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A redundant unit has been utilized in prior work for a T-type three-level inverter to handle multiple open-circuit faults, but the output was limited to two levels. In this article, equivalent switching state and improved virtual vector methods are proposed to complement switching states and provide better fault-tolerant performance. Under fault scenarios, equivalent replacement of unreachable switching states can be used to generate the target line-to-line voltages. If the replacement cannot be achieved by equivalent switching states, a virtual vector with redundant capability is employed to support two- and three-level modulations at the output. The rearranged switching sequences can be formed by employing virtual vectors and existing vectors. In addition, by reconstructing the virtual vectors, a balancing method is adopted to eliminate neutral point voltage offset in cases of multiple faults. Experimental tests verify the effectiveness and feasibility of the proposed method.

© 1986-2012 IEEE. 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.

This paper presents a transistor open-circuit fault diagnosis method based on an average model and post-fault reconfiguration algorithms for a grid-tied single-phase inverter that transfers power bi-directionally. The transistor open-circuit fault occurrence is detected by comparing the 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 the 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. Finally, experiments are carried out on a 550-W/10-kHz grid-tied single-phase inverter to verify the effectiveness.

The circulating current of modular multilevel converter (MMC) has a great impact on the power losses, the submodule (SM) capacitor voltage ripples, and the rating of power devices. Existing analysis of circulating current has considered the capacitor voltage and the modulation signal. 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 online calculation scheme by using the instantaneous information of MMC is proposed in this paper 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 mathematical analysis model is built and it reveals 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 six SMs 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 (OC) 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 closed-loop control are needed; thus, this method can be easily embedded in the system without extra sampling and circuits. Average PTP voltage deviations are chosen as diagnosis 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, diagnosis 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 diagnosis 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.

The modular multilevel converter (MMC) with nearest level modulation (NLM) is widely used in the high voltage applications for low switching frequency and easy implementation. Existing literature has not provided a complete submodule (SM) fault ride-through scheme for MMC with NLM. In this paper, a strategy including fault detection, localization, redundancy, and recovery is proposed to ensure continuous operation of MMC under IGBT open-circuit faults conditions. It only requires a few hardware and software resources. The features of MMC and SMs with three types of failures are studied, respectively. Based on these, the fault detection method is proposed by using a simple hardware circuit, thus high computation complexity is avoided. Since current fault localization schemes are limited to MMC with carrier phase shifted pulse width modulation, this paper further proposes a strategy for MMC with NLM to locate the faulty SM and identify the fault type. After this, the fault redundancy and the proposed fault recovery method are applied to eliminate the fault and then exit the failure state. Therefore, the ability of SM fault ride-through can be realized. Analysis of failure characteristics are verified in simulation. Experimental results based on a single-phase MMC prototype with 11 SMs per arm are presented to demonstrate the validity of the proposed fault ride-through strategy.

Inverters and loads can freely connect to any node in a meshed parallel inverters system, leading to variations of line impedance between two nodes and the structure of the electric network. More challenges exist for proportional load-sharing control in such a system. An improved proportional load-sharing strategy for a meshed parallel inverters system with complex impedances is proposed in this paper. The RL -type droop method is modified through injecting the power droop information of all other inverters, which can improve the load-sharing accuracy to some extent and completely compensate the voltage amplitude and frequency deviations caused by droop equations under accurate load-sharing conditions. Besides, an adaptive virtual resistance tuned by the integral of the linear combination of output active and reactive powers, is added to eliminate load-sharing errors. In addition, the central controller broadcasts the control variables to all inverters. This improves the simultaneity of control data and minimizes the mismatch of control delay. Three modified droop methods are compared with the small-signal analysis and simulation. Experimental results are presented to investigate the performance of the proposed control strategy.

This paper aims at improving transient performance of LCL-filter-based grid-connected inverters with active damping. Transient performance of inverters controlled by capacitor current feedback active damping (C2FAD) is analyzed, revealing that grid-currents of the inverters may have overshoot and long settling time when loads fluctuate. To improve transient response better than C2FAD do, a dual current feedback active damping (DCFAD) is proposed. The proposed DCFAD could realize virtual resistor connected in parallel with capacitor and virtual resistor connected in series with grid-side inductor, consequently, adding a control parameter to the algorithm to configure the poles effectively without adding sensors. Experimental results based on a single-phase grid-connected inverter prototype with LCL filter are presented to demonstrate that DCFAD could eliminate overshoot and reduce settling time, when comparing with C2FAD.

Modular multilevel converter (MMC) is one of the most potential converter structures for medium/high power/voltage applications. Since the electrical energy is stored in the capacitances of its sub-modules during the operation of the MMC, the output power quality is directly related to the capacitor voltage. Therefore, capacitor voltage balance is of great importance. In this paper, a capacitor voltage balancing strategy for the carrier phase-shifted modulation strategy is presented for MMC by rotating the carriers, which not only solves the problem of capacitor voltage balance, but also avoids complex closed-loop modulation and voltage sequencing. Considering the potential additional switching process during carrier rotation, the algorithm is further improved to find the best rotation period. The experiment verifies the propositions.

This paper proposes a fast multiple semiconductor open-circuit fault diagnosis method for a T-type three-level inverter. In this method, appropriate Boolean operations are selected to represent the relationship between voltages across the upper half-bridge switches and switching signals. Based on this, the current path in the circuit can be tracked under normal and abnormal conditions. Logical relationships are linked to corresponding switch states, so this method is applicable even under multiple open circuit faults. Diagnosis results can be obtained rapidly, with simple hardware. Misdiagnosis caused by delays in power devices and sampling circuits can be avoided if switching signal dead time is longer than the total delay. This diagnostic scheme is immune to load disturbances and dead times. Experimental results obtained under various conditions demonstrate the applicability and performance of the approach.

This paper designs an LLC converter with super-wide output voltage. A variable-mode control strategy with three operating modes is adopted for the converter, and a novel optimal parameters design method based on this strategy is proposed. The designed converter can achieve a super-wide output voltage range at various load without increasing circuit’s complexity, expanding the operating frequency and phase-shifted range. The magnetizing inductance is maximized to decrease magnetizing and conduction losses on condition that the requirements of wide gain and primary MOSFETs’ soft-switching are met, so the converter can maintain a high efficiency within the entire output voltage range. Finally, a 2.5kW experimental prototype with output voltage ranging from 25V to 150V is set up, by which the effectiveness of the strategy and the design method are proved.

Modular multilevel converters (MMC) can be used in many applications such as medium voltage multimegawatt drives and HVDC power transmission. Modulation strategy plays an important role for the good performance of MMC. In this paper, a submodule (SM) regrouping method of MMC is presented and based on this, a sampling time staggered space vector pulse width modulation (STS-SVM) technique is proposed for MMC. Each group has three SMs, which is irrespectively from three phases. Thus, the SVM scheme for the two-level converter can be applied in each group. For an MMC with n-SMs per arm, the two-level SVM for each group is staggered evenly over the sampling time, so as to achieve an equivalent switching frequency at the output. Therefore, the STS-SVM can be very easily expanded with the increasing of the SMs number, and also, the off-line or on-line effort/tasks about generating the SVM switching sequences are simplified significantly. Besides, the capacitor voltages of SMs can be balanced by rotating the gate signals of different SMs even without any capacitor voltage detecting or sorting because the even power distribution among SMs is realized. A three-phase MMC is simulated at both steady and dynamic states in MATLAB/Simulink and validate the performance of the proposition.

Inductive power transfer (IPT) system for electric vehicle battery charging has many advantages over the traditional plug-in system. EV battery can be regarded as a wide range load according to the charging profile. The charging process requires constant current (CC) stage and constant voltage (CV) stage. The charging profile for IPT system is influenced by inductively coupled transformer, compensation structure and operating frequency. This paper proposes an IPT topology which naturally obtains CC and CV output functions simultaneously without increasing the complexity of control system. The system switches between the two charging modes in succession along with the change of load condition. It operates under the CC mode when the load resistance is small, which corresponds to the beginning of battery charging state. With the increase of load resistance, it switches to CV mode at a load turning point after which the battery is under the state of cutoff voltage. As the system is completely compensated, zero phase angle input is achieved during the entire charging process, which realizes soft switching for the entire load range. A fixed frequency control method is used to adjust the output accurately. The experimental IPT system is established to verify the theoretical analysis.

This paper presents an average model-based transistor open-circuit fault diagnosis method for grid-tied single-phase inverters which transfer power bi-directionally. It is easy to detect whereas tricky to identify the faulty transistor, for two different faults may show the same characteristics. In order to solve this problem, the inverter is changed to boost mode after fault is detected, so that the diagnosis variable can show distinguishable characteristics to locate the faulty transistor. The main advantage of this method is that the fault can be diagnosed fast without adding extra circuits. Additionally, it is featured with strong robustness against power change and sampling error. Finally, the effectiveness of the proposed method is verified by experiments.

Owing to mismatched feeder impedances in an islanded microgrid, the conventional droop control method typically results in errors in reactive power sharing among distributed generation (DG) units. In this study, an improved droop control strategy based on secondary voltage control is proposed to enhance the reactive power sharing accuracy in an islanded microgrid. In a DG local controller, an integral term is introduced into the voltage droop function, in which the voltage compensation signal from the secondary voltage control is utilized as the common reactive power reference for each DG unit. Therefore, accurate reactive power sharing can be realized without any power information exchange among DG units or between DG units and the central controller. Meanwhile, the voltage deviation in the microgrid common bus is removed. Communication in the proposed strategy is simple to implement because the information of the voltage compensation signal is broadcasted from the central controller to each DG unit. The reactive power sharing accuracy is also not sensitive to time-delay mismatch in the communication channels. Simulation and experimental results are provided to validate the effectiveness of the proposed method.

A novel fast open-transistor diagnosis approach for voltage-source inverters (VSI) based on calculated bridge arm pole-to-pole (PTP) voltages is proposed in this paper. The theoretical PTP voltages can be calculated with duty cycle ratio and DC input, and the real PTP voltages can be calculated with output voltages and currents. When no fault occurs, there should be no difference between the theoretical and the real PTP voltages. Once fault happens, great deviations between these two calculated voltages will be incurred and can be utilized to detect and localize the fault. This method is able to achieve fast diagnosing speed, while no extra hardware circuits and complex calculation are needed, thus it can be embedded in the existing control system with little effort. Furthermore, strong robustness against operation variation makes this method feasible in diversified applications. Simulations and experiments have been carried out to validate the correctness and effectiveness of this method.

In this paper, a zero-current switching (ZCS) current source converter (CSC) topology is proposed by adding an auxiliary circuit between the dc side. During each commutating process, the auxiliary circuit is activated, so that the outgoing switches can be turned off with zero current switching. As a result, all active switches can reach ZCS operations. This feature makes it possible to increase the switching frequency to improve the line current quality where a small size of ac filter is required. That means the presented ZCS CSC topology is very suitable for some applications with high PWM frequency. Simulation gives the verification.

A current soft-commutation Space Vector Modulation (SVM) scheme for three-phase current source converter (CSC) is proposed in this paper. With the presented SVM, the incoming switch has a positive collector-to-emitter voltage while the outgoing switch has a negative collector-to-emitter voltage during current commutations. That is, natural soft-commutations for switching transitions are achieved with accelerated current-commutating processes and lessened durations. As a result, distortion of the line currents can be reduced. Besides, due to the lessened overlapping time, the switching frequency is expected to reach as high as possible to reduce the ac filter and improve the line current quality. Especially, since the presented SVM is based on a closed-loop strategy, soft commutations can be reached in CSC with any power factor operation. The experiment verifies the proposition.