Small Signal Stability Analysis Of Double-fed Induction Generator Variable Speed Pumped Storage Units

The construction process of a new type of power system with new energy as the main body is accelerating,and the characteristics of"double high"and"double peak"in the power system are becoming more prominent.The power grid needs more efficient,reliable,and flexible regulation of power sources to support the large-scale development of new energy,alleviate the pressure of system peak shaving,and maintain the safety and stability of the power grid.Variable speed pumped storage units are more suitable as regulation and management tools in new power systems due to their fast response speed,wide regulation range,and high operating efficiency.However,variable speed units also have characteristics such as more complex structure,stronger nonlinearity,and"isolation"from the power grid,which will inevitably have new impacts on the stability of the units and the power grid.This article takes the Double Fed Induction Generator based Variable Speed Pumped Storage(DFIG-VSPS)under power generation conditions as the research object,and studies the small signal stability of DFIG-VSPS and its grid connected system from the perspectives of a single machine infinite bus system and an interconnected power grid.The specific research work and results are as follows:(1)The complete mathematical model and overall control strategy under DFIG-VSPS power generation condition are established,and the differential equations describing the dynamic characteristics of each part of the system are derived.Through the linearization of the model,the Small-signal model under DFIG-VSPS power generation condition is established.(2)In a single machine infinite bus system,the eigenvalue method was used to calculate the eigenvalues of the system state matrix and the participation factors of each oscillation mode.The degree of participation of state variables in each mode was analyzed,and the relationship between the underdamped oscillation mode and state variables such as pump turbine speed,guide vane opening,water head,and transient electromotive force of doubly fed motors was obtained;Analyzed the influence of the water flow inertia time constant T_w and the power outer loop control of the rotor side converter on the system damping and small signal stability.Finally,the law was verified through time-domain simulation.(3)Based on a four machine two area system,four scenarios were set up to compare the changes in key oscillation modes before and after DFIG-VSPS was connected to the interconnected system;Explored the impact of reduced output and increased grid connection distance of DFIG-VSPS on the small disturbance stability of the system,and finally verified it through time-domain simulation.The research results indicate that DFIG-VSPS access can enhance the small interference stability of interconnected systems to a certain extent,and the improvement effect on inter regional oscillation mode is more prominent when accessing the power receiving end region;Adopting the connection method of adjusting the output of synchronous units is more conducive to improving the local oscillation mode.The larger the output of the DFIG-VSPS unit,the better the overall small disturbance stability of the system.Increasing the grid connection distance will worsen the small signal stability of the entire system.