Unified Modeling Of Integrated Energy System In Time Domain: Theory And Its Applications

To alleviate the global energy and environmental crises,the traditional energy system is gradually evolving into a hybrid energy system.In this context,taking the power system(PS)as the core,integrated energy systems(IES)are developed to combine electric power with gas and heat,thereby promoting multi-energy coordination and complementarity,facilitating renewable integration,and achieving the low-carbon transformation of the modern energy system.Compared with traditional energy systems,IES has the following advantages: 1)multi-energy flows can be cogenerated and converted through coupling equipment,which enhances the complementarity among different energy sectors and provides additional adjustable resources for economic and secure operation;2)the linepack of gas systems,as well as the thermal inertia of heating systems and loads,can naturally provide flexibility to the power system.This can promote the renewable integration and help flatten the load curve.3)cogeneration equipment can utilize various types of energy in a cascading manner,improving energy quality and efficiency while also reducing carbon emissions.However,from a physical perspective,the coupling modes of multi-energy networks,as well as the properties of multi-energy flows,are diverse.Moreover,the operating time scales and control methods of each system also differ.From a mathematical perspective,the IES model is described by a set of high-dimensional,nonlinear partial differential-algebraic equations.The diverse physical operating environments and complex mathematical forms present significant challenges for the coordinated operation of IES.Therefore,this thesis focuses on developing efficient and accurate models for IES,which lays the basis to facilitate dynamic simulations,economic dispatch,and security region analysis.These tools will support operational analysis of IES and help address the mentioned challenges.The main contents of this thesis are summarized as follows:(1)Unified modeling theory of electricity-gas-heat integrated energy system in time-domain.Firstly,a unified branch and network model is proposed for multi-energy flows,which considers their differences in physical characteristics,consistency in mathematical representations,and generality in directed energy network topologies.Then,the unified network model is further transformed into a two-port form to achieve dimensional reduction,based on the differences in operating modes of multi-energy networks.Thirdly,a unified per-unit transformation method is introduced to eliminate the dimensional and numerical differences of multi-energy state variables.Finally,according to the similarity of physical properties in coupling equipment,four types of generalized physical processes are constructed.This help to achieve the generalized modeling of different coupling equipment.(2)Dynamic simulation of integrated energy systems based on unified models in time domain.Firstly,the computational scale of the simulation model is reduced by reconstructing it based on the time-domain unified modeling theory,eliminating spatiallydiscrete elements and intermediate nodes in the gas systems(GS)and heating systems(HS).Secondly,two simulation frameworks are established according to different coupling modes in IESs: a decomposition-iterative framework for unidirectionally-coupled IESs and a sequential-united framework for bidirectionally-coupled IESs.Finally,the efficiency,accuracy,and convergence of the GS and HS models are further improved.This is achieved through the development of a variable-coefficient partial differential equation model for GS to ensure convergence and stability,and a fully analytical model for HS to eliminate approximation errors and discrete elements.(3)Economic dispatch of integrated energy systems based on unified models in time domain.Firstly,a general economic dispatch model is established for IESs,taking into account the dynamic characteristics and operational constraints of various coupling equipment and multi-energy networks.Secondly,based on the time-domain unified modeling theory,the dynamic energy flow constraints in the GS and HS models and operational constraints in coupling equipment models are equivalently transformed to reduce the scale of variables and equality constraints in the optimization problem.On this basis,the economic dispatch model of IES is reconstructed into an easy-to-expand form,with maintained accuracy and lower complexity.Its superiorities are finally verified in large-scale systems.(4)Security region analysis of integrated energy systems based on unified models in time domain.Firstly,the time-varying nature of the steady-state security region(SSR)in IES is analyzed,which emphasizes the significance of a novel sequential SSR model for IESs.Then,taking the different time scales of each subsystem into account,the sequential SSR is reformulated as the intersection of a time-varying SSR and a constant SSR.Using the time-domain unified modeling theory,an equivalence-based hyperplane method is designed for formulating the time-varying SSR.Finally,an efficient computational method for the sequential SSR is devised,which involves the offline computations of the constant SSR and online equivalence of the time-varying SSR.