Research On Oscillations Of Power System With DFIG-based Wind Farm Considering Induction Motor Loads

In recent years,China’s wind power industry has developed rapidly,and the total installed capacity has already ranked first in the world.However,large-scale grid-connected wind power has brought a series of stability problems to the power system.As the most important dynamic load in the power system,the induction motor is also the main factor affecting the stable operation of the power system.Therefore,it is of great theoretical and practical significance to study the problem of power system oscillation caused by induction motor and grid-connected wind power.To carry out the research,this paper takes the power system integrated with the aggregated load of multiple induction motors in parallel connection(Abbreviation:the aggregate load)and the doubly-fed induction generator(DFIG)-based wind farm as the research object.A closed-loop interconnected system model is established,wherein an induction motor or the aggregated load is modelled as "open loop feedback subsystem"and the rest of the power system is modelled as "open loop feedforward subsystem".Based on this model and according to open-loop mode resonance theory,the dynamic interaction between induction motors is analyzed.Besides,the low-frequency electromechanical power oscillation caused by the aggregated load is described and investigated.In addition,the strong interactions between the aggregated load and DFIG or the power system and their influence on power system oscillation are separately explored.The main works of the thesis are presented as follows:1)The reason for the degradation of power system stability is explained from the perspective of the open-loop mode resonance between induction motors.It is found that when two open-loop motor oscillation modes are close to each other in the complex plane,the open-loop mode resonance will be induced,resulting in a strong interaction between the two motors.At this time,the corresponding closed-loop motor oscillation modes are located on the opposite sides of the open-loop modes.Hence,one of the closed-loop modes will be located on the right side with respect to the open-loop modes.2)The low-frequency electromechanical power oscillation caused by the aggregated load is studied.When the aggregated load contains N motors,it is found that if the operating conditions of all the motors are same or similar,the open-loop oscillation modes of the aggregated load can be divided into two categories:one oscillation mode is independently located,and the remaining N-1 oscillation modes coincide or nearly coincide.The simulation results show that as the number of motors increases,due to the strong interaction between multiple motors,the independently located open-loop oscillation mode and its corresponding closed-loop oscillation mode will continuously move to the right side on the complex plane.This will result in power system low-frequency electromechanical oscillations and a large drop in the voltage magnitude of load bus in severe cases.3)The open-loop mode resonance theory is applied to analyze the strong dynamic interactions between the aggregated load and DFIG or the power system.It is found that when the open-loop aggregated load oscillation mode and the open-loop DFIG oscillation mode are close in the complex plane,the open-loop mode resonance will be triggered,and the strong interaction between the aggregated load and DFIG may cause the closed-loop mode damping to be negative.If the open-loop aggregated load oscillation mode and the open-loop electromechanical oscillation mode are under the condition of open-loop mode resonance,the strong interaction between the aggregated load and the power system may repel the closed-loop electromechanical oscillation mode to the right side of the imginary axis,thereby causing power angle instability of the power system.