The development and utilization of energy is a crucial part of human development history. In recent years, solar and wind energy have become the fastest growing renewable energy sources. With the continuous decline in power generation costs, they have become the main incremental players in electricity installation. With the introduction of the concept of “carbon neutrality”, under the 2060 carbon neutrality goal, the power industry is expected to change its traditional power generation models in order to reduce carbon emissions as much as possible. The future energy supply system will mainly focus on electricity, so the integration of energy storage technology and renewable energy generation technology will promote the development and implementation of renewable energy. Moreover, integrating the two into a joint system can reduce fluctuations and enhance the flexibility of the power system, effectively reducing carbon emissions while providing safe and stable electricity energy.
Solar energy is a green and renewable energy source that converts sunlight into electricity; Compared with other conventional energy sources, it has the following characteristics:
(1) Universality: Solar energy resources are distributed throughout the world, and more than two-thirds of China’s regions can utilize solar energy.
(2) Harmlessness: Solar energy does not have an impact on the environment from generation to utilization, which is beneficial for energy conservation and emission reduction.
(3) Massiveness: Solar energy reserves are enormous and abundant. According to statistics, the energy radiated by the sun to the Earth within a year is equivalent to the energy released by the combustion of 137 trillion tons of coal, which is more than 20000 times the total energy of various energy sources worldwide each year.
(4) Durability: The sun can exist for a long time, as long as there is still light on Earth, energy is inexhaustible.
The large land area of our country and sufficient lighting time are equivalent to having relatively abundant solar energy. According to estimated statistics, the annual acceptable solar radiation energy in the country is about 50 × At 1018kJ, the lowest annual solar radiation amount in various regions of the country is 335kJ/cm · Y, while the highest can reach 837kJ/cm · Y. Based on the analysis of the distribution of solar energy in China, most regions of the country receive relatively abundant solar energy, especially in high-altitude areas such as the Qinghai Tibet Plateau, where the atmosphere is relatively thin and can receive more direct sunlight. The solar energy loss is relatively small, and the lighting time is also long enough.
Based on the continuous exploration and research of new energy sources such as light and wind, foreign literature has proposed the concept of Distributed Generation (DG). Distributed power generation is a power generation method with many advantages, such as achieving local consumption, low cost for transmission and distribution facilities, and relatively small line losses. With the extensive development and utilization of new energy in China, and due to the unique energy output of new energy itself, the grid connection of new energy power generation will have a significant or minor impact on the large power grid, which is highly likely to affect the safe and stable operation of the power grid. Therefore, it is necessary to study the output characteristics of new energy itself and the impact of different grid connection methods on the operation of the power grid. The combination of intermittent distributed new energy sources with microgrids has been a hot topic in recent years, and the research on control strategies for microgrids is increasingly being pursued by scholars. Figure 1 shows a typical microgrid structure diagram containing photovoltaic, wind power, and diesel generators.
Microgrid system is a new network structure in the current power network, mainly composed of power generation units such as light energy, wind energy, micro gas turbines, fuel cells, energy storage equipment, loads, and control systems. The microgrid system has strong self-healing function, and through independent control, management, and protection functions, it can be connected to the large power grid through public connection points or operate independently. Microgrid is a completely new concept system compared to traditional power grids, and is based on a specific topology structure, combined with distributed power sources and their loads to form a network system. The proposal and implementation of microgrid systems can greatly improve the reliability of power supply and also greatly improve the utilization rate of electricity, which is an important direction for the development of future power grids.
Throughout the development history of global microgrid systems to this day, photovoltaic power generation is one of the most mature and widely used power generation methods in the field of new energy utilization. It plays an indispensable role in the field of power generation, whether in rural areas, cities, and industries. For example, solar street lights, traffic lights, and scenic area lighting, which are widely used, have good development prospects in solving special power supply and remote mountainous electricity demand. Therefore, every country around the world attaches great importance to the method of photovoltaic power generation and is vigorously promoting the implementation of photovoltaic power generation projects. China has provided more policy support for photovoltaic power generation, such as increasing government subsidies. It is precisely due to the government’s emphasis that photovoltaic power generation is becoming increasingly popular nationwide. We believe that in the future of low-carbon and environmental protection, it will gradually replace traditional power generation models.
Energy storage technology is a very important technology in microgrid systems, which helps to improve the stability of microgrid operation, enabling faster response to grid demand during peak loads, effectively reducing the impact, intermittency, and instability of new energy generation on the grid, and providing significant assistance for grid scheduling and forecasting. The energy storage system can store excess energy in the power grid system into the energy storage system. When the system needs to provide energy, the corresponding control system can effectively release the capacity in the coarse energy system, supplement the active and reactive power in the power grid, and ensure the stability of the power grid voltage level and frequency. Power electronics technology is an indispensable technical support in microgrid energy storage systems, as the development of power electronics technology enables the widespread application of high-power energy storage devices.
1. Current research status of photovoltaic power generation technology
Photovoltaic power generation is a power generation mode that can directly apply solar energy. In the mid-1950s, American scientists Chapin and Pearson first manufactured monocrystalline silicon solar cells for practical use at Bell Laboratories, which gave rise to practical photovoltaic power generation technology that converts solar energy into electricity. With the continuous improvement of ecological concepts, environmental protection, and sustainable development awareness from all sectors of society, as well as the continuous innovation of modern power electronics technology, photovoltaic power generation technology has developed rapidly in recent years, mainly manifested as follows:
(1) The total installed capacity of global photovoltaic power generation continues to increase. Figure 2 shows the cumulative installed capacity of global photovoltaics from 2007 to 2019. From 2007 to 2018, the new installed capacity of global photovoltaics continued to rise. In 2018, the new installed capacity of global photovoltaics exceeded 100GW, and by 2019, the new installed capacity had reached 121GW. The reason for the higher than expected new installed capacity internationally is that the sharp drop in component prices in 2019 has stimulated the international market, the comprehensive opening of the European market, and the growing demand for electricity from underdeveloped countries and regions. By the end of 2019, the cumulative photovoltaic installed capacity worldwide was 626GW, an increase of 54.57% compared to 2017. China’s 14th Five Year Plan also proposes that the proportion of non petrochemical energy will reach 20% of the national energy proportion by 2030. By 2021-2025, the installed capacity of photovoltaic and wind power in China is expected to reach 425-610GW, and the vast space for energy storage applications will be further enhanced with the widespread application of photovoltaic technology.
(2) Efforts are being made to improve the efficiency of photovoltaic power generation. On the one hand, research is being conducted on Maximum Power Point Tracking (MPPT) in photovoltaic power generation. A MPPT tracking technology based on improved sliding mode control is proposed, which uses corresponding sliding mode controllers and improves the nonlinear components to reduce system oscillations. A Memetic Salp swarm algorithm has been proposed, which can better consider the impact of shadow conditions and rapidly time-varying weather on MPPT through improvements to the algorithm. On the other hand, there are two main methods of mainstream photovoltaic grid connected inverters: voltage source grid connected inverters and current source grid connected inverters. At present, the research focus at home and abroad is mainly on the control methods of voltage source grid connected inverters, and corresponding control strategies have been proposed for voltage source photovoltaic inverters.
(3) Accelerate the exploration of new materials and processes. The Earth has abundant silicon resources, which provide a favorable foundation for the production and upgrading of photovoltaic power generation materials. Silicon based solar cells are the most important photovoltaic materials. Currently, for second-generation photovoltaic cells with thin film technology, manufacturing costs can be reduced by reducing the amount of silicon materials used in traditional photovoltaic cells. The third-generation photovoltaic cells are based on second-generation thin film technology to improve the efficiency of converting solar energy into electricity. The emergence of a series of new materials and technological research achievements has elevated the conversion efficiency and stability of thin film solar cells to a new level, and will provide more beneficial assistance for the future of humanity.
2. Research status of microgrids
Microgrids can more flexibly coordinate distributed power sources and leverage the inherent advantages of distributed generation, which has been applied and promoted worldwide. The concept of microgrids was first initiated and proposed by the American Association for Reliability Technology Solutions. A microgrid is a complete system that includes power loads and micro power sources, capable of providing the required energy to end users. The energy conversion of internal micro power sources is achieved through electronic devices, which are mainly composed of various power sources. For external large power grids, microgrids are an independent control unit that must meet users’ requirements for power quality, safety, and other related requirements. The United States has built 50% of the world’s microgrids, and in its announced “Grid 2030” strategic plan, it indicates that the development of microgrids in the United States will mainly focus on reliability, low cost, and intelligence.
European countries have been gradually researching microgrid systems since 1998, and in 2005, they released plans related to the “smart grid”. According to the actual situation of European countries, the future development of European power grids should start from the reliability, flexibility, economy, and other aspects of microgrids. Currently, many demonstration projects have been established.
In Japan, limited by local resources, research on microgrids has also started relatively early. The Japan New Energy and Industrial Technology Development Organization (NEDO) emerged as the times require. The country uses wind and solar energy as its main distributed energy sources and has established multiple microgrid test platforms in Kyoto, Okinawa Prefecture, and Aomori Prefecture. Based on the situation of China’s power system, microgrids can be considered as independent systems that provide available energy to certain surrounding users by optimizing and configuring local distributed microsources; Microgrid is also an independent system with self-control, protection, and management functions, which can not only meet the power supply needs of users, but also provide safe and stable power supply. It can also achieve isolated or grid connected operation through microgrid.
Although the research on microgrids in China started relatively late, with the continuous deepening of the concept of microgrids by scholars, a series of microgrid experimental platforms and demonstration projects have been established, and the utilization of various new energy microgrids has emerged like mushrooms after rain at present. Thus, sufficient and useful experience has been gained in key technologies such as the operation and control of microgrids, reliable power sources, and energy storage technologies, laying a solid foundation for the vigorous development of microgrids in China.
3. Overview of the development of microgrid control strategies
3.1 Development Overview of Micro Power Control
There are three main control methods for micro sources, including P-Q, V/f, and droop control. The purpose of P-Q control is to ensure that the active and reactive power output of distributed power generation (DG) is the same as the reference value, without adjusting voltage, frequency, etc. Its voltage and frequency are mainly determined and maintained by external conditions; V/f control is similar to traditional secondary frequency adjustment, in order to maintain the amplitude and frequency of the DG output voltage floating within a range; The droop control is similar to the main frequency regulation of traditional power systems, where there is a linear relationship between the active power output of DG and frequency, reactive power, and voltage amplitude. The power demand of the system can be adjusted without communication with other DGs, and the control method of micro power sources depends on the type of micro power source. For example, micro sources suitable for P-Q control can be of different types, either intermittent or continuous. The reason is that the energy utilization efficiency has been improved with the intervention of P-Q control. The requirements for micro sources in droop control and V/f control are relatively single, and in most cases, they can only be continuous. The main reason is that the entire system needs sufficient stable voltage when the microgrid operates in isolation Frequency to support the entire power system. In most cases, the control method of energy storage devices in microgrids is P-Q control.
3.2 Overview of the Development of Microgrid Integrated Control
Based on the development of microgrid control, the integrated control of microgrids mainly includes two types: master-slave control and peer-to-peer control. The master-slave strategy refers to distributed power sources as the master controller and another power source as the slave controller in the microgrid. After connecting to the microgrid, all DGs are controlled using P-Q indicators to ensure efficient and economical operation of the microgrid. After disconnecting from the large power grid, when the microgrid is in island operation mode, the main controller in the microgrid maintains the balance of frequency, voltage, and power of the entire system by using V/f control. In order to maximize the power allocated to the distribution system, other slave controllers still use P-Q control. In peer-to-peer control, the state of all DGs in the microgrid is the same, and the “plug and play” of the microgrid mainly relies on it to achieve, that is, adding micropower to the microgrid can automatically participate in the allocation of output power. Peer to peer control enables distributed generation to effectively respond to system changes without the need for other data sources or locations, thereby improving the reliability of the microgrid and reducing system operating costs. Compared to traditional control modes, more and more control methods have been proposed. From the perspective of frequency domain, the control strategies for fundamental and harmonic power of a single inverter in a microgrid were analyzed and reviewed. Reference [33] proposes a new interconnected converter control strategy for the interconnection of DC microgrids and AC microgrids.
4. Current research status of energy storage systems
With the promotion and application of various new energy sources and distributed power generation technologies, the permeability of multiple power sources is high, which has caused a significant impact on the stable operation of the entire system. The research on energy storage technology has become the key to solving this type of problem. According to the conversion forms of electrical energy, it can be mainly divided into four categories: mechanical energy storage, electromagnetic energy storage, battery energy storage, and phase change energy storage. Battery energy storage is currently one of the best energy storage technologies, supporting functions such as frequency regulation, black start, and load balancing. Due to a series of advantages such as mature technology, low cost, long lifespan, and limited application flexibility, battery energy storage has been widely used in power grid energy storage, and is also the main direction for the development of large-scale energy storage technology in the future. In recent years, the combination method and control mode of energy storage systems have also been a hot research topic. A hybrid energy storage system composed of batteries and supercapacitors was introduced, and the switching control strategy of microgrids was studied by optimizing the combination. Studied the control strategies adopted by the energy storage system in grid connected and island mode operation of microgrids. In response to the power fluctuations caused by large-scale centralized integration of wind power and photovoltaic power into the power grid, corresponding control strategies for energy storage systems were proposed based on model predictive control (MPC) and fluctuation rate intelligent segmented control smoothing time constants, and parameters such as battery state of charge (SOC) were introduced in the control process to ensure the health and stability of energy storage units. The concept of energy storage power station was proposed, and optimization and improvement plans were proposed for the control strategy of AGC/AVC and source network load system in the energy storage power station.
The commonly used control methods for energy storage systems in microgrids include P-Q control, V/f control, and droop control. The microgrid based on master-slave control can choose P-Q control and V/f control, and droop control is suitable for peer-to-peer control of microgrids. In the past decade, there has been increasing research on energy storage systems in China, and the application of energy storage in microgrids has received widespread recognition and in-depth research. The research on energy storage systems in stable operation, isolated operation, and dual mode switching control of microgrids has achieved many results, which is of profound significance for the healthy and stable development of the power grid.
5. Summary
A microgrid system with photovoltaic power generation units and battery energy storage was discussed, and the control strategy of the microgrid was studied. In grid connected mode, the microgrid is connected to the main grid, and the voltage and frequency of the microgrid are determined by the main grid. Due to the variability of photovoltaic power generation, the inverter of the photovoltaic power generation unit must operate under different solar irradiation conditions according to its MPPT control strategy. This article proposes an adaptive variable step size disturbance observation method based on the traditional variable step size disturbance observation method, and verifies its feasibility through simulation.
During the transition from grid connected mode to island mode, the imbalance between the output power of photovoltaic power generation units and the supply and demand of the microgrid can lead to grid frequency and voltage deviations. In island mode, due to the lack of support from the main grid and the variability of photovoltaic power generation, the voltage and frequency of the microgrid may experience significant fluctuations. The use of energy storage systems can stabilize the voltage and frequency of microgrids in a short period of time.
Therefore, it is necessary to study an effective control strategy to address the above issues, so that a microgrid containing photovoltaic power and battery energy storage can quickly control the voltage and frequency of the microgrid in grid connected and island operating modes, and achieve smooth transition between the two states.
A modeling analysis was conducted on the research of microgrids containing photovoltaic power sources and battery energy storage, mainly including photovoltaic arrays, inverter circuits, and equivalent models of battery energy storage. The control strategies and methods adopted in this paper were discussed. Studied the control strategy of microgrids in grid connected/island mode. Specific control methods were proposed based on the P-Q control mode and V/f control mode in the master-slave mode according to different operating modes. Based on Matlab/Simulink software, simulation verification was conducted on the control strategies of microgrids (including photovoltaic power generation systems and battery energy storage systems) in two modes.
During the simulation experiment, analysis and research were mainly conducted on the three states of battery charging, discharging, and idle under grid connection conditions, as well as the power grid operation between power generation unit splitting, load shedding, and grid connection and island mode conversion under island conditions.