| With the wide application of HVDC in China,the role of LCC(line commutated converter)is becoming increasingly prominent.The mathematical analytical model of LCC is the theoretical basis for analyzing its operation characteristics.The existing electromagnetic transient model uses numerical integration method to solve the switching circuit of power electronic devices,which has high accuracy but is difficult to be applied in high-dimensional complex systems.The quasi steady state model only uses the steady-state electrical quantity,which is fast but can not calculate high-order harmonics;In view of these shortcomings,this thesis uses the switching function method to model the converter switch port,which can accurately and quickly describe the transfer process of harmonics through the converter.At the same time,aiming at the problem that the traditional method can not deal with the three-phase switching asymmetry caused by harmonics,and can not consider the influence of the control system,an improved method is proposed,and an impedance modeling method based on the switching function is further proposed.The research content of this thesis can be divided into the following parts.Firstly,this thesis introduces the control level of conventional HVDC system,and emphasizes the key role of the pole control stage responsible for triggering angle and the valve control stage responsible for triggering pulse.Several control modes of rectifier inverter side in CIGRE standard test model and the mechanism of equal interval triggering trigger pulse based on synchronous phase are studied.On the basis of trigger pulse,the mathematical expression of switching function in steady state is derived.The model is established in PSCAD/EMTDC and compared with electromagnetic transient simulation results,which verifies the accuracy of steady state model and point out the necessity of establishing three-phase asymmetric model.Secondly,the characteristics of phase shift of three-phase voltage under asymmetric condition are analyzed.It is pointed out that there are high contents of negative sequence fundamental wave and third positive and negative sequence harmonic components in grid voltage under asymmetric fault.Based on the symmetrical component method,the formula of phase shift of commutation voltage considering harmonic is given.Because phase shift will change the actual conduction time of valve,the phase shift of commutation voltage will change the actual conduction time of valve,Based on the actual triggering angle,conduction offset angle and commutation angle of the valve,an improved switching function model suitable for asymmetric conditions is established in this paper.Simulation verification is carried out in CIGRE HVDC standard test model and ±500k V Niu Cong DC(double circuit on the same tower)project model.The simulation results show that the proposed method can accurately calculate the harmonic characteristics under asymmetric AC / DC conditions.In addition,the small signal stability criterion of two terminal system is studied by impedance analysis.In order to establish the frequency impedance model of the converter,the switching function in time domain is transformed into frequency domain.The frequency domain model of the output disturbance of the controller and PLL is studied.The theoretical DC impedance model of both sides is established by harmonic linearization method.In PSCAD/EMTDC,the actual DC impedance is obtained by inputting small disturbance harmonic current in DC line and measuring the voltage response of the same frequency.The accuracy of the theoretical impedance is verified by comparing the two results.The influence of changing the parameters of controller and PLL on the stability of small disturbance is analyzed.The simulation results in time domain verify the correctness of the impedance analysis theory based on the impedance model of switching function.Finally,the work of this thesis is summarized,and the further research of switching function modeling is prospected. |
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