Key Technologies and Related Research of Smart Microgrid

Smart microgrid refers to a small power generation and distribution system composed of distributed power sources, energy storage devices, energy conversion devices, related loads, monitoring and protection devices. It is an autonomous system capable of self-control, protection and management. It can run in parallel with the external power grid or in isolation.

Smart microgrid is a modernized and miniaturized form of large-scale power system, which can provide higher reliability of power supply, meet the growing needs of users more easily, utilize clean energy as much as possible and promote technological innovation. It is the intelligent optimization and management of multiple energy generation equipment and end-user equipment, which can maximize investment benefits while achieving sustainable development goals. The smart microgrid meets the higher development needs of the microgrid for future power, energy, environment and economy by adopting advanced power technology, communication technology, computer technology and control technology on the basis of realizing the existing functions of the microgrid.

1. Background of smart microgrid

At the beginning of the 21st century, several large and protracted power outages occurred successively around the world, which made the operation of large-scale power system centralized power generation difficult, and the disadvantages such as difficulty in meeting users' higher requirements for power quality became increasingly prominent. At the same time, the energy crisis and environmental pollution have also attracted global attention. It is obvious that these problems cannot be solved only by expanding the scale of the power grid. Therefore, distributed power generation has emerged as an effective supplement to centralized power generation. It has less pollution, high reliability, and energy efficiency. High, flexible installation location and many other advantages, effectively solve many potential problems of large-scale centralized power grid.

However, distributed power has the characteristics of intermittent, random, slow response, small inertia, etc., and is not easy to control. After grid connection, it is easy to cause voltage fluctuations and voltage flicker, especially when large-capacity distributed power is incorporated into medium and low voltage power distribution. It is difficult to realize the power balance of the distribution network and ensure the reliability and quality of power supply.

The concept of microgrid is proposed to solve the technical, market and policy problems brought about by large-scale and multi-type distributed power grid connection, and to maximize the advantages of distributed power generation technology in economy, energy and environment. Many countries such as Europe, the United States, and Japan have proposed the concept of micro-grids based on their actual conditions and actively carried out related research. So far, micro-grids have achieved fruitful results in theory and application, and are gradually developing in the direction of intelligence. Becoming an important organic part of the smart distribution network is an important part of the construction of the smart grid.

2. Concept and Features of Smart Microgrid

2.1 The concept of microgrid

In 2001, the United States Consortium for Electrical Reliability Technology Solutions (CERTS) first gave the definition of microgrid: a microgrid is a system composed of loads and micro power sources, which can provide electricity and heat at the same time; The power supply is mainly responsible for energy conversion by power electronic devices and provides necessary control; compared with the external large power grid, the microgrid is a single controlled unit, and can meet the user's requirements for power quality and power supply security at the same time. Subsequently, microgrids have grown significantly in many countries and their definitions have evolved. The definition given by the EU microgrid project is: use of primary energy; use of micro power sources, which are divided into three types: uncontrollable, partially controllable and fully controllable, and can be combined with cold, heat and electricity; equipped with energy storage devices; use electricity Electronics perform energy regulation. The concept given by R.H.Lasseter of the University of Wisconsin-Madison in the United States is that the microgrid is an independently controllable system composed of loads and micro power sources, which provides electricity and heat to the local area. In March 2009, my country's "Microgrid Technology System Research" working conference held by the State Grid Electric Power Research Institute defined the microgrid as: a microgrid refers to a small power station based on distributed power generation technology and relying on decentralized resources or users. Based on the combination of end-user power quality management and energy cascade utilization technology to form a small modular, decentralized energy supply network.

Although there are different definitions of microgrid at home and abroad, the common view is that microgrid is an integrated unit based on distributed power generation technology and integrated with energy storage devices, control devices and protection devices; it is close to user terminal loads; The input voltage level is the distribution network; it can work in both grid-connected and autonomous modes, improve power supply reliability, intensively apply distributed new energy, and improve power quality [3].

2.2 Concept of Smart Microgrid

As an important organic part of the smart distribution network, the microgrid is its basic requirement to realize intelligence in technology and equipment.

The concept of intelligent micro-grid has not yet been unified internationally. It is considered that the smart microgrid is a modernized and miniaturized form of the large-scale power system, which can provide higher power supply reliability, meet the growing needs of users more easily, make the most possible use of clean energy and promote technological innovation. Valence Energy believes that smart microgrid is the intelligent optimization and management of multiple energy generation equipment and end-user equipment, which can maximize investment benefits while achieving sustainable development goals. Chinese scholars believe that: smart microgrid is the intelligence of microgrid. By adopting advanced power technology, communication technology, computer technology and control technology, on the basis of realizing the existing functions of microgrid, it can meet the requirements of microgrid for future power, energy and environment. and higher economic development needs.

According to the definition of smart microgrid, it can be seen that smart microgrid has the following characteristics:

2.2.1 Grid-connected and independent operation modes. This is the essential difference between smart microgrid and distributed generation system with load. When the smart microgrid is connected to the grid, it can cut peaks and fill valleys for the large power grid, which is a strong support for the stable operation of the large power grid. When the power grid fails, the smart microgrid can quickly detach from the large power grid and operate independently, continuously supplying power to important loads such as governments, hospitals, and transportation hubs, improving the reliability of power supply.

2.2.2 Stability. When the microgrid operates independently, it can achieve power balance and voltage/frequency stability in steady state and transient state, effectively solve voltage and harmonic problems, and avoid the direct impact of intermittent power supply on the power quality of surrounding users.

2.2.3 compatible. Microgrid is the most effective way to realize the grid connection of distributed power sources. It integrates the original layout of scattered renewable energy, and smooths power fluctuations in real time through energy storage devices and control and protection devices to maintain supply and demand balance and system stability. It can effectively Overcome the random and intermittent shortcomings of distributed power, and solve the access problem of distributed power.

2.2.4 Flexible. The smart microgrid can be used as a controlled unit of the large power grid to realize the "plug and play" of distributed power generation, and can flexibly provide power quality with different prices and levels according to user needs to meet the diverse needs of users.

2.2.5 Economy. As an effective carrier of renewable energy power generation, the microgrid can significantly improve the utilization efficiency of renewable energy. At the same time, the combination of microgrid and small and medium-sized cogeneration can reduce the conversion of different energy forms by realizing temperature matching, cascade utilization and energy quality matching. , to meet the various needs of users for power supply, heating, cooling, humidity control and domestic water, thereby significantly improving energy utilization efficiency, optimizing energy structure, reducing pollution emissions, and achieving the goal of energy saving and consumption reduction, and the microgrid has realized market transactions and Unified management of asset allocation.

3. Basic structure and operation status of smart microgrid

The structure of the smart microgrid varies with different load requirements, but its basic units include distributed power sources (micro energy sources), energy storage devices, management systems, and loads. Most of the interfaces between micro energy sources and the grid are required to be based on power electronics technology Designed to ensure the flexibility and reliability of the microgrid operating as a single system.

The microgrid has two stable operating states of grid-connected and independent (isolated) and two corresponding transitional operating states--grid-connected transition state and micro-grid disconnected transition state.

4. Key issues and related research in smart microgrid

The smart microgrid can be regarded as a small power system, and its power supply security, protection and control issues also need attention, but there are many differences between the smart microgrid and the traditional power grid in terms of function and structure, so the focus of attention and research methods Also very different. In order to realize the seamless access of various distributed power sources, the smart micro-grid needs to solve a series of complex problems, mainly including: the operating characteristics of the smart micro-grid; the protection and control of the smart micro-grid itself; the grid-connected standard of the smart micro-grid; Operation control and energy management of smart microgrid.

4.1 Operating characteristics of smart microgrid

Since the smart microgrid has two operating states, grid-connected and islanded, its operating characteristics also include two aspects: one is its own operating characteristics when the smart microgrid operates in an isolated state. Various energy inputs (light, wind, hydrogen, natural gas, etc.), various energy outputs (electricity, heat, cold), and various energy conversion units (light/electricity, heat/electricity, wind/electricity, etc.) AC/DC/AC) and multiple operating states (grid-connected, independent) make the dynamic characteristics of the microgrid more complex than a single distributed generation system. In addition to the dynamic characteristics of each distributed generation unit, the network structure and The type of network (DC microgrid or AC microgrid) will also affect the dynamic characteristics of the microgrid to a certain extent; on the other hand, when the smart microgrid is connected to the grid, it interacts with the external grid. It involves the study of the mechanism and corresponding measures of the micro-grid on the stable operation of the voltage, power angle and frequency of the large power grid, the study of the interaction mechanism between the micro-grid and the control system of the large power grid and the fault process, and the power quality of the large power grid caused by the micro-grid. It is the theoretical basis to realize the safe and stable operation of the large power grid system including smart microgrid.

Some literatures have established a theoretical system for microgrid analysis at the theoretical level, including: the interaction mechanism between microgrid and large power grid, the function mechanism of distributed energy storage in microgrid, the simulation theory of Electric system planning theory, micro-grid optimization operation theory. There are also studies from the perspective of various operating states of the microgrid, and the interaction mechanism between the microgrid and the large power grid is divided into two categories: one is to explore the interaction between the microgrid and the external The generation mechanism and distribution law of steady-state power quality disturbances characterized by waveform distortion and random fluctuations at the distribution network and its public connection points; the second is to discuss the micro-grid grid connection and disconnection, micro-power supply switching, and load impact. In transient processes such as faults and faults, the generation mechanism and propagation characteristics of transient power quality disturbances in which the voltage or current deviates severely from its rated value or ideal waveform for a short time. In order to conduct in-depth theoretical and experimental research on the operating characteristics of the microgrid, a small laboratory microgrid system was established in some literature. The distributed power in this system adopts the photovoltaic simulation unit and the fan simulation unit, and is incorporated into the microgrid through the power electronic conversion device. The system uses a battery as an energy storage device, and is incorporated into the microgrid through a bidirectional inverter to maintain the transient power balance of the microgrid. When the micro-grid is connected to the network, the external grid voltage and frequency are used as reference, and the battery bidirectional inverter, photovoltaic grid-connected inverter and wind turbine grid-connected inverter adopt constant power control; when the island is running, the control strategy of the bidirectional inverter Switch to constant voltage and constant frequency control to provide microgrid voltage and frequency reference. Experimental results show that the system can work stably in network mode and island mode, and can realize smooth switching between the two, which improves the reliability of energy supply.

4.2 Protection and control of the smart microgrid itself

Control and protection devices are the basic components of a smart microgrid. The smart microgrid is an active network distributed over small and medium-sized power sources and loads. Its internal power flows in two directions, and the short-circuit current varies greatly between grid-connected and islanded operating states. Therefore, fundamental changes have occurred in its protection settings and mechanisms. The protection control should also be able to respond autonomously to events in the power grid based on local information. For example, for voltage drops, faults, power outages, etc., the generator should use local information to automatically switch to the independent operation mode, rather than the traditional way. Coordinated by grid dispatching. Studying the protection and control problems of the smart microgrid itself is the basis for smoothing power fluctuations in real time, maintaining supply-demand balance, and system stability.

Some literatures have analyzed the strategy of microgrid protection from a macro perspective: for the protection of AC microgrid, it is analyzed from the perspective of whether it depends on communication, and the idea of ​​constructing microgrid switching stations is proposed; Based on this, a microgrid-level communication network is constructed, and the current and voltage information of multiple places in the microgrid is used for comprehensive analysis and judgment, so as to realize the protection of the microgrid; two protection strategies for the independent and grid-connected operation of the microgrid are proposed. Design a unified protection strategy so that both independent and grid-connected operation protections are effective; the second is to set restrictive conditions so that only one protection is effective during independent or grid-connected operation.

4.3 Grid-connected standard of smart microgrid

At present, the international standards related to microgrid mainly include the technical regulations on grid connection of distributed power generation and power quality standards in various countries. reference suggestions.

4.4 Operation control and energy management of smart microgrid

Since the smart microgrid contains distributed power sources such as wind energy and solar energy, the randomness is relatively large, and the ability to withstand disturbances is relatively weak, especially in the island operation state, the operation security may face higher risks, so effective operation Control and energy optimization management are the core contents of smart microgrid research.

The key difference between the microgrid energy management system (EMS) and the traditional energy management system is: due to the integration of heat load and electric load in the microgrid, the microgrid EMS needs heat and electricity matching; it can freely exchange energy with the grid; the microgrid EMS can provide classification Under special circumstances, non-critical loads can be sacrificed or response to demand can be delayed, so as to provide high-quality power guarantee for critical loads.

Some studies have pointed out that there are three types of smart microgrid control methods that are relatively mature at present: "plug and play" and "peer-to-peer" control ideas based on power electronics technology; control based on power management systems; microgrid based on multi-agent technology Control Method. Some literature divides microgrid control into two levels: unit level and system level, pointing out that the research on microgrid control needs to model the unit level and system level controllers and management systems respectively, so as to establish the overall operation control and energy optimization management of the system Model. At present, the micro-grid EMS that has been developed can have thermal energy utilization; management of heating, ventilation and air-conditioning systems; control system; energy interaction with distribution network, providing reactive power support and hot backup; grading services to ensure power consumption of important loads, etc. Function. However, improvements are still needed in advanced energy management: development of advanced control strategies, coordination of user control systems; rapid demand-side response based on real-time electricity prices; improvement of monitoring systems, including intelligent early warning and market information; improvement of data collection and processing technology; rapid fault location , Isolation and service restoration technology; Network reconstruction and protection technology; Scheduling decision-making technology that comprehensively considers environmental benefits and economic benefits; Decision visualization technology.

5. Key technologies of smart microgrid

The realization of smart microgrid involves many technical fields, and the key technologies of smart microgrid are mainly summarized from the aspects of communication, sensing and measurement, analysis, and equipment.

5.1 Integrated communication system

Provides two basic functions to make the information exchange between the microgrid itself, multiple microgrids, and the microgrid and distribution network interactive in real time: a unified and open communication standard enables both parties to identify and reorganize information; Compatible physical media enable open communication facilities to connect various intelligent electronic devices (IEDs), smart meters, control centers, power electronic devices, protection systems and end customers, creating a "plug and play" environment.

Advanced communication technologies will be widely used in smart microgrids: banded power line (BPL) access technology is a broadband access technology connected to the home, using the medium and low voltage power lines of the existing AC distribution network to transmit Internet broadband data services can realize power services such as remote meter reading, load control, data analysis, power quality monitoring, equipment monitoring, and distributed power generation monitoring, as well as Internet broadband access, video transmission, and virus defense. , fault diagnosis and other user network services, it can be said that BPL technology is developing rapidly and will be the leading force in the future intelligent micro-grid communication technology; wireless communication technology, including the current rapid development of WiFi technology, emerging 3G and WiMax technology Mesh network (WMN) technology can be introduced into the microgrid to realize wireless communication between devices and energy management.

5.2 Advanced Sensing and Metering Technologies

Advanced sensing and metering technology based on digital communication technology can quickly collect and integrate data at each node of the network, diagnose the health and integrity of the smart micro-grid, and have functions such as automatic meter reading, consumption metering, and electricity theft detection , and can alleviate power congestion, provide demand-side response and new control strategies.

Advanced sensing technology is an important part of micro-grid intelligent technology and has a good application prospect. For example, a wireless sensor network is composed of dense, low-cost, and randomly distributed nodes, which has strong self-organization and fault tolerance capabilities, and will not cause the entire system to collapse due to the damage of some nodes in malicious attacks. Introducing wireless sensing technology into the microgrid can effectively improve the security defense capability of the microgrid and provide an effective basis for the microgrid to achieve autonomy.

At present, electromechanical meters are mainly used for electric energy measurement in China, and the fineness of user-side metering data collection is not enough, and the data is not fully utilized. Advanced Metering Infrastructure (AMI) has been widely used abroad, and smart meters have been installed in many countries. AMI can realize electric energy metering, record consumption information of "three meters" (electricity meter, gas meter and water meter), and can realize two-way communication for remote reading.

The microgrid integrates power producers and consumers, and needs to provide real-time power information to operating agencies to balance power supply and demand. As the link to realize the intelligentization of the power grid, especially the distribution network, AMI is indispensable in the intelligentization of the microgrid. In addition, the micro-grid includes commercial users, community users, and some small and medium-sized industrial users. User port technology, as a gateway to the user's room, also plays an active role in the micro-grid.

5.3 Advanced Analysis Techniques

Advanced analysis technology is the functionalization of advanced energy management. It is a tool to realize the autonomous operation of smart microgrid, including system performance monitoring and simulation, measurement analysis system, comprehensive forecast system, real-time power flow analysis and market simulation system.

5.3.1 System performance monitoring and simulation: monitor the electrical parameters of each node in the microgrid in real time; verify and improve the offline system model based on real-time data; determine the optimal strategy for microgrid stability/recovery based on online or simulated faults.

5.3.2 Measurement and analysis system: detect instantaneous value of voltage or current; analyze microgrid transient process; monitor microgrid emergency events; support real-time state estimation; improve microgrid dynamic model; provide a better data visualization platform.

5.4 Comprehensive Forecasting System

5.4.1 Real-time power flow analysis: visually display the restricted area of safe operation; give the optimal coordination plan, expand the safe operation area, reduce transmission congestion, optimize loss management, and improve system planning analysis.

5.4.2 Market simulation system: simulate various market factors (such as different characteristics of market members, dynamic learning ability, self-judgment and decision-making ability, and interaction among members) for microgrid economic analysis and control, and provide open program development The environment enables software upgrades and information sharing.

5.5 Advanced Equipment Technology

5.5.1 Advanced Power Electronics. At present, the application of power electronics technology in microgrid is reflected in: connecting distributed power supply and energy storage to the grid; providing local power supply control and protection; islanding/anti-islanding detection.

Advanced power electronics can greatly improve microgrid performance. For example: the unified power flow controller can comprehensively improve the reactive power compensation and power flow control of the microgrid; the static var compensator or dynamic var compensator of the distribution network can effectively provide voltage support, suppress voltage flicker, and ease distributed generation grid connection The impact; the fast transfer switch can provide stable power and realize the seamless transfer of the microgrid under the two modes of grid-connected and off-grid.

The rapid development of material technology accelerates the development of power electronics technology to high frequency and intelligence. The strengthening of the research and development and application of grid-connected interfaces will help reduce the cost of distributed power generation, and the improvement of interface control and harmonic control will help improve power quality and help improve the economical and stable operation level of microgrids.

5.5.2 Superconducting power technology. As a key forward-looking technology in the 21st century, superconducting power technology has entered the trial operation stage in cables, transformers, current limiters and energy storage. In particular, superconducting cables have been put into commercial operation in the United States in April 2008. One 138kV cable can meet the electricity demand of 300,000 households at the same time. Superconducting power technology is a new technology approach to solve problems such as power security, high-quality power supply, high-density power supply and high-efficiency power transmission. Introducing it into the microgrid can effectively guarantee high-quality power services, reduce power transmission losses, reduce land occupation, and reduce electromagnetic pollution , so as to provide a guarantee for the efficient operation of the microgrid.

5.5.3 New energy storage technology. Energy storage technology is an important part of the self-government of the microgrid. According to the form of energy conversion, it can be divided into four types: physical, electromagnetic, electrochemical and phase change energy storage. A large number of renewable energy generation units with high penetration rate are integrated in the microgrid. Due to its inherent randomness and intermittent nature, it will bring about problems such as voltage and frequency stability, low voltage ride through, power quality and economy. Due to the differences in power range, response time, conversion efficiency and technology maturity of various energy storage technologies, the development of joint control technology between different energy storage units is an important link to solve the above problems and realize the intelligent transformation of microgrids.

Smart microgrid is a small power system that integrates distributed power supply, energy storage device, energy conversion device, related loads, monitoring and protection devices. It has both grid-connected and independent operation capabilities, and is stable, compatible, flexible and economical. At present, the research on smart microgrid mainly focuses on its operating characteristics, protection and control, grid-connected standards, operation control and energy management. In my country, the smart microgrid will be widely used as an effective measure to make full use of renewable energy, save energy and reduce consumption, improve the level of electricity consumption in remote rural areas, and improve the ability of power grids to prevent and resist disasters.

As new things, smart grid and microgrid are constantly enriching and deepening their connotations. The smart grid with "strongness" as its material foundation, "interaction" as its core feature, and "smartness" as its technical support will play an increasingly important role in China's economic construction, energy utilization and environmental protection. As an important part of the smart grid, the microgrid plays the roles of power grid support, earthquake prevention and disaster reduction, energy efficiency improvement, energy conservation and consumption reduction, and rural electrification. Realizing intelligence is an objective requirement for the development of microgrids. By integrating advanced information technology, control technology and power technology, the smart microgrid can not only provide higher power reliability, meet various needs of users, but also maximize energy, economic and environmental benefits. A new organizational form of the smart distribution network in the future.