Research On Energy Flow Calculation Used The Unified Back/Forward Sweep Algorithm For Urban Integrated Energy Systems

Integrated energy systems(IES)is the carrier of the energy internet,which is of great significance for improving energy efficiency and achieving sustainable development.Energy flow calculation is one of the basic algorithms in IES analysis theory.The existing IES energy flow calculation model is relatively complex and the decoupled energy flow algorithms are not uniform.It is urgent to select a simplified IES model and unify the energy flow algorithm to improve the efficiency of IES energy flow analysis.Taking into account the core position of the power grid in IES,the back/forward sweep method commonly used in distribution networks is applied to the IES unified energy flow calculation.However,the traditional algorithm can’t effectively deal with the multi-PV node problem in modern power grid,and can’t be directly applied to the energy flow calculation of other heterogeneous energy grid.In this paper,a unified energy flow algorithm used back/forward sweep method for urban IES is proposed.The main research work is as follows:1)The steady-state and dynamic models of electric power,thermal power and natural gas network in urban integrated energy network are built.Considering the difference of transient time scale of energy flow in different networks,AC power flow steady-state model is adopted;Based on the steady-state model,the time delay caused by slow time scale is considered;The static model of gas network is suitable for the pressure of gas distribution network,while the dynamic model uses partial differential equation to describe the dynamic process.Combined with the coupling link,the whole model of urban IES is built,which lays the foundation for the unified method of energy flow calculation.2)A unified energy flow algorithm used back/forward sweep method for urban IES is proposed.In power flow calculation,the improved influence factor matrix method and superposition theorem are used to solve the problems of multi PV nodes and closed-loop operation in modern distribution network;In the calculation of thermal energy flow,the decoupling method of supply and return network is adopted,and the back/forward sweep method of thermal system is improved based on the idea of thermoelectric analogy;For the calculation of natural gas energy flow,based on the idea of loop solution and linearized gas flow compensation,the problem that the nonlinear gas network with loop cannot be solved by superposition is effectively solved,and an improved back/forward sweep method suitable for natural gas network is proposed;Based on the coupling link model,the unified energy flow algorithm used back/forward sweep method for urban IES is realized.3)The simulation results show that the proposed method is feasible and applicable.The improved algorithm in power flow calculation solves the problem that traditional back/forward sweep method can’t effectively deal with the problem of multi-PV nodes and ring network.The introduction of least square method not only ensures the accuracy of the algorithm,but also does not increase the calculation burden;The decoupling algorithm in thermal energy flow has good applicability to radiant heating network and converging regenerative network;The compensation airflow method of natural gas system can solve the problem of nonlinear system well,and has obvious advantages compared with gradient method.In the single period energy flow calculation of the comprehensive energy system,the coordination and interaction among the subsystems and the influence on wind power consumption are shown through static characteristics analysis.The multi-period one shows the dynamic change process of the load and coupling element output of each node in the urban integrated energy network.The proposed algorithm not only unifies the energy flow algorithm of urban comprehensive energy network,but also retains the advantages of traditional back/forward sweep method,such as good convergence,low initial value requirement and simple programming.For the solution of nonlinear gas network,the proposed compensation flow method can effectively deal with the loop network problem,and has obvious advantages compared with other algorithms.