| Jiangsu Power Grid is one of the provinces with the largest load demand in eastern China.The construction of the Ximeng-Taizhou UHV DC transmission line can not only supply electricity to Jiangsu,but also rationally use Inner Mongolia's clean energy to promote the development of new energy.Since the sending end system is far away from the North China main network,the supporting capacity of the North China main network to the transmitting end power grid is weak.This paper relies on the science and technology project of the State Grid Headquarters "Modeling Research and Development of Electromechanical Transient Simulation Capability of HVDC Transmission Control System",studies the working principle of DC system,the load characteristics of DC system and proposes to optimize the low-voltage current-limit control link to improve The improvement of the stability of the delivery system.According to the electromechanical transient simulation model of Ximeng-Taizhou,the electromagnetic transient simulation model under the real-time digital simulation platform is built,which is helpful for further research work.The main research work of this paper is as follows:The HVDC transmission system has the function of quickly controlling power,which can effectively improve the stability of the AC and DC system.Among them,the converter is one of the important components of the HVDC transmission system,with rectification and inverter functions.This paper first analyzes in detail the working principle of the converter and the commutation process of each valve.Furthermore,the main control links of the DC transmission system such as constant current,fixed gamma angle,constant voltage,minimum firing angle(RAML)and low voltage current limiting(VDCOL)are described.On this basis,the switching process of the transmission and reception control mode is analyzed,and the analytic formula of the DC load characteristic after the voltage change at both ends is derived by the DC quasi-steady-state model theory.Finally,the waveform of the DC load characteristic is plotted using the RTDS script function and compared with the analytically drawn curve.Aiming at the weak delivery characteristics of the Ximeng-Taizhou UHV DC transmission system,the low-voltage current-limit control link of the DC control system is proposed to improve the system stability during the recovery process of the weak-transmission system fault clearing system.The function of traditional low-voltage current limiting control in system restoration is analyzed in detail.The coefficient related to the short-circuit ratio of the sending end and the voltage of the receiving end commutation bus is proposed as the input value of the cut-off voltage in VDCOL,and then the slope of VDCOL varies with the voltage of the receiving end during the transient period.In this paper,the transient energy function of the weak-transmission single-machine infinity-DC is taken as an example to construct the transient energy function,and the effect of optimizing VDCOL on improving the stability of the system is analyzed.Due to the limitations of the electromechanical transient simulation software,this paper combines the actual electrical structure and parameters of the project to build an electromagnetic transient simulation model of the Ximeng-Taizhou UHVDC transmission system.The electrical parameter conversion process and its differences are analyzed in detail,and some details such as the division of the medium value network and the allocation of processor resources in the process of building the simulation model by RTDS are described.According to the initial power flow and short circuit current calculated by PSASP,the RTDS related parameters are adjusted to be within the allowable range of engineering error.Finally,this paper conducts the rated working condition test of electromagnetic transient simulation and compares the transient operating conditions of electromechanical transient and electromagnetic transient simulation model to verify the accuracy of the construction model. |
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