| In today's low-carbon,clean and sustainable energy strategy transformation,energy development will gradually replace fossil energy power generation with wind power,solar power and other renewable energy power generation,and energy consumption will continue to promote the transformation of electric energy substitution.As a result,the characteristics of the sources and loads connected to the power system(in a broad sense of the power system)with the power grid as the carrier will undergo tremendous changes.The source-grid-load relationship and interaction will become closer and more complex.At the same time,the traditional power supply-based support and regulation source,and the active power balance(dynamic balance)voltage level method between loads will gradually be replaced by a decentralized and distributed method.With the emergence of power electronic flexible control technology,the ability and characteristics of power grid voltage support and control have undergone essential changes.In this context,the power grid voltage regulation and stability issues will face severe challenges.Studying its static voltage stability assessment theory and application has important theoretical and practical significance.In the wide-area system environment,online voltage stability assessment can quickly obtain the voltage stability status,trend and margin.It is an important means for real-time monitoring and effective regulation of power grid voltage stability.The power balance(active power balance)between the source and load of the traditional power system is relatively controllable,and the change of the power part is slower than the change of the electrical state of the power grid.The change of the active power balance mode has a relatively small impact on the stability of the grid voltage.However,the energy transition has led to complex and fast-changing characteristics of active power balance between source and load.Voltage stability assessment methods that cannot accurately and quickly reflect the impact of changes in active power balance have become increasingly difficult to adapt to the new situation.Therefore,how to make full use of the existing rapid measurement technology to summarize and combine the relevant measurements,parameters,and source-grid-load relationships in the power grid based on the power system operation law and regulation law.Researching and establishing a grid equivalence method that can accurately and quickly reflect the source-grid-load correlation is the key and basis for ensuring the effectiveness of online voltage stability evaluation under the new situation.Thus,in the new situation where the active power balance between source and load is becoming increasingly complex and rapidly changing,this paper presents an in-depth theoretical study of the grid equivalence methods that takes into account the source-grid-load correlation,based on wide-area measurements and oriented towards the assessment of online voltage stability.The widely used black-box method of power grid Thevenin equivalent(difficult to connect with regulation)and the time-varying problem caused by the change of active power balance(resulting in the reduction of equivalent accuracy and accuracy range)are respectively carried out in-depth mechanism analysis and improvement.A Thevenin equivalent method is proposed that considers the source-grid-load correlation and can quickly and accurately reflect the influence of changes in the active power balance mode.Based on the above research,the modal decomposition grid equivalence method,which reflects the overall impact of source-grid-load power and regulation power transmission modes on grid voltage,and the grid reactive-voltage elasticity mapping method,which reflects the grid voltage support topology under active power balance changes,have been proposed respectively.The method is designed to provide accurate,rapid,multi-angle and visualised analysis and assessment of the increasingly complex grid voltage stability situation under the new situation of energy reform,and to provide a basis for decision making in grid operation and regulation.The contributions of this paper are as follows:(1)Aiming at the black-box Thevenin equivalent methods,only the overall equivalent parameters are identified,and it is difficult to know the reasons that affect the parameters and the mechanism that guides the changes of the parameters,which leads to the problem that the voltage stability assessment and the power grid regulation are difficult to connect.According to the nodal voltage equation,the influence of the power system on the Thevenin equivalent node voltage is analytically decomposed into three parts,namely power generation,load and the coupling between the two,so that the source-grid-load correlation characteristics can be reflected.On this basis,according to the characteristics of Thevenin equivalent circuit,the equivalent potential and equivalent impedance are decomposed,and the corresponding Thevenin equivalent circuit that takes into account this correlation characteristic is given,and the Thevenin equivalent based on the compensation method is proposed accordingly.Parameter identification method.This method can accurately identify the Thevenin equivalent parameters of the node,and can also quantitatively analyze the influence of the change characteristics of other nodes on its parameter composition.It is an extension of the Thevenin equivalent from point to surface.Therefore,this research can not only evaluate the static voltage stability,but also provide the basis for the implementation of preventive control strategies.The analysis of the simulation results of the calculation example shows the effectiveness of this method.In the wide-area measurement environment,the research has a good engineering application prospect.(2)The time-varying problem of the Thevenin equivalent circuit of the power grid caused by the change of active power balance has been deeply studied.Based on the circuit principle,it is analyzed and demonstrated that the generator node in the steady-state operation of the actual power grid has the nature of coupling between the voltage source and the controlled current source,so it cannot be directly applied to the conditions of Thevenin theorem.In this regard,based on the principle of superposition,the steady-state circuit of the power grid is decomposed into a ground-state circuit(ie,a reactive power disturbance balance circuit)and an active disturbance balance circuit.The generator node is then decomposed into a voltage source and a controlled current source.Based on the grid node voltage equations and Ohm's law,the Thevenin equivalent analytical models of the two decomposition circuits are respectively given.The superposition of the two forms the Thevenin equivalent of PQ disturbance decomposition.Due to the difference between the regulation and transmission of grid reactive power balance and active power balance,and their influence on node voltage.Compared with the traditional power grid Thevenin equivalent method,the Thevenin equivalent of PQ disturbance decomposition can not only fit the steady-state circuit of the power grid more accurately,but also provide more and more accurate information for static voltage stability analysis and control.The analysis of numerical examples shows the accuracy and effectiveness of this method.Under the condition of active power balance change,the accuracy and accuracy range of the Thevenin equivalent of the power grid can be significantly improved.(3)A modal decomposition grid equivalence method that considers the source-grid-load correlation is proposed.The Thevenin impedance matrices of the base state circuit(reactive disturbance circuit)and the active disturbance balance circuit in the PQ disturbance decomposition Thevenin equivalence model are decomposed separately by eigenvalues,so that two sets of decoupled single-source,single-load,single-branch modal circuits are created.One of the modal circuits of the ground state circuit represents a grid power transfer mode and a reactive power regulation transfer mode.One modal circuit of the active disturbance circuit represents an active power regulation transmission mode.The source and load in the modal circuit represent the generation aggregation and load aggregation in this mode respectively.A reactive-voltage stiffness index and an active-voltage stiffness index are proposed for power transfer modal circuits in reactive and active balance,respectively.The modal decomposition equivalence decomposes the source-grid-load equivalence from the perspective of modal analysis,which can reflect the grid structure and the overall impact characteristics of active-balance regulation and reactive-balance regulation on the grid voltage state.This helps to determine the dominant mode of power transfer affecting the voltage stability of the grid and provides a reference for grid structure assessment and planning.The analysis shows that the dominant mode of the base-state circuit reflects the key modes and characteristics of the grid power transmission,and that the reactive-voltage stiffness index and active-voltage stiffness index reflect the resistance of the grid voltage state to reactive power disturbance and active power disturbance respectively.(4)A voltage support topology resolution method based on the idea of elasticity mapping is proposed to assess the static voltage stability of the grid.In response to the changing active power balance model,the reactive-voltage stiffness index of the line is first established according to the line transmission power characteristics,and then the reactive-voltage elasticity mapping method of the grid is proposed by mapping the geographically distributed flat grid into a spatial longitudinal network distributed according to the reactive-voltage support topology based on the incremental reactive power transmission characteristics of the grid lines,while retaining the complete structural information of the grid.The voltage support topology of the grid can be visualised,making the static voltage stability assessment of the grid simple,intuitive and visual.On the basis of this,the reactive-voltage stiffness index is used to explain the mechanism and criteria of static bifurcation voltage instability in the grid under the reactive-voltage elasticity mapping.The analysis shows that the method is able to clearly,comprehensively and intuitively reflect the voltage support topology of the grid under active power balance changes,and can provide early warning and determination of the risk of static bifurcation voltage instability in the grid. |
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