Research On The Low Voltage Ride Through Control Technology For Grid-forming Converters Under Symmetrical Grid Faults

With the acceleration of the global energy green and low-carbon transformation process,the total installed capacity of new energy is increasing year by year,and a new power system with new energy as the main body will be formed in the future.Since the grid-forming converter is synchronized with the grid through a power control loop,it has similar external characteristics to synchronous generators and voltage source characteristics with low series impedance,thus avoiding resonance problem introduced by phase-locked loop,showing good adaptability and stability to weak power grids and even extremely weak power grids,independently building system voltage in weak power grids,and providing inertia,damping and active support for power grids.Therefore,grid-forming converter is considered to be a practical and effective solution to ensure the safe and stable operation of the new power system with new energy as the main body.However,after the grid faults,the grid-forming converter has some problems such as overcurrent and insufficient reactive power support capacity.On the one hand,as the grid-forming converter has the inherent characteristics of a voltage source,its output voltage is not affected by the grid faults and its output current will increase rapidly,thereby triggering the overcurrent protection and causing the new energy generating set to go off the grid.On the other hand,as the power control loop of the grid-forming converter has the characteristics of narrow bandwidth and slow response speed,its reactive current adjustment speed is slow,which may lead to low voltage ride-through failure.Therefore,this thesis studies the low voltage ride through control strategy for grid-forming converter in the case of the grid Symmetrical faults.The research contents are as follows:Firstly,the topology and mathematical model of grid-forming converter are analyzed.At the same time,according to the different bandwidth of power control loop and voltage control loop,the state space model of power outer loop and the impedance model of voltage inner loop are established respectively.According to the established mathematical model,the transient instability mechanism of grid-forming converter under symmetrical grid faults and the generation mechanism of transient impulse current and steady-state overload current are analyzed.Next,according to the requirements of fault current limitation and low voltage crossing for grid-forming converter,the dynamic requirement of virtual impedance under different grid voltage drop is analyzed,the virtual impedance control strategy of grid-forming converter is studied,and a method of simulating differential link at fundamental frequency is proposed to realize virtual inductor.The impedance model of grid-forming converter with virtual impedance is established,and the stability criterion of grid-forming converter based on impedance is used to analyze the stability.Then,the influence of virtual impedance on transient impulse current and the relationship between virtual impedance and active and reactive components of gridconnected current are analyzed.On this basis,a low voltage ride through control method of grid-forming converter based on adaptive virtual impedance is proposed,which makes the grid-forming converter has the ability of fault current limitation and fast reactive current injection without losing synchronization with the grid.The proposed control method consists of two parts.The initial value and the adaptive adjustment of virtual impedance are designed to realize the limitation of transient impulse current and the rapid injection of reactive current during fault initiation and recovery.Adjusting the power outer loop reference to achieve the limitation of steadystate overload current and the improvement of transient stability.Finally,the experimental platform of three-phase grid-forming converter is designed and built,and experiments are carried out on the experimental platform for the proposed control method.The experimental results show that the virtual impedance designed in this thesis can keep the grid-forming converter running stably even with a large virtual impedance that met different grid voltage drop depths.And the proposed low voltage ride through control method of grid-forming converter based on adaptive virtual impedance can limit the fault current to the allowable value under different grid voltage drop depths,realize the rapid injection of reactive current and enhance the transient stability of the system,so that the grid-forming converter has the ability of fault current limitation and low voltage crossing without losing synchronization with the power grid.