Calculation method of hybrid energy flow in electric-thermal integrated energy system

Abstract

The present invention discloses a method for calculating the mixed energy flow of an electric-thermal integrated energy system, including: obtaining the known state quantity of the mixed energy flow of the electric-thermal integrated energy system, determining the electric and thermal balance nodes of the electric-thermal integrated energy system; establishing a steady-state model of hydraulic and thermal working conditions of the thermal subsystem taking into account the heat loss of the water supply and return pipe network according to the structure of the thermal subsystem; establishing an AC power flow model of the power subsystem according to the power parameters of the power subsystem; establishing a thermoelectric conversion model of a cogeneration unit and constructing a mixed energy flow model of an electric-thermal integrated energy system with the cogeneration unit as a coupling device; setting the initial value of the state quantity to be calculated in the energy flow model of the electric-thermal integrated energy system, and using the Newton-Raphson method to decompose and iterate to solve the mixed energy flow model of the electric-thermal integrated energy system. The method can be used to quickly and accurately calculate the distribution of multiple energy flows in the electric-thermal integrated energy system, which is helpful for the investment planning and regulation and operation of the integrated energy system.

Description

Technical Field

[0001] The present invention belongs to the field of electric-thermal integrated energy systems, and in particular relates to a hybrid energy flow calculation method for electric-thermal integrated energy systems. Background Art

[0002] In order to cope with the increasingly severe problems of fossil energy shortage and environmental pollution, the energy Internet, which takes the power system as the core, the Internet and other cutting-edge information technologies as the basis, and includes multiple energy coupling and complementation, has come into being. The electric-heat integrated energy system with cogeneration units, electric boilers, and heat pumps as coupling equipment is one of the important manifestations of the energy Internet, realizing the conversion and interaction between electric energy and thermal energy in the system. As the main energy supply networks in the integrated energy system, how to effectively calculate the distribution of multiple energy flows in the power grid and heating pipeline network is the basis and basis for guiding the investment planning, operation and regulation, and energy trading of the integrated energy system.

[0003] At present, most of the research on the electric-thermal integrated energy system is centered on the power system, promoting the coordinated operation of electric and thermal energy and the absorption of wind power through electric boilers, heat storage, heat pumps and other equipment, or using a matrix-shaped energy hub model to simplify the multi-energy conversion process in the system. The above models all focus on the operation constraints of the power system, and only consider the thermal power balance condition for the thermal system, ignoring the impact of network constraints. In actual operation, the heating network itself has many limitations such as water flow balance, pressure balance, and heat balance, and their impact cannot be ignored. In the research on the calculation of hybrid energy flow in the electric-thermal integrated energy system, most of the existing commonly used solution methods have problems such as large calculation amount, excessive computer memory occupation, and difficulty in processing ring networks. Therefore, it is necessary to study the calculation method of hybrid energy flow in the integrated energy system containing electricity and heat. Summary of the invention

[0004] The purpose of the present invention is to provide a mixed energy flow calculation method for an electric-thermal integrated energy system. First, a hydraulic-thermal steady-state model of a thermal subsystem is established that takes into account flow, pressure, and heat balance; then, a decomposition and iterative mixed energy flow calculation method based on Newton's method is proposed in combination with the thermal-electric conversion model of the cogeneration unit and the AC flow model of the power subsystem. After determining the thermal output of the heat source in the electric-thermal integrated energy system, the electric power output of the computer unit can be calculated through the thermal-electric conversion model of the cogeneration unit, thereby obtaining a mixed energy flow solution for the electric-thermal integrated energy system. The present invention calculates the operating state quantities of various types of energy flows in the electric-thermal integrated energy system through the decomposition and iterative process of the state quantities of the thermal subsystem-power subsystem, thereby guiding the long-term investment planning and control operation strategy design of the electric-thermal integrated energy system in the region.

[0005] The technical solution adopted by the present invention is: a hybrid energy flow analysis and calculation method for an electric-thermal integrated energy system, and the specific operation steps are as follows:

[0006] Step 1: Identify the topological structure of the electric-thermal integrated energy system, wherein the electric-thermal integrated energy system is composed of an electric power subsystem, a thermal subsystem and a cogeneration unit, wherein the thermal subsystem is composed of a water supply network and a return network, and the cogeneration unit is a coupling device between the electric power subsystem and the thermal subsystem, and supplies power and heat to the electric power subsystem and the thermal subsystem at the same time;

[0007] Based on the topological structure of the electric-thermal integrated energy system, the known state quantities of the mixed energy flows of the electric subsystem and the thermal subsystem are obtained respectively, and the balance nodes of the electric subsystem and the thermal subsystem of the electric-thermal integrated energy system are set, wherein the known state information of the mixed energy flow includes the electric parameters of the electric subsystem, the hydraulic parameters and the thermal parameters of the thermal subsystem;

[0008] Step 2: Generate the thermal subsystem association reference matrix and loop matrix based on the thermal subsystem topology; establish a steady-state model of the hydraulic condition of the thermal subsystem based on the hydraulic parameters of the thermal subsystem; establish a steady-state model of the thermal condition of the thermal subsystem taking into account the heat loss of the water supply and return pipe network based on the thermal parameters of the thermal subsystem;

[0009] Step 3: Establish an AC power flow model of the power subsystem based on the power parameters of the power subsystem;

[0010] Step 4: Establish a mathematical model of heat-to-electricity conversion of the cogeneration unit, and construct a hybrid energy flow model of the electric-thermal integrated energy system with the cogeneration unit as the coupling device. The hybrid energy flow model consists of the following four parts in sequence: a steady-state model of the hydraulic condition of the thermal subsystem, a steady-state model of the thermal condition of the thermal subsystem, a heat-to-electricity conversion model of the cogeneration unit, and an AC power flow model of the power subsystem;

[0011] Step 5: Set the initial value of the unknown state quantity in the hybrid energy flow model of the electric-thermal integrated energy system, use the Newton-Raphson method to iteratively calculate the hybrid energy flow model of the electric-thermal integrated energy system in step 4, and output the calculation results of the final hybrid energy flow model of the electric-thermal integrated energy system.

[0012] The present invention is also characterized in that:

[0013] The specific derivation process of the steady-state model of hydraulic and thermal conditions of the thermal subsystem in step 2 is as follows:

[0014] The steady-state model of hydraulic conditions of the thermal subsystem is a nonlinear model including the node water flow balance equation and the circuit pressure drop balance equation.

[0015] The node water flow balance equation means that the mass flow of a node in the pipe network is equal to the difference between the mass flow of the branch flowing into the node and the mass flow of the branch flowing out of the node. The specific expression is:

[0016]

[0017] In the formula, and are the mass flow rates of the working fluid in branch j and branch k respectively; is the mass flow rate at node i; in i and out i They represent the set of branches flowing into and out of node i respectively; the superscripts pipe and node represent pipeline branches and nodes respectively;

[0018] The circuit pressure drop balance equation refers to any circuit in the heating network, in which the algebraic sum of the pressure drops on each branch is 0, which can be expressed as follows:

[0019]

[0020] In the formula, h j is the head loss on branch j; n loop Represents a collection of loops in a heating network;

[0021] The head loss h in formula (2) is j It can be calculated by the following formula:

[0022]

[0023] In the formula, s j is the resistance characteristic number of branch j, which indicates the head loss value when a water flow of 1 kg / h passes through branch j;

[0024] According to the topological structure of the thermal subsystem, the thermal subsystem association reference matrix A and loop matrix B are generated, and the steady-state model of the hydraulic condition of the thermal system is expressed in matrix form as follows:

[0025]

[0026] In the formula, m pipe and m node are the vectors composed of the mass flow of each branch and the mass flow of the node; h is the branch head loss vector; s is the vector composed of the resistance characteristic numbers of each branch.

[0027] The distribution of supply water temperature, return water temperature and heating power of each node in the thermal subsystem can be calculated by the steady-state model of the thermal working condition of the thermal subsystem. The distribution of supply water temperature and return water temperature directly reflects the quality of the heating effect.

[0028] The steady-state model of thermal subsystem thermal conditions includes pipe branch heat transfer equations and node temperature mixing equations.

[0029] Since the mass flow rate of the heating medium flowing in the thermal subsystem is stable, the convection between its molecules and the change of the internal energy of the medium with time are ignored, and only the axial change of the temperature of the medium is considered, the branch heat transfer equation can be obtained as follows:

[0030]

[0031] In the formula, They represent the mass flow rate of the working fluid in pipeline j and the temperature at the starting point x of the pipeline respectively; C P is the specific heat capacity of water; T a is the ambient temperature around the pipeline; R j is the total thermal resistance of the multi-layer insulation structure of pipeline j;

[0032] By separating the variables and integrating equation (5), we can get the average temperature of the working fluid at the end node of pipeline j:

[0033]

[0034] Where, T start , T end Respectively represent the temperature of the initial and final nodes of pipeline branch j; λ j is the thermal conductivity per unit length of pipe j, λ j =1 / R j ; l j is the length of pipe j;

[0035] The node temperature mixing equation indicates that in the thermal subsystem, the working fluids in all pipes flowing into the same node are fully mixed at the node, and then the temperature of the working fluids in all pipes flowing out of the node is equal to the mixing temperature. The specific expression is:

[0036]

[0037] Where, T i node represents the mixed temperature at node i; T end.j is the working fluid temperature at the end of branch j, which can be obtained by formula (6).

[0038] For the central heating network, the mass flow rate m of the heating medium in the pipeline is usually determined by using the hydraulic and thermal steady-state model of the thermal subsystem under the conditions of given heat source water supply temperature and load return water temperature. j , water supply temperature T of each load node s.i and the return water temperature T of the heat source node r.source ; Heat load at each node Φ i Under known conditions, the mass flow rate of each node except the heat source node It can be obtained by the following formula:

[0039]

[0040] Where, T s.i and T r.i are the supply water and return water temperatures of node i except the heat source node.

[0041] Step 3 The AC power flow model of the power subsystem is as follows:

[0042]

[0043] Where P i , Q i are the active power and reactive power injected into node i respectively; U i , U j are the voltage amplitudes of nodes i and j respectively; n is the number of nodes in the power subsystem; G ij and B ij are the conductance and susceptance of branch ij respectively; θ ij is the phase angle difference between node i and node j.

[0044] The cogeneration unit described in step 4 is the main heat source for centralized heating and generally works in two modes: heat-to-electricity mode and electricity-to-heat mode. The cogeneration unit for centralized heating in winter usually works in the heat-to-electricity mode, that is, the thermal power output is used to determine the unit's electrical power output. This can effectively avoid the waste of the unit's heating capacity, ensure the quality of heating, and when the power is insufficient, it is convenient to purchase electricity from the power grid. Therefore, this article uses the heat-to-electricity mode for the cogeneration unit. In this mode, the thermal power and electrical power output of the cogeneration unit satisfy the following relationship:

[0045]

[0046] Among them, Φ CHP Indicates thermal power, P CHP Indicates electrical power.

[0047] In summary, the overall process of the hybrid energy flow calculation method for the electric-thermal integrated energy system is as follows: Figure 1 shown.

[0048] According to the four components of the hybrid energy flow model of the electric-thermal integrated energy system, namely, the steady-state model of the hydraulic condition of the thermal subsystem, the steady-state model of the thermal condition of the thermal subsystem, the thermal-electric conversion model of the cogeneration unit and the AC power flow model of the power subsystem, the basic steps of the calculation method of the hybrid energy flow model of the electric-thermal integrated energy system are given as follows:

[0049] The first step is to calculate the steady-state model of hydraulic and thermal conditions of the thermal subsystem, and obtain the thermal power generated by the cogeneration unit at the heat source node of the thermal subsystem and the temperature and flow information of each node in the thermal subsystem;

[0050] The second step is to obtain the electrical power output of the cogeneration unit based on the thermal power of the cogeneration unit and the unit's thermal-electric conversion model;

[0051] The third step is to substitute the electric power output of the cogeneration unit into the AC power flow model of the power subsystem for calculation. If the convergence condition described in formula (11) is met, the calculation ends and the mixed energy flow solution of the electric-thermal integrated energy system is output. Otherwise, return to the first step to continue the iterative calculation.

[0052] ||ΔP (k) ,ΔQ (k) ||<ε(11)

[0053] In the formula, ||ΔP (k) ,ΔQ (k) || represents the power deviation vector ΔP of the power subsystem node (k) , ΔQ (k) The absolute value of the maximum component in ; k is the number of iterations; ε is the permissible iterative convergence error. When the per-unit value is used for calculation, ε=10 -4 .

[0054] The beneficial effects of the present invention are:

[0055] The hybrid energy flow calculation method of the electric-thermal integrated energy system proposed in the present invention can establish a steady-state model of the hydraulic and thermal working conditions of the thermal subsystem taking into account the heat loss of the water supply and return water pipe network by analyzing the topological structure of the thermal subsystem and its hydraulic and thermal characteristics; on this basis, combined with the hot spot conversion model of the cogeneration unit and the AC flow model of the power subsystem, a hybrid energy flow model of the electric-thermal integrated energy system with the cogeneration unit as the coupling device is constructed; finally, based on the Newton-Raphson method, a decomposition and iterative hybrid energy flow calculation method for the electric-thermal integrated energy system is proposed. After determining the thermal output of the heat source in the electric-thermal integrated energy system, the electric power output of the computer unit can be calculated by the thermoelectric conversion model of the cogeneration unit, and then the hybrid energy flow solution of the electric-thermal integrated energy system can be obtained. The method can be used to quickly and accurately calculate the distribution of various energy flows in the electric-thermal integrated energy system, which is helpful for the investment planning and regulation and operation of the integrated energy system.

[0056] The decomposition and iteration mixed energy flow calculation method proposed in the present invention can converge at a relatively fast speed for the two thermal subsystem structures (ring network and radiation type thermal network) in the electric-thermal integrated energy system, and has good network adaptability. In addition, for the electric-thermal integrated energy system coupled by the cogeneration unit, when the supply and demand of thermal energy changes, it will affect the operating state of the power subsystem. It is difficult to consider the interactive characteristics between different energy sources using the calculation method of the traditional independent system. The calculation method proposed in the present invention can give the distribution of mixed energy flows in the integrated energy system. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 It is a flow chart of the hybrid energy flow calculation method of the electric-thermal integrated energy system of the present invention;

[0058] Figure 2 It is a topological structure diagram of an electric-thermal integrated energy system in one embodiment of the present invention. DETAILED DESCRIPTION

[0059] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0060] The hybrid energy flow calculation method of the electric-thermal integrated energy system of the present invention is as follows: Figure 1 The specific steps are as follows:

[0061] Step 1: Identify the topological structure of the electric-thermal integrated energy system, wherein the electric-thermal integrated energy system is composed of an electric power subsystem, a thermal subsystem and a cogeneration unit, wherein the thermal subsystem is composed of a water supply network and a return network, and the cogeneration unit is a coupling device between the electric power subsystem and the thermal subsystem, and supplies power and heat to the electric power subsystem and the thermal subsystem at the same time;

[0062] Based on the topological structure of the electric-thermal integrated energy system, the known state quantities of the mixed energy flows of the power subsystem and the thermal subsystem are respectively obtained, and the balance nodes of the power subsystem and the thermal subsystem of the electric-thermal integrated energy system are set, wherein the known state information of the mixed energy flow includes the power parameters of the power subsystem, the hydraulic parameters and the thermal parameters of the thermal subsystem;

[0063] Step 2: Generate a reference matrix and a loop matrix of the thermal subsystem according to the topological structure of the thermal subsystem; establish a steady-state model of the hydraulic condition of the thermal subsystem based on the hydraulic parameters of the thermal subsystem; establish a steady-state model of the thermal condition of the thermal subsystem taking into account the heat loss of the water supply and return pipe networks based on the thermal parameters of the thermal subsystem;

[0064] Step 3: establishing an AC power flow model of the power subsystem according to the power parameters of the power subsystem;

[0065] Step 4: Establish a heat-to-electric conversion mathematical model of the cogeneration unit, and construct a hybrid energy flow model of the electric-thermal integrated energy system with the cogeneration unit as the coupling device. The hybrid energy flow model is composed of the following four parts in sequence: a hydraulic condition steady-state model of the thermal subsystem, a thermal condition steady-state model of the thermal subsystem, a heat-to-electric conversion model of the cogeneration unit, and an AC power flow model of the power subsystem;

[0066] Step 5: Set the initial value of the unknown state quantity in the hybrid energy flow model of the electric-thermal integrated energy system, use the Newton-Raphson method to iteratively calculate the hybrid energy flow model of the electric-thermal integrated energy system in step 4 and output the calculation result of the final hybrid energy flow model of the electric-thermal integrated energy system.

[0067] The specific derivation process of the steady-state model of hydraulic and thermal conditions of the thermal subsystem in step 2 is as follows:

[0068] The purpose of hydraulic calculation of heating pipe network is to obtain the mass flow and pressure distribution of working fluid in the network. The hydraulic model of thermal system is a nonlinear model including node water flow balance equation and loop pressure drop balance equation. Since the topological structure of water supply network and return network is the same, only water supply network is analyzed.

[0069] The node water flow balance equation means that the mass flow of a node in the pipe network is equal to the difference between the mass flow of the branch flowing into the node and the mass flow of the branch flowing out of the node. The specific expression is:

[0070]

[0071] In the formula, and are the mass flow rates of the working fluid in branch j and branch k respectively; is the mass flow rate at node i; in i and out i They represent the set of branches flowing into and out of node i respectively; the superscripts pipe and node represent pipeline branches and nodes respectively.

[0072] The circuit pressure drop balance equation refers to any circuit in the heating network, in which the algebraic sum of the pressure drops on each branch is 0, which can be expressed as follows:

[0073]

[0074] In the formula, h j is the head loss on branch j; n loop Represents a collection of circuits in a heating network.

[0075] The head loss h in formula (2) is j It can be calculated by the following formula:

[0076]

[0077] In the formula, s j is the resistance characteristic number of branch j, which indicates the head loss value when 1 kg / h water flow passes through branch j. According to graph theory, the associated reference matrix A and loop matrix B determined by the topological structure of the heating network are introduced, and the steady-state hydraulic model of the heating network is expressed in matrix form as

[0078]

[0079] The thermal model of the heating network includes the pipe branch heat transfer equation and the node temperature mixing equation.

[0080] The branch heat transfer equation is a partial differential equation that describes the dynamic change process of the working fluid temperature in the pipeline and the transmission delay. Since the flow rate of the heating working fluid in the pipeline is stable, the convection between its molecules and the change of the working fluid internal energy over time are ignored, and only the axial change of the working fluid temperature is considered, the steady-state heat transfer equation of the branch can be obtained as follows:

[0081]

[0082] In the formula, They represent the mass flow rate of the working fluid in pipeline j and the temperature at the starting point x of the pipeline respectively; C P is the specific heat capacity of water; T a is the ambient temperature around the pipeline; R j is the total thermal resistance of the multi-layer insulation structure of pipeline j.

[0083] By separating the variables and integrating equation (5), we can get the average temperature of the working fluid at the end node of pipeline j:

[0084]

[0085] Where, T start , T end Respectively represent the temperature of the initial and final nodes of pipeline branch j; λ j is the thermal conductivity per unit length of pipe j, λ j =1 / R j ; l j is the length of pipe j.

[0086] The node temperature mixing equation indicates that in the heating network, the working fluids in all pipes flowing into the same node are fully mixed at the node, and then the temperature of the working fluids in all pipes flowing out of the node is equal to the mixing temperature. The specific expression is:

[0087]

[0088] Where, T i noderepresents the mixed temperature at node i; T end.j is the working fluid temperature at the end of branch j, which can be obtained by formula (6).

[0089] For the central heating network, the mass flow rate m of the heating medium in the pipeline is usually determined by using hydraulic and thermal models under the conditions of given heat source water supply temperature and load return water temperature. j , water supply temperature T of each load node s.load And the return water temperature T of the heat source node r.source . Heat load at each node Φ i Under known conditions, the mass flow rate of each node except the heat source node It can be obtained by the following formula:

[0090]

[0091] The AC power flow model of the power subsystem in step 3 is as follows:

[0092]

[0093] Where P i , Q i are the active power and reactive power injected into node i respectively; U i , U j are the voltage amplitudes of nodes i and j respectively; n is the number of nodes in the power subsystem; G ij and B ij are the conductance and susceptance of branch ij respectively; θ ij is the phase angle difference between node i and node j.

[0094] In step 4, the mathematical model of heat-to-electricity conversion of the cogeneration unit as an electric-thermal coupling device can be expressed as follows:

[0095] As the main heat source for central heating, cogeneration units generally work in two modes: heat-to-electricity mode and electricity-to-heat mode. In winter, cogeneration units for central heating usually work in the heat-to-electricity mode, that is, the thermal power output is used to determine the unit's electrical power output. This can effectively avoid the waste of the unit's heat supply, ensure the quality of heating, and when the power is insufficient, it is convenient to purchase electricity from the power grid. Therefore, this article uses the heat-to-electricity mode for the cogeneration units. In this mode, the electricity and heat energy generated by the cogeneration unit satisfy the following relationship:

[0096]

[0097] The step 4 comprises the following sub-steps:

[0098] The hybrid energy flow calculation of the electric-thermal integrated energy system is divided into three parts: the heat network flow calculation link, the electric-thermal coupling link and the power grid flow calculation link. The basic steps of the solution are as follows: First, the heat network flow calculation is performed to obtain the thermal power generated by the cogeneration unit at the heat source node of the heat network and the temperature and flow information of each node in the system; secondly, the electrical power output of the unit is obtained according to the thermal power of the unit, and this value is substituted into the power grid flow model; finally, the power grid flow calculation is performed until the convergence conditions are met to obtain the hybrid energy flow solution of the system. As shown in the attached figure Figure 1 shown.

[0099] Example

[0100] The embodiment is a 7-node electric-heat integrated energy system constructed by coupling heat and power with a 3-node heating network and a 4-node distribution network. The topology is as follows: Figure 2 As shown. The cogeneration water supply temperature at the heat source is constant at 100°C, nodes 1 and 2 are heat load nodes with a thermal power of 0.3MW, the return water temperature at the load node is constant at 50°C, and the external environment temperature is 10°C. The thermal power output of the cogeneration unit in the figure is determined by the return water temperature and mass flow at node 3. Grid node 4 is a balancing node with a voltage amplitude of 1.02pu, node 3 is the access location of the cogeneration unit, a PV node with a voltage amplitude of 1.05pu, and the remaining nodes are PQ nodes with active loads of 0.15MW. Figure 2 The electric-heat integrated energy system works in the grid-connected mode, the cogeneration unit works in the heat-to-electricity mode, and node 3 is the balancing node of the heat network.

[0101] Using the method proposed in this paper, Figure 2 The system performs hybrid energy flow calculations, and the heat network model converges after 3 iterations, and passes the results to the power grid model for further calculations. After 4 iterations, the power grid flow converges. From the simulation results, it can be seen that the voltages of each node in the power system are between 1.0057pu and 1.05pu, and there is no voltage over-limit situation; due to the presence of a heat source at the heat network node 3, the load node with the lowest heating temperature in the thermal system is node 2 in the middle of the system.

Claims

1. A hybrid energy flow calculation method for an electric-thermal integrated energy system, characterized in that: The specific steps include: Step 1: Identify the topological structure of the electric-thermal integrated energy system, wherein the electric-thermal integrated energy system is composed of an electric power subsystem, a thermal subsystem and a cogeneration unit, wherein the thermal subsystem is composed of a water supply network and a return network, and the cogeneration unit is a coupling device between the electric power subsystem and the thermal subsystem, and supplies power and heat to the electric power subsystem and the thermal subsystem at the same time; Based on the topological structure of the electric-thermal integrated energy system, the known state quantities of the mixed energy flows of the power subsystem and the thermal subsystem are respectively obtained, and the balance nodes of the power subsystem and the thermal subsystem of the electric-thermal integrated energy system are set, wherein the known state information of the mixed energy flow includes the power parameters of the power subsystem, the hydraulic parameters and the thermal parameters of the thermal subsystem; Step 2: Generate a reference matrix and a loop matrix of the thermal subsystem according to the topological structure of the thermal subsystem; establish a steady-state model of the hydraulic condition of the thermal subsystem based on the hydraulic parameters of the thermal subsystem; establish a steady-state model of the thermal condition of the thermal subsystem taking into account the heat loss of the water supply and return pipe networks based on the thermal parameters of the thermal subsystem; The specific derivation process of the steady-state model of hydraulic working condition and the steady-state model of thermal working condition of the thermal subsystem in step 2 is as follows: The hydraulic steady-state model of the thermal subsystem is a nonlinear model including the node water flow balance equation and the circuit pressure drop balance equation; The node water flow balance equation means that the mass flow of a node in the pipe network is equal to the difference between the mass flow of the branch flowing into the node and the mass flow of the branch flowing out of the node. The specific expression is: In the formula, and are the mass flow rates of the working fluid in branch j and branch k respectively; is the mass flow rate at node i; in i and out i They represent the set of branches flowing into and out of node i respectively; the superscripts pipe and node represent pipeline branches and nodes respectively; The circuit pressure drop balance equation refers to any circuit in the heating network, in which the algebraic sum of the pressure drops on each branch is 0, which can be expressed as follows: In the formula, h j is the head loss on branch j; n loop Represents a collection of loops in a heating network; The head loss h in formula (2) is j It can be calculated by the following formula: In the formula, s j is the resistance characteristic number of branch j, which indicates the head loss value when a water flow of 1 kg / h passes through branch j; Mathematical model of thermoelectric conversion According to the topological structure of the thermal subsystem, the thermal subsystem association reference matrix A and loop matrix B are generated, and the hydraulic steady-state model of the thermal system is expressed in matrix form as follows: In the formula, m pipe and m node are the vectors composed of the mass flow of each branch and the mass flow of the node; h is the branch head loss vector; s is the vector composed of the resistance characteristic number of each branch; The distribution of supply water temperature, return water temperature and heating power of each node in the thermal subsystem can be calculated by the thermal working condition steady-state model of the thermal subsystem. The distribution of supply water temperature and return water temperature directly reflects the quality of heating effect. The specific derivation process of the steady-state model of thermal conditions in the thermal subsystem is as follows: The steady-state model of thermal conditions of the thermal subsystem includes the pipe branch heat transfer equation and the node temperature mixed equation; Since the mass flow rate of the heating medium flowing in the thermal subsystem is stable, the convection between its molecules and the change of the internal energy of the medium with time are ignored, and only the axial change of the temperature of the medium is considered, the branch heat transfer equation can be obtained as follows: In the formula, They represent the mass flow rate of the working fluid in pipeline j and the temperature at the starting point x of the pipeline respectively; C P is the specific heat capacity of water; T a is the ambient temperature around the pipeline; R j is the total thermal resistance of the multi-layer insulation structure of pipeline j; By separating the variables and integrating equation (5), we can get the average temperature of the working fluid at the end node of pipeline j: Where, T start , T end Respectively represent the temperature of the initial and final nodes of pipeline branch j; λ j is the thermal conductivity per unit length of pipe j, λ j =1 / R j ; l j is the length of pipe j; The node temperature mixing equation shows that in the thermal subsystem, the working fluids in all pipes flowing into the same node are fully mixed at the node, and then the temperature of the working fluids in all pipes flowing out of the node is equal to the mixing temperature. The specific expression is: Where, T i node represents the mixed temperature at node i; T end.j is the working fluid temperature at the end of branch j, which can be obtained by formula (6); For the central heating network, under the conditions of given heat source water supply temperature and load return water temperature, the hydraulic and thermal steady-state model of the thermal subsystem is used to determine the mass flow rate m of the heating medium in the pipeline. j , water supply temperature T of each load node s.i and the return water temperature T of the heat source node r.source ; Heat load at each node Φ i Under known conditions, the mass flow rate of each node except the heat source node It can be obtained by the following formula: Where, T s.i and T r.i are the supply and return water temperatures of node i except the heat source node; Step 3: establishing an AC power flow model of the power subsystem according to the power parameters of the power subsystem; Step 4: Establish a heat-to-electric conversion mathematical model of the cogeneration unit, and construct a hybrid energy flow model of the electric-thermal integrated energy system with the cogeneration unit as the coupling device. The hybrid energy flow model is composed of the following four parts in sequence: a hydraulic condition steady-state model of the thermal subsystem, a thermal condition steady-state model of the thermal subsystem, a heat-to-electric conversion model of the cogeneration unit, and an AC power flow model of the power subsystem; Step 5: Set the initial value of the unknown state quantity in the hybrid energy flow model of the electric-thermal integrated energy system, use the Newton-Raphson method to iteratively calculate the hybrid energy flow model of the electric-thermal integrated energy system in step 4 and output the calculation result of the final hybrid energy flow model of the electric-thermal integrated energy system.

2. The hybrid energy flow calculation method of the electric-thermal integrated energy system according to claim 1 is characterized in that: The AC power flow model of the power subsystem in step 3 is as follows: Where P i , Q i are the active power and reactive power injected into node i respectively; U i , U j are the voltage amplitudes of nodes i and j respectively; n is the number of nodes in the power subsystem; G ij and B ij are the conductance and susceptance values ​​of branch ij respectively; θ ij is the phase angle difference between node i and node j.

3. The hybrid energy flow calculation method of the electric-thermal integrated energy system according to claim 1 is characterized in that: The heat-to-electricity conversion mathematical model of the cogeneration unit in step 4 can be expressed as follows: In the heat-to-electricity mode, the thermal power and electrical power output of the cogeneration unit satisfy the following relationship: Among them, Φ CHP Indicates thermal power, P CHP Indicates electrical power.

4. The hybrid energy flow calculation method of the electric-thermal integrated energy system according to claim 1 is characterized in that: Step 5 is as follows: According to the four components of the hybrid energy flow model of the electric-thermal integrated energy system, namely, the steady-state model of the hydraulic condition of the thermal subsystem, the steady-state model of the thermal condition of the thermal subsystem, the thermal-electric conversion model of the cogeneration unit and the AC power flow model of the power subsystem, the basic steps of the calculation method of the hybrid energy flow model of the electric-thermal integrated energy system are given as follows: The first step is to calculate the steady-state model of hydraulic and thermal conditions of the thermal subsystem, and obtain the thermal power generated by the cogeneration unit at the heat source node of the thermal subsystem and the temperature and flow information of each node in the thermal subsystem; The second step is to obtain the electrical power output of the cogeneration unit based on the thermal power of the cogeneration unit and the unit's thermal-electric conversion model; The third step is to substitute the electric power output of the cogeneration unit into the AC power flow model of the power subsystem for calculation. If the convergence condition of formula (11) is met, the calculation ends and the mixed energy flow solution of the electric-thermal integrated energy system is output. Otherwise, return to the first step to continue the iterative calculation. ||ΔP (k) ,ΔQ (k) ||<ε (11) In the formula, ||ΔP (k) ,ΔQ (k) || represents the power deviation vector ΔP of the power subsystem node (k) , ΔQ (k) The absolute value of the maximum component in ; k is the number of iterations; ε is the permissible iterative convergence error. When the per-unit value is used for calculation, ε=10 -4 .

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