Microgrid technology

Energy is the driving force of modern society and economic development, and the lifeline of the existence and reproduction of human life. The gradual depletion of traditional fossil energy has brought the energy crisis closer. China's energy industry in the 21st century will be a new and transformative new industry for the sustainable development of energy resources and environmental protection. Therefore, rationally adjusting the energy structure, vigorously developing renewable energy and other new energy sources, and taking the road of diversified clean energy development is China. The only way for sustainable social development.

The microgrid is a new type of network structure, which is a system unit composed of a set of micro power supply, load, energy storage system and control device. The power sources in the microgrid are mostly distributed power sources with small capacity, that is, small units containing power electronic interfaces, including micro gas turbines, fuel cells, photovoltaic cells, small wind turbines, and energy storage devices such as supercapacitors, flywheels, and batteries. They are connected to the user side and have the characteristics of low cost, low voltage and low pollution. The development and extension of microgrid can promote large-scale access of distributed power and renewable energy, and achieve high-reliability supply of multiple energy sources. It is an effective way to realize active distribution network, making traditional power grids Smart grid transition.

1. The meaning and research dynamics of microgrid

At present, many countries in the world have carried out micro-grid research, and based on the actual problems of the domestic power system, put forward their respective micro-grid concepts and development goals. As a new technology field, the development of microgrids in different countries presents different characteristics.

1.1 Research on American Microgrid

The ERTS (Consortium for Electric Reliability Technology Solutions) cooperative organization consists of research organizations such as the US Power Group and the Berkeley Lawrence National Laboratory. It has conducted special research on microgrid technology with funding from the US Department of Energy and the California Energy Commission. The basic concept of microgrid defined by CERTS: This is a collection of loads and micropower supplies. The micropower supply operates in a system that provides both electrical and thermal power, and most of these micropower supplies must be power electronic and provide the required flexibility to ensure operation in an integrated system that is controlled Flexibility enables the microgrid to act as a controlled unit for large power systems to accommodate the reliability and safety requirements of local loads. The microgrid defined by CERTS proposes a new approach to a distributed power access system that is completely different from the previous one. The traditional approach focuses on the impact of distributed power on network performance when considering distributed power access systems. When there is a problem with the power grid in the traditional way, it is necessary to ensure that the networked distributed power supply is automatically shut down, so as not to adversely affect the power grid. The microgrid defined by CERTS should be designed such that when the main grid fails, the microgrid and the main grid are seamlessly disjointed or run into islands. Once the fault is removed, it can be reconnected with the main grid. The advantage of this kind of microgrid is that it is regarded as a self-control entity in the power distribution system connected to it, ensuring the uninterrupted power supply of important users, improving the reliability of power supply, reducing feeder loss, and supporting local voltage. Correction effect. Therefore, the microgrid not only avoids some negative effects of traditional distributed generation on the distribution network, but also plays a certain supporting role for the distribution network of the microgrid access point.

1.2 Research on European microgrid

Europe proposes to make full use of distributed energy, intelligent technology, advanced power electronics technology to achieve efficient and close integration of centralized power supply and distributed power generation, and actively encourage the community to participate in the power market and jointly promote the development of the power grid. Microgrid has become an important component of Europe's future power grid due to its intelligence and diversified energy utilization. At present, the theory of operation, control, protection, safety and communication of microgrids has been initially formed in Europe, and these theories have been verified on the laboratory microgrid platform. Its follow-up tasks will focus on researching more advanced control strategies, developing appropriate standards, and establishing demonstration projects. That is, it is actively preparing for the large-scale access of distributed power and renewable energy and the initial transition of the traditional power grid to the smart grid.

1.3 Research on Japanese Microgrid

Based on the reality that domestic energy is increasingly scarce and the load is increasing, Japan has launched microgrid research, but its development goals are mainly aimed at diversifying energy supply, reducing pollution, and satisfying users' individualized power needs. Japanese scholars have also proposed flexible reliability and intelligent energy supply system (FRIENDS). The main idea is to add some flexible AC transmission system devices in the distribution network, and use the controller to quickly and flexibly control the performance to achieve energy distribution to the distribution network. The structure is optimized and meets the various power quality requirements of the user. At present, Japan has adopted this system as one of the important implementation forms of its microgrid.

1.4 Understanding of the microgrid

Based on the above concepts, micro gas turbines with a power range below 100 kW in the microgrid will be widely used. It has high speed (50,000 to 100,000/min) and air bearing. The high frequency (about 1000Hz) AC power needs to be converted into 50Hz power frequency AC power supply through AC-DC-AC link, but the combustion process. The resulting NOx will still have an adverse impact on the city's environmental protection. Due to its high efficiency and low emission, fuel cells are naturally suitable as power sources for microgrids. Especially high temperature MCFCs and SOFCs are more suitable for power generation, but they are more expensive and less practical. Photovoltaic power generation, small wind power and biomass power generation are good power options. Batteries, flywheels and supercapacitors are important energy storage components for microgrids. The waste heat recovery device is also one of the important components. It is precisely because the utilization of waste heat improves the efficiency of energy utilization, because hot water or hot steam is not as easy and economical to transport as long as electricity, and the structure of the microgrid can The heat source is closer to the heat load.

2. The significance of China's development of microgrid

The microgrid is also of great significance to the development of China's power system and national economy.

(1) The microgrid can improve the safety and reliability of the power system and contribute to the building of the disaster resistance capability of the power system. At present, the development of China's power industry has entered the stage of large power grid, high voltage, long distance and large capacity. The six regional power grids have been interconnected and the grid structure is increasingly complex. Realizing the exchange and interconnection between regions, in theory, we can play an inter-regional accident support and backup role to achieve optimal allocation of power resources. However, large-scale AC synchronous grids have large-area low-frequency oscillations and instability, and their dynamic stability accidents are difficult to control, resulting in a large possibility of large-scale power outages. On the other hand, after the separation of the plant and the network, the market interests are diversified, the contradiction between the plant and the network increases, and the coordination of the plant network is more difficult. In particular, the safety management of the grid equipment is not in place, posing a threat to the safe and stable operation of the power system. Compared with the conventional centralized power supply station, the microgrid can be combined with the existing power system to form a new system with high efficiency and flexibility. It has the following advantages: no need to build a substation, it can avoid or delay the increase of transmission and distribution costs, no or very Low transmission and distribution loss can reduce the cost of end users; Miniaturization, low requirements for construction, no occupation of transmission corridors, short construction period, high efficiency and flexibility, can quickly cope with short-term surge in power demand, high reliability of power supply At the same time, it can also reduce pollution to the environment.

(2) Microgrid can promote the integration of renewable energy distributed generation, which is conducive to the development of renewable energy in China. The large number of distributed power grids on the edge of power system management may cause the power system to be uncontrollable, unsafe and unstable, thus affecting grid operation and power market transactions. Therefore, distributed power generation faces many technical obstacles and doubts. Micro-grid can give full play to the advantages of distributed generation and eliminate the impact and negative impact of distributed generation on the grid. It is a new concept, using the system to solve the problems caused by distributed generation. By combining a micro power supply, energy storage device and load with similar geographical proximity for coordinated control, the microgrid presents a single controllable set of “grid-friendly” to the distribution network, which can exchange energy with the large power grid. It can operate independently when the grid fails.

(3) The microgrid can improve the reliability of power supply and the quality of power, which is conducive to improving the service level of power grid enterprises. The microgrid can provide differentiated electrical energy according to the needs of the end users, and classify the load according to the different needs of the microgrid users for the power supply to form a pyramid-shaped load structure. The idea of ​​load grading reflects the characteristics of the individualized power supply of the microgrid. The application of the microgrid is beneficial to the grid enterprises to provide different power quality and power supply reliability to different end users.

(4) Microgrid can delay grid investment, reduce network loss, and help build a conservation-oriented society. The traditional power supply method is the power generated by a centralized large-scale power plant, which is powered by the long-distance, multi-stage transmission of the power system, that is, “local consumption”, thus effectively reducing the power to centralized large-scale power plants. The dependence of production and the loss of long-distance power transmission and multi-stage transmission, thus delaying grid investment and reducing network loss.

(5) The microgrid can alleviate poverty and is conducive to the construction of a new socialist countryside. The microgrid can effectively solve the problem of long transmission distance, low power, large line loss and expensive construction substation in the current power supply in western China, and provide strong support for power supply in areas that are difficult to cover in China's remote and conventional power grids. .

3. Microgrid structure

Relative to the power system, the microgrid is similar to a separate control unit, each of which has a sophisticated plug-and-play function. For each micro power supply, the most critical is its own interface, control, protection and voltage control of the microgrid, power flow control and maintain its operational stability. An important function is the smooth transfer between the networking operation of the microgrid and the operation mode of the island. In the microgrid, in order to prevent the impact of the microgrid and the distribution network on the internal load of the microgrid, the power distribution structure of the microgrid needs to be redesigned, and the unimportant load is connected to the same feeder, important or sensitive. The load is connected to another feeder. Distributed power supplies, energy storage components and corresponding control, regulation and protection devices are installed on the feeders connected to the sensitive load. In this way, when the microgrid and the main network are unpacked, some unimportant loads can be removed by the isolation device, but normal and continuous operation of some important loads can still be guaranteed.

The microgrid has functions such as control, coordination, and management, and is implemented by the following systems.

(1) Micro power supply controller The micro power grid mainly relies on the micro power supply controller to adjust the feeder power flow, the bus voltage level and the solution of the main network, and the grid operation. Due to the pull-and-plug function of the micro power supply, the control relies mainly on the local signal and the response is on the order of milliseconds.

(2) Protection Coordinator The saturation coordinator is applicable to both the fault of the primary network and the failure of the microgrid. When the main network fails, the protection coordinator should isolate the important load in the microgrid from the main network as soon as possible. In some cases, the important load in the microgrid allows the voltage to sag for a short time. After taking certain compensation measures, the microgrid can not be separated from the main network. When a fault occurs in the microgrid, the protection should isolate the fault segment to the smallest possible extent. (3) Energy Manager The energy manager schedules the system according to the preset values ​​of voltage and power, and the corresponding time is minute.

4. Microgrid related technologies

Microgrid is a new type of network structure Microgrid technology has become the cutting-edge technology for the development of power systems.

(1) Hardware research of microgrid

The implementation of the microgrid requires advanced equipment for support. This includes the development, transmission, transformation, distribution and use of the microgrid. To this end, it is necessary to develop smart meters, vector measurement units, wide-area measurement systems, etc., to develop appropriate hardware devices, so that the micro-grid has the ability to plug and play. Develop new distributed energy controllers to ensure efficient operation of the microgrid.

(2) Microgrid Modeling Research

Develop static and dynamic simulation tools for low-voltage asymmetric microgrids that can be used to control inverters; establish models of components within the microgrid, including models of distributed power and load; establish an overall model of the microgrid, including the overall model structure, Equivalent static model, equivalent motor model, etc.

(3) Study on the impact of microgrid on large power grid

The access of the microgrid will inevitably affect the large power grid. It needs to be studied: the stability analysis of the microgrid under grid-connected and island operations; the impact of the microgrid on the operation of the large grid, including regional and wide-area effects; The grid can improve the power grid reliability, network loss and environment; the impact of microgrid development on the development of the grid.

Micro-power supplies in microgrids, such as wind power and photovoltaic power generation, mostly use fully-controlled inverters. The introduction of these power electronic devices is likely to bring about some harmonic problems. For the microgrid harmonic problem needs further discussion and research.

(4) Microgrid control strategy

There is an optimal state between the microgrid and the large grid, in which the microgrid and the large grid can operate efficiently and stably. The goal of controlling the microgrid is to achieve optimal control of the microgrid. To this end, we must study microgrid control technology, including: coordinated control between micro-power supplies, intelligent control and optimal control of power electronic equipment, coordinated control between micro-grid and main network, etc., research on isolated islands and interconnected Operational concepts, agent-based control strategies, local black start strategies, control strategies based on advanced communication technologies, etc.; research to create new network design concepts, including the application of new protection schemes.

(5) Other

The implementation of the power grid also needs a lot of support: the technical and commercial agreement standards of the microgrid need to be established; the integration of various microgrids in technology and business needs to be completed; the existing small generator sets need to be integrated into the microgrid Feasibility analysis; need to establish microgrid demonstration project and experimental test system.

5. Key technologies of the microgrid

The emergence of microgrid will fundamentally change the way traditional grids cope with load growth, which has great potential in reducing energy consumption and improving power system reliability and flexibility. At present, microgrid technology has become the cutting-edge technology for the development of power systems.

5.1 Microgrid control function

The basic requirements of the microgrid control function include: the new micro power supply does not change the original equipment when it is connected, the microgrid solution is parallel and fast and seamless, and the reactive power and active power must be independently controlled, voltage sag and The system imbalance can be corrected to accommodate the dynamic demands of the load in the microgrid. The control functions of the microgrid mainly include the following:

(1) Basic active and reactive power control (PQ control). Since the micro power supply is mostly of the power electronic type, the control and adjustment of the active power and the reactive power can be separately performed, and the reactive power can be controlled by adjusting the voltage amplitude of the inverter, and the voltage of the inverter and the network voltage can be adjusted. Phase angle to control the working power.

(2) Voltage adjustment based on adjustment. It is not appropriate to use PQ control when there is a large amount of micro power supply. If local voltage control is not performed, voltage or reactive oscillation may occur. The voltage control must ensure that there is no reactive current circulation between the power supplies. In large power grids, this is not the case because the impedance between the power supplies is relatively large. In the microgrid, as long as the voltage setting has a small error, a large reactive circulating current may be generated, so that the voltage value of the micro power source exceeds the standard. Therefore, it is necessary to determine the setting value of the voltage according to whether the current generated by the micro power source is capacitive or inductive, and the voltage setting value is reduced when the current is generated, and the voltage setting value is increased when the inductive current is generated.

(3) Fast load tracking and energy storage. In a large power grid, the initial energy balance when a new load is connected depends on the inertia of the system, mainly because the large generator is inertia, and only the system frequency is slightly reduced (almost undetectable). Since the inertia of the generator in the microgrid is small, the response time constant of some power sources (such as fuel cells) is very long (10~200s). Therefore, when the microgrid and the main network are decomposed into island operations, batteries and supercapacitors must be provided. Energy storage equipment such as flywheels is equivalent to increasing the inertia of some systems to maintain the normal operation of the grid.

(4) Frequency adjustment control. When the microgrid is operated as an island, frequency adjustment control should be adopted to change the load ratio of each unit so that the respective output can be adjusted according to a certain proportion without exceeding the standard.

5.2 Microgrid protection

The microgrid structure puts forward some special requirements for relay protection. The main factors that must be considered are the following: 1 The distribution network is generally radial. Due to the micro power supply, the current flowing through the protection device may have a single The direction becomes bidirectional; 2 once the micro-grid is operated, the short-circuit capacity will have a large change, affecting the normal operation of some of the original relay protection devices; 3 changing the way of the original single distributed generation access to the grid One of the original intentions of the microgrid is to maintain as much as possible that some important loads can operate normally in the event of a grid failure without interrupting the power supply. These must use some fast-acting switches to replace the original relatively slow-moving switches. . These can change the original protection devices and strategies.

5.3 Microgrid connected to the grid

The protection scheme is determined according to the demand of the load in the microgrid, that is, the protection is based on the sensitivity of the load (such as the semiconductor manufacturing industrial load or the general commercial load) to the voltage change and the control standard. If the fault occurs in the distribution network, a high-speed switch type isolation device (SD) is used to isolate the important sensitive load in the microgrid from the fault as quickly as possible. At this time, the DR (or DER) in the microgrid should not be tripped to ensure that the critical load can still be supplied normally (heating) after the fault isolation. When the fault occurs in the microgrid, in addition to the above-mentioned isolation device coordination, so as not to affect the upper feeder load. Once the distribution network returns to normal, the microgrid should be reconnected to the grid either automatically or manually by measuring and comparing the amplitude and angle of the voltage across the SD. If there is only one micro power supply in the microgrid, of course, it is allowed to resynchronize the grid by manual method. However, if there are multiple micro power sources in multiple locations in multiple locations in the microgrid, it must be considered to be resynchronized in an automatic manner. network.

5.4 Microgrid island operation

When the grid island is operating, the coordination of the protection devices in the microgrid is particularly important in order to keep the isolated fault zones as small as possible. In particular, since the power supply of the microgrid is mostly power electronic equipment, the power generated is connected to the network through the inverter, and only a small short-circuit current is provided during the fault (for example, 2 is under normal load current), which is difficult. Start a conventional overcurrent protection device. Therefore, the protection device and strategy should be modified accordingly, such as impedance type, zero sequence current type, differential type or voltage type relay protection device. In addition, the grounding system of the microgrid must be carefully designed to avoid malfunction of the relay protection when the microgrid is disconnected.

5.5 Microgrid Energy Management System

A microgrid is defined as a collection of power generation and load, while a typical load includes not only electrical loads, but also hot and cold loads, ie, combined heat and power and combined heat and power. Therefore, the microgrid not only needs to generate electricity, but also uses the waste heat of power generation to improve the overall efficiency. The purpose of the Energy Management System (EMS) is to make decisions to optimally utilize the electricity and heat (cold) generated by power generation. The decision is based on local equipment's demand for heat, climate, electricity, and fuel costs.

Dispatching control function of energy management system: The energy management system is for the whole microgrid, which is system level. The first task is to clearly distinguish between equipment control and system control, so that their respective functions and functions are simple and clear. The speed, frequency, terminal voltage of the microturbine, power factor of the generator (micro power supply), etc. should be controlled by the micro power supply, and they are based on the local signal. In the CERTS model, the EMS only schedules the system's currents and voltages. Fuel cost, power generation cost, electricity price, and climatic conditions must be considered when tidal scheduling. The EMS only controls the voltage amplitude of some key busbars in the microgrid and is completed by the controllers of multiple micropower sources. The busbar voltage associated with the distribution network should be controlled by the dispatching system of the associated distribution network.

In addition to the above basic functions, EMS has other functions. For example, when the microgrid and the distribution network are disconnected, the microgrid should be equipped with a fast load shedding function to balance the power generation and load in the microgrid; For electric and thermal loads, the scheduling should be balanced at the same time. Under normal circumstances, the principle of “heating the electricity” is adopted, that is, the power consumption is scheduled to meet the user's demand for heat load; the micro-grid should be equipped with some energy storage. Equipment, such as batteries, super capacitors, flywheels, etc.

The function of EMS should naturally first address the needs of the microgrid, such as power flow and voltage regulation, power quality and reliability, improve operational efficiency and economy, reduce pollution emissions, etc., but in the long run it can also provide some to the distribution network. Auxiliary services and reliability services, especially the microgrid as an integral part of the smart grid, can play a role in load response. In addition, since the microgrid itself is located on the user side, these users may be central business districts (CBDs), schools, factories, etc. They have process control systems for heating, ventilation, air conditioning, etc., and future EMS (Heating Ventilation and Air Conditioning) , HVAC) It is possible to become the total dispatch system for these systems as well as local power generation, energy storage, etc.

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