# Enhanced Control Strategy for Three-Level T-Type Converters in Hybrid Power-to-X Systems
## 5 Verification
### Experimental Results  
The experimental setup for a three-level, three-phase T-type converter with an _LCL_ filter prototype was designed and tested, as shown in Figure 15. It can be seen that the proposed control was implemented digitally using the Delfino F28335 control card. The resonant controllers that are employed in the proposed control algorithm are very sensitive to the discretization process due to their narrow band and infinite gain. A slight displacement of the resonant poles causes a significant loss of performance. At the first stage, the power source supplies DC 800 [V], which is split into two capacitors, each holding 400 [V]. These capacitors act as the DC link and serve as the power source for the T-type inverter. The proposed T-type inverter consists of two main switching blocks: Si-IGBT transistors and SiC-MOSFET transistors. In each phase, two Si-IGBT transistors function as a bidirectional switch, alternating the voltage polarity at the load with a 50 [Hz] frequency while operating at a switching frequency of 50 [KHz]. The SiC-MOSFET transistors are responsible for generating the PWM signal, ensuring that the output voltage maintains the same average value as a sine wave. This topology is based on a buck converter with a 50 [KHz] switching frequency, allowing efficient power conversion with reduced losses. The output voltage of the T-type inverter is connected to an _LCL_ filter, which effectively attenuates high-frequency harmonics generated by the square wave switching process. To minimize power losses associated with transistor switching, SiC-MOSFET transistors were selected, as they enable higher switching frequencies, reducing the required size of the _LCL_ filter inductors and capacitors, thereby optimizing system efficiency and compactness. Recent advancements in switching devices, such as GaN transistors, have further improved power electronics by offering even lower conduction and switching losses. However, in this system, SiC MOSFETs (Mouser Electronics, Mansfield, TX, USA) were chosen over GaN transistors due to their superior performance in high-power applications, better thermal management, and higher voltage handling capabilities, making them more suitable for the 800 [V] input voltage used in this setup. The discrete version of the proposed controllers (\(C_{1}(s)\) and \(C_{2}(s)\)) have been derived using first-order hold (FOH) transformation, which is the most accurate discretization method for resonant controllers. System parameters are the same as simulations and are given in Table 1. The converter was connected to a 10 kW power load. Figure 16a shows the actual experimental values of all three phases, including the RMS values of capacitor voltages and inverter currents and fundamental frequency. Figure 16b presents the time-domain signals of the DC-link capacitors voltages, confirming they are balanced as anticipated, along with the capacitor voltage of phase \(a\) and the current of phase \(b\). In addition, Figure 17 compares results from the simulation (right) and experiment (left). Both the desired voltage levels and sinusoidal current are achieved with precision using the proposed dual-loop control and DC-link capacitor voltage balancing strategy, as demonstrated by identical results. To further evaluate the performance of the proposed control strategy, a comparative analysis was conducted using conventional PI controllers instead of the resonant controllers. The experimental results are presented in Figure 18 and reveal that when PI controllers are applied, both gain and phase errors are introduced. Specifically, the measured results display the output voltage of phase \(a\) and the current of phase \(b\), which ideally should have a 120\({}^{\circ}\) phase shift as shown in the proposed control results in Figure 17, yet an undesired 180\({}^{\circ}\) shift is observed as shown in Figure 18. Additionally, harmonic distortion is evident in both voltage and current waveforms, leading to signal distortion and reduced power quality.