Interface System And Its Controlfor Accessing Microgrid To Grid Based On Back To Back VSC With Energy Storage

Microgrid is a new technology of comprehensive utilization of renewable energy distributed generation system.The key problem of microgrid integration and application is to suppress the intermittence and randomness of microgrid output.It is ofevident application value and practical significance to smooth the power fluctuations of microgrid by a flexible interconnection interface based on back to back voltage source converter(BTB-VSC)containing energy storage.In this dissertation,the control of the flexible grid-tied interface system and its related technologies are investigated by means of theorical analyses,the simmulation and experiment validations.The main contents and the related conclusions of this dissertation are as follows:(1)Based on the analysis of the BTB-VSC and the energy storage to stabilize the microgrid power fluctuation,a microgrid access to grid interface system structure is proposed.The fundamental frequency mathematical model of the interface system and the low frequency mathematical model under the synchronous rotating coordinate system are established under the conditions of voltage balance and voltage imbalance.The results show that the power transmission and power flow direction can be effectively maintained by controlling the charging and discharging of the energy storage when the microgrid side power fluctuates.When the three-phase voltage is unbalanced or contains background harmonics,the double frequency fluctuation will appear in the dc voltage and the active power transmission,which will affect the battery charge and discharge current,and produce harmonics.(2)An auxiliary-excited active tuned hybrid power filter(ATHPF)is studied and improved,and a multi-harmonic tuning suppression strategy based on voltage balance detuning detection and the principle of dc side voltage optimization are proposed.The relationship between the DC-side voltage and the rated voltage of grid,the tuning order of LC filter,the capacitance and the rated harmonic current are determined,and the output voltage and power of auxiliary-excited power is reduced.The simulation and experimental results show that the designed multi-harmonic tuning control strategy can effectively suppress multiple harmonic currents and is not easily affected by the system impedance,so as to achieve the purpose of suppressing multiple harmonics and detuning under the condition of greatly reducing the voltage and capacity of the active part.(3)In order to improve the response speedand the stability of energy storage unit,the L-LLC resonant bidirectional DC-DC converter(L-LLC-BDC)and its control strategy are studied.The compound optimal trajectory control strategy is proposed by analyzing the operating mode and output gain characteristics of L-LLC-BDC.The trajectory equations under different operating modes are deduced.The converter achieves steady state in the shortest time by predictingthe conducting time of switch when the load changes.It realizes zero-voltage switching and zero-current switching for input switches and output rectifiers respectively.Besides,it not only significantly reduces the computational complexity and further increase switching frequency,but also improve dynamic response of the converter remarkably.(4)In the case of voltage balance on the microgrid side,a cascade control strategy based on power follow and a switching control strategy for energy storage multi-stage charge and discharge are proposed,and the overall control scheme of the grid-tied interface system is designed.The vector current control is introduced into the control of the grid-tied interface system to improve the fastness of control and current limiting capability.The results of simulation and experiment show that both converters can adjust active and reactive power independently at the same time.When the power on the microgrid side fluctuates,the power on the distribution network side and the direction of power flow can be stabilized by controlling the charging and discharging power of the battery.It effectively reduces the influenceof microgrid side faults on grid and has good dynamic performance.(5)In the case of voltage unbalance on the microgrid side,a voltage unbalance suppression strategy based on equivalent impedance using proportional-resonant(PR)anda unbalanced suppression strategy based on voltage compensation feedback linearization are proposed,and the overall control scheme of the grid-tied interface system is designed.The performance of two voltage unbalance suppression strategies is compared and the applicability of the two strategies is analyzed.The simulation and experimental results show that the control of the interface system can stabilize the power level and power flow direction of the grid,and the proposed unbalanced suppression strategy can effectively suppress the three-phase voltage unbalance at the microgrid side.Meanwhile it can support the short-time voltage sag and realize the suppression of harmonic voltage and reactive power compensation.(6)The physical interface system is established.On this platform,the experimental research on the power fluctuation,harmonics and three-phase unbalance suppression when the microgrid is connected to the grid is carried out.The results show that the adverse effects of the microgrid can be effectively eliminated when the microgrid is connected to the grid by using the interface system,and the excess power of the microgrid can be transmitted to the grid,which provides a reference physical platform for the future construction and research of the microgrid.