Overview of Micro-grid Energy Management System Research Status

Qing Qina; Zhixin Chenb, Xiaohui Wangc College of Electricity and Information Engineering, Beijing University of Civil Engineering and Architecture

Beijing, China a qinqing2010@sina.com, bchenzhixin@bucea.edu.cn, cwangxiaohui@bucea.edu.cn

Abstract-Micro-grid has gradually become a new method for developed countries to solve many problems in the traditional power system. In this paper, the concept of micro-grid and energy management system is introduced firstly, and then the important roles of the application of energy management technology in micro-grid system are described. Combined the literature, a summary is made on the development status of micro-grid energy management system at home and abroad, then the corresponding feasible solutions to the problem of key technical problems was proposed. Finally, the outlook of the development trend of micro-grid energy management system in the future is made.

Keywords- Micro-grid ; Energy management system; Distributed power energy; Storage system

I. INTRODUCTION With the intensification of the global energy crisis and

environmental degradation, micro-grid will become an important development direction of the grid for its intelligence, flexibility, environmental protection and the diversification of energy utilization in the future. Energy Management System (EMS), as an important module in the scheduling system, is also an important part to the micro-grid technology. It provides data management, monitoring, control and optimization for micro-grid dispatch control center, it can be said that micro-grid provides a guarantee for stable and efficient operation of the micro-grid.

II. THE CONCEPT OF MICRO-GRID AND MICROGRID ENERGY MANAGEMENT SYSTEM

The concept of micro-grid was first proposed by the United States[1].Consortium for Electric Reliability Technology Solutions (CERTS) puts forward a clear definition of the concept of micro-grid in 2002 and the basic structure of the micro-grid(see Fig.1) was given as well. It organizes the distributed power whose generating capacity is less than five hundreds KW and energy storage devices together to provide electricity to the users who live nearby. The major part of CERTS micro-grid is power electronic devices with the main features of flexibility and intelligence, and it can achieve "plug and play" control technology by it. At present, the voltage level and range of applications of micro-grid is constantly expanding and has been raised from megawatt to tens of megawatts, it is expected to be further developed in the rural distribution network.

Micro-grid Energy Management System (MGEMS) is a comprehensive automated system based on computer and is

mainly used for a micro-grid scheduling management center. It combined the advanced IT technology and can optimize management of distributed power and energy storage devices within the micro-grid. Relative to the traditional grid energy management system, MGEMS is more targeted, it can validly manage and configure many resources in the micro-grids that we study on, and accordingly it can improve the use efficiency of the all kinds of energies in the micro-grids.

III. THE FUNCTION OF MICRO-GRID ENERGY MANAGEMENT SYSTEM

Micro-grid is the advanced mode of distributed generation, it is composed of distributed power supply, energy storage device, load, power electronic device and the energy management system. In the practical application of micro-grid, we should consider not only the stability of its power supply, but also its economic operation. In the economic operation of electric power dispatching, the micro-grid can refer to the knowledge and experience of the power grid to optimize the design in the principle of the scheduler, power trading, and resource optimization.

The micro-grid optimization can be executed by the micro-grid energy management system. Energy management system can use location information to meet the local heat, cold, electricity demand, quality of the electric power grid, special requirements, demand side management and so on, so as to determine micro-grid distributed generation system configuration operation and distribution network are required to provide a total amount of energy, and also can make schedule through the integration of real time electrovalence, environment and electric demand forecasting information.

In order to minimum the global energy consumption, the micro-grid energy management system (MGEMS) needs to schedule the generator in time. Its major functions are as follows: providing their own power and voltage set values for

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each micro-power controller; meeting the demand of electric load; satisfying the operation of treaties between the micro-grid and grid; minimizing emissions and system losses; maximizing the efficiency of micro-power run; providing logic and control during the micro-grid island and re-grid.

IV. RESEARCH OF THE MICROGRID ENERGY MANAGEMENT SYSTEM, THE PROBLEMS AND THE CORRESPONDING SOLUTION

A. Research of the microgrid energy management system 1) Research abroad

At present, there are already relevant literatures that describe the micro energy management research [2-4]. One research based on the central controller and hierarchical control of micro energy management strategy and analysis two kinds of market policy in micro operation was proposed in literature[2].One research based on the photovoltaic, wind power and battery single-phase AC of energy management strategy and the cost of optimize the system operation and service life of battery was proposed in literature[3].A distributed power supply side model was proposed in literature[4].There is a comparison between the installation and running costs of distributed generation and supply of the electricity sector cost in the model and a discussion of the optimal investment problem.

2) Research home Here are three mainstreams of domestic micro-grid energy

management practices:

a) Micro-grid management based on multi-agent system In the control of multi-agent system, to achieve

coordination and control of micro-grid, we should get know the purpose of each micro source supply for different power supply purpose micro power control requirements are different, such as those power supply like photovoltaic and wind power require itself generate electricity first for their control strategy is different from the control strategy of micro gas turbine.

The idea of multi-agent system is divide large scale system into a number of small communication systems through a certain method, so it will have high autonomy. The core of this system is a centralized management replaced by an intelligent, distributed management. Compared with the centralized management, it has the following advantages:①Multi-Agent allows different subjects to choose a different optimization algorithm, which that can improve the efficiency of the whole optimization process.② The amount of information of the multi-agent control system is much larger than that of a centralized control system.

b) Adaptive management of micro-grid based on the voltage autonomous

The adaptive management of micro-grid based on the voltage autonomous is a distributed control system though carrying out unit controls to protect the balance of power and energy storage devices of the system and mainly for the entire micro sources in the micro-grid. In the micro-grid, both the active power and reactive power are functions of the node

voltage, thus the micro-grid can be controlled through the node voltage to meet the needs of load requirements in micro-grid.

The adaptive management of micro-grid based on the voltage autonomous is also a micro-power voltage control system which mainly takes the normal fluctuations of the energy storage components and characteristics of different types into consideration. The main contents are as follows:①We need to find a continuous power supply for a long time in the micro-power controllable power and let the voltage regulation mode responsible for the active balance within the micro-grid. The reactive power of the micro-power are seen as settings and though control the local micro-grid node voltage by adjusting the active power output to ensure the energy storage device was charged.②As the energy storage element has a faster adjust speed, the energy storage components in micro-grid that usually work in the voltage source mode can stable micro-grid node voltage through regulating the output voltage amplitude of the inverter.③The uncontrollable power in the micro-grid is needed to be considered as a negative active load.

c) Energy management strategy based on the micro-grid inverter

The energy management strategy based on the micro-grid inverter is a control mode which responses to the control strategy rapidly according to the system state, especially the state transitions of micro-grid operation.

The thought of energy management strategy based on the micro-grid inverter is that micro-grid will convert to the island operation state when there is some fault in the large power grid. At this situation, voltage and frequency regulation in the micro-grid depend mainly on one or several micro-powers, so how to control these micro-powers is the problem of control strategy; When the micro-grid is operating in grid connected state, voltage and frequency of the micro-grid are mainly determined by the large power grids. The output power of micro-grid micro source are in their max state when the entire micro-grid can be considered as a controllable power supply or a controllable load, and the micro of the micro-grid power supply is not involved in the task of the tune pressure and frequency modulation(FM)task.

Since there are a large number of distributed sources in micro-grid, in order to switch the states successfully between the isolated operation of the micro-grid and grid operation, and the purposes of voltage and frequency adjustment when the micro-grid isolated operation, the control method holds the opinion that the center of the inverter micro-grid by measuring the local node information to control the output of the micro-power and energy storage system, in which can load and network changes in the structure of the nodes near the inverter, in which we can take the following control methods: (1) the P / Q control; (2) adjusting the slip control; (3) Voltage / frequency (V / f) control.

B. The main problems and their relevantly solutions The number of micro-grid distributed sources is increasing

and their positions are also getting more and more decentralized, which resulted in the monitoring of the micro-grid is more and more complex. At present, there are following questions with the micro-grid energy management system.

1) Communication problems: Because there are so many points which needed to be

monitored and so many real-time data needed to be collected, so it increases the difficulty of communication in the micro-grid and it is not propitious for controller to make the right decisions. In order to ensure that micro-grid can provide a flexible power mode and high-quality supply services, it is very important to have enough monitoring and control points and perfect communication system by means of uploading the collected information and issue the control information in time in order to realize real-time control between the main power grid and micro-grid and its power source.

The corresponding solution: Strengthen the communication channel construction. Advanced communications technology can be used in micro-grid, such as: using optical internet protocol (IP) network, band-pass limiter (BPL) carrier, broadband wireless network technology to solve the problem of real-time transmission and two-way inter action of information in the micro-grid.

2) Access control problem: In the micro-grid, the control problem is not just a major

problem, but also a key issue. The control of the micro-grid[5] include the control strategy of grid-connection and distributed power control within the micro-grid. The control between micro-grid and main grid requires the ability of real-time. When the power produced by micro-grid cannot meet the needs of users, the system needs to switch to the main grid in time without removing the loads; when the power produced by micro-grid satisfy the needs of users or faults occur in the main grid, micro-grid can be disconnected from the main network and achieve an independent run. For the distributed sources in the micro-grid, because the electricity generated by conventional thermal power and biomass is relatively stable, they can select the traditional access method; for intermittent power generation, they are affected by the climate and the season significantly, and there are significantly uncertainty, which will bring frequent access and resection to the micro-grid .On this basis, real-time interface technology is requested to eliminate the distributed power of access to the power grid and the influence of resection.

The corresponding solutions are as follows: ①Developing a new interface control technology. At present, there are three kinds of typical control method:"Plug and play" technology based on power electronic devices; the control based on the power management system; control technology based on multi -agent technology[6-7].For the three methods, they all have some progress in different degrees, and it is believed that these three technologies will be more mature with the continuous progress of the study.② Developing coordination and control module of distributed power. The coordination and control module of distributed power is used to optimize the micro-grid inner

power and achieve coordination for distributed power control. There are following coordination and control methods we can take in the micro-grid, when the power generated by intermittent power generation equipment is beyond the needs of load, the energy storage should be detected to find whether it has finished charging, if not, then it continue until complete charging, if so, the generating capacity of stably power supply (adjustable power) is needed to be reduced; when the power generated by intermittent power generation equipment is lower than the need of load, the energy storage should be detected to find the energy storage to find whether it can provide electricity, if so, energy storages are inserted to send power, if not, the generating capacity of stably power supply should be increased, if the load demand cannot be met, the system is needed to be switched to main network.

3) The unpredictable character of new and renewable energy power [8]:

since there are so many intermittent power supply in the distributed power , such as solar energy, wind energy, tidal whose generation are affected by the weather and season apparently, they presents lots of unpredictable problems which is bad for micro-grid to supply power stably.

The corresponding solution is to develop new energy and renewable resources prediction module. According to the position where micro-grid is located, a climate database based on the local climate information collected in advanced can be compiled to predict the power generated by intermittent energy sources. Thereby it is good for micro-grid to take strategy for its stable operation.

V. THE DEVELOPMENT DIRECTION OF MICRO-GRID ENERGY MANAGEMENT

With the continuous development of network technology and increasing progress of computer technology, the functions of micro-grid energy management system are constantly updated and improved. The trends of its development can be summarized as the following aspects [ 9-10 ].

A. Application of 3-D visualization technique With the enlargement of power grid scale, the power

system is becoming more complicated, and the type and number of data that generated in the operation of the system are increasing. In the network wiring diagram, the traditional expression way using digital tagging method has the advantages of high display speed and accurate display data, however it is not intuitive enough and difficult to meet the need of network real-time monitoring because it is hard to pick key information from a large number of data.

Comparing to conventional two-dimensional display,3D visualization technology makes full use of the space of dimension third, strong expression ability, more efficient, intuitive expression power of large amounts of data, so it is convenient for operators to have a holistic grasp to understand the current problems in the system quickly, so as to make a response in a short time to ensure the micro grid work stably and efficiently.

B. Information integration With the rapid development of internet technology, the

WEB application plays a more and more important part in the energy management system (EMS).And WEB which integrates the technology of internet, object-oriented technology and the distributed computer technology makes the data acquisition and monitoring more open. It can be visited by more management system, control system and user access, which is conducive to expand the application range of the system in real time and reduce the maintenance workload.

C. The application of artificial intelligence technology With constant fusion between various software and

artificial intelligence technology in MGEMS, for example, the modification of weather in load forecast, pick up special event, security constraints and optimal power flow calculation function constraint processing. Power market software for artificial intelligence applications also have further requirements, such as: transmission capacity analysis, trade matching and information filtering and so on.

VI. CONCLUSION With the continuous expansion of the size of the main

power system, the micro-grid which has enormous potential that reducing energy consumption, improving power system reliability and flexibility will fundamentally change the traditional way that power grid mainly response to load growth. Micro-grid energy management system, as an important part of the micro-grid technology, plays an important role in effective and stable operation of the micro-grid. How to make full advantage of new energy, and how to manage and configure the distributed power are important problems in the microgrid microgrid energy management system.

In this paper, the concept of micro-grid and energy management system and research of the microgrid energy management system at home and abroad are introduced. With the expanding scale of micro-grid, micro-grid energy management system is also gradually reveals many problems, such as, communication problems, access control problem, the unpredictable character of new and renewable energy power. Their corresponding solutions are showed in this paper as well.

Finally, the outlook of the development trend of micro-grid energy management system in the future is made.

With the intensification of the global energy crisis and environmental degradation, distributed power(DG) and micro-grid technology as effective solutions provide methods to solve the problem. There is no doubt that for a long period of time, main power grids is still the mainstream, and the micro-grid as a supplement to the large grid there has a huge potential and bright prospect.

ACKNOWLEDGMENT This work is supported by Science Research Fund of

Beijing University of Civil Engineering and Architecture and Foundation for Beijing municipal Talents.

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