Distributed energy is in a stage of rapid development in China. With the help of policies and capital, during the 13th Five-Year Plan period, China's distributed photovoltaic projects will be fully blossomed and enter the fast lane of accelerated development.
The development of renewable energy and the construction of sustainable energy systems have become an inevitable trend in the development of the energy industry. Due to the proximity of power users and the realization of power consumption, distributed power can save power and transformation investment and operation costs, reduce line loss of centralized transmission; complement power supply of large power grids, reduce power grid capacity, and improve peak and valley performance of power grids. The advantages of improving power supply reliability and reducing environmental pollution have been widely concerned, especially in recent years, photovoltaic and wind power generation have been rapidly developed. However, due to the uncontrollability and stochastic volatility of renewable energy generation, the increase in permeability has also increased the negative impact on the stability of the power system.
In order to adapt to the rapid development of distributed energy, a new distributed energy organization and structure, microgrid, has been proposed in recent years. A microgrid is a collection of distributed power sources, energy storage power supplies, loads, and monitoring and protection devices. The microgrid contains various power sources in the form of micro-power sources such as distributed photovoltaics, decentralized wind power, small gas turbines and batteries. It is also equipped with an energy management system that can solve voltage control and power flow control through data acquisition and connection to the energy management system. A series of problems such as protection control. There are different types of loads in the microgrid, different energy supply strategies need to be adopted, and the requirements of power quality and power supply safety can be satisfied at the same time.
In addition, the microgrid is a single controlled unit relative to the external large grid. Through the connection between the main isolator and the microgrid, grid-connected operation with the main grid can be realized, and the power quality of the main grid can be improved.
According to the microgrid itself and combined with distributed power, the microgrid has the following characteristics:
(1) It is a new type of energy network supply and management technology that coordinates the distributed distributed power sources to ensure the reliability and safety of the distribution network, facilitates the access of renewable energy systems, and realizes user requirements. Side management and maximization of existing energy and resources;
(2) It has flexible operation mode and schedulable performance, and can realize switching between two modes: grid-connected operation and island-running operation. The flexible parallel operation mode with large power grid can play the role of cutting peaks and filling valleys. The contradiction between the power grid and the distributed power source, fully exploiting the value and benefits of distributed energy for the power grid and users;
(3) Realizing local conversion and consumption of energy, improving power generation efficiency, and reducing losses due to long-distance transmission;
(4) Through the coordination and cooperation between the relevant control devices, various forms of energy such as electricity, heat, cold and gas can be provided to the user at the same time.
(1) Power electronics technology
According to the special needs of the microgrid, it is necessary to study the applicable power electronics technology and develop some new types of power electronic equipment, such as converters, static switches and power quality controllers.
First of all, the emergence of converters is of great significance for the transformation, transmission and storage of electrical energy in distributed power sources. According to the use of primary energy, the distributed power source can be divided into a DC source type distributed power source and a rectified high frequency AC source type distributed power source, the former including solar energy, a battery and a fuel cell, etc., the latter including a miniature Gas turbines and wind turbines, etc. In the final stage of the use process, these two distributed power supplies are converted into standard power frequency AC power to supply power to the grid or load, that is, the converter (rectifier, inverter) needs to be connected to the microgrid system.
Because the converter has the characteristics of fast response, small inertia and strong overcurrent capability, the control concept of the microgrid energy management is very different from the conventional system. At the same time, inverters suitable for microgrids need to have the functions of conventional inverters and can operate in parallel. They also need to have some control functions according to the special requirements of the microgrid system, such as active-frequency droop control and voltage-none. Power droop control function. Therefore, the operational control of the inverter has become an important aspect in the research of the microgrid.
Then there is the static switch. The static switch is placed at the common connection point between the connected piconet and the main network. When some disturbances such as main network failure or power quality event occur, it can automatically switch the piconet to the island/autonomous operation state; when the fault is After the exclusion, it can also automatically reconnect the micro network and the main network. In addition to the above switching functions, the static switch also needs to provide protection, measurement and communication functions provided by relays, DSPs and other hardware components in the conventional power system.
There is also a need for a related power quality control system. The access of any distributed power generation unit will have certain influence on the power quality in the system. If it is not properly controlled, it will be detrimental to the voltage waveform, frequency and power factor, especially the volatility of primary energy such as scenery. Also, electronic loads are susceptible to transients, drops, harmonics, momentary interruptions, and other disturbances.
(2) Communication technology
The operation of the microgrid is based on the collection of distributed energy unit information with different characteristics, and the connection is established between devices with different response characteristics, and is realized by mutual communication between the controllers at the distribution network level, the micro network level, and the unit level. The characteristics of distributed energy units with power electronic devices as interfaces are quite different from those of conventional synchronous machines, so higher requirements are placed on the reliability and speed of communication technologies. Communication technology is also directly related to the ability of the micro-network to provide ancillary services faster.
(3) Monitoring and control system
In order to be able to integrate with the existing power system, the normal operation of the microgrid needs to be realized through coordination and cooperation between the following three levels of monitoring and control systems with the support of the communication network.
a. Unit level: The local controller of each distributed power generation unit and load adjusts the system voltage and frequency to facilitate the stable operation of the microgrid system.
b. Micro-network level: The micro-network central controller is the interface between the primary network and the micro-network. On the one hand, it interacts with the upper-layer distribution network level controller, and interacts with the local controllers of the lower layer on the other hand.
c. Distribution network level: distribution network controller for controlling the area containing one or more micro-networks; market controller, responsible for the function of the electricity market in each specific area. These two controllers belong to a hierarchical system on the micro-network, and realize the scheduling function of the distribution network at the primary network.
(4) Fault detection and protection
The microgrid protection problem is very different from the traditional distribution network protection, which is typically manifested in the following aspects:
a. The microgrid has different short-circuit currents in the two operation modes of grid-connected operation and island operation, and the difference is very large.
b. The grounding mode of the power supply and related loads in the microgrid has a great influence on the magnitude of the fault current.
c. After adding distributed energy, the bidirectional tidal current characteristics of the microgrid also have new requirements for the direction of microgrid protection.
d. When a fault occurs, the different isolation strategies of the microgrid and the distribution network need to be combined with the microgrid protection, which is also the key to microgrid protection.
Therefore, under the premise of combining the microgrid and distribution network isolation strategy and different grounding methods, a new microgrid protection technology is explored, so that the microgrid can be quickly perceived in the grid-connected operation mode or island operation. External faults, while ensuring the selectivity, speed, sensitivity and reliability of protection, are the basic requirements for microgrid protection.
Microgrid operation managementThe operation mode of the microgrid, the power market and energy policy adopted, the type of distributed energy in the system, the permeability, the load characteristics, and the requirements for power quality are quite different from those of the conventional power system. Therefore, it is necessary to develop a reasonable control strategy for the operation scheduling and energy optimization management system between the distributed energy units within the microgrid system, between the individual micro-networks and the main network, or even between multiple micro-networks, in order to ensure the security of the micro-network. , stability and reliability for efficient and economical operation.
When the microgrid system is in the grid-connected state, how to interact with the main network is related to the adopted market strategy. A market strategy is that the microgrid uses the internal distributed energy unit to try to meet the load demand within the network, and can absorb power from the main network but does not output power to the main network; the other is to allow the micro net to participate in open power. In the market, power can be exchanged freely with the main network, and in addition to the participating power generation units participating in the bidding, the demand side can also participate in market transactions.
In addition, the micro network can also participate in the auxiliary service market. The microgrid can provide auxiliary services such as frequency modulation, peak shaving, voltage regulation, standby, black start, etc. while providing energy to the main network; or operate in a controlled load form to control the load and power factor. The network system is important when it is overloaded.
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