What are the impacts of the microgrid on the distribution network system?
The National Development and Reform Commission and the National Energy Administration issued the "Notice on the List of New Energy Microgrid Demonstration Projects", Development and Reform Energy  No. 870, and 28 new energy microgrid demonstration projects were approved. The new PV installations brought by this batch of projects are 899MW, and the newly added electric energy storage capacity is over 150MW. In addition, various types of energy such as heat storage, wind power and other types of energy are also reflected.
The National Development and Reform Commission of the National Development and Reform Commission issued a notice on the issuance of the "Trial Measures for Promoting the Construction of Grid-Connected Microgrids" to issue Energy  No. 1339. The methods and ideas for the efficient construction of energy production and consumption systems for grid-connected microgrids are proposed.
Since the independent type is separated from the distribution network, only the influence of the grid-connected micro-grid on the distribution network is analyzed here. The advantage of the grid-connected microgrid is that it can solve the problems caused by the large-scale access of distributed renewable energy to the grid. The access of distributed power sources has changed the original single, radial network structure of the distribution network, and its large-scale application will have a greater impact on distribution network planning, power quality, relay protection and reliability. The use of microgrid technology is conducive to the solution of the above problems.
1. Improvement of power quality
In the power system, the users who are sensitive to the quality of the power supply originally had many new requirements, and the power supply quality has been put forward. Therefore, the power grid company needs to continuously improve the power quality. The access of the microgrid has the following improvements to the power quality of the distribution network:
(1) When the grid peak load or some emergency situations, the micro grid can increase the output quickly and provide emergency support for part of the load.
(2) Distributed power sources such as photovoltaic power generation systems and wind power generation systems are affected by natural weather, and the output power is volatility, randomness, and intermittent. In this regard, the microgrid can realize the power balance adjustment in the microgrid through comprehensive control of the controllable power source such as the gas turbine and the energy storage device, and reduce the adverse influence of the intermittent distributed power source on the power grid.
(3) In the microgrid, distributed power and power quality regulators can achieve optimal configuration and unified control, and even integrated multiplexing technology can be used to improve equipment utilization efficiency.
2. The complexity of relay protection
The different types of distributed power supplies included in the microgrid make them exhibit completely different fault characteristics in the event of an external short-circuit fault. In general, distributed power sources can be divided into two categories, one is a distributed power supply based on a rotating generator, and the other is a distributed power supply with an inverter interface. When the current limiting device is not installed at the common connection point of the microgrid, the short-circuit current that the microgrid can provide externally is directly related to the type of internal distributed power supply.
At present, the operating structure of China's medium and low voltage distribution network is generally a radiation-type power supply network with single-sided power supply. Distribution line feeder protection is generally configured with traditional three-stage current protection. After the microgrid is connected, the distribution network is changed from a single-ended power supply system to a double-ended power supply system. At this time, protection devices need to be installed at both ends of the feeder connected to the microgrid, and directional components must be added as appropriate. Therefore, the protection configuration scheme in the grid-connected microgrid must fully consider the impact of the large difference in the magnitude of the short-circuit current in different operating states.
3. Reliability improvement
Users on the traditional low-voltage distribution network are highly susceptible to faults on the feeder line, and the recovery time is long. The lack of necessary countermeasures for sudden failures is difficult to meet the requirements for reliability. With its advanced monitoring and control technology, the microgrid can detect faults in the upper feeder or microgrid internal lines and components in real time, or power quality problems. In order to ensure that the power supply of the users in the network is not affected or the faults in the micro-grid do not affect other users on the upper feeder, if necessary, the micro-grid control system will control the micro-grid from the main network, only by the internal distributed power supply and energy storage equipment. Power is supplied to form a small power supply network to supply power to users in the network. The flexible switching and complementary functions between the main network power supply and the internal power supply of the microgrid greatly increase the reliability of the power supply. Since the microgrid can reduce the load on the medium voltage line, it can also help the reliability of other users on the medium voltage network and the entire distribution network. Under the fault, especially in the case of power outage caused by natural disasters, the microgrid According to the specific situation, it can bear the power supply or black start function to the important load outside the micro grid.
The microgrid has small capacity, good stability, strong flexibility, adaptability to the grid, and can improve the stability of voltage and frequency when faults occur. At the same time, the microgrid has the characteristics of self-management and self-control, which can be connected to the grid and independently operated, and can realize the flexible exchange of two-way energy with the grid. Compared with European and American countries, there are still some gaps in China. It is urgent to further study the key technologies of micro-grid, explore a low-carbon energy development road suitable for China's national conditions, and promote the comprehensive application of intelligent micro-grid in China.