Research On External Network Static Equivalent Based On Sensitivity Consistency

Because of the large scale of interconnected networks, the voltage-grading anddistrict-dividing feature of grid operation&management and the outdated developmentof grid data unified platform, it’s hard to achieve integrated analysis anddecision-making in the large-scale interconnected networks, which brings severelypotential security issues for power system operation. The static equivalent theory ofexternal networks is working as a key technology that guarantees the efficiency ofsteady-state analysis and decision-making for the independent sub-network.In case of the internal network remains the same state, important information andinfluence factors of external network can be covered by power flow states, but when thestate of internal network changes, the sensitivity information indicating the state, powerflow transformation among the border buses and the comprehensiveness of equivalentcomponent type should come into consideration. But the existing equivalent methodsignore the consistency of sensitivities between variables before and after equivalence.Therefore, focus on this point, this paper deal with the problem in the following threeaspects:①Based on the most widely used three kinds of Ward equivalent methods, analysiswhether they are able to ensure the consistency of the sensitivity before and after theequivalent both in theory and simulation.Firstly, the sensitivity expressions ofnon-generator node voltages with regard to generator node voltages and non-generatornode voltages with regard to non-generator injected currents are derived based on nodalvoltage equations. Then, the prerequisite under which Ward equivalent satisfies thesensitivity consistency is acquired by comparing the developed sensitivity expressions.Finally, the sensitivity error of the three kinds of Ward equivalent methods is discussedin IEEE14-bus system, IEEE39-bus system and an actual power system with661buses.We can find the sensitivity error is so large.②A new method on static external equivalent network which considering theconsistency of the sensitivity before and after equivalence is proposed: Thecomputational formulas of the equivalent parameters are derived based on theconsistency of power flow states, power flow transformation among border buses andsensitivities between variables before and after equivalence. Finally, the static securityanalysis of IEEE14-bus system, IEEE39-bus system and actual power system with661 buses shows the accuracy and effectiveness of the proposed method.③A new method on static external equivalent network which considering theconsistency of the sensitivity and comprehensiveness of component types is proposed:This method by keeping the generator branch, the line and transformer branch,grounding branch to retain the corresponding characteristic of different elements ofexternal network, thus further improving the accuracy of equivalents. Thecomputational formulas of the equivalent parameters are derived based on theconsistency of power flow states, power flow transformation among border buses andsensitivities between variables before and after equivalence. Finally, the static securityanalysis of the two new methods using the IEEE39system whose grounding branch hasbigger influence on the internal network shows the correctness and effectiveness of thesecond new method.