Steady Stability Analysis On Sending End Of Energy Base HVDC Transmission System With Local Grid

The characteristics of long-distance,large-capacity and low-loss of HVDC technology can well alleviate the contradiction between the reverse distribution of electric energy production and consumption.They are widely used in such projects as large-scale hydropower and thermal power bases,interconnection of regional power grids,and so on.Although DC transmission effectively isolates two AC systems,there is no inherent stability problem.However,the power characteristics of DC systems will have a profound impact on the AC systems to which they are connected.Moreover,the normal operation of DC systems depends on the support of AC systems.The limits of capability,the level of static stability and the level of transient stability are all constrained by the strength of the AC system.Due to the interaction between AC and DC,the system faces many problems in terms of security and stability.With the actual construction of DC,there are a large number of sending end systems consisting of energy bases and rectifying converter stations,which can not make contact with the AC main network and often need to be operated in an isolated island or connected to a weaker local power grid,Its grid structure is weak,outstanding stability problems.However,in the past,the strength of DC system was weaker and the operating conditions of DC inverter side converter were more severe.In the past,the research on the stability of AC-DC system mainly focused on the inverter side and the rectifier side research is less.In this paper,the static stability problem of DC power delivery system considering local power grids is analyzed.First,a quasi-steady state model of DC power delivery system considering local power grids is constructed according to the simplified equivalent method of DC system construction and AC system.Correspondingly,Based on this,the operating characteristics of the DC sending system are analyzed.It is pointed out that the static stability of the system depends on the relationship between the operating point and the limit of transmission power.According to the static stability reserve coefficient and DC modulation margin index The influence of different short-circuit ratio on the static stability of DC delivery system is analyzed.Then the critical short-circuit ratio index of DC power delivery system in local power grid is proposed and its analytical formula is derived.Based on the analytic formula of short-circuit ratio index and critical short-circuit ratio index,the influence mechanism of local power grid on direct current sending system of energy base is analyzed.The results show that the type of local grid,the relative scale of energy base and the load composition will affect the delivery The static stability of the system and the static voltage stability index calculation and timing simulation analysis in BPA further verily the validity of the above analysis.