Research On External Network Static Equivalent Method And Its Application In State Estimation

Under the environment of the electricity market,due to the technical reasons or the commercial confidentiality requirements of the power market,it is impossible to realize the real-time synchronous sharing of the complete power flow basic data between the subsystems of the interconnected power grid.When the external network equivalent parameters are calculated only based on the real-time measurement information at the inner network and the border nodes,the existing gray-box equivalent methods require the known external network typical operating mode data,and assume that there is a similar or consistent size relationship between the current state and typical mode equivalent parameters.Considering that the relationship between the two in practice is uncertain,even not at all to meet the gray-box equivalent constraint requirements,then great equivalent errors may be produced.Therefore,the black-box static equivalence of external network is the key technology to ensure the accurate security analysis of independent power flow for interconnected subnet.The existing black-box equivalent method generally has serious problems with unstable parameter estimation and low equivalence accuracy.For this point,the main research work of this paper are as follows:(1)A static equivalent method of black-box external network based on the improved branch-outage simulation DC method is proposed.Firstly,the external network is equivalent to simplified Ward model for the common two-port interconnected grid.Then the quadratic term of the network susceptance increment multiplied by the voltage phase angle increment is added to the original incremental power flow model of the branch-outage simulation DC method,which is used to construct the linear relationship between the equivalent transfer susceptance and the voltage phase angle increments at boundary-associated nodes.And based on the linear relationship,the transfer susceptance can be efficiently acquired.Finally,the IEEE 39-bus two-port network is used to test the proposed method.The correctness and effectiveness of the proposed method are verified by the analyses of test results.(2)A static equivalent method of black-box external network based on the branch-outage simulation sensitivity method is proposed.Firstly,the external network is equivalent to simplified Ward model.Then the linear relationship is developed between equivalent transfer admittance and the voltage amplitude and the phase Angle increments at boundary-associated nodes using the sensitivity of branch-outage power flow.And based on the linear relationship,the transfer conductance and susceptance parameters can be efficiently acquired by least square estimation method.This method can be applied to the equivalent calculation of two-port and three-port networks,and has a wider scope of application.Finally,the IEEE 39-bus two-port and three-port networks,as well as Guangdong-Hong Kong two-port network,are used to test the proposed method.Comparing the two new methods,it is verified that the second new method has higher equivalent precision and better equivalence effect.(3)Two kinds of state estimation methods,which consider the modified extended Ward equivalent of the external network,are proposed.One is the state estimation method of considering the extended Ward equivalent of the external network without the need for external network to provide the state information.When the outer network is equal to the extended Ward model,and if the state of the inner network is estimated,the outer network measurement information only includes the voltage amplitude and active output of the equivalent generators of the outer network.The voltage amplitude is equal to the voltage amplitude of the boundary node and the active output is equal to zero.The other is the state estimation method of considering the extended voltage-source-branch Ward-equivalent-circuit model of the external network.The method is based on the existing extended voltage-source-branch Ward-equivalent-circuit model,and presents the calculation method of equivalent impedance parameters and state parameters.Based on this model,it is not necessary to provide any status information of external network when performing intranet state estimation.The two proposed modified methods both avoid the mismatch problem of the state between the external network and inner network.Finally,the IEEE 39-bus two-port network is used to test the proposed method.The effectiveness of the proposed method in improving the accuracy of the independent state estimation of the internal network with an external network equivalent model is verified by the analyses of test results.And through the practical comparison of the two proposed methods,it is verified that the second method is more convenient for splicing with the existing state estimation program of the actual system.