Research On Influence Mechanism Of DFIGs’ Power Characteristic On Power System Angle Stability

Power angle stability has been one of the most important issues for electrical engineers,which has been researched persistently.With the integration of new energies such as wind power and solar power,power system introduces many power supply types which are not comprised of synchronous machines.The validity of traditional power angle stability method needs to be verified.As one of the most popular kinds of wind turbines,doubly fed induction generator(DFIG)has unique working principle and power characteristic compared with synchronous generator,which brings in more difficulty for power system stability analysis.In order to figure out this issue,the main works of this thesis are as follows:(1)DFIG’s equivalent model applied to analyze power angle stability is discussed.Firstly,the steady state models of wind turbine,transmission shaft,generator,rotor side converter and grid side converter are established.Secondly,the transient characteristic of DFIG is analyzed.According to the detailed steady and transient models,the thesis summarizes DFIG’s power characteristic and establishes DFIG’s Norton equivalent model.(2)Equivalent method of power system with DFIG’s integration is proposed.Firstly,extended equal area criterion(EEAC)is used to simplify the multi-machine power system.Then the equivalence of multi-machine system and equal single machine infinite bus system is proved.The way DFIGs influence power angle stability is analyzed,and the equivalent single machine model with DFIG’s integration is proposed.Finally,the simulation of 10 machines model and single machine model with DFIG’s integration are carried out to prove the above conclusion.(3)Power angle stability of power system with DFIG’s integration is discussed.Dividing the transient state into 2 periods,the thesis analyses DFIG’s power characteristics during fault and after fault.Using DFIG’s equivalent ground admittance model,the thesis establishes admittance matrix of power system.On the basis of the matrix,the influence of DFIG’s active and reactive power on synchronous generators’ transient stability.The simulation of 10 machines model with DFIG’s integration is carried out to prove the above conclusion in the end.(4)In order to improve the transient power angle stability,the thesis proposes new DFIG’s power control method.Firstly,the value ranges of DFIG’s active and reactive power during the fault and after the fault are calculated.According to the value ranges and the conclusion of DFIG’s characteristics,the thesis proposes reference values of DFIG’s active and reactive power during the whole transient process as well as the principle of generator tripping.Simulation is carried out to prove the effectiveness of the new control method in improving rotor angle stability and decreasing the amount of tripped synchronous generators.