# Voltage Ride through Control Strategy of Modular Multilevel Converter under Unbalanced Voltage Sag
## 1 Introduction  
Recently, the modular multilevel converter (MMC) has gained more importance for different applications such as high-voltage direct current (HVDC) and renewable power generation systems [1; 2]. They can provide high reliability and good harmonic performance in grid-connected converters [3]. In the structure of an MMC, a high number of semiconductor switches and capacitors are employed. A small part of the active power is absorbed by the MMC to retain the voltage of the submodule (SM) capacitors constant as well as compensate the active power losses. However, the voltage of the capacitor can create an imbalance due to a mismatch in conduction and the switching power losses. Balancing the SM capacitor voltages has been a major technical challenge in the latest research [4; 5]. Currently, many kinds of literature on voltage balancing techniques are available. These can be sorted into distributed voltage control approaches and centralized SM selection control approaches [6]. Both contain controlling capacitor voltages based on the control of power electronic switches during the controlled pre-charge and normal operation stages of the power system [7]. Moreover, there have been investigations regarding the execution of an advanced current controller for the MMC. A combination of model predictive control (MPC) and classical control has previously been presented [8] for the MMC's voltages balancing. In this approach, an arranging method is utilized to accomplish SM capacitor voltage balancing and other control objectives are realized with an MPC strategy. Thus, the number of switching states and computational load on the MPC strategy is considerably reduced [9]. However, the computational complication is one of the main matters in MPC. A new capacitor static voltage balance method based on self-power supply control has beenpresented elsewhere [10]. In this technique, the SM capacitor voltages are organized from low to high and the minimum SM capacitor voltage is controlled by the regulating of the self-power supply input current. However, the time required for voltage balance will be longer, when an MMC has more SMs. The literature [11; 12; 13] uses different methods to reduce the capacitance-voltage deviation between SMs. In Reference [14], the SM capacitor voltage was balanced by a carrier-based pulse width modulation method. This method can better reduce the capacitance-voltage deviation between SMs, but it is necessary to add a controller to each SM for control, which increases the economic cost and system complexity. In References [15; 16; 17], if the SM capacitor voltage deviation was within the acceptable range of the system, the SM switching frequency was reduced by different methods. In Reference [16], the SM operating state was determined according to the direction of the bridge arm current and change trend of the reference voltage. In Reference [17], a method of fixing the switching frequency was proposed. In all the aforementioned studies, an inclusive model for assessment of the MMC during unbalanced voltage conditions was absent. Therefore, it is crucial to report on the behavior of the MMC under different voltage disturbances in distribution power systems. Hence, some published studies have taken a look into the control and analysis of the renewable power generation system during faulty conditions in an unbalanced network. In Reference [11], a grid-connected wind-photovoltaic (PV) hybrid system with battery energy storages was offered in order to increase the integration proportion of renewable energies. In this study, a control strategy was developed to insulate the hybrid power system during a grid fault. Moreover, the study in Reference [12] offered a symmetrical components control algorithm, proposing it for a four-leg, three-phase, grid-connected voltage source converter used for renewable energy resources with wind-solar-battery units. A mechanism of negative and zero sequences injection based on the control of (d, q) current coordinates was introduced.