## 1 Introduction  
Traditional three-phase, three-wire inverters can cause undervoltage or overvoltage in one phase when carrying unbalanced loads [1; 2; 3]. To improve the output voltage balance of three-phase inverters with unbalanced loads, the main methods used in the literature can be divided into the following four categories: combined three-phase inverters, three-phase four-bridge arm inverters, D/Y transformers on the inverter and load side and split-capacitor, three-phase, four-wire inverters. Three-phase, four-wire split-capacitor inverters can also be widely used in grid-connected systems for power quality improvement [4; 5; 6]. Literature [4] proposes a current-feedback-based impedance shaping strategy to suppress the high-frequency resonance caused by the split-sequence control. Literature [5] analyses the three-phase four-wire split-capacitor inverter and three-phase four-bridge-arm inverter and also presents several cases of small-signal instability caused by the positive sequence, negative sequence or zero sequence. Literature [6] proposes an impedance shaping method based on complex filters and combined differential elements to improve the stability of grid-connected systems. But the above-mentioned literature does not give a method of selecting the splitting capacitor on the DC-side. Literature [7; 8] proposes a combined three-phaseinverter with a circuit using three independent H-bridges so that the inter-phase circuits are independent of each other, with sigmoid _P-f_ sag control in literature [7] and split-sequence and repetitive control in literature [8]. Literature [9; 10] proposes a three-phase four-bridge arm inverter with the neutral point of the three-phase load connected to the mid-point of the fourth bridge arm as a means of controlling unbalanced currents; literature [9] uses an improved positive-sequence, negative-sequence and zero-sequence independent control strategy that is applicable to the three-phase four-bridge arm, and literature [10] proposes a control specific to the three-phase four-bridge arm. Literature [11; 12] proposes the insertion of a D/Y transformer between the conventional three-phase, three-wire inverter and the load to achieve electrical isolation between the input and output. Literature [11] controls the positive and negative sequence components of the voltage, while literature [12] uses split-phase control. All of the above-mentioned control methods achieved good control results, but their circuit structures are relatively complex. Literature [13; 14; 15] proposes split-capacitor three-phase four-wire inverters, where the neutral point of the three-phase load is connected to the mid-point of the DC-side capacitor to provide a path for the unbalanced current. Literature [13] uses a DSTATCOM control method based on a dual-loop controller. Literature [14] compensates for negative and zero sequence currents, and literature [15] uses split-sequence sag control. The above control enables the inverter to output a three-phase-balanced voltage. In the above references, the circuit structure used is simple, but the modelling and analysis of the control method is relatively cumbersome, and none of them analyses the value of the capacitance of the DC-side division.  
In this paper, a split capacitor three-phase four-wire inverter is used. The appropriate DC-side capacitance is selected by means of calculation and analysis. And the use of a feedforward + feedback + RMS three-closed-loop composite control strategy means that the output voltage meets national standards. The effectiveness of this method has been verified through simulation and experimentation. The circuit structure is simple, the cost is low, the control method is simple, and the system requirements can be met at the same time.