1. Architecture of microgrids
The microgrid architecture consists of two parts: communication architecture and basic architecture. The microgrid communication architecture generally consists of three parts: controllable load and load controller, controllable power supply and micro power supply controller, and central controller. The basic architecture consists of a load group and several radial feeders, where the radial microgrid has a common connection point (PCC) with the main distribution system, and each feeder is equipped with a power flow controller and a short circuit breaker. Due to the different types of buses, microgrids can be divided into DC bus microgrids, AC bus microgrids, and AC DC bus microgrids. The DC bus microgrid is the architecture of the microgrid that this article focuses on.
DC bus microgrid: Distributed power sources, energy storage devices, loads, and other equipment are directly connected to the DC bus. Bidirectional DC/DC converters, DC/AC converters, AC/DC converters, and transformers can be used to output AC or DC power that meets the load requirements. The PCC switch controls the switching of the microgrid between two operating modes, namely island mode and grid connected mode.
Energy storage systems play a crucial role in microgrids. In microgrids, the electricity generated by new energy sources such as photovoltaic power generation and wind power generation is unstable, with randomness and volatility, which reduces the reliability of power supply quality and leads to significant fluctuations in input power. Energy storage systems can enhance the safety, economy, and reliability of power systems. Connecting energy storage systems to microgrids can effectively address the adverse effects of distributed generation on microgrids; When the load is at a low point, the energy storage system can store excess electricity in other forms of energy; When the load is at its peak, the energy storage system converts other forms of energy into electrical energy for release. Thus achieving the effect of peak shaving and valley filling in the microgrid, improving the utilization rate of generator units and the reliability of microgrid power supply, reducing power generation costs, and filling the problem of distributed generation fluctuations and microgrid peak valley differences.
The energy storage system can serve as both a load and a power generation end in a microgrid, achieving bidirectional energy flow. There are two ways for microgrids to achieve peak shaving and valley filling effects: firstly, considering the economic benefits of microgrids, utilizing the difference in peak valley electricity prices to gain benefits. Although there is no clear goal of peak shaving and valley filling effects, this effect has been achieved to a certain extent; The second is to directly control the power generation process of the energy storage system, directly achieving the goal of peak shaving and valley filling.
2. Analysis of commonly used energy storage systems in microgrids
Energy storage systems are an important component of microgrids, and countries around the world are developing energy storage technologies. However, the development level of different types of energy storage technologies varies. Some are in the research stage, some are in the early stages, and some are in the commercial operation stage. Below is a summary of various energy storage systems, as shown in Table.
| Type of energy storage system | Pumped energy storage | Compressed air energy storage | Flywheel energy storage | Electrochemical energy storage | Superconducting electromagnetic energy storage | Supercapacitor | Molten salt thermal storage and energy storage |
| Operational principle | Realize the mutual conversion of kinetic energy and electrical energy | Using electricity to compress air in a closed environment | Interaction between electric motor and flywheel | Utilizing chemical reactions to convert electrical energy into chemical energy | Utilizing coils made of superconducting materials to achieve energy storage | Utilizing the attraction between electrodes and electrolytes to achieve energy storage | Using heat transfer media in conjunction with the sun for use |
| Advantage | Long operating life | Efficient | High power density and long service life | Different materials have different advantages | High power density and short response time | Long cycle life, green and environmentally friendly | Good safety performance and high thermal capacity |
| Disadvantage | Not widely used | Affected by regional terrain | Large capacity energy with narrow application range | Different materials have different drawbacks | Low energy density and high cost | Low energy density and high cost | Highly affected by natural environment |
(1) Pumped energy storage is a traditional energy storage technology and is currently a relatively mature one. It is in commercial operation, with a lifespan of up to 40 years and a capacity of up to kilowatts per megawatt. The establishment of pumped storage requires high terrain and water resources, therefore it cannot be widely used.
(2) Compressed air energy storage technology is a new type of energy storage technology at present, which compresses and stores the air in a high-pressure sealed environment by utilizing the remaining electricity generated during the low load period of the microgrid, and then releases it during the peak electricity consumption period to drive power generation. Its energy storage efficiency is nearly 70%, which is relatively high, but it will be affected by different terrains in different regions.
(3) Flywheel energy storage technology is a storage method that converts electrical energy into kinetic energy; Its advantages are high power density and long service life, while its disadvantage is low energy density. Therefore, it is suitable for storage of small capacity energy and has quickly achieved commercial promotion and application; Large capacity energy storage is currently in the experimental stage.
(4) Electrochemical energy storage, also known as battery energy storage, converts electrical energy into chemical energy, stores electrical energy, and converts chemical energy into electrical energy during discharge to utilize electrical energy. There are many types of battery energy storage and they are composed of various energy storage materials. Different energy storage materials result in different characteristics and applications of battery energy storage. The following will introduce the battery energy storage of different materials. The electrode of a lead-acid battery is composed of lead, its corresponding oxide (such as lead dioxide), and the electrolyte of a sulfuric acid solution. Due to the long application time and mature technical level of lead-acid batteries, which have the advantages of simple maintenance and high safety performance, lead-acid batteries are not suitable for use in low-temperature environments due to their high environmental pollution and low energy density. Therefore, lead-acid batteries are also developing in a new direction, represented by lead-carbon batteries. By adding carbon materials to the negative electrode of lead-acid batteries, they have become a new type of energy storage battery. This battery has minimal environmental pollution. Long cycle life and high charging rate, but the battery is prone to water loss and the principle of carbon material action is not clear, so it is still in the trial operation stage. A liquid flow battery is a type of battery in which active substances are stored in the electrolyte, which has fluidity and achieves charging and discharging functions through electrochemical reactions. Although it has no pollution to the environment and a long cycle life, its charging and discharging efficiency is generally only 60%, and it occupies a large area, which limits the application of liquid flow batteries, mainly used for frequency control. At present, lithium batteries are the most widely used and account for the largest proportion of applications in microgrids. There are many domestic applications of lithium iron phosphate batteries, such as the energy storage demonstration power station of Southern Power Grid and the Zhangbei Wind and Wind Power Storage Power Station of State Grid of China, which both use lithium iron phosphate batteries. Lithium batteries have high cycling efficiency, up to 90%, fast response speed, high energy density, long service life, and no toxic substances. Therefore, it has a wide range of applications.
(5) Superconducting electromagnetic energy storage technology is a type of energy storage technology that utilizes electromagnetic energy storage devices to store and release energy. The device is made of a coil made of superconducting materials, and the microgrid is excited by a converter. Then, a magnetic field is generated in the coil and energy is stored. When in use, the energy is then fed into the microgrid through an inverter. This energy storage technology utilizes the zero resistance characteristic of superconductors to achieve lossless energy storage within the coil. Its technology has advantages such as fast response, small size, high power density, and improved power supply quality. However, its widespread application is limited due to high cost and overload protection issues.
(6) Supercapacitors are also a new type of energy storage device, located between traditional capacitors and rechargeable batteries. This device has the characteristics of fast charging and discharging of capacitors and energy storage of batteries. Utilize the attraction between electrodes and electrolytes to form capacitors for energy storage. This technology has advantages such as high power density, short response time, long cycle life, maintenance free, and green environmental protection, but its storage energy density is low and its usage cost is high.
(7) Molten salt thermal storage is an emerging energy storage technology that utilizes raw materials such as nitrates as heat transfer media for energy storage. It is used in conjunction with solar thermal power generation systems to achieve energy storage and release. This technology has advantages such as high thermal capacity and good safety performance, but solar energy resources are limited by natural factors such as climate and geographical environment, so its future application is not yet clear.
In summary, battery energy storage systems is a good choice for peak shaving and valley filling in microgrids.
The research mainly focuses on the peak shaving and valley filling of battery energy storage systems. On the other hand, compared with large microgrid power systems, battery energy storage systems has relatively small energy storage capacity and charging and discharging power. At present, larger capacity can only reach the level of megawatts, and its corresponding capacity is only a few megawatt hours. Therefore, battery energy storage technology is not yet applicable to larger microgrids. At home and abroad, there have been demonstration projects of microgrid systems and some related commercial applications, with the maximum daily operating load basically controlled within a few megawatts. The participation of battery energy storage systems in peak shaving and valley filling of microgrid systems can effectively improve the load curve of the microgrid, reduce the peak valley difference of the microgrid load, greatly improve the utilization rate of power generation equipment, thereby improving the power supply reliability and overall operating efficiency of the microgrid. Moreover, battery energy storage systems has many advantages in lithium batteries and is widely used in various applications. Therefore, in this article, battery energy storage systems using lithium-ion batteries as energy storage materials is used as the research object for peak shaving and valley filling in microgrids.
3. Composition and simplified model of battery energy storage system
The battery energy storage systems structure diagram is shown in Figure 1. Each individual battery string is formed by series/parallel connection, and each battery string is formed by series/parallel connection to form a battery energy storage systems. The battery energy storage systems is directly connected to the DC/DC converter or DC/AC converter through switch H, and the energy conversion between the battery energy storage systems and the microgrid is achieved by the converter. By increasing the series connection of battery cells or battery strings to increase the voltage of battery energy storage systems, and increasing the parallel connection of battery cells or battery strings to increase the current of battery energy storage systems. Therefore, increasing the series/parallel connection of battery cells or battery strings can improve the capacity of battery energy storage systems and achieve its energy exchange ability with the outside world. In practice, battery strings are divided into two categories: series type battery strings as shown in Figure 2 (a) and parallel type battery strings as shown in Figure 2 (b). In summary, the modeling of battery energy storage systems is actually formed through simple series/parallel swapping of series and parallel batteries.
Assuming that S represents the series model of batteries, P represents the parallel model of batteries, and y represents the single cell model of batteries, the series type battery string shown in Figure 2 (a) can be obtained, and its formula is shown in the formula.
In the formula, π 1 is the parameter of the series battery string; πππΆ is the state of charge of the single battery; πΌπΌππΌππΌπππΆππππΆπΊππΆ is the current of single battery a; πΌπ is the current of single battery b πππΆπ is the current of single battery b πππΆ;, πΌπ is the current of single cell 2a, and πππΆπ is the current of single cell 2a.
The parallel battery string shown in Figure 2 (b) can be obtained, as shown in the formula.
In the formula, π 2 is the parameter of the series battery string.
For battery energy storage systems, the relationship between current and voltage is not simple linear. When the voltage of the battery string is equal, the current may not be equal, and vice versa.
4. Working principle and model analysis of battery energy storage system
Battery energy storage systems can serve as both a load and a power source device in microgrids. In practical work, its working method is to first charge and then discharge. When charging battery energy storage systems, it receives electrical energy from the microgrid for storage as the load of the microgrid; When battery energy storage systems is discharged, it serves as the power source in the power load and outputs voltage that meets the frequency waveform through power electronic devices.
Battery energy storage systems is mainly composed of five parts: energy storage battery, inverter, transformer, control unit, and Battery Management System BMS. The basic structure diagram of battery energy storage systems is shown in Figure 3. After being connected in series and parallel, the energy storage battery is connected to the microgrid through inverters and transformers. The current balance control strategy of power compensation through unbalanced current controls the inverter to adjust the switching of DC voltage and AC voltage, allowing the energy storage battery to store and release electrical energy, achieving peak shaving and valley filling of the load. The main function of BMS is to intelligently manage energy storage batteries and monitor their operating status, prevent overcharging and discharging, extend the service life of energy storage batteries, and thereby reduce the cost of microgrids. Considering the economy of microgrids while achieving good load peak shaving and valley filling.
Understand the working principle and system structure of battery energy storage systems. The charging and discharging process of battery energy storage systems involves the conversion of electrical and chemical energy, energy storage and release, and thus peak shaving and valley filling of power loads. Therefore, it is necessary to have an understanding of the battery energy storage systems model.
5. SOC and capacity relationship of battery energy storage system
Battery energy storage systems stores excess electrical energy through chemical energy, and achieves the charging and discharging of battery energy storage systems through the mutual conversion between chemical energy and electrical energy. For battery energy storage systems of different materials, although their chemical reaction equations are different, the basic principles are similar, all aimed at achieving the storage and release of electrical energy.
In practical applications, there are usually three equivalent circuit models for battery energy storage systems : a constant current current source model, an ideal voltage source model, and a power source model. In fact, battery energy storage systems is formed by individual battery cells/battery strings passing through series/parallel and corresponding power electronic devices. Once the voltage of the battery energy storage systems is determined, it will not change. However, this article focuses more on studying the power output of battery energy storage systems. Compared to the above three equivalent models, it is simpler and can be widely applied in practice.
The internal structure of battery energy storage systems is very complex, requiring consideration of its chemical reactions and thermal effects; However, in the application of battery energy storage systems in peak shaving and valley filling, only external electrical characteristics need to be considered, so as long as corresponding electrical models are established. battery energy storage systems exchanges energy with the microgrid through converters and inverters, while BMS monitors the battery energy storage systems. The control unit controls the energy storage battery and BMS. However, in practical microgrid applications, only the input and output of battery energy storage systems power need to be considered. There are three working states during battery energy storage systems operation, namely discharge, charging, and resting state.ππ is the charging power of battery energy storage systems, ππ>0; ππ is the discharge power of battery energy storage systems, ππ<0; ππ’ is the battery energy storage systems static electrical power, ππ’=0.
The energy storage capacity of battery energy storage systems is limited, so the impact of capacity needs to be considered. The capacity formula of battery energy storage systems is shown in the formula.
In the formula: πΈπ‘ is the energy stored by battery energy storage systems at the moment π‘ (kWh); ππππ π , π‘ is the external power of battery energy storage systems at the first moment (kW); π is the corresponding charge and discharge coefficient. When π is greater than 1 during discharge, there is a loss in battery energy storage systems during the discharge process. When π is less than 1 during charging, it indicates that there is also a certain loss during the charging process.
In practical microgrids, πππΆ is often used to evaluate the energy state of battery energy storage systems, and the specific formula is shown in the formula.
In the formula: πΆ is the rated capacity of battery energy storage systems (kWh); πΈ is the energy stored by the current battery energy storage systems (kWh); πππΆ is the percentage of the current battery energy storage systems capacity πΈ to the rated capacity C; Therefore, calculate the time interval of each time period as shown in the formula.
In practical engineering applications, the power of battery energy storage systems cannot be continuously measured, and the significance of continuous strategy is not significant; Therefore, discretize the formula as shown in the formula.
In the formula, β π‘ is the interval time between sampling (h).
Research is conducted on the composition and characteristic analysis of battery energy storage systems in the optimization control strategy of peak shaving and valley filling for island operating microgrids. Firstly, the architecture of microgrids is analyzed, with a focus on the role of energy storage systems in DC microgrids. Secondly, analyze the application of various energy storage components in the microgrid, and analyze the working principle, advantages and disadvantages, and select battery energy storage systems as the energy storage component of the microgrid. Finally, a large capacity battery energy storage systems composed of single cells or battery strings through series/parallel connection is studied. Its working principle is the mutual conversion between electrical and chemical energy, thereby achieving peak shaving and valley filling of power loads, as well as the relationship between the SOC and capacity of battery energy storage systems.