Study On Smooth Switching And Off-grid Stable Operation Control Of Hybrid Microgrid

The shortage of traditional energy and the serious environmental pollution have promoted the development and utilization of new energy.In order to adjust the existing energy structure,the hybrid microgrid including photovoltaic,gas engine and energy sources is gradually developed.The cooperation of multiple distributed energy can not only make full use of the traditional primary energy such as wind,light,coal,oil and natural gas,but also conform to the sustainable development concept of energy.Due to the different output characteristics of distributed generation in hybrid microgrid,especially the wind and solar power generation system is greatly affected by natural factors,with strong randomness and weak reliability.The stability of hybrid microgrid is guaranteed by the large power grid.Once it is off-grid,the stability will be greatly reduced.Therefore,it is of great significance to study the fast switching technology aiming at the stable operation of isolated grid and ensure the independent and stable power supply after off-grid.It has a long-term development prospect in improving the utilization efficiency of distributed energy.This thesis carries out the research on hybrid microgrid including photovoltaic,gas engine and energy storage system.First of all,a dynamic simulation model with practical engineering significance is established according to the actual situation of each equipment,source and load in the project demonstration park.In the energy storage system,PQ control is adopted for the special electric energy storage under grid connection,VF control is adopted for the main energy storage under off-grid,and PQ control is maintained for the secondary energy storage;PQ control is adopted for the productive energy storage.The photovoltaic power generation system adopts bipolar generation control,the front pole adopts maximum power point tracking(MPPT)control,and the rear pole adopts PQ control;the gas turbine is improved on the basis of the general Rowen model,and the single shaft gas turbine power generation system model is established.Secondly,for the switching from grid connected to off-grid,a smooth switching strategy is proposed considering the rate characteristics and dynamic PQ control.After unscheduled off-grid,the high rate discharge from energy storage is used to reduce the deviation between voltage and frequency and rated value.Then,according to the characteristics of inverter overload rate,the load shedding area is divided,and the load is switched smoothly between multiple areas.In each load shedding transient oscillation process,using virtual droop and virtual inertia control,the dynamic PQ control from energy storage is proposed to reduce the impact caused by load shedding.It is planned to cut off unimportant loads in advance and connect gas turbines in advance to reduce the impact caused by the imbalance of supply and demand.Simulation results show that the proposed strategy can significantly reduce the voltage overshoot caused by load shedding.Finally,in view of the random fluctuation of load and renewable energy,a multisource coordinated control strategy based on model predictive control(MPC)and low pass filtering(LPF)is proposed after off-grid operation.According to the planned value of load fluctuation,the planned output value of gas turbine and energy storage is obtained through MPC rolling optimization,and then the low-frequency components are divided into gas turbines through low-pass filtering to dispatch their output ahead of schedule.The remaining high-frequency components and the gas engine response bias are superimposed on the energy storage.The final simulation results verify that the proposed strategy can effectively reduce the tracking plan value from the energy storage SOC.The regulation pressure of energy storage realizes the coordinated control of multiple power sources,ensures the safe and reliable power supply of important loads and makes full use of renewable energy output.