Open-Circuit Fault Diagnosis Method of Neutral Point Clamped Three-Level Grid-Connected Inverter Based on Adaptive Sliding Mode Observer

Neutral-point-clamped (NPC) three-level grid-connected inverter plays a pivotal role as an effective interface connecting new energy generation system and grid. However, the failure of any switch will cause the inverter to fail to work normally, resulting in significant economic losses and safety accidents. Some open-circuit fault diagnosis methods of NPC three-level inverters are proposed in recent years. However, these methods are heavily dependent on the quantity of the data, have a long diagnosis time, or only realize single switch fault diagnosis. This paper proposed a diagnosis method based on an adaptive sliding mode observer for single-switch and double-switches faults. In this method, the mixed logic dynamic models of the inverter in the normal state and the open-circuit fault state were established at first, respectively. Then, an adaptive sliding mode observer with fast convergence and significant chattering suppression was designed to estimate the three-phase current in normal states. Subsequently, a fault detection method based on the current form factor was proposed using the output current of the actual system and the observer. Finally, the fault localization variable was constructed according to the current residual for fault localization. On this basis, the open-circuit fault diagnosis of double switches was realized by establishing an adaptive sliding mode observer of the previous fault state. A hardware-in-loop experiment platform is built to verify the effectiveness and robustness of the proposed method. When the inverter works normally, the fault detection variable Ma is smaller than the adaptive threshold Ta, and the detection variable ηa =ηb =ηc . When Qa1 open-circuit fault occurs, the output currents are distorted to a certain extent, and the detection variable Ma exceeds the adaptive threshold Ta. Meanwhile, the detection variable ηa＞ηb ,ηc indicates that fault occurs in phase-a of the inverter. At the same time, the fault location variable Pa=3/2 can locate the open-circuit fault according to the fault diagnosis criteria. After the Qa1 fault is detected, replace the switch function σ a1 ′ in the Qa1 fault state with the original switch function σ a , and the adaptive sliding mode observer under Qa1 fault state is constructed. Then the fault detection variable Ma is lower than the adaptive threshold Ta, which verifies the accuracy of the first fault diagnosis result. When an open-circuit fault occurs in Qa3, the fault detection variable Ma begins to exceed the adaptive threshold Ta, and the fault detection variable ηa also increases and is greater than ηb and ηc . In contrast, the fault location variable meets Pa=−3/2, indicating that fault occurs in Qa3. It can be seen that the diagnosis method can realize the open-circuit fault diagnosis of double switches. In addition, after the grid voltage is increased from 220V to 305V, although the fault detection variables Ma, Mb and Mc all fluctuate to a certain extent, they are still far less than their corresponding adaptive thresholds and will not produce false alarms. Therefore, the robustness and reliability of the proposed diagnosis method are verified. Through experimental verification and analysis, the proposed fault diagnosis method has the following advantages. (1) An adaptive sliding mode observer is established. An adaptive sliding mode observer with fast convergence speed and significant chattering suppression is constructed by designing a new adaptive reaching law. (2) A method for fault detection and localization is proposed. According to the designed current form factor, two fault detection feature quantities are constructed, and the adaptive threshold is designed. Meanwhile, the fault localization variable is constructed by combining the current residuals for fault localization, and the fault diagnosis results are more accurate and robust. (3) A double switches fault diagnosis method is proposed. After the first fault occurs, the adaptive sliding mode observer of the previous fault state is established by replacing the original switch function with the fault switch function, which verifies the accuracy of the first fault diagnosis and diagnoses the double switch open-circuit fault. © 2023 Chinese Machine Press. All rights reserved.