Transient Stability Analysis Of Power System Integrated With DD-PMSG Wind Generators Based On Averaged Converter Model

The stability of the power system is prominently affected by the grid-connected wind power generators which are employed to replace the traditional thermal plants.The direct drive wind generators have received a lot of attention in bulk wind power generation due to the superior grid compatibility.In this paper,the power system model with direct drive wind power generator is constructed.The transient stability of the system is studied by the time domain simulation method and the energy function method.First,the mathematical model of the power system is established,including the wind turbine,the permanent magnet synchronous generator and the averaged converter model.Accordoing to wind velocity,the maximum power point tracking control and constant power control are combined in the control strategy,which is verified on a single machine system.Second,based on the energy function for the structure-preserving model and the concept of port energy,an energy function for power system with the direct drive wind power generator is proposed.The wind generator is represented by the equivalent dynamic load at the grid-connection bus and the potential energy due to wind power is deacribed by the load voltage and power,thus the inner detailes of the generator is ignored in energy function construction.The procedure of the potential energy boundary surface method is clarified in detail.Then,the multi-machine power system model with direct drive wind generator is established,including a three-machine nine-bus system model and a ten-machine 39-bus system model.The effectiveness of the control strategy is further verified on these systems under the wind speed disturbances.Finally,transient stability of the power system under the grid fault is studied by the time-domain simulation method and the potential energy boundary surface method.(i)By observing the swing curves,the critical clearing time of the fault is obtained by the time domain simulation method.(ii)By using the energy function proposed in this paper,the energy during and after the fault is calculated,and the monotonically decreasing property of the energy function is verified,i.e.the qualification of the proposed energy function is verified.(iii)After the system energy function is validated,the critical clearing time of the fault is obtained by the potential energy boundary surface method,which takes the maximal potential energy on the persistent fault-on trajectory as the critical energy.(iv)Taking the critical clearing time obtained by the time domain simulation method as the reference,the effefectiveness of the energy function method is analyzed.In the model and the contingencies studied in this paper,the potential energy boundary surface method may be conservative or aggressive,and more conservative results are obtained than the aggressive ones.(v)The transient stability under different permeability is also studied by comparison.At the grid-connection point,the synchronous machine connenected in parallel with the wind generator is represented by two types of models: the one with reduced mechanical power(Model I),and the one with modified equivalent synchronous generator parameters(Model II).For these two systems under grid fault,when Model I is used,the transient stability is improved with increasing permeability.However,when Model II is used,the transient stability is reduced with higher permeability.