Research For Stability Analysis Of HVDC System Sending Terminal Connected With Renewable Generation

With the development of human society and science and technology,traditional fossil energy is difficult to meet the needs of continued development,and people are gradually focusing on the development and utilization of new energy sources such as solar energy,wind energy and biomass energy.In China,the “Three North” region has extremely abundant wind resources,and the local load cannot consume these energy sources.For example,economic centers such as the Yangtze River Delta region urgently need energy supply,so large-scale transfer and deployment of electric energy is crucial in China.The advantages of HVDC transmission system due to its low power loss are the main situation of energy deployment in China.The line commutated high voltage direct current transmission system(LCC-HVDC)system using thyristors is widely used in high voltage and ultra high voltage direct current transmission projects at home and abroad due to its good reliability.The end of the HVDC system that inputs power from the outside is the transmitting end,and the end that supplies power to the load is the receiving end.Under normal circumstances,the sending end can be normally merged into the AC grid,but as the large-scale new energy power generation base is also integrated into the AC grid,the equivalent grid impedance becomes larger,the grid strength becomes weaker,and the HVDC system is highly likely to have harmonic instability.A series of problems,even affecting the operation of new energy power generation bases.The general quasi-steady state model can only analyze the small signal disturbance of the HVDC system under steady state conditions,and cannot find the potential resonance point between the HVDC system and the grid impedance.However,the impedance frequency model has a clear physical concept and is often used for frequency analysis.The stability criterion of the grid-connected system can be found by the stability criterion of the control theory.Therefore,it is necessary to study the stability of the grid-connected system from the perspective of impedance analysis.Moreover,when the commutation failure of the HVDC system occurs,the change of the equivalent impedance of the system may cause the originally stable system to be unstable.In response to these problems,this paper has done the following research: 1)The design of HVDC system functions and control methods can significantly affect system impedance changes.This paper first analyzes the similarities and differences of the control methods of the LCC-HVDC system.Then,the control strategy of using the constant current and the minimum firing angle control and the fixed voltage is adopted at the receiving end,and the external characteristic curve equation of the whole system is obtained,which is used to establish the impedance model.2)Most of the LCC-HVDC systems in operation in the world use the 6-pulse converter as the most basic unit.In order to analyze the stability of the grid connection of the HVDC system,this paper firstly establishes the input impedance model of the ideal 6-pulse rectifier according to the switching function theory and Fourier transform,and further corrects the impedance model according to the previously selected control method.The input impedance model of the HVDC system.3)After obtaining the input impedance model of the HVDC system,the stability of the grid connection of the HVDC system is studied by using the Nyquist stability criterion based on the impedance ratio.By using the analysis method,the resonant frequency generated by different sending end control parameters can be accurately and effectively found,and the resonant frequency point can be avoided when designing parameters,and the stability of the grid-connected system can be maintained.4)A short-circuit ground fault in the AC system and the DC system will cause the AC voltage to drop and the DC current to rise,which may cause the inverter to commutate.This paper analyzes the change of the equivalent impedance of the DC system during the single commutation failure.The change of the impedance will further cause the input impedance of the HVDC system to change,which may generate a new resonance point with the grid impedance.5)All models,control methods and analysis methods have been verified by simulation.