Research On Optimal Energy Flow And Optimal Operation Of Electricity-natural Gas Integrated Energy Systems

Recently,due to the shortage of fossil energy and environmental problems,the coordinate operation of electricity,natural gas,and heat energy systems has been deeply studied to improve the efficiency of energy utilization.Researchers have studied the multi-energy integrated systems and established some engineering projects.With the large amount integration of gasfired units(GFUs),power systems and natural gas systems are interconnected closely.It is necessary to establish a unified energy flow model according to the multi-time scales and multiuncertainties,study the impact of information attacks on systems and defense strategies.By studing the static and dynamic energy flow models and optimization problems in the operation stage,provide analytical tools and improve the energy utilization efficiency,ensure the economic operation and information security of the system.The main research works in this paper are summarized as follows:(1)A static OEF model of AC/DC-natural gas integrated energy system and the solution method are proposed.First,the steady state power flow model of AC/DC power systems based on rectangular coordinate system is conduceted.The nonlinear gas flow model is transformend into a convex model by second order cone(SOC)relaxations.The gas supply uncertainty of gas fired units(GFUs)is modeled by Gaussian distribution function.Finally,Benders decomposition approach is utilized to solve the master-sub bi-level problem with uncertainty.The effectiveness of the proposed method is verified by simulation results on a modified IEEE 118-bus test system and Belgian 20-node gas system.Restuls indicate that the proposed convex relaxation gas model could improve the computational efficiency while ensure the accuracy of the solutions.Moreover,simulation results indicate that the limitation of gas supply for GFUs could be mitigated by thermal generators,the generation adjustments are increase with the uncertainty level of gas supplies.(2)A dynamic OEF model for electricity-gas integrated energy system with natural gas inertia and wind power uncertainty is presented.The nonlinear dynamic transmission model of natural gas system is transformed into a linear model by using the reformulation linearization technique(RLT)and modified convex relaxations,respectively.The characteristics of gas inertia during the process of gas dynamic transmission are analyzed.The long response time between gas supply and consumption caused by gas inertia is quantified as a time delay number according to the geographical proterty.Then,the dynamic OEF with wind power uncertainty is transformed into a deterministic optimization problem through information gap decision theory(IGDT).Extensive simulations are carried out on 6-bus power system-6 node gas system and IEEE 39-bus test system-20 node gas system.Simulation results show that the proposed linear gas transmission model could provide good accuracy and computational performances compared with Taylor expansion method and segmental linearization method.In addition,the pipeline gas flow is more stable due to the transmission delay caused by gas inertia.The real gas flows and nodal presusures are quite differernt from the ideal value,which could cause the over limit of nodal pressures.Thus,the gas inertia should be incorporated in the dispatch process.(3)The optimal location-allocation strategy of GFUs in the transmission network of power systems is proposed.A modified loading ability index is proposed and regarded as the objective function of the upper level optimization problem to optimize the location of GFUs.The operational cost and overall power losses is utilized as the objective function of the lower level optimization problem to optimize the installed capacity of GFUs.The overall optimization model mainly consists of steady state operational constraints,loading margin constraints and location-allocation limitations.The original mixed integer nonlinear programming(MINLP)model is replaced by mixed integer second order cone programming(MISOCP)by convex power flow formulations and it is solved by Gurobi.The effectiveness of the proposed method is verified by extensive simulations on IEEE 14,30,57,118-bus test systems.Simulation results indicate that the MISOCP model has higher computational efficiency than MINLP model and it is easier to find feasible solutions.The proposed optimization strategy could effectively improve the maximum loading capability and reduce the active power loss compared with the random placement strategy for GFUs.(4)To mitigate the unwilling results caused by false data injection attacks,an optimal attack-defense strategy for integrated energy system is proposed.First of all,a bi-level load redistribution(LR)attack model for integrated energy system is presented,which could overload multiple power lines and gas pipelines.Then,defenders establish defense strategy by optimically encrypting a part of load measurements.The nonlinear logical constraints are linearized by bigM method and the bi-level problem is transformed into a single level problem by Karush-KuhnTucker(KKT)conditions.Case studies on IEEE 118-bus test system and 14 node gas system show that the proposed defense strategy could avoid the overloading of power branches and gas pipelines caused by the LR attack.