Study Of Dynamic Interaction Analysis In Weak Sending-end Power System With High Proportion Power Electronics Apparatus

With the national strategic demands gradually promoted on highly developing renewable energies,extending the scope of resource allocation optimization,saving the energy of the loads and improving its efficiency,it is an inevitable trend for power system to be dominated by power electronics.In particularly,for those power systems weakly-supported sending-end system connected with large-scale and centralized renewable energy,the dynamic problems in multiple timescale are being highlighted,especially the oscillations in DC voltage control(DVC)timescale.However,it still stays in a fledging period for the studies on the dynamic problems in weak power systems with multi-type power electronics apparatus.Though some works have been done aiming at the dynamic problems of specific two-or three-end systems in AC current control timescale,their conclusion are of limited application scope.Besides,for the previous analyzing approaches,the physical mechanisms behind the stability of multiple apparatus' interactions could not be revealed clearly.Hence,this paper focuses on analyzing the coupling interaction of multiple apparatus in weakly sending-end system in DVC timescale.The research contains two aspects: on one hand,from the complex problem in practical weakly sending-end system,this paper analyzes the interactions among multiple apparatus and controllers in DVC timescale,explores the laws of the dynamics in the systems and influence factors,and explains the instability mechanism.On the other hand,based on the existing stability analysis methods,this paper further presents a generic approach on analyzing the interactions in the power system with multiple apparatus.The concrete contents are as follows:(1)The basic research approach is illustrated.For large-scale system analysis,a unified modeling method based on amplitude/phase motion equation,a thinking for analyzing interactions based on self-stabilizing and en-stabilizing,and an optimizing idea for multi-type apparatus are introduced.For multi-type apparatus,unified amplitude/phase motion equation models,which with uniform input/output states and inner structural features,are built after remaining the corresponding pivotal energy storage elements' states.Based on these,the thinking for depicting the interactions by self-stabilizing and en-stabilizing is illustrated in detail.And under the concept of en-stabilizing,the analytical approach for the multi-controller precision reintegration and multi-apparatus series expansion approximation are systematically explained.(2)The uniform linearized motion equation models for synchronous generator(SG),voltage-source converter(VSC)and line-commutated converter high voltage direct current(LCC HVDC)system in DVC timescale.Firstly,the motion equation of SG is developed to intuitively understanding the rotor speed variation.Then,considering the different control strategies of VSC,linearized motion equation models with uniform input/output states and inner structural features are built.The characteristics of VSC with different control strategies are quantitatively compared.In the same way,the uniform linearized motion equation model of LCC HVDC system is developed.It illustrates that the similarities and differences between VSC-based apparatus and LCC-based apparatus in terms of the forms and essences.Finally,the proposed motion equation models in DVC timescale are verified by time-domain simulation,which lay groundworks to the following analysis.(3)A basic thinking for analyzing the interactions among multiple controllers,(namely the multi-controller precision reintegration)is proposed.Based on this,the laws for the influences of interaction between VSC and SG and the interaction between VSC and LCC on the stability of connected weak grid are summarized.Firstly,the thinking of multi-controller precision reintegration is introduced.And its advantages for the system with several apparatus are illustrated.Then,based on the concepts of self-stabilizing and en-stabilizing,the factors' influence of VSC on the sub-synchronous oscillations on SG's shafting are discussed.Next,the en-stabilizing transform function from VSC to SG is reintegrated.The en-stabilizing influences when single controller acting and multiple controllers interacting are extracted and interpreted.Finally,the interaction between VSC and LCC HVDC is analyzed under the concept of self-/en-stabilizing.It is pointed out that the oscillation frequencies of system will change in a wide range with the grid strength,controller parameters,power levels and so on,which is an essential distinguished from the traditional power system.(4)A series expansion approximation method is proposed and applied in analyzing the interactions among multiple apparatus in typical weak sending-end system.Firstly,under the concept of en-stabilizing,the definition of the series expansion approximation method and its basic properties are introduced.A generic method for constructing the series expansion approximation of multiple apparatus interaction under the certain conditions is proposed.Then,the definition and concept of series expansion approximation of multiple apparatus interaction are explained with concrete forms.And the mathematical meaning of approximately expansion form is intuitively explained by approaching the original system with physical iterative loop.Finally,the series expansion approximation method is applied in analyzing the interaction in a typical system with multi-type apparatus.(5)An additional frequency control strategy is proposed to solve the stability problem when the VSC synchronized phase-locked loop(PLL)is connected into an extremely weak grid.On the bases of this scenario,the stability of VSC and its adaptation in islanded grid are analyzed.A black start scheme of VSC in islanded grid is proposed,whose transient responses under grid faults are also primarily verified.