With the rapid development of the national economy, electricity demand has also increased rapidly, and the power grid structure has become increasingly complex. The traditional power grid structure of centralized power generation and long-distance transmission of large power grids is also facing more and more challenges. The cost is high, the operation is complex, and it isn’t easy to adapt to users’ requirements for high-quality, high-reliability power and diversified power supply needs. The multiple, widespread, massive outages of the 2000s also made people realize that some other model of power system development was needed to deal with the problem so that the grid could operate more safely and cost-effectively.
At the same time, the smart grid with distributed power sources as the central unit, as an essential part of the smart grid, has also received more and more attention. However, when the power system fails, the distributed power supply must be immediately out of operation, limiting the power supply’s role. Scholars have proposed the microgrid concept to harness the benefits of distributed power.
The concept of Microgrid
Microgrid refers to a small power generation and distribution system composed of distributed power sources, energy storage devices, energy conversion devices, loads, monitoring, and protection devices.
A microgrid is a concept relative to the traditional large power grid. It refers to a network composed of multiple distributed power sources and their related loads according to a particular topology structure related to the conventional power grid through static switches. The development and extension of microgrids can fully promote large-scale access to distributed power and renewable energy and achieve a highly reliable supply of various forms of energy to loads. Smart grid transition. The proposal of Microgrid aims to realize the flexible and efficient application of distributed power and solve the problem of grid connection of a large number of various forms of distributed energy.
Microgrid: Classification and application scenarios
Macroscopic classification: There are roughly two types of microgrids, independent and grid-connected.
① An independent microgrid is a power supply method that can be constructed and operated independently and has nothing to do with the external power grid. An autonomous microgrid can ensure the balance of power generation and power supply in the Microgrid and achieve the stability of power supply in the grid.
② The grid-connected Microgrid is networked with the external power grid. When the external power grid fails, it must be disconnected from the external grid so that the power supply and energy storage system in the grid can be controlled so that the power supply of essential power loads can be normal.
Microscopic classification: There are the following five types of microgrids.
① DC microgrid: distributed power sources, energy storage devices, loads, etc., are all connected to the DC bus, and the DC network is then connected to the external AC grid through the power electronic inverter device. The DC microgrid can provide electrical energy to AC and DC loads of different voltage levels through the power electronic conversion device. The energy storage device can adjust the distributed power supply’s fluctuation and the DC side’s load.
② AC Microgrid: distributed power sources, energy storage devices, etc., are all connected to the AC bus through power electronic devices. At present, the AC microgrid is still the main form of Microgrid. The Microgrid can transition between grid-connected and islanding modes by switching at the PCC.
③ AC-DC hybrid microgrid: It contains both AC bus and DC bus, which can directly supply power to both AC and DC loads.
④ Medium-voltage distribution branch line microgrid: a microgrid that effectively integrates distributed power sources and loads based on medium-voltage distribution branch lines.
⑤ Low-voltage micro-grid: A micro-grid formed by properly integrating users’ distributed power sources and loads at low-voltage voltage levels. Most of these micro-grids are owned by electricity or energy users and are relatively small in scale.
We comprehensively analyze microgrids’ characteristics and types and their suitable application scenarios.
① application in the offshore island microgrid
In the microgrid system, various energy sources operate cooperatively in different seasons and periods. Multi-energy complementarity has also become the best configuration of the offshore island power system. In summer, you can enjoy longer sunshine hours, and with less rain in summer, the utilization rate of photovoltaic systems is also improved. However, wind power and hydropower output in summer are not ideal due to the weather. In winter, when rainfall increases on the island, small hydroelectric generators can also become the primary source of electricity. The control system of the Microgrid can monitor the operation of the power generation facility, optimize the charging and discharging cycle of the battery, and automatically start the diesel generator in the event of a power shortage.
Microgrids will significantly improve the quality of power consumption. Off-grid island microgrids can meet the electricity demands of modern life.
② Application of Microgrid in remote areas
In addition to improving the existing power supply system, off-grid microgrids are essential to realizing electricity popularization in areas without electricity. A distributed energy system with photovoltaics as the mainstay and diesel generators as a backup can solve the problem of electricity consumption in these remote areas. Unlike ordinary AC power, the household microgrid system operates with DC power, avoiding the energy loss caused by the AC-DC conversion between photovoltaics, batteries, and home appliances. The cost of the whole system is lower than that of setting up the grid, and the power supply is more reliable. Distributed photovoltaics, energy storage batteries, and diesel generators form a microgrid system.
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Microgrids can provide users with reliable power while reducing electricity costs. When diesel prices rise, replacing photovoltaics makes the system more economical.
③ application in an urban community microgrid.
If off-grid microgrids are a must for achieving universal access to electricity in islands and remote areas, developing microgrid systems in cities covered by reliable grids will be the icing on the cake. The grid-connected Microgrid can switch freely between the two states of networking and independent operation. When the power grid fails, the Microgrid can choose to disconnect from the power grid to ensure the safety of the energy supply for users in the region. The Microgrid can also obtain additional economic benefits when connected to the network by selling excess power and responding to demand.
03 Application Pain Points
① The development of the microgrid industry is unbalanced. Especially when doing demonstration projects, the performance of the equipment is quite different, and it is necessary to deal with the equipment manufacturers frequently.
② The construction of the Microgrid is multi-unit participation, and many non-technical factors need to be considered, such as the source of funds, the control of the Microgrid, and who will build it.
③ The technical threshold of Microgrid is relatively high, and the span is rather large.
④ The technical service scope of Microgrid is relatively narrow. Because microgrids do not have large-scale commercial applications, manufacturers will not invest a lot of research and development efforts, resulting in the technical services of microgrids cannot keep up.
Currently, relevant national policies encourage and support microgrids in electricity distribution and sales businesses. Since the incremental distribution network is still in the pilot stage, microgrid distribution and electricity sales will face more difficulties. For example, how users will pay the Microgrid company, what additional fees would be included, and whether users still need to pay the transmission fee of the national power grid. Also, there isn’t a straightforward way to set prices for trading electricity between the Microgrid and the outside grid.
The diversity of distributed power sources and the complexity of microgrid operation make microgrids different from traditional power systems. Below we discussed the critical technologies of microgrids in detail.
① Planning and design:
Microgrid planning and design aim to determine the system structure and equipment configuration based on the complete analysis and prediction of load demand and renewable energy resources and according to specific goals and system constraints to achieve system economy and environmental protection as much as possible.
② Running optimization:
The energy management system formulates the operation optimization strategy of the Microgrid based on known various operation information. The purpose is to make decisions according to different optimal operation objectives and constraints based on data such as distributed power output forecast, energy demand in the Microgrid, and market information, and formulate microgrid operation scheduling strategies in real-time. And flexible scheduling of loads to achieve optimal operation of the system.
③ Protection control:
The fault conditions involved in microgrid protection can be divided into microgrid external faults and microgrid internal faults. Among them, the internal faults of the Microgrid have different fault characteristics and protection methods in the two modes of Microgrid grid-connected operation and remote operation. They are closely related to the control method of distributed power in the Microgrid.
④ Simulation experiment:
Digital simulation is one of the primary means of microgrid research, which provides:
- Necessary tools and strong technical support for the study of microgrid operation mechanisms.
- Planning and design.
- Optimal operation.
- Protection and control.
- Other issues.
The time constants of different devices and control systems in the Microgrid are pretty different, and the entire microgrid system presents the characteristics of solid rigidity. The digital simulation technology should realize the whole process simulation of the electromagnetic transient process of microsecond-level rapid change, the transient electromechanical process of millisecond-level change, the medium and long-term dynamic process of second-level to minute-level change, and the steady-state operation process of the system.
02 Control method
At the same time, there are three more common microgrid control methods on the market.
① Control methods based on concepts such as power electronics technology
The process gives unbalanced power to each unit based on the control needs of the Microgrid and the droop characteristics of the generator. It has the advantages of simplicity, reliability, and easy implementation.
② Control based on the energy management system
This method uses different control modules to control the active power and reactive power, respectively, which well meets the various control requirements of the Microgrid. The control method thus improves the control performance.
③ Microgrid control based on multi-agent technology
This method applies the multi-agent technology in the computer field to the Microgrid, and the characteristics of the agent’s autonomy and spontaneity can well adapt to and meet the requirements of the decentralized control of the Microgrid.
The development trend of Microgrid
It is understood that the voltage covered by the Microgrid in the future will be higher and higher, and the capacity will be more significant. In the end, the scale of microgrids will become larger and larger, which will make it more convenient for residents to use electricity. Microgrid projects are still in the demonstration stage, many technical issues still need to be further studied, and the marketization process is slowly taking steps.
However, through the investigation of Microgrids and careful consideration of their development, we believe that the characteristics of micro-grids meet the needs of the development of the power industry, and the development of Microgrid has excellent application value:
① The form of renewable energy power generation dominated by photovoltaics, wind energy, and biomass will be an essential power source component of the Microgrid.
The development potential of renewable energy is enormous. Suppose the distributed energy generation technology, energy storage, and power electronic control technology near the load point can be well combined to form a microgrid. In that case, renewable energy will give full play to its significant potential.
② Microgrid plays an essential role in improving the reliability of power supply and the power grid’s power quality.
Electricity is an indispensable booster for the country’s economy. Microgrids can classify loads according to users’ different power supply needs to provide customers with a personalized power supply in the most appropriate situation.
③ As a supplement to the power grid, the Microgrid can also improve the stability and security of the power system and better face emergencies.
Combining the Microgrid with the current power system can form a new system that is highly efficient and flexible compared to conventional centralized generators. In the unique design of the Microgrid, there is no need to build too many distribution stations. The transmission lines are significantly reduced, and the failure rate is also reduced considerably. On the one hand, the small-scale power supply can be set flexibly, and the construction period is short, which can not only cope with the rapid, short-term surge in power demand but also does not occupy the power corridor.
④ Microgrid has excellent economy and flexibility.
The Microgrid can control the operation of the distributed power supply. When the Microgrid is operated reasonably and correctly, it can be smoothly controlled to switch between the grid-connected and isolated grid operation modes. It can be adjusted according to the power market to obtain better results. That is, when the load of the enormous power grid is heavy, the Microgrid operates in parallel with the grid. When the market load is low, the Microgrid works like an isolated grid, using distributed power sources to supply power to nearby loads.
⑤ Microgrids can delay grid investment and reduce power loss, which aligns with the call to build a conservation-minded society.
Microgrid users get their electricity from power plants close to where they use it. This is called “local consumption.” So, it can reduce the need for centralized large-scale power plants and the loss of long-distance and multi-level power transmission. Building small-scale wind farms, solar power plants, and other power sources based on renewable energy in the local area to form a micro-grid can ensure residents’ electricity consumption and reduce unnecessary investment.
According to our analogy, the energy network in the future is like a living body: a large, intelligent grid is a blood vessel, and a microgrid is an indispensable capillary terminal, enabling the grid to absorb a large amount of renewable distributed energy, thereby promoting the efficiency of green energy.