DC Microgrid - Promoting Energy Conservation and Emission Reduction, Energy Sustainable Development

The DC microgrid is a microgrid composed of DC, which is an important part of the future intelligent power distribution system and is of great significance for promoting energy conservation and emission reduction and realizing sustainable energy development. Compared with AC microgrids, DC microgrids can more efficiently and reliably receive distributed renewable energy generation systems such as wind and light, energy storage units, electric vehicles and other DC power loads. This paper first sorts out the research status of relevant technologies and experimental systems in the field of DC microgrids in domestic and foreign academia and industry; Finally, the development and application prospects of DC microgrid are prospected from the aspects of AC-DC hybrid microgrid, AC-DC hybrid distribution network and energy Internet.

1. Introduction to DC Microgrid

Microgrid technology represents the development trend of distributed energy supply systems in the future and is an important part of the future intelligent power distribution system. It is of great significance to promote energy conservation and emission reduction and achieve sustainable energy development. The National Energy Administration has recently successively issued documents such as "Guiding Opinions on Promoting the Construction of New Energy Micro-grid Demonstration Projects" and "Notice on the Action Plan for Construction and Transformation of Distribution Networks (15-22} Years), pointing out that new energy should be actively developed and greatly improved. The ability of the distribution network to accept new energy, distributed power and multiple loads, accelerate the construction of new energy micro-grid demonstration projects, and explore micro-grid technology and operation management systems that adapt to the development of new energy.

Most of the electric energy generated by photovoltaics, fans, fuel cells, and battery energy storage units in the microgrid is direct current or non-power frequency alternating current; common electrical equipment, such as personal computers, mobile phones, LED lighting, variable air conditioners, and electric vehicles, are all passed through The corresponding adapter becomes a DC drive. If the above power generation unit or load is connected to the AC microgrid, it needs to pass through a multi-level energy conversion device composed of power electronic converters such as DC-DC, DC-AC and AC-DC. The power grid will save part of the AC-DC conversion device, reducing costs and losses. The DC bus voltage is the only standard to measure the balance of active power in the system. There are no problems in the system like frequency stability and reactive power in the AC system. The DC microgrid can also be connected in parallel with the existing AC microgrid or distribution network through a bidirectional DC-AC converter, and can effectively isolate AC side disturbances or faults, ensuring highly reliable power supply for loads in the DC system. Therefore, the research and development of the DC microgrid system has attracted extensive attention from the industrial and academic circles at home and abroad.

At present, relevant technical research, experimental systems and demonstration projects in the field of DC microgrid at home and abroad have been gradually carried out. Related technology research mainly involves key equipment such as DC microgrid power electronic converters and DC circuit breakers, operation control technology, protection and energy management systems, etc. International mainstream magazines in the electrical field, such as IEEE power electronics and smart grid, successively published special issues on "Smart DC Power Distribution/Microgrid" in 2013 and 2014. In June 2015, IEEE held a conference in Atlanta, USA. Organized the first International Conference on DC Microgrid, and introduced the latest research progress of DC microgrid related research technologies and engineering practices. In China, there are already a number of National Natural Science Foundation projects in this field (such as "Transient Characteristics Analysis and Control Strategy Research of DC Microgrid (51207001)", "Research on Coordinated Control and Stability of DC Microgrid (51307140), "Research on Distributed DC DVR System for DC Microgrid (51307117)", "Research on AC-DC Hybrid Power Distribution Operation Control for Efficient Power Supply and Multi-terminal Mutual Support (51407177)", "Layered Distributed Cooperative Control of DC Microgrid and Stability Research (51507109), etc.) received project support.

In terms of experimental systems and demonstration projects, in 2007, the CPES Center of Virginia Tech put forward the "sustainablebuilding initiative (SBI)" research plan, which mainly provides electricity for future residences and buildings. In 2011, the University of North Carolina in the United States proposed the FREEDM system structure, which is based on DC power supply to build a future automatic and flexible renewable energy transmission and management network. In 2012, a 3-year research project called "DC Components and Grid" (DCC+G) jointly launched by universities and enterprises in Germany, the Netherlands and other countries aims to design and develop high-efficiency semiconductor and power electronics technologies. Energy-efficient buildings based on 380V DC power distribution system. In 2014, the China-Denmark Renewable Energy Cooperation Project on Smart DC Microgrid was jointly launched by Denmark’s Aalborg University, North China Electric Power University, Institute of Electrical Engineering, Chinese Academy of Sciences, and State Grid Corporation of China, aiming to promote smart DC microgrid technology in future residential buildings. and industrial parks and other aspects of the development and application. In China, a number of "863 projects" of the Ministry of Science and Technology received project approval support. Among them, the national 863 project "Research and Application of Key Technology of Intelligent Power Distribution Based on Flexible DC" undertaken by Shenzhen Power Supply Bureau was officially launched in 2013. The research focus is on DC The flexible DC power distribution technology with solid-state transformer as the core to realize flexible control and rapid management of voltage and power between high and low voltage DC distribution networks or micro-grids; the national 863 project "High Density Distributed Energy Connection" The key technology of AC-DC hybrid microgrid was officially launched in 2015. The project mainly focuses on the access of high-density distributed renewable energy, and focuses on overcoming theoretical and technical difficulties such as grid configuration optimization and stability control of the AC-DC hybrid microgrid system.

2. Topology of DC Microgrid

DC microgrid is suitable for future smart homes, commercial buildings, and typical DC microgrid structures in industrial parks. The system can include intermittent distributed power sources such as photovoltaics and wind power, controllable distributed power sources such as micro gas turbines and fuel cells, and batteries. Energy storage, energy storage units such as flywheels or supercapacitors, and local AC/DC loads. If the DC microgrid can be interconnected with the external AC grid, it can be connected to the AC system through a bidirectional DC-AC converter.

In the future DC microgrid, in order to further improve the flexibility and reliability of the DC system power supply, to adapt to different voltage levels of distributed power supply, energy storage system and load access, a bipolar three-wire structure can be adopted. According to the different outlet forms of the neutral line, two bidirectional DC-AC converters with the same capacity are used at the main DC system and AC system interconnection ports of the bipolar three-wire power supply system, or two energy storage units in the DC system are connected through DC-DC transformers. In fact, there are two independent power supply circuits inside the DC system, which have high reliability, but two sets of full-power power electronic converters are required, and the cost is higher. By leading out the neutral line at the midpoint of the DC bus capacitor, in the case of distributed power supply or unbalanced load between positive and negative poles. In order to solve the above problems, the DC microgrid can form a bipolar three-wire system through a voltage balancer]. The application of the voltage balancer is not limited by the operation mode of the DC microgrid (grid-connected operation or independent operation), and can be flexibly incorporated into the output port of the DC-AC or DC-DC converter; at the same time, the DC bus voltage of the DC microgrid Control (such as controlled by DC-AC when connected to the grid, controlled by energy storage DC-DC when running independently) and positive/negative voltage balance control (controlled by a voltage balancer) are completely decoupled, compared with the use of NPC topology The DC microgrid has more flexible control and higher reliability.

3. Optimal planning of DC microgrid

The optimization planning problem of DC microgrid is the core problem that needs to be solved in the design stage of microgrid. The quality of the optimal configuration scheme will directly determine whether the system can operate safely and economically. The purpose of optimization planning is to determine the system structure and equipment configuration (including distributed power supply, energy storage equipment type, etc.) , capacity and location), and optimize quantitative indicators such as system economy, reliability and environmental protection as much as possible. Therefore, the optimal configuration of DC microgrid is a typical optimization problem, including three major elements: optimization variables, objective functions and constraints.

In the optimal configuration of DC microgrid, the planning and design problem is highly coupled with its operation optimization strategy. The influence of the operation optimization method must be fully considered during planning. Therefore, the optimization variables include distributed power sources, energy storage equipment type, location and capacity. In addition to parameters, microgrid operation strategies and related parameters can also be used as optimization variables to be decided.

Optimization objectives can be roughly divided into three categories: economical objectives, reliability objectives and environmental objectives. The economy is mainly evaluated from the aspects of investment cost, cost-effectiveness, and investment payback period; the reliability evaluation index needs to be able to reflect the operation status of the system and its equipment, as well as the impact on the user's power supply; the environmental protection is mainly evaluated from the perspective of emission reduction benefits, pollution gas Emissions, fossil fuel consumption and the proportion of renewable energy power generation are considered. For the grid-connected DC microgrid with AC and DC interconnection, the optimal configuration problem also needs to consider the grid-connected performance index. The DC microgrid is connected to the AC system (large grid or AC microgrid) through a bidirectional DC-AC converter, and the AC and DC systems can interact with each other to achieve mutual support. When optimizing the configuration of the grid-connected DC microgrid, indicators such as the self-balancing rate and the utilization rate of AC-DC interconnection devices can be used to evaluate its grid-connected performance. When optimizing the configuration, according to the different optimization requirements of the DC microgrid, the corresponding evaluation indicators can be set to comprehensively evaluate the performance of the microgrid.

4. Grounding method of DC microgrid

The grounding method of the DC microgrid system has a great impact on ground fault detection, fault current magnitude, personal and equipment safety, and also affects the configuration of protection schemes. According to the definition of DC system grounding type in IEC60364-1, it can be divided into 3 types: TT, IT and TN. Among them, TT means direct grounding at the DC bus (it can be positive pole, negative pole or neutral point, which is neutral grounding). The exposed conductive part is directly grounded, and the two grounding points are electrically independent of each other; IT means that the DC bus (which can be positive, negative or neutral point) is not grounded or grounded through high impedance, and the exposed conductive part of the electrical device is directly grounded ;TN means that the DC bus (which can be positive, negative or neutral) is directly grounded, and the exposed conductive parts of all electrical equipment are connected to the protective line and connected to the above-mentioned grounding point.

5. Protection equipment for DC microgrid

Fuses and DC circuit breakers are two commonly used protection devices in DC microgrids. Among them, the fuse is a switching device that integrates an overcurrent relay protection device and a breaking device. According to the current exceeding the specified value for a period of time, the melt is melted by the heat generated by itself, thereby disconnecting the circuit. The choice of fuse is mainly based on the protection characteristics of the load and the size of the short-circuit current to select the type of fuse. Fuses have the advantages of simple structure, convenient use, and low price, and are widely used in low-voltage systems.

DC circuit breakers mainly include mechanical DC circuit breakers, solid state DC circuit breakers and hybrid DC circuit breakers based on the combination of the two according to the different current breaking methods. The mechanical DC circuit breaker is mainly composed of an auxiliary oscillating circuit composed of an AC circuit breaker and RLC components, and the zero crossing point is artificially generated by means of the auxiliary oscillating circuit. The on-state loss of the mechanical DC circuit breaker is low, but the ability to quickly cut off the fault current is not strong (at present, the fastest still needs tens of milliseconds). In recent years, the DC solid-state circuit breaker composed entirely of controllable semiconductor devices has attracted widespread attention for its advantages of several millisecond-level breaking capacity, no contact, and no arcing when breaking. Compared with mechanical DC circuit breakers, solid-state DC circuit breakers cut off the fault current faster, but the on-state loss is relatively large and the cost is high. The hybrid DC circuit breaker uses a fast mechanical switch to conduct the normal operating current, and the solid-state power electronic device breaks the short-circuit current, which effectively combines the advantages of small on-state loss of the mechanical circuit breaker and fast breaking speed of the solid-state circuit breaker. In the future, with the rapid development and cost reduction of semiconductor devices, solid-state DC circuit breakers and hybrid DC circuit breakers will be applied in DC microgrids and DC power distribution systems.

For DC microgrids, multi-section or multi-terminal complex DC microgrids, DC circuit breakers with the functions of quickly breaking DC fault currents and isolating faults are crucial to ensure the safe and reliable operation of the system. Therefore, how to improve the breaking speed and breaking capacity of the DC circuit breaker is the main challenge faced by the research and development of the DC circuit breaker.

The DC bus in the DC microgrid usually contains a large capacity bus capacitor. When there is a fault between poles, the transient short-circuit impact current caused by the instantaneous discharge of the bus capacitor may cause the malfunction of the DC circuit breaker in the system, resulting in the loss of selectivity of the protection system, and the power failure of equipment such as excessive distributed power sources or loads. Consequences such as a reduction in the ability to coordinate with protective equipment. In order to avoid excessive instantaneous short-circuit current and reduce malfunction of the DC circuit breaker, a fault current limiting device can be used to cooperate with the DC circuit breaker.

6. Prospects for the development of key technologies in DC microgrids

The future development direction of key technologies of DC microgrid is as follows:

6.1 At present, distributed power sources, energy storage units, and AC/DC loads are all connected to the DC microgrid through conventional power electronic devices, which generally have problems such as single function and lack of plug-and-play. The research and development are more efficient and reliable, as well as modular and intelligent The simplified plug-and-play multifunctional converter interface or power exchanger will be a research method worthy of further exploration by industry and academia.

6.2 In order to cope with high-density distributed energy and multiple loads connected to AC-DC hybrid microgrids, how to comprehensively consider factors such as system grid structure design, optimal configuration of source-grid-load-storage, and close coupling between operation and planning are key issues for future AC and DC hybrid microgrids. An important topic in the optimization planning direction of DC hybrid microgrid.

6.3 In terms of operation control, how to improve the robust autonomy performance of the equipment-level control system and the reliability, flexibility and scalability of the system-level control system, and how to comprehensively coordinate the operation control technology and intelligent protection technology is the key to the future DC microgrid. Important theoretical research and technical development direction of energy management and operation control system.

6.4 In terms of DC microgrid protection technology, it is worth exploring to develop DC circuit breakers with more decisive breaking speed, higher breaking capacity, and more efficient and reliable DC circuit breakers; new DC power distribution protection technologies based on fault current limiting cutting-edge topics.