Research On Improvement Of External Network Equivalent Based On Sensitivity Consistency

In the large scale analysis of interconnected networks, static equivalent technology has become an important analysis tool for its advantage of reducing the scale of analysis and calculation and protecting trade secrets. In order to guarantee the accuracy of equivalent method, all information of external network which relates to the future study should be kept during equivalent. Compared to the equivalent method which can only keep the information of power flow state, the existing equivalent method which can consider the consistency of the powser flow state and sensitivity before and after equivalence has effectively improve the accuracy of equivalent. However, this external network equivalent based on sensitivity consistency cannot be used in the system which the border node is PV node. Also, the equivalent of ground branches in that method is not reasonable enough. Besides, that equivalent method does not keep the information of external limits. Focus on the above deficiency of the existing external network equivalent based on sensitivity consistency, the following improvement is proposed in this paper.(1) An improved method of the existing external network equivalent based on sensitivity consistency is proposed by pushing it suitable for the system which the border node is PV node. Considering the border node is PV node, two types of sensitivity of generator node injected currents are kept consistent before and after equivalent in the paper. Then the equivalent parameters are derived based on the consistent of power flow states and sensitivities before and after equivalence. Combining with the above improved method in this paper and the existing external network equivalent based on sensitivity consistency, a new equivalent method of wind farm is also proposed which can suitable for the system of different border node types and can keep the consistent of power flow states and sensitivities. Besides, weighted sensitivity method was used to correct the equivalent grounding branch in the new equivalent method of wind farm. Finally, the static security analysis of system which is composed of a wind farm and IEEE14-bus system shows the accuracy and effectiveness of the proposed improved method and wind farm equivalent method.(2) An improved method of the existing external network equivalent based on sensitivity consistency is proposed by considering the limits of external network. This method obtains the equivalent network by the existing external network equivalent method based on sensitivity consistency. Then the available capability which keeps the information of various limits of original external network, including power balance equations, voltage limits, generation power limits and line power limits, is calculated by optimization model. And the power limit of equivalent network can be derived by the consistent of available capability before and after equivalent. Besides, based on the above improved method, a new equivalent optimal power flow model of interconnected network is also proposed. Finally, the optimal power flow analysis of IEEE39-bus system and actual power system with 661 buses shows the accuracy and effectiveness of the proposed improved method and optimal power flow model.