The proposal of the microgrid system aims to realize the flexible and efficient application of distributed power and solve the problem of grid connection of a large number and various forms of distributed power generation.
Developing and expanding microgrid systems can fully facilitate large-scale access to distributed power and renewable energy. Achieving a highly reliable supply of multiple forms of energy is an effective way to realize active distribution networks, and make the transition from traditional power grids to smart grids.
1. What is a microgrid
Microgrid, also known as microgrid, refers to a small power generation and distribution system consisting of distributed power sources, energy storage devices, energy conversion devices, loads, monitoring and protection devices, etc.
A microgrid system is an autonomous system that enables self-control, protection, and management. As a complete power system, it relies on its own control and management of energy supply to achieve power balance control, system operation optimization, fault detection and protection, power quality governance and other functions.
● The composition of the microgrid
Most of the power sources in the microgrid system are distributed power sources with small capacity. That is, small units with electronic power interfaces, including micro-gas turbines, fuel cells, solar battery, small wind turbines, and energy storage devices such as super capacitors, flywheels and batteries. They are connected to the user side and have the characteristics of low cost, low voltage and low pollution.
● The significance of microgrids
Due to the dual pressures of environmental protection and energy depletion, it is important to vigorously develop clean renewable energy. The development potential and benefits space of efficient distributed energy industry are enormous.
The need to improve the reliability and quality of power supplies drives the establishment of corresponding power supplies close to load centres. Centralized control of the entire power grid can be achieved through the microgrid system controller, and there is no need for a distributed local controller.
The local distributed power supply and load controller is used to realize the transient control of the microgrid system, and the centralized energy management system of the microgrid system realizes the steady-state safety and economic operation analysis. The centralized energy management system of the microgrid system uses a weak communication connection with the local controller.
2. Features of microgrid system
The microgrid system system has become a large-scale, nonlinear, multi-constrained and multi-time multi-dimensional complex system due to the large number of distributed power sources with different characteristics. It has complex system characteristics such as complexity, nonlinearity, adaptability, openness, spatial hierarchy, organization and self-organization, and dynamic evolution.
It belongs to a special complex giant system with many variables, complex operating mechanisms and significant uncertainties. Therefore, the research and development and application of microgrid system technology should first have a comprehensive understanding and mastery of its typical characteristics.
The construction concept of the microgrid system is to configure the distributed power supply close to the home energy storage for power supply, and the transmission distance is relatively short. Its load characteristics, distributed power layout, power quality requirements and other factors determine the main aspects of the microgrid system.
Such as capacity scale and voltage level, structure mode and control mode. Moreover, it presents characteristics that are different from traditional power systems.
● Capacity and voltage level
The capacity scale of the microgrid system is relatively small, and its voltage level is often low voltage or medium voltage level.
● Structural pattern
The microgrid system can be divided into three different structures according to the power supply system: AC, DC and AC/DC mixed, and the technology is mature, and the structure of the AC microgrid system is widely used.
● Control mode
There are two main types of microgrids: peer-to-peer control mode and master-slave control mode, and master-slave control mode is still the mainstream of microgrid experimental systems and demonstration projects.
3. Key technologies for microgrid system
Microgrids have very complex dynamic operating characteristics and energy management problems due to the large number of distributed power sources with different characteristics. This leads to a complex coordination operation and control mechanism of the microgrid system.
In order to effectively coordinate and control the distributed power sources to achieve and ensure the safe, stable, economical and reliable operation of the system. Key technologies such as microgrid planning and design, microgrid operation control, microgrid protection and microgrid economic operation of microgrid from the perspective of system.
● Planning and design of microgrid systems
It is the first stage of research and development of microgrid system, and is an important foundation to ensure the safe, stable, economical and reliable operation of the system.
Its purpose is to achieve and meet the system investment cost objectives of safe and stable operation and minimize the system investment cost during the planning period by optimizing the selection of system structure and power supply configuration under the condition of meeting the stable operation of the system and the load demand under its jurisdiction.
● Covered content
The planning and design of microgrid mainly covers a variety of hybrid distributed power combination types with complementary energy characteristics, selection, operation mode, optimization objectives, operation strategies and constraints, optimization algorithms and system network structure design of distributed power sources.
4. Operation control of the microgrid system
It is the key technology to realize and ensure the safe, stable, economical and reliable operation of microgrid system, and it is to realize coordination and cooperation optimization between two different levels of unit-level distributed power supply control and system-level microgrid control.
- Covered content
The operation control technology of microgrid mainly includes the operation control mode of microgrid, operation control strategy, control method of each distributed power source (including inverter control), and coordinated control method between multiple distributed power sources.
● Protection of microgrid systems
It refers to a key technology that can quickly identify, locate and cut off faults and restore the safe and stable operation of the system when the microgrid fails.
- Covered content
The faults of the grid-connected operation mode are mainly reflected in the external faults and internal faults of the microgrid, and the faults are eliminated according to the corresponding fault protection strategies selected by different fault locations. The faults of off-grid operation mode are mainly reflected in power supply faults and feeder faults, and then power protection or feeder protection methods are used to eliminate the faults. It can also be used with off grid solar batteries.
● Economic operation of microgrid systems
It is the key technology of the third stage to realize and ensure the economic and environmental benefits of microgrid system, and it is also the final embodiment of testing the advantages and disadvantages of microgrid planning, design and control schemes.
- Covered content
The economic operation optimization of microgrids should fully consider important factors such as operation mode, optimization of dispatching objectives, economic and environmental benefits, optimization algorithms, optimization of dispatching strategies and operation constraints of microgrids, so as to maximize the use of renewable energy, reduce greenhouse gas emissions and reduce system power costs on the basis of meeting the quality of load power supply and power supply reliability.
Microgrid system is a new type of autonomous power system that realizes the large-scale application of distributed generation technology and effectively improves the reliability of system power supply. Microgrids are a complex system with nonlinearity, multiple constraints, multiple time scales and multiple dimensions.
The characteristics and key technologies of microgrid systems should be deeply understood from a system perspective in order to scientifically study and develop and apply microgrids. In order to achieve the goal of safe, stable, reliable and economical operation of the entire microgrid system.
On the basis of comprehensive consideration of the typical characteristics of microgrids, appropriate design principles should be followed, and the key technologies of microgrids should be reasonably selected.