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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.

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.

Higher power applications require a multilevel converter to meet high power ratings. A multimodule current-source converter (CSC) can provide a multilevel current waveforms and at the same time eliminate the bulky transformer at the ac side. However, it is required for the multimodule CSC to use multiple dc-link inductors for the purpose of power balance among different modules. Thus, one key issue in the design of the multimodule CSC is to balance the current through different dc-link inductors. This paper focuses on studying the control strategy of the multimodule CSC and a generalized current balancing method is presented. As presented, zero vectors are distributed based on the deviation of the dc-link inductor currents and the comparison of the capacitor voltages. Therefore, the current imbalance problems, not only those among the upper inductors but also those among the lower inductors are solved. Moreover, since the current balancing algorithm is not dependent on module numbers, the presented control strategy is very suitable for modularization. Especially, with the feedback of the capacitor voltages, the resonance arising from the inductor-capacitor (LC) filter is damped. In addition, a design method of the dc-link inductors is also derived. Finally, simulation and experimental results show the validity of the propositions.

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.

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.

This paper presents a current soft-commutation pulsewidth modulation (PWM) scheme for three-phase current source converter. With the presented PWM scheme, the incoming switch has a positive collector-to-emitter voltage while the outgoing switch has a negative collector-to-emitter voltage during current commutations. Therefore, natural soft commutations for switching transitions are achieved, while current-commutating durations are also lessened. Moreover, some particular advantages can also be obtained such as reduced switching frequency, high dc current utilization ratio, and in-phase transfer between line currents and their references. For more efficient control, this paper then designs logic equations for mapping out the assignments from references to gating signals, allowing a simple implementation using a digital signal processor or a programmable logic device. The theoretical analysis is verified by experimental results.

This paper aims to explore the pulsewidth modulation (PWM) schemes for three-phase current source converter, and a generic six-step direct PWM (SS-DPWM) scheme is proposed. In the SS-DPWM, one line cycle is divided into six 60° intervals, so called as ?six-step.? During each interval, only two of the three references, referred to as the Master and Slave references, respectively, compare with the carriers to directly generate the required switching patterns instead of using the transformation from bilogic signals to trilogic ones, as used in the conventional sinusoidal PWM (SPWM). In this paper, the variety of the Slave reference for each interval is identified, and many sets of modulating waveforms are presented for the SS-DPWM, resulting in very flexible and expanded modulations. The mechanism of the SS-DPWM is also presented to develop its modulating characteristics, and it is shown that, comparing with the conventional SPWM, the SS-DPWM has lower switching frequency, higher dc current utilization ratio, and in-phase transfer between the line currents and their references. A prototype has been built in the lab and the validity of the theoretical analysis is verified by the experimental results.

This paper explores the issue of constructing multilevel current source converter (MCSC) topologies using the duality principle. Three circuit cells for MCSCs are derived from the applications of the duality on the three common leg circuit cells of multilevel voltage source converters (MVSCs). Based on the presented three cells, three families of MCSC topologies, which are also corresponding to three families of MVSCs, are constructed. Analytical results show that these derived MCSCs and their corresponding MVSCs present analogous features such as the ac side currents/voltages and inductor/capacitor currents/voltages, though they are not direct dual structures. Some experimental results are given to verify the presented MCSC structures.

Three-phase inverter has been widely applied in various applications, and lots of control schemes are investigated for the inverter powering nonlinear load, aiming to achieve favorable performance. However, the model currently proposed for inverter is based on linear load, which is not suitable for condition with nonlinear load. In this paper, a novel modeling method for three-phase inverter powering nonlinear load is proposed. Then, the mathematical derivation and performance analysis of the proposed model are presented in detail. Based on the proposed model, a control scheme is further presented, incorporating PI control and capacitor current feedback. In addtion, the second-order system damping theory and phase-plane analysis method are well developed to describe the steady-state and dynamic performance of the proposed model and control scheme. Compared with conventional dual-loop control, low THD of the ouput voltage could be acheived by means of the proposed scheme, which is perfectly validated by the simulation results.

Quantitative Risk Assessment（QRA）, as a new technique, is widely applied in construction and operation for oil and gas pipelines. However QRA technology has usually been applied for land use or making maintenance plan during the construction or the pipeline operation respectively. Thus it has less been applied for station design of pipeline. This paper analyzes the risk of gas station in China by using QRA technology and provides suggestion for station design according to the assessment result.

LCL filter presents a significant attenuation of switching harmonics, however its resonance peak influences the system stability. Passive damping reduces the switching current harmonics attenuation and increases the extra losses. Nevertheless, traditional active damping method often needs additional sensors or increases the complexity of algorithm. According the Nyquist stability criterion, a novel active damping method is proposed in this paper based on the grid current feedback compensation which adds no more sensors and easy to control. Two kinds of current compensation transfer function are presented. Moreover, the design principles and the range of stable system with active damping are derived step by step. Finally, it compares the damping effect of the different compensation methods through experiment and verifies the theoretical analysis at the same time.

An improved switching loss calculation method for IGBT switching on, switching off and the diode reverse recovery energy losses in neutral point clamped inverter is proposed in this paper. A model of linear instantaneous switching on and off waveforms is presented to approximate actual switching process. The IGBT switching on and off duration, the changing rates of current di/dt and collector-emitter voltage dv/dt all can be calculated at different load currents with the given IGBT parameters. Besides, the proposed method is apparently different from the linear waveform method used before which assuming switching time to be a constant value. In addition, compared with the traditional mathematical equation that uses measured data to fit a formula, the equation proposed here is based on physical model and it can indicate the relationship between gate drive resistor, the IGBT capacitance, trans-conductance and switching process. An inverter employing a switching pattern as sinusoidal PWM (SPWM) was designed and experiment was carried out to test the switching waveforms under different output currents. Switching loss under different currents was also measured and compared with calculated value using mathematical equation proposed in this paper.

The grid-connected VSC system with LCL filter has resonance problems which will magnify the harmonics near the resonant frequency and result in a low stability margin and complex design of current regulators. In this paper, a novel control strategy is proposed by introducing the feedforward compensatory controls of state variables and grid voltage, which can not only lessen the system order from the third-order to the first-order but also reject the current distortions arising from grid voltage disturbances, such as surge, sag or harmonics. Therefore, with the proposed strategy, the system stability margin is enlarged and the design of current regulators is simplified greatly. Besides, a sensorless solution is also proposed to lower the cost of the system. Especially, with the sensorless solution, the system can also avoid disturbances from the local load. Theoretical analysis simulation and experimental results verified the proposition.

A resonant current-fed push-pull converter with active clamping is investigated for vehicle inverter application in this paper. A communal clamping capacitor and a voltage doubler topology are employed. Moreover, two kinds of resonances to realize ZVS ON of MOSFETs or to remove reverse-recovery problem of secondary diodes are analyzed and an optimized resonance is utilized for volume restriction. At last, a 150W prototype with 10V to 16V input voltage, 360V output voltage is implemented to verify the design.

In this paper, a three-phase high power factor Multilevel Current-Source Rectifier (MCSR) is developed. The structure of the given MCSR consists of three modules of current-source, three-phase six-valve converters and some sharing-current inductors. The operation characteristics of the MCSR are investigated. In the MCSR, Carrier Phase-Shifted SPWM is used. Harmonic characteristics of the ac input current and the dc output voltage and current are analyzed in detail. It is shown that, many good features can be achieved such as equivalent higher carrier frequency at the ac side of the MCSR even with lower switching frequency. Furthermore, dc output voltage has no low harmonic components, and the power factor of the ac side is easy to control to be unit. A fully digital solution of tri-logic SPWM gating signals for the MCSR is studied based on the DSP and FPGA platform. At last, simulated and experimental results are given to demonstrate the proposition.

Three-phase current source PWM converters require tri-logic PWM control technique. From the point of view of digital control, implementation of tri-logic PWM, especially for multi-module current source converters (CSCpsilas), is much more complex than that for a single CSC. Aiming at digital controls of multi-module CSCpsilas, the digital solution of tri-logic SPWM scheme is investigated in this paper. A tri-logic carrier phase-shifted SPWM control system is presented based on DSP and FPGA hardware platform. The fully digital control of the multi-module CSCpsilas is realized. The experiment on a three-module current source inverter (CSI) with carrier phase-shifted SPWM gives the verification.

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.

Multilevel converters offer benefits of reduced harmonics and increased power ratings compared to their two level counterparts, but to date most of the research in this area has been directed towards voltage source inverters rather than current source inverters. Partly, this is because DC link capacitors are a more effective energy storage medium than link inductors, with less losses and easier operating requirements. However, for higher power applications, and with improvements in inductors using superconducting magnetic energy storage systems (SMES), current source inverters can have particular advantages in some circumstances. Hence it is useful to explore how a multilevel CSI system might be managed. This paper presents a 6-switch single-phase 5-level current source inverter, and compares its topology to a previously reported 8-switch system. For both converter topologies, it is identified that control of the intermediate 1/2-level DC link current is a major challenge, and a closed loop strategy is proposed to control this current under varying load conditions. Simulation and experimental results are presented to confirm the operation of the new 6-switch topology under PWM control, and to verify that the 1/2-level link current can be maintained at half of the total DC current with very little current ripple under differing load conditions.