## 1 Introduction  
In recent years, the depletion of fossil fuels and environmental pollution have become important matters of global concern because they cause the depletion of energy resources and global warming. Accordingly, the demand for the use of renewable energy sources, such as wind power, solar energy, and biomass, has rapidly increased [1; 2; 3]. To meet the demand and reduce the problems associated with the depletion of fossil fuels and environmental pollution, numerous studies have been conducted for green energy and renewable energy development [4; 5; 6]. The common renewable energy systems are wind power generation (WPG), which uses wind turbines, and solar energy generation (SEG), which uses photovoltaic cells. However, sources such as wind power and solar energy might be unreliable and unpredictable because of changes in environmental conditions. Therefore, energy storage systems (ESSs) are used for conserving energy generated by the renewable energy sources in battery systems. The grid-connected ESS usually generates and supplies power by connecting to a grid. It is used for conserving the additional energy with a reasonable cost, such as at night. Moreover, it can improve the energy quality and maximize its efficiency by supplying the conserved energy on requirement. The ESS has been made commercially available as typical renewable energy and energy conservation systems [7; 8; 9; 10; 11]. Power generation systems connected to the grid can have a negative impact on the grid in case the power scale of the systems becomes large. Therefore, several countries suggest grid-code regulations to mitigate the negative impacts on the grid. Grid-code regulations differ from country to country; Germany's grid-code regulations are more rigorous than the grid-code regulations of other countries. In general, the grid-code regulations define that systems such as WPG and SEG have to remain connected to the grid when the voltage drops for a specified time and support the grid with a reactive current. This requirement, known as a low-voltage ride-through (LVRT), is needed to avoid grid blackouts. The LVRT requirement is crucial for grid-code regulations. Figure 1 shows the LVRT requirements in Germany and China. The requirements are different regarding a fault duration and the injection quantity of the reactive power depending on the voltage drop ratio. This requirement ensures that the system connected to the grid operates properly when the grid voltage drops [12; 13; 14; 15; 16; 17; 18]. The LVRT requirement contributes to the recovery of the grid voltage by supplying the reactive current in the designated voltage range. In order to satisfy the LVRT requirement, a proper control of the reactive current in the grid-connected ESS is necessary. To control the reactive power and the current for the grid-connected inverters in the WPG, SEG, and ESS is crucial. Numerous studies have been conducted for issues related to the reactive power and current compensation capabilities of such systems [19; 20; 21; 22; 23; 24; 25]. The WPG systems using wind turbines and the SEG systems using photovoltaic cells usually have a large power scale. Therefore, the LVRT requirement needs to be thoroughly implemented in these systems. The LVRT control strategies researched recently comply with the LVRT requirement. In addition, these strategies focus on the additional operations that are effective and economical, which include the continuously stable operations with dynamic breakers in the WPG systems [26; 27; 28; 29; 30] and the maximum power generation with low voltages in the SEG systems [31; 32; 33]. These methods for additional operations are presented by considering the characteristics of the source type. The research on the LVRT control strategies for the WPG and SEG systems is actively progressed, however, the research on the LVRT control strategy for the grid-connected ESSs is insufficient.