| In China,energy bases and load centers are distributed in reverse.In order to meet the urgent needs of energy transmission and power supply of load centers,the conventional HVDC transmission technology which is suitable for long-distance and large-capacity transmission has been vigorously developed.Conventional HVDC converter station is a typical power electronic conversion equipment,and its characteristics which is different from synchronous machines have brought many new dynamic problems to the safe and stable operation of AC/DC systems,such as small-signal stability problems of conventional HVDC systems,the dynamic mechanism of the voltage amplitude of the receiving-end power grid,the controlled commutation failures of the inverter station in the dynamic process of power system(commutation failures caused by the change of trigger pulse time after control action),and so on.In terms of the small-signal stability of conventional HVDC systems,the current methods can judge the small-signal stability of the system,but there are deficiencies in understanding the mechanism of small-signal stability.In terms of the dynamic mechanism of the voltage amplitude in the receiving-end power grid,the current research work has established the relationship between reactive power and voltage amplitude,but the physical components behind the voltage amplitude and their driving forces are insufficiently understood.Regarding the controlled commutation failures of the inverter station in the dynamic process of the system,the current research work has focused on the commutation failure mechanism,influencing factor analysis and suppression measures,and has made progress.However,it is necessary to deeply understand the bottom layer control of the converter station,that is,pulse triggering.principle,to study(multiple)controlled commutation failures.To this end,based on the understanding of the dynamic process of the power system with conventional HVDC transmission and the method of motion equation modeling,a converter station excitationresponse relationship model is established to study the dynamic problems related to conventional HVDC transmission listed above.The specific content is as follows:(1)Based on the understanding of the dynamic process of the power system and the method of modeling the motion equation,the small-signal power excitation-internal current response relationship model of the conventional HVDC converter station is established.First,it is pointed out that the output of the converter station is a current rotation vector with a certain amplitude,angular frequency and phase in the steady state and dynamic process(without commutation failure).Then,based on the modeling method of motion equation,by comprehensively considering the control in the converter control layer and commutation process of the rectifier station/inverter station,the small-signal power excitation-internal current response relationship of the rectifier station/inverter station is modeled.Finally,by comparing the dynamic response of the proposed small-signal power excitation-internal current response model with the detailed time-domain simulation model after the system is disturbed,it is verified that the built model can be applied to small-signal dynamic analysis of the conventional HVDC system in the converter control layer.(2)Based on the established small-signal power excitation-internal current response relationship model of converter station,different factors(DC current filter parameters,DC current control parameters,phase-locked loop parameters,DC voltage control parameters)which affect the system's small-signal oscillation are analyzed in the single converter station grid-connected system,the two-end HVDC transmission system and the two-infeed HVDC transmission system.In addition,by imposing disturbances on the sending-end power grid and observing the changes in the output power of the receiving-end inverter station,it shows that the disturbance will propagate between the sending-receiving power grids through conventional HVDC transmission.(3)Based on the understanding of the dynamic process of the power system and the method of modeling the motion equation,a conventional HVDC transmission inverter station power excitation-terminal voltage response relationship model for the understanding of the voltage amplitude dynamic mechanism in the receiving end power grid is established.The error of the command power and the feedback power is used as the excitation,and the terminal voltage amplitude and phase are used as the response,including three sets of power balances.The model reflects the command and feedback reactive power,the active power delivered by the inverter station and the active power absorbed by the grid are balanced at all times.In the dynamic process of the system,the inverter station(under the extinction angle control mode)exists the positive feedback mechanism between voltage amplitude and reactive power which is not conducive to the stability of system voltage amplitude,it has been verified by simulation.(4)Based on the understanding of equipment and power system dynamic process,the voltage amplitude-frequency excitation/current amplitude-frequency response relationship model of inverter station is established,and the controlled commutation failures problem of inverter station in the dynamic process of power system is studied.First,it points out the necessity of studying the controlled commutation failures of the inverter station from the dynamic process of the power system,and analyzes the mechanism of the successful commutation/controlled commutation failure of the inverter station from the perspective of supply and demand balance.The established voltage amplitude-frequency excitation/current amplitude-frequency response relationship model of inverter station reflects the influence of commutation related quantities on the commutation process.Based on this model,it analyzes the influence of the phase-locked loop(PLL)dynamics on the commutation process of the inverter station under the strong power grid at the receiving end,and proposes to intuitively understand the influence of PLL dynamics on the commutation process from the perspective of commutation voltage rotation vector and PLL rotation vector motion.In addition,the corresponding suppression measures are proposed based on the idea of compensation and their effectiveness is verified by simulation.A new problem of controlled commutation failures under weak power grids at the receiving end is proposed.Different from the case of strong power grids,the commutation voltage frequency is unknown in the dynamic process of the system,and the time interval between pulse trigger time and negative zero crossing point of commutation voltage is also unknown..The influence of the dynamic trigger angle command value of the inverter station on the commutation process is analyzed,and the phenomenon that the small-signal oscillation divergence of the extinction angle control loop causes multiple controlled commutation failures of the inverter station is found. |
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