State Analysis And Simulation Method Of Integrated Energy Systems

Nowadays,the increase of global demand for energy,the growing shortage for fossil fuels and the deteriorating environment have posed a key challenge to the development of energy sector.In order to relieve the dependence on fossil fuels,improve energy utilization efficiency and reduce the emission of greenhouse gases and other pollutants,the energy industry has raised its awareness about the integration of renewable energy sources,energy co-generation &cascade utilization and the coordinated operation of multiple energy supply systems.Integrated energy systems,as the combination of electric power,natural gas,heating and cooling systems which covers the generation,transmission,distribution,conversion and storage of various energy supply systems,have been receiving more and more attention from both the academia and the industry during the past decades.Compared with the traditional electric power systems,integrated energy systems consist of more types of energy and more interactions between different energy sectors along with various time scales in operation,which is the main challenge in the research of this area.The coupling relationships and the larger number of variables and constraints inside the integrated energy systems make the relevant study with more computation burden and difficulties.Furthermore,the timescales of power,gas and heat networks are different and the dynamics inside the pipelines shall not be neglected,therefore it is demanded to utilize different mathematical modelling according to the specific requirement of the study.Working as the tool for providing the fundamental operation data,the state analysis and simulation method,namely,state estimation,steady-state energy flow analysis and dynamic simulation,should adapt well to the complexity of network and different operation time scales.However,the current research in this field is still expecting more improvement.The current state estimation often utilizes the simplified steadystate model,which often leads to large deviations in state estimation,hindering the operation of energy management systems.The conventional energy flow analysis methods are usually slow in their convergence process and some are quite dependent on the selection of initial point,difficult to perform energy flow analysis of integrated energy system in a stable way.The dynamic simulation in integrated energy systems normally uses differential equations that bring great computation burden and difficulties,where a faster but still accurate computation method is in need.Therefore,this thesis conducts the study on the state analysis and simulation methods of integrated energy systems,and the main contents of this thesis are as follows:(1)A state estimation algorithm of integrated energy systems considering the dynamics of district heating networks is developed,along with a decentralized computation framework based on the asynchronous alternating direction method of multipliers.The proposed algorithm makes advantage of the quasi-dynamic district heating network model and the higher measurement redundancy brought by the joint state estimation to cut down the measurement residuals.In the meantime,the asynchronous computation method can guarantee the data privacy of subsystems in the integrated energy system while maintaining high computation efficiency and fast computation speed.Case study shows that the proposed algorithm is effective and can quickly generate the accurate and comprehensive estimation result of integrated energy systems.(2)A steady-state energy flow analysis method for the integrated energy systems is developed based on the holomorphic embedding method.This method aims to avoid the computation disadvantages of commonly-used methods,such as high sensitiveness to the selection of initial point and spending too many iterations to meet the convergence demand,and perform a fast and accurate energy flow computation by reconstructing the energy flow equations with holomorphic functions.Case study shows that the proposed method can generate the same energy flow solution as the Newton Raphson method while conserving low sensitiveness to the initial value and costing less iterations.(3)A dynamic simulation method for the transient process of natural gas networks is developed,based on which a dynamic energy flow analysis method for the integrated gas and electricity systems is proposed.This method first constructs a set of algebraic differential equations to depict the dynamics of gas flow inside the pipelines,guaranteeing the accurate calculation of system’s operating states.Then the energy flow equations are further constructed with the time-embedded holomorphic functions that reflect the time-varying state variables and solved to obtain the varying operating states.The proposed method can easily acquire the values of state variables at any time during the simulation period,showing higher flexibility and efficiency in computation performance.Case study shows that the proposed method can effectively generate a continuous-time profiling of the transient process towards the new steady state after a disturbance occurs.The method’s computation errors are within reasonable range and this method is more efficient than the conventional dynamic simulation tools.