# Hybrid Harmonic Suppression Method at DC Link of Series-Connected 18-Pulse Rectifier
## 5 Simulation and Experiment Validation  
To verify the above analysis, some experiments were carried out. The parameters of the rectifier are described as follows.  
1. The turns ratio of the main transformer meets that \(N\):\(N_{11}\):\(N_{12}\):\(N_{21}\):\(N_{31}\):\(N_{32}\) = 5\(\sqrt{3}\):0.742:0.395:1:0.742:0.395.
2. The injection transformer turns ratio meets that \(N_{7}\):\(N_{8}\) = 0.0118.
3. The RMS of the power supply is 220 V, and the frequency is 50 Hz.  
Figure 10 describes the prototype of the proposed multipulse rectifier. In Figure 10, the control circuit is a combination of the sampling circuit, the drive circuit, and the rapid control typing (RCP).  
When without the suppression method, the input voltage of the rectifier is described as Figure 11. The input voltage in Figure 11 has 18 steps per cycle, the simulated THD of input voltage is 8.7%. Because of the leakage inductance of the main transformer, the experimental value is about 6.8%.  
When using the suppression method, Figure 12 shows the input voltages of the rectifier. In Figure 12, the input voltages are sinusoids, the simulated THD of input voltage is 3.6%. Similarly, because of the leakage inductance of the main transformer, the experimental THD of input voltage is about 2.4%.  
Figure 11: Input voltage of the 18-pulse rectifier without the reduction method. (**a**) Simulation result. (**b**) Experimental result.  
Figure 10: The prototype of the proposed rectifier.  
Figure 13 expresses the input currents when without suppression method. In Figure 13, the simulated THD is about 7.4%, and the experimental THD is about 5.8%.  
Figure 14 shows the input currents when using the hybrid harmonic reduction method. In Figure 14, the simulated THD is about 3.1%, and the experimental value is about 2.7%.  
From Figures 11-14, after using the harmonic suppression method, the simulated THD of input voltage decreases from 8.7 to 3.6%, and the experimental THD decreases from 6.8 to 2.4%; the simulated THD of input current decreases from 7.4 to 3.1%, and the experimental THD decreases from 5.8 to 2.7%. Therefore, the hybrid harmonic reduction method can suppress harmonics of the input side.  
Figure 14: Input current of the converter with the reduction method. (a) Simulation result. (b) Experimental result.  
Figure 12: Input voltage of the proposed rectifier with the reduction method. (**a**) Simulation result. (**b**) Experimental result.  
Figure 13: Input current of the rectifier without the reduction method. (**a**) Simulation result. (**b**) Experimental result.  
From Figures 16 and 17, the amplitude of the current is around 8.5 A, the amplitude of the voltage is 2.4 V. Therefore, the capacity of each injection transformer is about 10 W, which is about 1% of the load power. Because of the leakage inductance of the main transformer, the experimental results of current \(i_{\text{x1}}\) and current \(i_{\text{z2}}\) in Figure 16b have some distortion. Because of the mode switch of VD1 and VD2, the experimental results of the injection voltages in Figure 17b have some spikes in the switching instant.  
Figure 18a describes the load voltage and load current of the proposed rectifier when the load resistance changes from 180 to 90 \(\Omega\). Figure 18b shows the input current of A phase when the load changes from 180 to 90 \(\Omega\). From Figure 18a it can be obtained that the output voltage remains constant while the output current has a step change. From Figure 18b, it can be acquired that the input current rises gently when the load is switching, but the THD value of the input does not change.