Transient Analysis Of Weak Synchronous Sending End Power Grid After DC Blocking

With the operation of 17 ultra high voltage direct current(UHVDC)projects in China's “Eight AC and nine DC lines”,large-scale high voltage direct current(HVDC)cross-region transmission and AC/DC hybrid transmission make the dynamic characteristics of power system more complex.The “strong DC and weak AC” grid structure has been formed in some areas.However,the weak synchronous power grids have poor toughness,low stability,and many uncertainties.Failure of the AC system may cause continuous failure of the DC system.In order to reduce the impact of continuous power fluctuations in the system,DC blocking of the system with continuous commutation failure is required.But the power redundancy of the sending end and the lack of power at the receiving end caused by DC blocking will threaten the stability of the power grid.So far many scholars at home and abroad have made mature research on transient stability control technology in strong power grids with traditional power generation modes.It is a lack of understanding and mechanism of the AC-DC hybrid interaction in weak synchronous grids composed of renewable energy,which are widely used a large number of power electronic equipment and transmitted by DC.Therefore,this paper focuses on the dynamic characteristics of weak synchronous sending end power grid after DC blocking.Firstly,the inverter commutation process of DC system is introduced in detail,and the reasons for the commutation failure are illustrated.At the same time,the DC block process caused by continuous commutation failure is analyzed.In order to reduce the influence of DC system power redundancy on the weak synchronous grid at the sending end during DC blocking,taking two consecutive commutation failures as the criterion,a suitable DC block is selected from the four different strategies commonly used in engineering.Secondly,the theoretical analysis of the short-circuit ratio of the weak synchronous grid and the low inertia time constant of the generator is carried out,and the transient characteristics of the weak synchronous grid after DC block is analyzed.To solve the problem that many nodes in large-scale AC-DC hybrid system and complex structure with variable power electronic components in DC system,the reasons for the difference in simulation accuracy of DC system between the electro-mechanical transient model and the electromagnetic transient model are analyzed,when the rectifier station uses constant current control and the inverter station adopts the constant fixed-off angle/constant voltage control strategy in HVDC system.Considering the precision,simulation and efficiency of simulation,an electromechanical-electromagnetic hybrid simulation modeling method suitable for AC/DC large power grid analysis is selected.It is composed of electromechanical model for AC system and electromagnetic model for DC system with a large number of power electronic devices.Combined with the characteristics of weak synchronous grid,the transient behavior of the weak synchronous grid after DC blocking is simulated.The factors affecting the stability of the weak synchronous grid are summarized.It will provide a more efficient and accurate simulation modern for AC/DC hybrid systems with a large number of power electronic devices,and provide reference for the study of the instability mechanism of weak synchronous networks.