| The frequency stability of the isolated microgrid with high proportion of renewable energy sources has faced great challenge because of its small rotating mass and low inertia.With the urgent demand of improving the system inertia,the virtual synchronous generator(VSG)technology has been developed.Up to now,there are still four issues to overcome in the field of VSG researches.(1)The angular frequency deviation and its change rate are not fully exploited when adjusts the virtual inertia of voltage-controlled VSG.(2)It is difficult to solve the overcurrent problem of voltage-controlled VSG efficiently through the existing control methods.(3)There are still some limitations and drawbacks of the existing secondary frequency control methods of voltage-controlled VSG.(4)The secondary frequency control system of microgrid with multi-type VSGs is not yet designed.Thus,this paper focuses on the frequency control of VSG for an isolated microgrid,and mainly discusses the adaptive inertia control and secondary frequency control of VSG.The main contents are as follows.1)An adaptive virtual inertia control method for voltage-controlled VSG is proposed.In this paper,we analyze the states of angular frequency deviation and its change rate during the frequency dynamic process in the disturbed system.Then two functional relationships between them and virtual inertia are established respectively,which are weighted combined to adjust the virtual inertia adaptively.The controlment sensitivity is significantly improved for the reason that,through the hyperbolic sine function,the collected angular frequency deviation and its change rate can be typically amplified.In addition,by introducing the sigmoid function,the regulating range of the virtual inertia is selected so that the oscillation of VSG output power in the control process is suppressed.The effectiveness of the proposed inertia control is verified by different simulation tests.The results show that the proposed method can reduce the frequency fluctuation and suppress the oscillation of VSG output power.2)A dual-mode switching control method between voltage-controlled VSG and currentcontrolled VSG is proposed.In this paper,we adequately describe the control methods of mutually switching modes of voltage-controlled VSG and current-controlled VSG according to its fundamental theories and switching conditions between different modes.We also summarize the implementation steps of the dual-mode switching control method.The effectiveness of the proposed dual-mode switching control method is verified by simulation tests.3)A distributed secondary frequency control method of VSG,which is suitable for voltage-controlled VSG without overcurrent,is presented.Based on the fundamental reason of system frequency fluctuation,we transform the frequency recovery problem into the rebalancing problem of the active power of the whole system.Then we establish the optimization objective function of the active power imbalance of the system.Additionally,with the systematic active power imbalance is estimated by the frequency deviation and its change rate of each VSG,the active power sharing among multiple voltage-controlled VSGs can be realized.Simulation results support that,the proposed secondary frequency control method can tolerate certain communication delay,recover the frequency more quickly and reduce the frequency fluctuation significantly comparing with the existing methods.4)Considering that the method discussed in 3)is limited when overcurrent of the voltage-controlled VSG occurs,a centralized secondary frequency control method based on the improved active disturbance rejection control for multi-type VSGs is proposed.We illustrate the framework of centralized control method as well as the idea of centralized control method for voltage-controlled VSG with overcurrent or not.Then by figuring out the existing problem of the extended state observer in the traditional active disturbance rejection control,we design an improved extended state observer using the gaussian error function.Compared with the traditional extended state observer,the proposed one performs better for observation.To further satisfy the balance of active power,we design a centralized secondary frequency control system based on improved active disturbance rejection control.The proposed system is able to guarantee the power sharing among the voltage-controlled VSGs and realize the recovery of system frequency because it can estimate and compensate the unbalance of active power in real time and then allocate the unbalanced power to each voltage-controlled VSG according to the ratio of their capacity.Compared with the secondary frequency control proposed in 3)and the centralized secondary frequency control based on traditional active disturbance rejection control,the proposed centralized control method can reduce frequency fluctuation effectively and improve frequency stability significantly. |
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