## 3 Principle of the T-Type Three-Level Inverter  
In recent years, The T-type three-level inverter has been widely used in photovoltaic (PV) inverters, power factor correction (PFC) rectifiers, automatic rectification, and other low-voltage systems [13]. Figure 4 shows a simplified circuit for a T-type inverter. T-type inverters use the same switches as conventional two-level inverters because these switches (\(S_{x1}(x=a,b,c)\) and \(S_{x4}\)) have to block the full DC link voltage. A bidirectional switch is connected between the neutral point and each output. Unlike the half-bridge switches (\(S_{x1}\) and \(S_{x4}\)), the bidirectional switches (\(S_{x2}\) and \(S_{x3}\)) only need to block half the DC link voltage. Therefore, the devices with lower voltage ratings can be used. A neutral point clamp inverter (NPC) uses two switches connected in series to block the voltage across the DC link. On the other hand, T-type inverters use a single switch to block the voltage across the DC link. Therefore, the T-type inverter conduction loss is greatly reduced compared to that of the NPC inverter. The T-type inverter reduces switching losses and switching noise because the neutral switch operates below half the DC link voltage. Therefore, the total losses in a T-type inverter are the lowest among those of the two-level, NPC, and T-type inverters at medium switching frequencies (4-30 kHz).  
Considering the switch combinations based on Table 1, this configuration allows three voltage levels related to neutral 0 to be generated at the output of phase x.  
For the T-type inverter, the number of switching states is 27, including 19 different voltage vectors, as shown in Figure 5, where some switching states are redundant and generate the same voltage vector. Compared with the two-level inverter, the three-level T-type inverter shows a higher number of switching states. Greater possible incentive mechanisms allow for additional degrees of freedom, and several combinations of cost functions can be considered [14].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \(S_{x1}\) & \(S_{x2}\) & \(S_{x3}\) & \(S_{x4}\) & \(S_{x}\) & \(\mathbf{U_{out}}\) \\ \hline on & on & off & off & P & \(+U_{dc}/2\) \\
0 & on & on & off & 0 & 0 \\ off & off & on & on & N & \(-U_{dc}/2\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Switching states in one phase of the inverter.  
Figure 4: T-type inverter.  
The general predictive control scheme is adopted in the T-type inverter [15]. The behavior of the system can be predicted for every possible switching state of this converter. We choose to minimize the switching state for a given cost function for use in the next sampling interval. Compared with the two-level inverter, the T-type inverter has a higher number of switching states. A larger set of possible drivers allows for additional degrees of freedom, and several combinations of cost functions can be considered. Considering the requirements of the control system, the characteristic and its applications of this topology are studied and compared with the classical PWM linear control algorithm, including load current reference tracking, balancing the DC link capacitor voltage, reducing the switching frequency, and so on [16]. With the development of microprocessor technology [17], the predictive control has been widely applied in power systems. Finite control set model predictive control (FCS-MPC) is more suitable for power electronic converters. The controller is intuitive and easy to implement, and can be used in a number of systems with constraints and/or non-linearly.  
In this article, the control strategy is summarized as follows: 1. Define a cost function; 2. Create a transformer model and its possible switch states; and 3. Create a grid model for forecasting. A discrete time model of the load is required for the predicting behavior of the evaluated variable by a cost function, that is, the grid current.