Hybrid Pulse Width Modulation Strategy of a High-Frequency Link Three-Phase Four-Leg Matrix Converter Based on Compensation Theory  
[PERSON]  
1 Key Laboratory of Modern Power System Simulation and Control & Renewable Energy Technology, Ministry of Education (Northeast Electric Power University), Jilin 132012, China; [EMAIL_ADDRESS] (F.W. ); [EMAIL_ADDRESS] (H.P. ); [EMAIL_ADDRESS] (S.L. )12  
and various devices have higher and higher requirements for quality of power system input [1; 2]. The contradiction between these two aspects has become remarkable. Thus, it is necessary to study the control of three-phase inverter under unbalanced grid conditions. In actual situations, the inverter is often required to have the ability to supply power to unbalanced load. For the three-phase three-wire inverter topology, when the three-phase load imbalance occurs, the phase current also becomes unbalanced, resulting in a deviation of the neutral point potential of the load, output three-phase unbalanced hazardous voltage, thereby affecting the normal operation of the power grid. This is not overcome by the three-leg inverter's own topology. The three-phase four-leg inverter is widely studied in recent years. It uses three-phase four-wire power supply topology. Compared with ordinary three-phase three-leg inverters, it has one more output bridge leg phase N, which can connect to the load neutral point to control the load neutral point potential, so that the three-phase symmetrical balanced voltage can be output under unbalanced load conditions [3; 4; 5; 6]. In order to protect the safety of the equipment and human body, the inverter needs to be electrically isolated from the input side and output side. However, the ordinary inverter topology itself does not have the function of isolating electrical. It is necessary to add a frequency transformer between the inverter and load. This will increase the volume and weight of the transformer, the noise pollution of the transformer is serious, the dynamic response performance of the system is poor when load fluctuates [7; 8]. The high-frequency link (HFL) three-phase four-leg matrix converter (MC) is a new type of inverter topology developed on the basis of ordinary three-phase four-leg inverter. It can satisfy the increasing demand of high power density and does not affect the performance and reliability of the inverter. In the topology, a high-frequency isolation transformer is used to transfer energy, so that it not only has the function of electrical isolation, but also reduces the volume and mass of the system through the frequency rise of the transformer. At the same time, it inherits the advantages of the MC, such as sinusoidal input, bidirectional energy flow, adjustable power factor, and no need for a large capacity energy storage element [9; 10; 11]. For the HFL three-phase four-leg MC, [PERSON] proposed a closed-loop control strategy based on the symmetrical component method [12]. The four bridge leg of the MC is controlled as a whole, which leads to large switching losses. When the load fluctuates, the dynamic response performance is insensitive, and the harmonic distortion is large. A mathematically cumbersome coordinate transformation is required, which increases the complexity and difficulty of implementation. An HFL three-phase four-leg MC modulation strategy with a separate control of the front three bridge leg and fourth bridge leg is adopted to improve the utilization efficiency of the switch [13]. However, in order to obtain good anti-interference and followability, additional adjustment systems are needed to reduce control errors. This not only increases the cost of the system, but also its practicality is limited by the adjustment accuracy. [PERSON] adopted a three-dimensional space vector PWM (3D-SVPWM) control strategy to maintain low harmonic distortion of the output voltage under a variable unbalanced load [14]. However, the concept of three-dimensional space vector is introduced into this method, which is more complex in both control and implementation method.