thirdly,the different application scenarios of the MMC-HESS are discussed.For two typical application scenarios,different control strategies in the coordination layer are proposed respectively.For the wind power grid-connected scenario,the average value method and lowfrequency filtering method are used for calculating the reference power and power distribution.Then the effect of smoothing the fluctuating power is achieved.Aiming at the system frequency instability caused by the high penetration of renewable energies,a power distribution strategy with SC energy management is proposed based on the frequency characteristics.Aforementioned control strategies are both verified through PSCAD/EMTDC simulations.Finally,the advantages of proposed MMC-HESS are summarized,and the future engineering application prospects are discussed and prospected. |
