| Based on the unequal distribution of energy resources and load demand in China,the increasingly complex basic national conditions of energy types and the outstanding advantages of DC transmission technology,AC/DC power grids show strong construction requirements.With the rapid development of AC/DC power grids,new characteristics such as the coupling between AC and DC power grids,the control characteristics of converters,and the increase in energy types have emerged.Power flow calculation is one of the basic methods for power grid analysis and control.Because of the above-mentioned characteristics,the power flow analysis of AC/DC power grids is complicated to solve,and the degree of nonlinearity is high,which makes it difficult to quickly estimate the state and optimize the analysis.Therefore,this paper studies the fast power flow algorithm and its uncertain power flow analysis of two types of AC/DC power grids,namely flexible AC and DC power grids and traditional DC power transmission systems,as follows.First,the basic structure and steady-state characteristics of VSC and LCC converters are outlined.By re-dividing the limits of AC and DC subsystems and the selection of state variables in flexible AC/DC power grids,the AC and DC subsystems are decoupled and multiple iterations of power flow solving are avoided.The decoupling of the traditional DC transmission system mainly uses the voltage regulation function of the converter transformer.Assuming that the no-load DC voltage is constant and the control angle of the converter is in the rated state,the AC and DC subsystems are decoupled.Therefore,power flow decoupling algorithms are established for the two AC/DC power grid forms mentioned above,which have good convergence performance and a concise model,which lays the foundation for subsequent linearized power flow analysis.Secondly,a linear power flow algorithm for AC/DC power grid is proposed,which takes into account the voltage amplitude-reactive power relationship.The non-linear analysis of the power flow model of the flexible AC/DC power grid and the traditional DC power transmission system is carried out,and a linearization method of the power flow model is established based on the operation characteristics of the power grid.A qualitative analysis of the errors with other linearization methods is performed,which proves the accuracy of the linearized power flow algorithm.At the same time,multiple case studies are used to verify the fastness and high accuracy of the results.This linear power flow model provides the basis for fast state estimation and optimization analysis of flexible AC and DC power grids and traditional DC transmission systems.Finally,based on the linearized power flow algorithm of AC/DC power grids established in this paper,an affine algorithm was used to complete the fast calculation of AC/DC power grid uncertain power flows.The linear power flow model avoids non-affine operations such as trigonometric functions,thereby solving the problem of error term accumulation caused by the non-linear operation of the affine algorithm,which makes the conservative interval of the estimated value of the uncertain power flow result smaller,that is,closer to the true distribution.Secondly,the affine algorithm is applied to power flow calculation.The affine number and noise element can be used for traceability to define the influence index of uncertain input variables,so as to quantify the influence of uncertain input variables on the state of the system.This will allow staff to quickly perform system security analysis.This AC/DC power grid uncertainty flow method not only absorbs the advantages of fast and accurate linearized power flow model,but also avoids the disadvantages of the affine number algorithm in the non-linear operation.It can provide the staff with fast state interval estimation,which has high practical value. |
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