In recent years, the scale of DC distributed power sources such as photovoltaics and fuel cells has been continuously expanding, and the DC load of large computers and data centers has been increasing. The medium voltage DC distribution network has gradually become one of the key technologies to promote the high proportion of new energy consumption and improve power supply reliability. However, the high proportion of new energy integration has gradually highlighted the power generation and consumption contradiction between distributed power sources and loads in terms of time and space, affecting the stability and operational reliability of medium voltage DC distribution networks. In the above context, battery energy storage can suppress fluctuations in new energy and become an important means to solve the high proportion of new energy consumption and maintain the safety and stability of the power grid, which has received widespread attention in the industry.
With the development of battery energy storage technology, its cost continues to decrease. To effectively address the challenges posed by the high proportion of new energy integration to the safe operation of the power grid, the capacity and scale of battery energy storage will continue to grow. Traditional battery energy storage systems connect battery cells in series and parallel to form large capacity battery clusters, which are connected to the AC power grid through a two-level converter. This method has poor reliability, and the system output capacity and capacity utilization are limited by the “short board effect”. In addition, uneven internal thermal effects may lead to the risk of thermal runaway, greatly increasing the difficulty of battery management and monitoring. In contrast, the Modular Multilevel Converter Battery Energy Storage System disperses battery energy storage units into sub modules, which has the advantages of high efficiency, high reliability, and easy scalability. In addition, the modular structure of Modular multilevel converter-battery energy storage system facilitates the management, status monitoring, and maintenance of battery energy storage units, and has broad prospects in large-scale grid connected battery energy storage systems on the grid side.
Regarding Modular multilevel converter-battery energy storage system, domestic and foreign scholars have conducted extensive research on topology, State of Charge (SOC) balancing, fault-tolerant control, and other aspects. A isolated three port DC-DC converter submodule topology is proposed to suppress voltage fluctuations in submodule capacitors. A compact integration technology for battery thermal management system was developed by directly embedding the battery into the half bridge submodule. Derived the maximum power difference that can be used to balance SOC between Modular multilevel converter-battery energy storage system submodules under different operating conditions. A SOC balancing control method for Modular multilevel converter-battery energy storage system under unbalanced power grid voltage conditions has been proposed. A fault-tolerant control method for Modular multilevel converter-battery energy storage system based on switch signal redistribution was proposed. However, the above literature did not consider the impact of charge throughput on the operation control of Modular multilevel converter-battery energy storage system.
The charge throughput is characterized by the integral of the absolute value of battery current, which is one of the important factors affecting battery life. A decay model was established for the capacity and internal resistance of lithium batteries with respect to charge throughput. It is pointed out that lithium iron phosphate batteries used in new energy generation scenarios have microcirculation, which affects the battery cycle life. Reference [16] points out that charge throughput and temperature are important factors affecting battery life. Under the premise of constant charge throughput and temperature, the impact of battery current ripple on battery life can be almost ignored. The above literature indicates that charge throughput is an important factor affecting battery life. A variable DC bus voltage control method based on Modular multilevel converter-battery energy storage system is proposed to suppress the charge throughput of battery energy storage systems. This method improves the control strategy and to some extent suppresses the charge throughput of the battery, but is only applicable to single-phase Modular multilevel converter-battery energy storage system with suspended DC buses. In medium voltage direct current distribution networks, the voltage of the Modular multilevel converter-battery energy storage system DC bus is constant, and the power flow on the AC and DC sides is complex and variable. Existing methods rely on changes in the DC bus voltage and do not consider the complex power flow including AC side power, DC side power, and energy storage side power, making it difficult to directly apply Modular multilevel converter-battery energy storage system in medium voltage direct current distribution networks. The charge throughput of Modular multilevel converter-battery energy storage system is directly related to the instantaneous power of the bridge arm. At present, there are literature studies on the impact of second harmonic circulation injection on the power of MMC bridge arms, in order to achieve various control objectives.
By injecting a second harmonic circulating current and adjusting the instantaneous power of the bridge arm, the fluctuation of the capacitor voltage in the MMC submodule was suppressed. A mixed injection strategy of second harmonic current and third harmonic voltage is proposed for the over modulation condition of hybrid MMC to reduce voltage fluctuations in sub module capacitors. Reference [20] proposes an optimal second harmonic circulating current injection strategy that suppresses energy fluctuations in the bridge arm while solving the dynamic balancing problem of sub module capacitor voltage. By adjusting the injection component of the second harmonic circulating current, the peak current of the bridge arm in MMC was suppressed, and the current carrying capacity of MMC was improved. The existing circulation injection methods have not considered the impact of battery energy storage power on the charge throughput of Modular multilevel converter-battery energy storage system, and cannot effectively suppress the charge throughput of Modular multilevel converter-battery energy storage system. In addition, there is currently no literature studying the mechanism of the interaction between the second harmonic circulation component and the charge throughput of Modular multilevel converter-battery energy storage system. How to inject a second harmonic circulating current to adjust the instantaneous power of the bridge arm and suppress the charge throughput of Modular multilevel converter-battery energy storage system under various operating conditions is an urgent problem that needs to be solved.
A Modular multilevel converter-battery energy storage system charge throughput suppression strategy based on dynamic second harmonic circulation injection is proposed in response to the above background. Firstly, the mathematical model of Modular multilevel converter-battery energy storage system in the medium voltage direct current distribution network was analyzed; Secondly, based on the definition of charge throughput and the mathematical model of Modular multilevel converter-battery energy storage system, the mechanism of Modular multilevel converter-battery energy storage system charge throughput generation is explained, and the influencing factors of charge throughput are analyzed; On this basis, a charge throughput suppression strategy based on double frequency circulating current injection is proposed, with the goal of minimizing the system charge throughput and selecting the optimal parameters of the double frequency circulating current under different operating conditions; Finally, the effectiveness of the proposed charge throughput suppression strategy was verified through simulation and experiments under various operating conditions.