| With the rapid development of ultra-high direct current(UHVDC)transmission project,the insufficient dynamic reactive power,poor voltage stability and low inertia moment problems are becoming more and more prominent.As a kind of dynamic reactive power device,the new type synchronous condenser(SC)developed in China has the advantages of large capacity,strong instantaneous reactive power support ability,fast transient reactive power response and providing inertias,which has a broad application prospect in the UHV AC/DC hybrid system.In this context,this thesis focuses on the configuration of the SC used in receiving end,SC’s loss of excitation protection and the setting value optimization of the AC protection with SC’s connection.The main work is summarized as follows:(1)Aiming at the problem that the configuration method of the SC connected at the receiving end lacks theoretical analysis,for the system transient voltage stability,SC’s configuration methods in single DC receiving end and multi-infeed area are proposed.Firstly,through qualitative analysis of HVDC’s transient reactive power behavior,the adverse effect of its reactive power demand on the system voltage stability is clarified.Then,SC’s reactive power mathematic model under different transient states is established.Based on the transient voltage stability demand of single DC point,a practical calculation method of SC’s capacity configuration in single point is proposed.Moreover,taking the system transient voltage stability and investment cost as the optimization objectives,the SC’s configuration method in multi-infeed area is proposed based on NSGA-II algorithm.(2)Since the sub-transient process of the SC with loss of excitation has been ignored in the existing study,leading to the incomplete conclusion of SC’s loss of excitation effect,the influence of SC’s loss of excitation fault on UHV AC/DC hybrid system is clarified.The reactive power behavior of the loss of excitation SC under system’s different conditions is established and the sub-transient reactive power ability of loss of excitation SC is verified.Based on the actual UHVDC system model,the influence of SC’s loss of excitation on UHVDC’s normal condition,transient low voltage condition at the receiving end and transient over voltage condition at the sending end is analyzed,the working conditions of the system with the different actions of SC’s loss of excitation protection are compared,the results show the sub-transient reactive power of loss of excitation SC is helpful to reduce commutation failure and over voltage level.Finally,based on the conclusion,some suggestions are proposed for SC’s loss of excitation protection.(3)Due to the existing loss of excitation protection’s poor performances in sensitivity and utilization of the advantage of loss of excitation SC,the new loss of excitation protection based on the excitation current difference feature is proposed for SC.Firstly,the deficiency of the existing loss of excitation protection strategy in fault identification and protection action mode is clarified.Then,the difference of the excitation current between the measurable actual value and the calculated value obtained by excitation magnetomotive force under the loss of excitation is put forward as the fault feature.Moreover,the current difference based loss of excitation index and criterion are proposed.With the optimization of the protection’s action,SC’s new loss of excitation protection strategy is further proposed.Finally,based on the actual UHVDC system model,the new protection strategy is verified to be effective in accurate fault identification,reducing commutation failure and decreasing overvoltage level.(4)Since the setting value research of the AC protection with SC’s large-scale access is still absent,the influence of SC’s connection on AC protection is analyzed and the setting value optimization strategy of AC protection is proposed.Firstly,SC’s short circuit feature is illustrated.Then,with the fault’s different locations,SC’s different configurations,capacities and excitation conditions are considered,SC’s influence on the system’s short circuit is studied.Finally,based on the operation principles of distance protection,zero sequence current protection and current differential protection,the impact of SC on the performance of various protections is analyzed,and the corresponding setting optimization strategy is proposed and verified through the East China Power Grid model. |
