## 5 Experiment Results  
An experiment was performed to verify the feasibility of the proposed control method that is applied in a 3-phase AC/DC PWM converter for a DC distribution system. The configurations of the experimental system and power stacks are shown in Figures 22-24, and the experimental parameters are shown in Table 1.  
Figure 21: Waveforms of simulation results when frequency suddenly change (**a**) the DC-link voltage, \(d\) and \(q\)-axis voltage in synchronous reference frame with conventional synchronous reference frame-phase locked loop (SRF-PLL) (**b**) the DC-link voltage, \(d\) and \(q\)-axis voltage in synchronous reference frame with proposed control method. Figure 23: Experiment setup 1 of the 3-phase AC/DC PWM converter. Figure 22: Configuration of the experiment system. MC: Magnetic Contactor. Figure 25 shows the waveforms for the operating sequence of a 3-phase AC/DC PWM converter for DC distribution. To prevent an excessive inrush current to the capacitor, the dc-link capacitor is charged while using an initial charging circuit consisting of magnetic switch and a resistor. After the initial charging is completed, the main magnetic switch is activated and the voltage is charged to the capacitor up to the input phase voltage peak value of 311 V\({}_{\text{dc}}\). The controller then starts to operate and the voltage increases up to 700 V\({}_{\text{dc}}\), which is the voltage reference, and the operation maintains steady state. As the load capacity increases, the output current gradually increases and the voltage remains constant, despite the increasing load. Figure 24: Experiment setup 2 of the 3-phase AC/DC PWM converter. SMPS: switched-mode power supply; DSP: digital signal processor; MCCB: molded case circuit breaker. Figure 25: Waveform of 3-phase AC/DC PWM converter operation. Figure 26 shows the input phase current waveform at the grid side when a load of 12.3 kW is applied, which is the largest capacity used of those shown in Figure 25. Since the load capacity is lower than that of the designed capacity of 50 kW, the current waveform includes the harmonics. Figure 27 shows the grid side input \(a\) voltage and current waveforms. The phase angle of the input phase voltage was extracted while using the positive sequence voltage and phase detector using SOGI-FL without the need to use the PLL of the conventional control or the PLL used for the coordinate transformation of the phase current. In addition, the power factor is controlled to be 1 by adjusting the _d_-axis current command in the synchronous reference frame, which is a reactive power component. The phase detection performance of SOGI-FLL is then verified by experiment waveform. Figure 28 shows the 3-phase voltage waveform when a voltage drop (about 10%) occurs on the \(b\) phase. Figure 28b shows the expanded waveform of Figure 28a when the voltage drop occurs, and Figure 28c shows the waveform of the 3-phase positive sequence input voltage extracted while using SOGI-FLL. The amplitude of the 3-phase positive sequence voltage decreases due to the voltage drop. Even though the input 3-phase voltage is unbalanced, the extracted positive sequence voltage using SOGI-FLL maintains the balanced condition. Figure 27: Waveform of input phase \(a\) voltage and current. Figure 29 shows the positive and negative sequence \(\alpha\beta\) voltage and phase angle \(\theta\), before and after the \(b\) phase voltage drop. In the case of the \(\alpha\beta\) voltage in the stationary reference frame, the amplitude and phase angle of the positive sequence component are always the same as these of the grid side. While the appearance of the negative sequence component is negligible at the steady state, the amplitude varies depending on the distortion in the unbalance condition. The phase angle \(\theta\) of the positive sequence is always rotated in the same direction. In the case of a negative sequence, the magnitude, direction, and shape of the phase angle change depending on the balance, unbalance, and distortion.