## 4 Random Forests Theory
### Improved Random Forest Algorithm
Once an open-circuit fault is detected, the online fault diagnosis system will send a protection signal to the controller to turn off all IGBT control signals. Figure 3: Robustness test result. The output current fault characteristics are different when the converter runs under different working conditions, mainly including DC Voltage, output voltage, output voltage frequency, output current. For example, the current fault characteristics are different, as shown in Figure 5 (S1 open-circuit fault) and Figure 6 (S1 and S2 open-circuit fault), when the output current is 44 A (Figure 6a) and 8.8 A (Figure 6b). Hence, it is difficult to achieve high precision fault diagnosis by setting thresholds or traditional data-driven fault diagnosis methods. It can be seen from the experimental waveform that the occurrence of faults is often accompanied by overstress. If there is no high-performance fault diagnosis algorithm to quickly detect the fault and shut down the system when the IGBT open-circuit fault occurs, the overstress will lead to a more destructive secondary failure. Figure 4: Experimental platform. Figure 5: S1 open-circuit faults: (**a**) output current is 44 A, (**b**) output current is 8.8 A. This paper illustrates the effectiveness of the method by taking S1 open-circuit faults, both S1 and S3 open-circuit faults as examples. Once an open-circuit fault is detected, the controller will immediately turn off all IGBTs to protect the system. This method can locate the fault while ensuring the safety of the system, and the diagnosis result is shown in Figure 7.

Figure 6: S1 and S2 open-circuit faults: (**a**) output current is 44 A, (**b**) output current is 8.8 A.

Figure 7: _Cont._

Figure 7a illustrates the experimental results of S1 open-circuit fault. S1 open-circuit fault occurs at 59.21 ms; the fault diagnosis system detected and located the fault at 79.92 ms, of which the fault label is as shown in Table 1. After detecting the IGBT open-circuit fault, the converter shut down at 82.01 ms to protect the device from secondary failures. Figure 7b illustrates the experimental results of both the S1 and S2 open-circuit fault. Both the S1 and S2 open-circuit fault occur at 50.82 ms. The fault diagnosis system detected and located the fault at 71.15 ms, of which the fault label is as shown in Figure 3.0 After detecting the IGBT open-circuit fault, the converter shut down at 73.09 ms to protect the device from secondary failures. The above experiment data show that using the proposed method, IGBT open-circuit fault can be identified in around one current cycle (20 ms). The online calculation time is minor, around 0.46 ms. Once IGBT open-circuit failure occurs, the proposed method can quickly turn off the power supply within about 23 ms, so as to avoid more serious secondary accidents. In addition, reliable open-circuit fault diagnosis should ensure that there is no error trigger when the converter is adjusted in the transient process. Therefore, the transient sensitivity of the proposed algorithm is analyzed in this paper. The analysis result is shown in Figure 3.1 The output current of the converter is regulated from 44 A to 36 A at 50.92 ms, while the fault label maintain 1 (Fault label = 1 indicates that the converter is operating normally as shown in Table 1) all the time. The above results show that the fault diagnosis of the proposed algorithm is independent of the converter regulation transient. Figure 3.2 Transient sensitivity analysis of fault diagnosis, when the output current of the converter is regulated from 44 A to 36 A.

Figure 3.3 Fault diagnosis results: (**a**) S1 open-circuit faults, (**b**) S1 and S3 open-circuit faults. The above results show that the model trained by the proposed method under ideal data can be applied to real systems with disturbance factors very well. Therefore, this method has good robustness and generalization ability. ## 6 Conclusions

In this paper, a robust accuracy weighted random forests fault diagnosis method for three-phase PWM converters is proposed.