Zhao Xia, Yang Lun, Yan Xiaobin, and Yan Wei, researchers of the State Key Laboratory of Power Transmission and Distribution Equipment and System Safety and New Technologies (Chongqing University), wrote in the 3rd issue of the Journal of Electrotechnics in 2018 that the integrated energy system Under the framework, the power and natural gas systems are closely coupled and it is necessary to carry out the energy flow calculation of the electro-pneumatic integrated system.

The existing energy-flow calculation methods of electro-gas integrated systems mostly use Newton's method as the core algorithm, and have strong dependence on the initial value. However, the existing research has not effectively solved the initial value problem of natural gas systems; in addition, the existing researches on electro-gas The analysis of interaction characteristics is also insufficient.

For this reason, this paper proposes to improve existing research from two aspects of energy flow algorithm and interaction characteristics. In terms of algorithm, combined with the global search of genetic algorithm (GA) and the local search ability of Newton method, a hybrid GA-Newton method for energy flow calculation of electro-pneumatic integrated system is proposed. In terms of interaction characteristics, two coupling methods of gas turbine and P2G are considered. The two types of electro-pneumatic interaction modes, "generative by electricity" and "fixed by electricity", were proposed.

The two examples of 14-node natural gas system and IEEE-57 node system and 14-node natural gas system are used to verify the effectiveness of the proposed improved method for the analysis of energy flow and interaction characteristics of the electro-gas integrated system. .

With the increasingly prominent energy and environmental issues, the Integrated Energy System (IES), which is based on the integration of multiple heterogeneous energy sources and integrated with each other, has become the development trend and main carrier of the future energy system [1-3]. Compared with other primary energy sources such as coal and petroleum, natural gas is economically environmentally friendly and has abundant reserves. Natural gas power generation has the outstanding advantages of high efficiency, low carbon, and fast response [4], and the installed capacity continues to increase [5].

On the other hand, the development of the Power to Gas (P2G) technology [6] has made large-scale energy storage possible, not only providing new ideas for the consumption of renewable energy, but also further promoting the power system and natural gas systems. coupling. Therefore, the electric-natural gas integrated system (hereinafter referred to as electric-gas system or Electric-Gas System, EGS) has become a typical way of comprehensive energy utilization and a key component of IES.

Energy flow calculation is the core of the steady-state analysis of electro-gas systems, and is also the basis for other related research of EGS. Around the energy flow calculation of the electro-gas system, a large number of studies have been conducted at home and abroad [7-16]. Literature [7] is based on the simplified model of natural gas system and generalizes the power flow calculation method of power system to the energy flow calculation of natural gas system. The literature [8,9] considers the influence of ambient temperature on the operation of natural gas system and establishes the electro-gas system. Energy flow calculation model, and propose a unified solution method for energy flow calculation of electro-gas system.

The literature [10] considers the bidirectional flow of energy flow in electro-gas systems, and studies the calculation methods for standardization of energy flow in electro-gas systems. The literature [11] analyzes the node method and the loop method for energy flow calculation of natural gas systems, and proposes An order-solving method for energy flow calculation of electro-gas systems; literature [12,13] taking into account the randomness of wind speed and load, using Monte Carlo simulation method for probabilistic power flow analysis of electro-gas systems. On the basis of the energy flow analysis of electro-gas systems, literature [14-16] further studies the energy flow calculation of electro-thermal-gas systems.

The essence of the energy flow calculation in the electro-gas system is the simultaneous solution of the power flow equation of the power system and the steady-state airflow equation of the natural gas system. From the solution strategy, the aforementioned energy flow calculation methods can be divided into two major categories: the unified solution method [8-10,12-15] and the sequential solution method [11,16], but they all use the Newton method as their core algorithm. . Newton method is an excellent local search algorithm [17], but its dependence on initial value is high. Improper selection of initial value will directly affect the convergence of the algorithm.

Although the loop method has the advantages of insensitivity to initial values â€‹â€‹and good convergence, the loop method is more complicated, especially for natural gas networks containing gas-driven compressors [7, 11], and therefore its application is not extensive. On the contrary, the nodal equation equation is simple and widely used, but it requires high initial value. It is relatively easy to select the initial value of the power system power flow problem (generally, the flat-start method can be adopted). Regarding the initial value problem of the energy flow calculation of the natural gas system, although some studies [7-8, 14] point out that the pressure of the natural gas system needs to be carefully selected. The initial value, but the existing research does not give an effective solution, and rely more on engineering experience to determine the initial value.

The literature [8,14] proposed that the initial value of the pressure of the node should be selected according to the pressure difference between the first and the end of the natural gas pipeline of 5% to 10%. However, this method does not allow the determination of the direction of all the airflow before the energy flow calculation. ) does not apply; literature [10] proposes to take the initial value of all pressures to be taken as 0, however, when there is a pipe with unknown pressure at both ends in the network, this method will cause the corresponding element of the Jacobian matrix to be divided by zero. The problem.

On the other hand, the above study considers the adoption of gas-fired units [8-11,14], electric-driven compressors [12,13], energy hubs [12,13,16] and P2G [10, 13] and other coupling methods, but the research mainly focuses on the establishment of energy-flow calculation models for electro-gas systems, and the analysis of the interaction characteristics between electro-gas systems and their impact is not sufficient.

In view of the above problems, this paper improves existing research from two aspects of energy flow algorithm and interaction characteristics. In terms of algorithm, this paper proposes a global search capability based on genetic algorithm (GA) [18] to solve the initial value problem of natural gas system, and takes the sequential solution method as the basic framework to construct a hybrid GA-based energy flow calculation for electro-gas systems. Newton method; interaction characteristics, considering the gas-fired unit and P2G two kinds of coupling methods, put forward two kinds of interaction modes: "air-powered electricity" and "electricity-based gas". The effectiveness of the proposed method for verifying the energy flow calculation and interaction characteristics of the electro-gas system was verified using electro-gas systems consisting of 14-node natural gas systems and IEEE-57 and 14-node natural gas systems, respectively.

An Improved Method for Calculation of Energy Flow in Electro-gas Comprehensive Energy System

Figure 3 EGS flow calculation process

**in conclusion**

In this paper, the energy flow calculation method of the existing electro-gas system is improved from two aspects of energy flow algorithm and electro-pneumatic interaction characteristics. Aiming at the initial value problem of natural gas system, a hybrid GA-Newton method was proposed to calculate the energy flow of electro-gas system. According to the operation mode of electro-gas system, two kinds of interactions were proposed: â€œsetting electricity with gasâ€ and â€œsetting with gas with electricityâ€. mode. Two examples are used to verify the effectiveness of the proposed method. The results of the example show that:

1) The hybrid GA-Newton method proposed in this paper can effectively solve the initial value problem of the natural gas system and is applicable to the energy flow calculation of the electro-gas system. Compared with the conventional Newton method, the proposed algorithm has good convergence, fast calculation speed and good adaptability to the change of the air load.

2) The two types of electro-pneumatic interaction modes referred to in this paper as "gas-based electricity" and "electricity-based gas" have certain complementary characteristics. The two modes and their cooperation are expected to improve the wind power absorption capacity and low carbonization level of the power system.

It is necessary to point out that this paper focuses on the hybrid GA-Newton algorithm for energy flow calculation of electro-gas systems, and discusses two kinds of interaction modes, "Gas-setting with electricity" and "Gain-setting with electricity," and their applicable scenarios. The next step will be to conduct in-depth research on the optimization of the two interaction modes.

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