Chun-Yuan Ning1, Jun-Jie Shang1, Thi-Xuan-Huong Nguyen2, Duc-Tinh Pham3,4
1Fujian Provincial Key Laboratory of Big Data Mining and Applications, Fujian University of Technology, Fujian Province 350118, China
2Haiphong University of Management and Technology, Haiphong, Vietnam
3Center of Information Technology, Hanoi University of Industry, Hanoi, Vietnam;
4Graduate University of Science and Technology, Vietnam Academy of Science and Technology, Vietnam
[email protected],[email protected], [email protected], [email protected]
Abstract. This paper presents the microgrid in terms of its structures, operation mode, optimal configuration, and other aspects are described, and the optimal configuration model, solution algorithm, and other status reviews. We also re- view the research direction of the planning and design method of microgrid prospects from the microgrid itself, integrated energy grid, and coordinated planning with the distribution network.
Keywords: Distributed generation; Micropower grid; Optimization algorithm
1 Introduction
The traditional high-voltage transmission mode with large capacity, concentration, and long-distance show the disadvantages of vehicles with high operating costs, diffi- culty in operation, and large regulating capacity [1]. Distributed generation has many advantages, such as less pollution, high reliability, high energy utilization efficiency, flexible installation location, etc., and effectively solves many potential problems of sizeable centralized power grid [2]. However, when the distributed power supply is connected to the power grid, it will significantly impact the power grid. To solve this problem, the concept of microgrid was proposed [3].
Microgrids combine generators, loads, energy storage devices, and control devices to form a single controllable unit that simultaneously supplies electricity and heat to users[4]. The micropower supply in the microgrid is connected to the user side, which has the characteristics of low cost, low voltage, and low pollution. The microgrid can be connected to the large grid or run separately when the grid is out of order or in need.
This paper reviewes the development and application of microgrid technology, for the micropower grid are summarized first, and then focus on microgrid control strategy, energy storage, protection mechanism, and outlines the research on the fundamental techniques such as the planning design, to raise some issues which need to solve, and the future development of microgrid technology was discussed.
2 The basic structure of a microgrid
A microgrid, or small power grid, is small compared with the traditional large power grid, but its function is not impaired. The microgrid concept has not been unified, and different visions of the microgrid have been proposed in other countries according to their actual conditions [4]. Generally, the microgrid consists of a distributed power supply, load, energy storage device, control device, and other parts. It can realize two operation modes, namely isolated island (directly connected to users), connected to the grid (connected to a large power grid), and smoothly switch between them. The composition and functions of the microgrid are shown in Figure 1.
Fig. 1. Structure and function of microgrid
From the system level, a microgrid is a kind of modern power electronics technology that combines equipment and devices. From the perspective of a large-scale power grid, the micropower grid is a micro and controllable small power grid with high sen- sitivity and can quickly control other devices and equipment. From the perspective of users, micro-grid can meet many requirements of users, including reducing power loss, saving cost, improving voltage stability, etc., which can be realized through mi- cro-grid technology [5].
3 The operation mode of microgrid
A microgrid can be regarded as either a small power system or a virtual power source or load in a distribution network. Micro-grid can be divided into the grid- connected mode and isolated mode according to its operation mode [6].
3.1 Grid-connected mode
In the grid-connected mode, the purpose of control is to rationally utilize the re- sources and equipment in the microgrid and meet the needs of the upper grid for some auxiliary services of the microgrid through the reasonable dispatch of distributed en- ergy within the microgrid and the coordination of the relationship between the mi-
User
Bulk power sys- Mi-
Distributed Genera- tion
Load
Energy storage de-
Control device
crogrid and the external grid. At an appropriate time, in the grid-connected mode, if there is surplus power in the micro-grid, it can be used as a powerful model to sell surplus power to the external grid through the retail market on the distribution net- work side. In addition to all distributed power units, controllable resources on the demand side can also participate in the market bidding [7-8].
3.2 Isolated mode
When the microgrid operates independently, the voltage and frequency deviations caused by fluctuations in renewable energy and load are usually compensated by the local control of distributed power within the microgrid. Microgrid energy manage- ment system's main function is through the charging and discharging management of energy storage system, the adjustable output of the distributed power supply, such as fuel cells, diesel generator scheduling, control of the load side, ensure the power grid in power generation and the demand of real-time power balance, to prevent over- charge and discharge of the battery, ensure the long-term and stable operation of the microgrid [9-10]
The mutual transformation among various operating states of the microgrid is shown in Figure 2.
Fig. 2. Operation state of microgrid
Restore/connect control
Optimized and coordinated control
Restoration of transition state of mi- crogrid
Transition state of grid connection Disassembly
transition state of microgrid
Microgrid outage state
Operation state of grid connection of microgrid
Isolated operation state of microgrid
Emergency de-alignment
control Grid
control
Restore control
4 Optimal configuration of microgrid
4.1 A microgrid model
In the modeling of micro-grid planning and design, reasonable optimization variables, objective functions, and constraints should be selected from different perspectives, such as technology, economy, and environment, according to load demand and dis- tributed energy, and based on the quasi-steady operation model of each device, to form a mathematical description of the planning and design problem [11-13]. Gener- ally, it can be expressed in the following form:
(1) where: X represents the optimization vector; F is the target function; Ω said the feasi- ble solution space; G and H respectively represent the set of functions constituted by equality constraints and inequality constraints. Due to the differences in design objec- tives, distributed power supply types, and operation characteristics in the planning and design stage, different microgrids' model details differ significantly.
Generally speaking, optimization variables of microgrid planning and design mainly include models [14-16], capacity [15-17], and location [18-21] of distributed power supply, energy storage device, and equipment contained in the cold/heat/power con- nection system, etc. The tilt Angle of the PHOTOVOLTAIC array, the fan wheel hub's height, the type of scheduling strategy, and the position of the contact switch in the micro-grid [22-30] can also be used as variables to be decided.
The objectives of micro-grid planning and design can be the minimization of total system cost [32-37,39-42,44-46,48-54], the maximization of net investment income [39,46,48-52], the minimization of pollutant emissions [32,33,40-42,48], the maximi- zation of system power supply reliability [32,35,40,48], the minimization of system network loss [31,39,49,50], and the minimization of fuel consumption [34,35,37- 39,41,48,50-52].In the economic analysis of the micro-grid, the capital investment mainly includes the initial investment cost of distributed power supply, energy stor- age, controller [32,34,35,40,48], operation and maintenance cost, equipment replace- ment cost, fuel cost, sewage penalty [32,34,35,48], power outage penalty [48], power purchase cost [34,35,37-39,44,47-52] and so on. Project revenue mainly comes from the sale of electricity [34,3537-39,44,47-52].
Because the influence of system operation optimization strategy needs to be consid- ered in the planning and design of micro-grid, the constraints that are considered in the system operation strategy usually need to be taken into account when formulating the constraints. In addition, some constraints of the planning design problem itself need to be considered. Constraint conditions mainly include: balance constraint of power (electricity, cold and heat) of micro-grid [31-39,41,42,44-47,48-52];Power flow constraint [31,33,35,39,46,48,51], thermal stability constraint [45,49,51], voltage
constraint [31,33-35,39,46,49,51], tie line power constraint [31,32,35-39,41, 45,48- 50];Equipment operation constraints [37-39,45,48];Other constraints, such as tie line power fluctuation constraints [31,32,35-39,41,42,45,48-50], etc.
4.2 Solutions to Microgrid Model
In essence, the problem of microgrid planning and design is a multi-scene, multi- objective, nonlinear, mixed-integer, and uncertain comprehensive planning problem.
In order to solve the micro-grid planning and design problems, the application of enumeration method [32,36,38,39], mixed integer programming method [31,33- 35,37,47,48,52], heuristic algorithm [32,43-46,49,50,53,54] and hybrid algorithm [36,39,42,48,51] have been studied respectively. To show the solution algorithm of the micro-grid planning and design problem more intuitively, the optimization algo- rithm commonly used in the literature is given in Table 1
Table 1. Standard solving algorithms used in the planning and design of microgrids
Common solving algorithm
Heuristic algorithm
The enumeration method[32,36,38,39]
Particle swarm optimization[32,37,45,47,48,50-54] and its improved algorithm[32,38,45,47,48,50,51]
Genetic algorithm[31,34,43,44,48,54] and its improved algo- rithm[31,34,43,44,48]
CS[33]、BA[35]、AFSA[36]
Simulated annealing algorithm
Hybrid al- gorithm
Particle swarm optimization[32,38,45,47,48,50-54] + Quad- ratic programming[32,38,45,47,48,50,51]
Analog programming+ Tabu Search
Differential evolutionary algorithm+ Fuzzy multi-objective algorithm
5 Research’s Status and Prospect
There have been much researches on the planning and design of micro-grid, but there are still many key technologies that need to be further studied systematically.
(1) Planning and design research of the micro-grid itself. The existing research re- sults are relatively simple, and a comprehensive systematic and scientific planning and design method is urgently needed to be established
(2) Coordinated planning of distribution network and microgrid. The influence of microgrid access should also be considered in the planning and expansion planning of the distribution network itself.
(3) Economic analysis and planning of energy storage system. In the market envi- ronment, the long-term economic analysis of different roles of the energy storage system and the influence of life on its economic performance still lack convincing demonstration.
(4) The planning and design of micro-grid, including cold and hot power supply systems. For microgrids with comprehensive energy network characteristics, there is still a lack of detailed analysis of the coupling characteristics between the optimal ratio of cooling, heating, and electricity corresponding to different structures and dif- ferent energy flows.
6 Conclusion
In this paper, domestic and foreign scholars' latest research achievements in microgrid planning and design methods are reviewed from the theoretical perspective. Based on the micro-grid planning and design method's main contents, the modeling method and solving algorithm involved in the current micro-grid planning and design research are expounded, and the possible research directions in the future have prospected. With the continuous deepening and improvement of the planning and design methods of microgrids, the microgrid will play a greater value in practical application.
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