A Summary of the Main Contents and Existing Problems in the Study of Multi-Energy Coupling in Energy Internet

Multi-energy flow (MEF) coupling is one of the key features of the energy Internet and integrated energy systems that are different from smart grids. With the increasing coupling of heterogeneous energy flow, the system characteristics of coupling are becoming more and more obvious and more complicated. The modeling, analysis and control methods of traditional single flow systems have not been applied directly. Therefore, it is necessary to study the modeling of multi-energy flow coupling, the power flow analysis, optimization and control method of heterogeneous energy flow, which plays the role of multi-energy flow synergy to avoid the adverse effects of coupling. This paper summarizes the current research situation of energy Internet at home and abroad from the aspects of modeling of multi-energy flow, power flow calculation and optimal dispatching, and analyzes the existing problems in the research of these aspects.

The Static Stability Region of an Integrated Electricity-Gas System Considering Voltage and Gas Pressure

In an integrated electricity-gas system(IEGS),load fluctuations affect not only the voltage in the power system but also the gas pressure in the natural gas system.The static voltage stability region(SVSR)method is a tool for analyzing the overall static voltage stability in a power system.However,in an IEGS,the SVSR boundary may be overly optimistic because the gas pressure may collapse before the voltage collapses.Thus,the SVSR method cannot be directly applied to an IEGS.In this paper,the concept of the SVSR is extended to the IEGS-static stability region(SSR)while considering voltage and gas pressure.First,criteria for static gas pressure stability in a natural gas system are proposed,based on the static voltage stability criteria in a power system.Then,the IEGS-SSR is defined as a set of active power injections that satisfies multi-energy flow(MEF)equations and static voltage and gas pressure stability constraints in the active power injection space of natural gas-fired generator units(NGUs).To determine the IEGSSSR,a continuation MEF(CMEF)method is employed to trace the boundary point in one specific NGU scheduling direction.A multidimensional hyperplane sampling method is also proposed to sample the NGU scheduling directions evenly.The obtained boundary points are further used to form the IEGSSSR in three-dimensional(3D)space via a Delaunay triangulation hypersurface fitting method.Finally,the numerical results of typical case studies are presented to demonstrate that the proposed method can effectively form the IEGS-SSR,providing a tool for IEGS online monitoring and dispatching.

Simulation and Analysis of Cascading Faults in Integrated Heat and Electricity Systems Considering Degradation Characteristics

Cascading faults have been identified as the primary cause of multiple power outages in recent years.With the emergence of integrated energy systems(IES),the conventional approach to analyzing power grid cascading faults is no longer appropriate.A cascading fault analysis method considering multi-energy coupling characteristics is of vital importance.In this study,an innovative analysis method for cascading faults in integrated heat and electricity systems(IHES)is proposed.It considers the degradation characteristics of transmission and energy supply com-ponents in the system to address the impact of component aging on cascading faults.Firstly,degradation models for the current carrying capacity of transmission lines,the water carrying capacity and insulation performance of thermal pipelines,as well as the performance of energy supply equipment during aging,are developed.Secondly,a simulation process for cascading faults in the IHES is proposed.It utilizes an overload-dominated development model to predict the propagation path of cascading faults while also considering network islanding,electric-heating rescheduling,and load shedding.The propagation of cascading faults is reflected in the form of fault chains.Finally,the results of cascading faults under different aging levels are analyzed through numerical examples,thereby verifying the effectiveness and rationality of the proposed model and method.

Fast Decoupled Multi-energy Flow Calculation for Integrated Energy System

In recent years,as a promising option to improve the overall efficiency of energy utilization and absorptive capacity of renewable energies,the integrated energy system(IES)has raised great interest in academies and industries.Multi-energy flow(MF)calculation,which differs from the traditional power flow calculation,plays a basic role in analyzing IES.MF calculation based on Newton-Raphson method has been proposed in literature,but its calculation efficiency is not high.In this paper,a fast decoupled MF(FDMF)calculation method for IES is proposed.Its main idea is to replace the original Jacobian matrix of MF calculation based on Newton-Raphson method with a diagonal and constant Jacobian matrix by the transformation.The simulations demonstrate that the proposed FDMF method can increase the calculation efficiency by at least 4 times with high calculation accuracy.

An integrated multi-energy flow calculation method for electricity-gas-thermal integrated energy systems

The modeling and multi-energy flow calculation of an integrated energy system (IES) are the bases of its operation and planning. This paper establishes the models of various energy sub-systems and the coupling equipment for an electricity-gas-thermal IES, and an integrated multi-energy flow calculation model of the IES is constructed. A simplified calculation method for the compressor model in a natural gas network, one which is not included in a loop and works in constant compression ratio mode, is also proposed based on the concept of model reduction. In addition, a numerical conversion method for dealing with the conflict between nominal value and per unit value in the multi-energy flow calculation of IES is described. A case study is given to verify the correctness and speed of the proposed method, and the electricity-gas-thermal coupling interaction characteristics among sub-systems are studied.

Optimization of Urban Multi-energy Flow Systems Considering Seasonal Peak Shaving of Natural Gas

In order to alleviate the shortage of natural gas supply in winter,relevant policies have been issued to promote the construction of gas peak-shaving and storage facilities.Largescale gas storage can transfer the supply-demand relationship of natural gas in time sequence,which has great potential in improving the economy and reliabillity of urban multi-energy flow systems.Addressing this issue,this paper proposes a mid-and long-term energy optimization method for urban multi-energy flow system that considers seasonal peak shaving of natural gas.First,the energy supply and demand features of different energy subsystems are analyzed.Then,a network model of the electricity-gas-heat multi-energy flow system is established.Considering the time-of-use electricity price mechanism and the seasonal fluctuations of the natural gas price,a mid-and long-term energy optimization model maximizing the annual economic revenue is established.The alternative direction multiplier method with Gaussian back substitution(ADMM-GBS)algorithm is used to solve the optimal dispatch model.Finally,the proposed method is verified by employing the actual data of the demonstration zone in Yangzhong City,China.The simulation results show that the proposed method is effective.

System Modeling and Optimal Dispatching of Multi-energy Microgrid with Energy Storage

The coordinated operation and comprehensive utilization of multi-energy sources require systematic research.A multi-energy microgrid(MEMG)is a coupling system with multiple inputs and outputs.In this paper,a system model based on unified energy flows is proposed to describe the static relationship,and an analogue energy storage model is proposed to represent the time-dependency characteristics of energy transfer processes.Then,the optimal dispatching model of an MEMG is established as a mixed-integer linear programming(MILP)problem using piecewise linear approximation and convex relaxation.Finally,the system model and optimal dispatching method are validated in an MEMG,including district electricity,natural gas and heat supply,and renewable generation.The proposed model and method provide an effective way for the energy flow analysis and optimization of MEMGs.

A Reliability Assessment of an Integrated Energy System Based on Coupling Energy Flow and Thermal Inertia

With the wide application of integrated energy systems(IES),the degree of coupling between different types of energy sources is further strengthened,and the mechanism of fault development tends to be complicated.Therefore,in order to improve the accuracy and practicability of the reliability assessment of IESs,a sequential simulation reliability assessment method considering multi-energy flow and thermal inertia is proposed in this paper.In this method,the IES structure model is constructed with the combined cooling,heating and power(CCHP)unit as the core equipment,combining with the new energy source to realize the comprehensive energy power flow calculation considering any loss.Then,a load reduction optimization model is established,considering the importance of load and operational economy,in the system status analysis.Furthermore,the heat supply reliability index is corrected taking into account the transmission delay characteristics and terminal thermal inertia.A case study based on the 20-node and the analysis of the influence factors demonstrates the validity of the proposed method.

Day-ahead Risk-constrained Stochastic Scheduling of Multi-energy System

As an increasing penetration of renewable energy sources can potentially impact voltage profile and compromise system security,the security continues to be the most critical concern in power system operations.A risk-constrained stochastic scheduling model is proposed to leverage the latent scheduling capacity of a multi-energy system to seek an economic operation solution while maintaining system operation risk level against uncertain renewable generation.Overvoltage risk constraints,as compared to the straightforward voltage boundary limits,are incorporated into the stochastic scheduling model to guarantee the operation security and economics.Linearized AC power flow model is applied to enable overvoltage risk assessment within the coordinated scheduling model.The proposed stochastic scheduling model is tackled via the improved progressive hedging approach with an enhanced relax-round-polish process,which overcomes the convergence issues of the traditional progressive hedging in handling nonconvex stochastic scheduling model with binary variables on both stages.Numerical simulation results of IEEE 30-bus system and IEEE 118-bus system illustrate the efficacy of the proposed model in ensuring voltage security and improving economic operation of systems.

Comprehensive Optimization-based Techno-economic Assessment of Hybrid Renewable Electricity-hydrogen Virtual Power Plants

Hydrogen is being considered as an important option to contribute to energy system decarbonization.However,currently its production from renewables is expensive compared with the methods that utilize fossil fuels.This paper proposes a comprehensive optimization-based techno-economic assessment of a hybrid renewable electricity-hydrogen virtual power plant(VPP)that boosts its business case by co-optimizing across multiple markets and contractual services to maximize its profits and eventually deliver hydrogen at a lower net cost.Additionally,multiple possible investment options are considered.Case studies of VPP placement in a renewable-rich,congested area of the Australian network and based on real market data and relevant sensitivities show that multi-market participation can significantly boost the business case for cleaner hydrogen.This highlights the importance of value stacking for driving down the cost of cleaner hydrogen.Due to the participation in multiple markets,all VPP configurations considered are found to be economically viable for a hydrogen price of 3 AUD$/kg(2.25 USD$/kg),which has been identified as a threshold value for Australia to export hydrogen at a competitive price.Additionally,if the high price volatility that has been seen in gas prices in 2022(and by extension electricity prices)continues,the flexibility of hybrid VPPs will further improve their business cases.