Research On Transient Stability Of Power System Integrated With PMSGs

Wind turbine with direct-driven permanent magnet synchronous generator (PMSG) has high reliability and low maintenance cost. PMSGs have been commonly used in wind power generation.Due to the integration of large scale PMSGs, the synchronous power system will couple with lots of asynchronous power sources possessing power electronic interfaces. The structure and operation characteristics of a PMSG are different from those of a traditional synchronous generator and other types of wind turbine generators. Therefore, new impacts on the power system operation including power system transient stability can be made by the integration of PMSGs. The power system transient stability is an essential factor to measure the security of power system operation. The study on the transient stability of power systems with PMSGs has important practical values. In this paper, the influence mechanism of integrating PMSGs on power system transient stability and the change rules of system transient stability after integrating PMSGs were studied. Based on the research results, some suggestions about the operation of power systems with PMSGs have been proposed. The main research contents are concluded as follows:The complete electromechanical transient model of a PMSG was established. Modeling objects include all the components and controllers of a PMSG. The components of a PMSG contain the braking resistor used for low voltage ride through of a PMSG. The controllers of a PMSG contain the damping controller to damp the self-excited oscillations of wind turbine rotors. The research results can provide models for further studying the transient characteristics of a PMSG.The impact of shafting models on the transient analysis results of a PMSG was studied. When the shafting of a PMSG is simulated by double-mass model and single mass model respectively, the difference in the transient variations of the generator speed of a PMSG was analyzed theoretically based on the principle of energy conservation. Afterwards, the impact of the difference in the generator speed variations on the transient analysis results of a PMSG was studied considering the transient characteristics of a PMSG with and without damping controllers. A selection principle of shafting models, which is appropriate for the transient analysis of a PMSG, was proposed. The theoretical analysis was verified by simulations. The proposed principle will help researcher adopt appropriate shafting models to analyze the transient behaviors of a PMSG. It is beneficial to guarantee the analysis accuracy and is convenient to the analysis process.Based on the accurate analysis of the transient characteristics of a PMSG,the operating status and transient stability mechanism of a power system with PMSGs were studied. The concept description of the transient stability of power systems with PMSGs was proposed. The study on the transient stability of power systems with PMSGs mainly focuses on the grid side transient response of a PMSG and its impact on the rotor angle stability of traditional synchronous generators. Afterwards, the transient responses of the grid side power and current outputs of a PMSG were analyzed in detail. The key factors to determine the grid side response of a PMSG were found out. And the model of a PMSG was simplified accordingly. Different models of a PMSG were compared by simulations. The research results show that the controlled current source model should be preferentially adopted to simulate a PMSG during transient analysis. This conclusion can make the impact analysis of integrating PMSGs become easier.A theoretical analysis method to study the impact of integrating PMSGs was proposed based on the extended equal area criterion. The impact of integrating PMSGs on the transient stability of multi-machine power systems was analyzed by using the proposed method. The situation of integrating PMSGs, which may have the biggest impact on the system transient stability, was considered. Two different multi-machine simulation cases were used to verify the theoretical analysis. Some conclusions have been gotten from the research. In the situation of integrating PMSGs nearby the critical machines, the system transient stability will be degraded when wind power is balanced by reducing the active power output of the remaining machines, but the system transient stability will be improved when wind power is balanced by the critical machines. These conclusions can be references for the researchers to assess the transient stability of power systems with PMSGs. The proposed analysis method offers a novel way to theoretically study the transient stability of power systems with PMSGs and can provide theoretical explanations to the impact of integrating PMSGs.The impact of the reactive power control mode of a PMSG on the system transient stability was studied. The specific influence of integrating PMSGs with reactive power support control mode in the system transient stability was analyzed by using the proposed theoretical analysis method. When PMSGs adopt different reactive power control modes, the system transient stability was also compared theoretically. Some conclusions have been achieved from the research and were verified by simulations. Due to the capacity limitation of converters of a PMSG, the reactive power control mode of a PMSG is not a key factor to determine that whether the impact of integrating PMSGs is positive or negative. But when PMSGs adopt reactive power support control mode, the system transient stability will be better compared with the situation of PMSGs adopting unity power factor control mode. In the end, the influence mechanism of integrating PMSGs in the transient stability of multi-machine power systems and the main factors to determine the impact of integrating PMSGs were concluded based on the research contents of the whole paper. Some suggestions about the operation of power systems with PMSGs were proposed accordingly to fully play the positive impact of integrating PMSGs on the system transient stability. The research results of this paper will have beneficial effects on the security and stability of power systems integrated with lots of PMSGs.