Control Strategy Of MMC Type Solid State Transformer Considering Power Quality Optimization Of Distribution Network

With the advancement of the energy Internet process,when a high proportion of renewable energy is connected to the distribution network through a large number of power electronic devices,the emergence of harmonic pollution and current three phase imbalance have led to a sharp deterioration in power quality,at the same time,the voltage three phase imbalance in the distribution network also deteriorates the quality of grid connected current.When a large capacity DC load and a high permeability distributed generators are connected to the low voltage side of a solid state transformer,the high voltage grid connected port tends to show the characteristics of low inertia and poor damping,which reduces its grid connected friendliness and reliability of system operation.This thesis is based on the research of Modular Multilevel Converter(MMC)type solid state transformer,the main contents are as follows:The topological structure of the input stage,isolation stage,and output stage of the MMC type solid state transformer is analyzed and the mathematical model is systematically deduced,the conventional control strategy and modulation method of the MMC solid state transformer are introduced.the simulation model under conventional control is established to simulate the sudden rise and drop of voltage,which verifies the strong voltage adaptability of MMC type solid state transformer,and also lays the foundation for the control strategy considering the optimization of power quality.Aiming at the three phase imbalance of the grid side voltage,the shortcomings of conventional control strategies are analyzed,and a delay cancellation method was proposed to realize the input stage double sequence control strategy for separation of positive and negative sequences.a first order all pass filter is used to realize the T/4delay cancellation technique and the separation of positive and negative sequence in two phase static coordinate system,and the positive and negative sequence current is control without static difference in the two phase rotating coordinate system,which effectively suppresses the double frequency fluctuation in the DC voltage and the negative sequence current component in the input current when grid side voltage unbalanced.In view of the non ideal situations such as unbalanced voltage three phase caused by resistive grounding fault and harmonic pollution on the grid side,the principle of low order harmonics and unbalance is analyzed,in order to eliminate the delay of phase locked loop,filter and operator,and speed up the system response,a MMC type solid state transformer input stage control strategy based on the improved instantaneous symmetrical component current sequence control is proposed,the positive and negative sequence can be separated without the use of a phase locked loop,which effectively controls the negative sequence current component and harmonic current component under unbalanced and harmonic conditions.Aiming at the three phase unbalance and harmonics of the distribution network,and in order to improve the output stage’s ability to carry unbalanced and non linear loads,a control strategy for the input stage and output stage of the MMC solid state transformer based on the Proportional Integral Quasi Resonant(PIQR)controller is proposed,which enhances the robustness of the system and simplifies the control structure.The PIQR controller is used in the input stage current inner loop,when there is an unbalance fault on the grid side,only the positive sequence current and the negative sequence current need to be adjusted in the synchronous rotating coordinate system,it can maintain the current three phase symmetry and eliminate the double frequency fluctuation of the DC link.When harmonic pollution occurs on the grid side,the quasi-resonant controller is used to adjust the AC component of a specific frequency to reduce the harmonic content in the input current.The PIQR controller is used in the voltage outer loop of the output stage,which can effectively suppress the negative sequence voltage component when the load is unbalanced,and improve the voltage quality,when the nonlinear load is input,it can show a certain low order harmonic suppression ability.In view of the system inertia and damping are further reduced when the high proportion of renewable energy connected to the distribution network through power electronic devices,and in order to enable the solid state transformer to provide the necessary voltage and frequency support capabilities to the upper power grid,the equivalent principle of MMC and synchronous generator is deduced,and an MMC solid state transformer controlled by virtual synchronous generator(VSG)technology is proposed,which enhances the inertia and damping of the input stage(grid connected interface)of the MMC solid state transformer.At the same time,an energy storage device is configured in the low voltage DC bus,and the primary frequency regulation of the upper level power grid is assisted by changing the charging and discharging power of the energy storage.The input stage provides voltage support to the upper power grid by controlling the reactive power,so that the MMC type solid state transformer also has a certain voltage regulation capability.Finally,combining the advantages of the input stage VSG control technology and the current dual sequence control strategy of the MMC solid state transformer,the input stage VSG current balance control strategy is designed,which can maintain the three phase balance of the input current when the grid side voltage is unbalanced.