1.1 Background and Significance of the Research Topic
With the continuous deepening of China’s “double carbon” strategy, China’s clean energy has developed rapidly, the installed scale of renewable energy has continued to increase, and the proportion of fossil energy power generation has continued to decline. From 2014 to 2023, the proportion of thermal power installation has decreased by 20%, while the proportion of renewable power generation has increased significantly by nearly 27%, accounting for about 36% of the total installed capacity. It is expected that by 2025, the combined installed capacity of wind power and photovoltaic power will account for 38% of the country’s total power generation installed capacity, and by 2030, the total installed capacity of solar and wind power in China will exceed 1.2 billion kilowatts. By 2050, the combined installed capacity of wind power and photovoltaic power will account for 69%. Vigorously promoting the transformation of the energy structure, further accelerating the development and utilization of renewable energy, and constructing a new power system with a high proportion of renewable energy are the evolutionary paths of China’s future energy system.
With the rapid development of renewable energy power generation, a large number of new problems have also emerged. Renewable energy power generation is greatly affected by the natural environment, has strong fluctuations, is not stable, is difficult to continuously output a determined power, and cannot arrange power generation plans according to predictions. Direct access to the grid will lead to a mismatch between power and load within the grid. The resulting consumption problem increases the difficulty of grid operation and dispatching, and renewable energy power generation equipment is forced to abandon wind or light, resulting in energy waste and seriously endangering the safe and stable operation of the power system. In December 2023, the utilization rates of wind power and photovoltaic power in China were 97.0% and 97.1% respectively; from January to December 2023, the utilization rates of wind power and photovoltaic power in China were 97.3% and 98.0% respectively. In the 12 months of 2023 in the Mengxi region, the utilization rate of wind power was only 93.2%, which was the region with the most serious wind abandonment; followed by Qinghai, Hebei, and Gansu, the utilization rate of wind power in 2023 was not higher than 95.0%. The utilization rate of photovoltaic power in Tibet was only 78.0% from January to December, which was the most serious region of light abandonment; the phenomenon of light abandonment in Qinghai was also very serious, and the photovoltaic utilization rate in 2022 was 91.4%. These problems are the crux of the difficulties encountered in the in-depth development and large-scale application of renewable energy.
The microgrid is an independent and controllable system composed of renewable energy, energy storage systems, energy conversion equipment, AC and DC loads. It has the advantages of autonomous operation, multi-energy complementarity, optimized management, and coordinated control, and can effectively improve the development level and scale of renewable energy. The energy storage system is the basis for the stable operation of the microgrid. It can not only alleviate the system fluctuations caused by the access of wind and solar energy, but also avoid the energy waste caused by the abandonment of wind and light. In addition, energy storage can also participate in grid peak shaving and frequency modulation, and peak-valley arbitrage services, and is a key support for the rapid development of new energy. In order to promote the high-quality development of energy storage technology, many national departments have successively introduced a number of policies and measures. At the beginning of 2022, the National Energy Administration officially released the “Implementation Plan for New Energy Storage Development during the 14th Five-Year Plan”, deploying the key tasks, goals, and implementation paths for the development of new energy storage. Among the new energy storage, battery energy storage has many advantages such as short construction input time in the early stage, fast production time, low requirements for the construction site, and fast response to scheduling instructions, and has become the fastest-growing and most potential energy storage technology in the current era.
Large-scale BESS is generally composed of multiple BESUs, and each BESU is composed of a large number of single batteries. Due to the influence of objective factors such as the uneven coating, composition, and impurity content during the battery manufacturing process, there are inherent differences in the same batch of the same type of batteries, resulting in inherent differences in the energy storage batteries when they are grouped at the factory. As the operating time progresses, these tiny differences will have a cumulative effect, leading to various types of inconsistencies within the battery energy storage unit. However, in the process of participating in the power regulation of the microgrid, the BESU is often simplified as a large single battery, ignoring the internal characteristics of the BESU, and the inconsistency of the batteries inside the BESU will lead to a reduction in the actual available capacity of the BESU. Therefore, for the internal consistency problem of the BESU, based on the internal characteristics of the energy storage unit, studying the balanced management and power distribution control strategy of the energy storage unit is the core to improve the regulation ability of the BESS, extend the service life of the BESS, and improve the level of safe operation. It is also the key to promoting the continuous development of battery energy storage technology.
1.2 Research Status at Home and Abroad
1.2.1 Classification and Performance Comparison of Energy Storage Technologies
The energy storage system can not only stabilize the fluctuations caused by the integration of new energy into the grid, but also effectively deal with peak power supply, save grid investment, and compensate for the poor frequency modulation ability of new energy power generation. It is a key prerequisite for wind, solar, and other renewable energy power generation. In recent years, China’s energy storage industry has continued to maintain a high-speed development trend. The continuous improvement of the policy system supporting energy storage, the significant breakthroughs in energy storage technology, the strong global market demand, the continuous improvement of various business models, and the accelerated creation of energy storage standards have provided strong support for the high-speed development of the industry. According to the incomplete statistics of the Global Energy Storage Database of the China Energy Research Society Energy Storage Special Committee/Zhongguancun Energy Storage Industry Technology Alliance (CNESA), by the end of 2022, the cumulative installed capacity of global power storage projects in operation had reached 237.2GW, with an annual growth rate of 15%. The newly added installed capacity of global power storage projects in 2022 was 30.7GW, a year-on-year increase of 98%. The cumulative installed capacity of China’s power storage projects in operation had reached 59.8GW, accounting for 25% of the total global market scale, with an annual growth rate of 38%. The newly added installed capacity of power storage projects in China exceeded 15GW for the first time, reaching 16.5GW.
At present, electrochemical energy storage, mechanical energy storage, and electromagnetic energy storage are the main forms of energy storage technologies. Among them, electromagnetic energy storage is a technology that realizes the storage and release of electrical energy by converting electrical energy and electromagnetic energy, and the main devices include supercapacitor energy storage devices and superconducting energy storage devices; electrochemical energy storage refers to the storage and release of electrical energy by manipulating bidirectional chemical transformations, and common energy storage batteries include flow batteries, lead-acid batteries, and lithium-ion batteries; mechanical energy storage is the conversion of electrical energy into mechanical energy for storage, and then the conversion of mechanical energy into electrical energy to meet the demand for energy release, mainly including pumped storage, compressed air energy storage, and flywheel energy storage. At present, the most important energy storage technology in China is mechanical energy storage, among which pumped storage accounts for about 77.1% of China’s cumulative installed capacity of the power storage market in 2022, for the first time lower than 80%, with a new scale of 9.1GW and a year-on-year growth of 75%. Although the technology of pumped storage is very mature and has been applied on a large scale, its selection of construction sites is very strict, and many factors need to be considered. Moreover, the time and economic costs invested in the early stage are high, and there are significant limitations. Superconducting energy storage has the advantages of long cycle life and low self-discharge rate, but its high construction, operation, and maintenance costs in the medium term make it difficult to achieve commercial use due to poor economy; the cycle life of supercapacitor energy storage is long, and the temperature characteristics are good, but the energy density is low, and the self-discharge rate is high, which is not suitable for systems with high energy demand; the storage efficiency of compressed air energy storage is greatly affected by temperature, and it is only suitable for large-scale wind farms, and is not the focus of domestic energy storage technology application and research; the manufacturing process of flywheel energy storage requires high precision and complicated steps, and it is also difficult to be applied on a large scale.
With the gradual maturity of electrochemical energy storage technology, its many advantages, such as short construction input time in the early stage, fast production time, low requirements for the construction site, and fast response to scheduling instructions, have gradually gained favor. And with the improvement of the battery production process, the cycle life, cell capacity, and production cost of energy storage batteries have made great progress. Electrochemical energy storage has become the “star of tomorrow” in energy storage and has become the fastest-growing energy storage technology so far. By the end of 2022, the cumulative installed capacity of new energy storage worldwide had reached 45.7GW, nearly twice that of the same period last year, with an annual growth rate of 80%. Among them, electrochemical energy storage accounted for 97.0%; the cumulative installed capacity of new energy storage in China exceeded 10GW for the first time, reaching 13.1GW/27.1GWh, with a power scale annual growth rate of 128% and an energy scale annual growth rate of 141%. Among them, electrochemical energy storage accounted for 98.3%; the development trend of China’s electrochemical energy storage in the past decade is shown in Figure 1-1.
Lithium-ion batteries have the advantages of fast power speed, high energy density, and flexible response compared with other batteries. By the end of 2022, the global installed capacity of lithium-ion battery energy storage was 43.1GW, accounting for 94.4% of the global new energy storage installed capacity; the installed capacity of lithium-ion battery energy storage in China was 12.31GW, accounting for 94.0% of the installed capacity of China’s new energy storage. And with the continuous reduction of materials and production costs, lithium-ion batteries have become the battery with the greatest development potential, the widest range of applications, and the most promising to become the main energy storage battery. The comparison of mainstream batteries is shown in Table 1-1.
Table 1-1 Comparison of mainstream batteries
| Battery Type | Lithium-ion Battery | Lead-acid Battery | Sodium-ion Battery | Flow Battery |
|---|---|---|---|---|
| Service Life | 5 – 15 years | 5 – 15 years | 10 – 15 years | 5 – 20 years |
| Main Advantages | Fast power speed, high energy density, flexible response | Mature technology, high cost performance | High energy density, fast response speed, high cycle life | Large storage capacity, deep charge and discharge |
| Main Disadvantages | Safety issues, high cost at present, battery life and inconsistency problems | Low energy density, cannot be deeply charged and discharged, short cycle life, environmental issues | High environmental requirements | High requirements for ambient temperature, high requirements for auxiliary equipment |
| Development Status | Industrial application, high status | Industrial application | Industrial application, rapid development | Industrial application |
1.2.2 Research Status of Battery Equalization Technology
The consistency of energy storage batteries directly affects the actual available capacity and safety of the energy storage unit. With the large-scale application of energy storage batteries, the battery equalization technology has developed rapidly. Battery equalization is a method of eliminating the energy difference caused by the different manufacturing processes and usage environments of batteries through specific technical means to improve the consistency of single batteries in the battery pack. At present, according to the energy transfer direction and energy processing method of the battery, it is divided into energy dissipative equalization and energy non-dissipative equalization.
The dissipative equalization, also called passive equalization, mainly consumes the excess energy in the battery pack by discharging the bypass resistor to heat, so as to achieve the purpose of consistency within the battery pack. This equalization method is simple and easy to implement without the need to design complex control strategies and control circuits. However, in order to avoid safety problems caused by excessive heat release, the size of the equalization current must be strictly limited, so it is not suitable for large-scale energy storage battery packs.
The non-dissipative equalization, also called active equalization, actively transfers the inconsistent energy in the battery pack through the equalization strategy and the equalization circuit to the lower monomer or the battery pack, and the lower monomer or the battery pack absorbs the energy supplemented from the inside or outside of the pack to achieve the energy consistency within or between the battery packs. This is the focus of academic research, but at present, there are still problems such as high cost of the equalization circuit and complex equalization control.
According to the different energy storage elements, the non-dissipative equalization can be divided into the following types:
- Capacitor type: The capacitor is used as a transfer station for energy. The excess energy is first transferred to the capacitor, and when a low-energy monomer appears, the stored energy is transferred to it to keep the battery consistent. However, the capacitor-type equalization circuit has a low efficiency and a slow equalization process because it can only transfer energy between adjacent batteries.
- Inductor/transformer type: The excess energy of the high-energy battery is transferred to the transformer or inductor through mutual inductance, and then transferred to the other batteries or battery packs through mutual inductance. The opposite is true for the low-energy battery equalization. The inductor/transformer-type equalization circuit has a large equalization current, fast speed, but the circuit design is expensive, occupies a large space, and has poor scalability.
- Transformer type: The excess energy of the high-energy battery is transferred to the transformer or inductor through mutual inductance, and then transferred to the other batteries or battery packs through mutual inductance. The opposite is true for the low-energy battery equalization. The inductor/transformer-type equalization circuit has a large equalization current, fast speed, but the circuit design is expensive, occupies a large space, and has poor scalability.
- Converter type: The converter equalization uses a DC conversion circuit such as Buck, Buck-Boost, Cuk, and bidirectional flyback circuit as the basis to form an equalization circuit. This type of equalization topology has the advantages of large equalization current, fast equalization speed, and high equalization efficiency, but the circuit design is complex, and the number of components is large, resulting in high costs.
The active bidirectional equalization of the non-dissipative type is the key research direction of the energy storage equalization technology, and the equalization topology based on the converter is also a hotspot in the industry. In reference [28], a Cuk equalization structure is designed to improve the circuit integration effect; in references [29] and [30], a bidirectional DC/DC converter is used to improve the equalization efficiency and effect, but the number of components is large and the cost is high; in reference [31], a Cuk and Buck-Boost combined equalization structure is designed, and in reference [32], an energy distribution module combined with a bidirectional DC-DC converter is used. Both reduce the number of circuit components and cost, but the efficiency of a single equalizer is low. Therefore, if an appropriate control strategy to improve the equalization efficiency of a single equalizer can be proposed, it can not only effectively reduce the cost, but also greatly reduce the adverse effect of the low efficiency of a single equalizer.
1.2.3 Research Status of Power Control of Energy Storage Systems
Large-scale BESS is generally composed of multiple BESUs, and the battery is connected in series and parallel to form a group to obtain a higher voltage level and a larger storage capacity. In the process of participating in the power regulation of the microgrid, ignoring the internal characteristics of the BESU and simplifying the BESU as a large single battery will cause the battery inconsistency within the BESU, resulting in a reduction in the actual available capacity of the BESU. In the Zhangbei energy storage power station, after 2 years of operation, the battery groups composed of the same batch and the same type of lithium-ion batteries had aggravated the inconsistency within the group. The SOC range (the difference between the highest SOC and the lowest SOC in the group) of a certain battery group increased from 1% two years ago to 25%, the capacity of the monomer battery with the highest SOH in the group attenuated by 2.5%, and the capacity of the monomer battery with the lowest SOH attenuated by 11.9%. The attenuation amplitude was much larger than that of the monomer battery with the highest SOH, and the SOH inconsistency intensified, seriously weakening the actual available capacity, regulation ability, and cycle life of the battery group.
In the application of the microgrid, the operating power of BESS participating in the regulation is usually less than its rated power, so there is a certain degree of freedom in the distribution ratio of the regulated power among the energy storage units, and there will also be differences between the operating modes of the energy storage units and the internal monomer batteries. Therefore, it is necessary to consider not only the inconsistency between the energy storage units, but also the inconsistency of the internal batteries of the energy storage units. Based on the internal characteristics of each energy storage unit, the power control strategy of the energy storage system should be formulated to improve the power regulation ability of the energy storage system. In reference [42], a double-layer optimization management strategy for BESS is proposed to improve the fluctuation range of the energy state of the energy storage unit. In reference [43], the power of the energy storage system is distributed according to the energy state of each energy storage unit to reduce the energy state difference of each energy storage unit, but it accelerates the decay rate of the life of BESS and increases the energy conversion loss. In reference [44], by adjusting the SOC reference value of the battery unit, the charging and discharging amount of the battery unit is controlled to achieve the purpose of different action times of the battery units with different SOH. In reference [45], aiming at the problem of deteriorating SOC consistency of the battery pack and overcharging/over-discharging of the single battery, the change law of the battery pack consistency is studied, and a BESS power control strategy considering the impact of battery pack consistency is proposed, but the article regards BESS as an energy storage unit and does not consider the power distribution problem between the units. In reference [46], taking the lowest system loss and the optimal SOC balance degree as the optimization goal, the BESS power distribution strategy based on the VP-ADE algorithm is given, but the impact of the health state of the energy storage unit on the internal characteristics of the flow battery is not considered.
1.3 Main Research Contents of This Paper
The power generation of wind and solar energy is characterized by strong fluctuations, instability, difficulty in continuously outputting a determined power, and inability to arrange power generation plans according to predictions. When it is directly connected to the grid, it will lead to a mismatch between power and load within the grid, seriously endangering the stable operation of the grid. The battery energy storage system, with its flexible working characteristics, can effectively stabilize the fluctuations caused by the access of renewable energy to the grid and improve the stability and safety level of the grid operation. In the process of stabilizing the irregular renewable power fluctuations, the internal batteries of the energy storage unit will inevitably experience inconsistency, leading to a reduction in the actual available capacity and cycle life of the energy storage unit, and a deterioration in the regulation ability of the energy storage system. To address these issues, the main work completed in this paper is as follows:
- Working principle and state estimation of lithium iron phosphate batteries: Firstly, the working characteristics of lithium iron phosphate batteries are analyzed, and a second-order Thevenin model is selected and established among various battery models as the equivalent model of lithium-ion batteries. The SOC and SOH estimation methods are analyzed, the SOH estimation method based on the weighted accumulation of discharge capacity is selected, and the influence of discharge depth and discharge capacity on SOH is considered. The discharge capacity at each discharge depth is converted to the maximum discharge capacity that lithium-ion can release, and the SOH of the battery is estimated based on the accumulated results of the converted discharge capacity. In terms of SOC estimation, the disadvantages of the traditional ampere-hour integration method and the open-circuit voltage method are analyzed, and the combination of the two methods is used to estimate SOC to avoid the SOC estimation error caused by the disadvantages of the two estimation methods and improve the estimation accuracy, laying a theoretical foundation for the subsequent analysis of the internal inconsistency of the energy storage unit and the formulation of the power control strategy of the energy storage system.
- Research on the inconsistency of energy storage units: This paper analyzes the causes and manifestations of the internal inconsistency of the lithium iron phosphate energy storage unit, proposes methods to improve the consistency of the batteries, introduces the battery equalization technology, proposes the active equalization control architecture based on the auxiliary power, analyzes the working principle of the equalization topology, and proposes the small-period equalization control strategy based on SOC to improve the equalization efficiency and available capacity of the energy storage unit.
- BESS power distribution strategy considering the internal state consistency of the energy storage unit: This paper studies the relationship between the SOH difference and the SOC inconsistency within the energy storage unit caused by the irregular charging and discharging of BESS in the microgrid, and proposes a BESS power distribution strategy considering the consistency of SOH and SOC by combining the charging and discharging priority ranking and the AMPSO algorithm. A common DC bus-type centralized microgrid grid-connected demonstration platform including BESS, wind/solar power generation, electric vehicles, and conventional loads is used to verify the correctness and effectiveness of the proposed power distribution control strategy.