| There will be power fluctuations and voltage stability problems of power system when large-scale wind farm is concentrated connected to grid, the effective solution is to use an energy storage device and reactive power compensation device, but there will be area and cost problems when both were installed respectively, so the device integrated active and reactive power compensation with four-quadrant compensation capability is researched in this paper, the energy storage system is combine to reactive power compensation device, which can smooth the power fluctuation and stabilize voltage simultaneously. Integrated compensation device is consisted of modular multilevel converter (MMC) and energy storage units.Firstly, operational mechanism of MMC, including modulation strategy and sub-module capacitor voltage balance control strategy are introduced in this paper; various energy storage technologies and their application are analyzed and compared, especially for wind farms use, considering the energy storage characteristics and economic issues, lithium battery is chosen as energy storage unit, and a simulation model of lithium battery is established.Secondly, the way energy storage device access to DC side of MMC is researched, the advantages and disadvantages of each access method are analyzed, considering the complexity of the control strategy, efficiency and economy and other issues, direct DC/DC link is used to parallel the batteries with DC side of sub-module, the control strategy DC/DC link is introduced, then the MMC-BESS simulation model which consisted MMC and lithium batteries is established.Thirdly, state of charge (SOC) balance control strategy is proposed to balance different SOC of the battery sub-modules; the circulating current is analyzed in situation of different power output, and circulating current suppressing method is given; then overall control strategy of the MMC-BESS is given, a simulation model is built in PSCAD/EMTDC, the feasibility of proposed control method is validated by simulation results.Finally, the MMC-BESS is applied in wind farm integration, according to the active and reactive power demand of wind farm, the active and reactive power control strategies of MMC-BESS are given; when the system is in transient or unstable operation state, the coordination active and reactive power control strategy based on filter and weighted slide is proposed, can smooth the power fluctuations and improve voltage of bus effectively. |
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