This article considers fault detection in the case of a three-phase three-wire (3P3W) inverter, when only two current sensors are used to save cost or due to a faulty current sensor. With two current sensors, there is no current method addressing the diagnosis of both IGBT open-circuit (OC) faults and current sensor faults. In order to solve this problem, this article proposes a method which innovatively combines two kinds of diagnosis variables, line voltage deviations and phase voltage deviations. The unique faulty characteristics of diagnosis variables for each fault are extracted and utilized to distinguish the fault. Using an average model, the method only needs the signals already available in the controller. Both IGBT OC faults and current sensor faults can be detected quickly in inverter mode and rectifier mode, so that the converter can be protected in a timely way to avoid further damages. In addition, error-adaptive thresholds are adopted to make the method robust. Effects such as system unbalance are analyzed to ensure that the method is robust and feasible. Simulation and experimental results are used to verify and validate the effectiveness of the method.

The circulating current of modular multilevel converter (MMC) has a great impact on the power losses, the submodule capacitor voltage ripples and the rating of power devices. Existing analysis of circulating current has considered the capacitor voltage. In this paper, the effect of semiconductor devices on the circulating current is further presented. In order to realize the different circulating current injection targets under different conditions, a unified on-line calculation scheme by using the instantaneous information of MMC is proposed to generate the circulating current injection references. It avoids detecting the amplitude and phase of the output current or using an extensive lookup table. Besides, a concise analysis model is built and it reveal the impact on the capacitor voltage ripple and the RMS value of arm current with different injection references, thus guide the determination of the injection ratio. Experimental results based on a three-phase MMC prototype with 6 submodules per arm are presented to demonstrate the analysis of circulating current and the proposed circulating current injection references generating method is verified by different circulating current control targets with different loads.

This paper presents a model-based method to diagnose single and multiple transistor open-circuit faults in grid-tied three-phase voltage-source inverters (VSIs). The method is based on calculated average bridge arm pole-to-pole (PTP) voltages and error-adaptive thresholds. Only existing signals for close-loop control are needed, thus this method can be embedded in system easily without extra sampling and circuits. Average PTP voltage deviations are chosen as diagnostic variables, which show considerable distinction quickly after fault. Consequently, fast fault diagnosis speed can be achieved. The fault diagnosis time for single fault can be as short as two switching periods. Besides, diagnostic variables show the same faulty characteristics in inverter mode and rectifier mode, thus this method is effective in both modes. Moreover, for the first time the variation of inductance caused by conducted current is considered to obtain a more accurate variable-inductance model, and the thresholds are updated according to mathematically estimated diagnostic variable calculation errors from sampling error, inductance error, dead time and delay time, which maintains high robustness as well as fast speed. Finally, the effectiveness of the proposed method is validated with experiments.

T-type multilevel $\rm (T^{2}ML)$ inverters feature low total harmonic distortion and high efficiency. Reliability improvement is particularly important for $\rm T^{2}ML$ inverters because they have high power device count. Methods that provide $\rm T^{2}ML$ inverters with the ability to tolerate simultaneous failures of multiple switches in several legs have not been addressed in the existing literature. This paper proposes a redundant unit that can be shared across phases in a T-type three-level inverter topology to manage open-circuit faults in multiple legs. The redundant unit includes two switches and six diodes, with the extra active switches connected by diodes to each middle switch of the base inverter. Under fault conditions, a standby parallel leg consisting of middle switches and the added redundant devices can be formed in each phase of the inverter to provide an extra current path connecting the DC bus and loads. The middle switches can share the redundant unit simultaneously to replace faulted halfbridge switches. By these means, open-circuit faults in multiple legs can be tolerated simultaneously and the rated output capacity can be maintained during faulted operation. Experimental tests are given to verify the effectiveness and feasibility of this proposed fault-tolerant method.

A Generalized three-phase multilevel current source inverter (CSI) structure is proposed in this paper. The novel multilevel CSI topology consists of n modules of current-source, three-phase six-valve converters and 2(n-1) sharing-current inductors. These inductors divide the dc link current into n-l different current ratings, and these current ratings are used to make up of 2n+1 current levels at the output of the CSI. In this structure, the bulky transformers in the ac side of the conventional CSI are eliminated, and current equilibrium problem of sharing-current inductors can be solved by redundant switching combination states. The operation principle and control strategy of the five-level inverter are analyzed in detail. The implementation of carrier phase-shifted SPWM scheme is investigated for digital control of the novel multi-module CSI structure. Simulated and experimental results demonstrate the proposition.

This paper presents a transistor open-circuit fault diagnosis method based on average model and post-fault reconfiguration algorithms for grid-tied single-phase inverter that transfers power bi-directionally. The transistor open-circuit fault occurrence is detected by comparing average bridge arm pole-to-pole voltage deviation and threshold first. Then, the exact faulty transistor is identified by developed algorithms based on post-fault reconfiguration of control (PFRC) and of utilizing redundant leg (PFRUR). These two fault identification algorithms are suitable for different situations. Compared with the PFRUR, the PFRC can identify the faulty transistor without help of redundant leg, but the identification time is longer. For inverters equipped with redundant leg already, the PFRUR can achieve faster identification speed, so that inverters in reliability-oriented applications can return to normal operation after fault with short interruption. With average model, this method only requires existing signals sampled for control, thus it can be embedded in system easily without adding extra sensors and diagnosis circuits. The sampling frequency can be as low as switching frequency. Furthermore, this method is suitable for both inverters with L or LCL filters, so that the method can be applied more widely. Finally, experiments are carried out on a 550W/10kHz grid-tied single-phase inverter to verify the effectiveness.