## 1 Introduction  
Compared with the two-level converter (inverter), multilevel converters possess many properties that make them highly applied on an increasing number of occasions, e.g., high power level, negligible total harmonic distortions (THD) [1], high bus voltage, in the staircase waveform free of bulky filters, modular configuration with fault tolerance, etc. [2, 3]. Among the multilevel converters, the three-level converter has the most extensive applications, such as electric motor drives, wind power generation, and so on [3]. The unbalanced neutral-point voltage (NPV) in the three-level converter means unequal voltage across the two series capacitors on the DC bus. Factors causing this problem include but are not limited to [4, 5, 6, 7] unbalanced three-phase parameters (voltage, inductance, power switch, etc. ), unequal load resistance values or different capacitance values of the two series capacitors, and low-frequency harmonics in neutral-point current on the DC side. The unbalanced NPV will lead to uneven voltage stresses on the main power devices and series capacitors, which can easily lead to device damage [8, 9, 10]; it will increase the harmonic components in the current and degrade the current quality; in addition, it may lead to degradation of the three-level characteristics. Many scholars have studied the NPV problem and proposed various methods, which can be classified into two categories: the hardware method [11, 12, 13] and software method [8, 14, 15, 16, 17, 18, 19]. The hardware method is generally applied only in the three-level inverter and is achieved by replacing the capacitor of the DC bus with DC power supply [11]. NPV can also be controlled by adding hardware circuitry to control or adjust the current [12; 13] flowing to the neutral point of the DC side. In general, the hardware method is less flexible, more costly, and more different regarding the control effect in controlling different types of unbalanced NPV problems. The software method is widely adopted, with the advantages of high flexibility and low cost in terms of the NPV control. The NPV control strategy introduced in [14] is based on a zero-sequence voltage injection, which controls the rapid reduction of NPV deviation by predicting the change of voltage difference on the two capacitors on the DC side in advance, but its control accuracy is poor, and control effect varies under different operating states. In [15; 16; 17], a virtual space vector pulse width modulation (VSVPWM)-based NPV control strategy is proposed, which increases the switching frequency and decreases the efficiency compared with the traditional space vector pulse width modulation (SVPWM)-based control strategy. In [18], a controller that superimposes a triplet frequency harmonic component to the neutral-point current is given, which is able to control NPV balance in the full range of the power factor, but its shortcoming is that the input to the controller is a triplet frequency component, which leads to a complex design process of the controller. A hybrid space vector modulation method is introduced in [19], which is implemented using a seven-segment partitioning method, which has an increased number of switching operations and higher losses compared to traditional space vector-based modulation method. In summary, various NPV control strategies implemented using software methods are able to achieve balanced control of NPV under certain conditions, but also have their own shortcomings. Therefore, it is still necessary and meaningful to study an NPV control strategy with a flexible and simple implementation process, fast adjustment speed, high accuracy, and low impact on the three-phase current quality. In this paper, the NPV control of the three-phase VIENNA circuit is studied. Through the analysis of the role and effect of small-voltage vector on NPV in the VIENNA circuit, an NPV control strategy is proposed by dynamically adjusting the selection method of small-voltage vector.