This paper proposes a hybrid islanding detection method for inverter-based distributed generation units. Firstly, this paper carries out a comprehensive characteristic analysis and obtains design principles for the hybrid method in inverter-based DGs. Then, based on these principles, the proposed method combines the passive method of voltage unbalance and total harmonic distortion(VU/THD) detection and the active method of bilateral reactive power variation(BRPV). In specific, the BRPV method is only triggered when the islanding condition is suspected by VU/THD method. Doing so, the islanding detection performance can be improved significantly without reducing the power quality. In addition, this paper modifies the conventional VU/THD method to realize fast and accurate detection, and the threshold setting principle is analyzed for the first time based on equivalent circuit approach. Comparison analysis reveals that the proposed method has a more satisfactory islanding detection performance for inverter-based distributed generation units. Simulation and experimental results under various conditions based on IEEE Std. 929 and IEEE Std. 1547 were carried out to verify the islanding detection performance of the proposed method.

This paper proposes a fault diagnosis method to diagnose multiple transistor open-circuit faults in a T-type three-level inverter. In this method, a finite-state machine(FSM) tracks state transitions caused by abnormal fault-linked current paths, and rough set theory(RST) is employed to optimize and obtain a minimum set of variables necessary to distinguish state transitions under various fault scenarios. After applying RST, voltage state variables expressed by Boolean logic relationships are adopted in the FSM to identify faults. This can also effectively reflect state transitions between single and multiple fault cases. The approach is immune to load disturbances and dead times. Through logic relationships, a circuit is designed for fast online fault location to minimize the impact of sampling frequency on diagnosis. Factors that affect diagnosis time and accuracy are considered and analyzed to ensure the reliability of the proposed method. Experimental results obtained under various conditions verify the effectiveness of this approach.

In this paper, an input-parallel output-series(IPOS) LLC resonant converter with a coupled transformer and current sharing capability is proposed for high-gain high-efficiency applications such as the renewable energy grid connection. The coupled transformer is utilized to reduce the core size and core loss by implementing the magnetic flux cancellation method, which achieves a natural balance of output voltage simultaneously. The current sharing between the converter modules is achieved by a bridge wire that interconnects the resonant tanks of different modules. Furthermore, the current sharing inside a module is also achieved by the proposed compensation current based on the coordination of compensation inductor and delay control. A 200 kHz 100 V/1.2 kV 3.6 kW two-module IPOS LLC resonant converter prototype is built to verify the proposed method. The peak efficiency can reach as high as 97.5%, and the current sharing between modules and inside a module are well presented.

Switched mode pulse power supply is a promising technique for high-power quasi-continuous laser driver. Contrast to lossy linear laser drivers, switched mode laser drivers can achieve higher efficiency. However, many challenges have been proposed, such as fast pulse edge, low current ripple. This paper proposes a multiphase interleaved pulse power supply with energy recovery and inductive storage(MIEF-PPS). The basic concept of the topology is the inclusion of a multiphase converter with pulse forming circuits to the converter system, which decouples the current slew rate and current ripple. Using an inductive storage technology and pulse forming circuits, a shorter pulse current rising time is obtained. The inductor energy is fed back to the input source not discharged to the load, resulting in a fast pulse trailing edge and energy saving. Thus the pulse current response observed when using this proposed technique is found to be much faster when compared to the conventional interleaved buck driver. Moreover, a pre-charge method is proposed to overcome the challenge of digitally controlling the inductive storage. The proposed topology was simulated in MATLAB/Simulink and validated against the experimental results of a laboratory prototype, 360 W dual-interleaved pulsed power supply.

Three-phase DC/AC power electronics converter systems used in battery-powered variable-speed drive systems or employed in three-phase mains-supplied battery charger applications usually feature two power conversion stages. In both cases, typically a DC/DC stage is attached to a three-phase DC/ AC stage in order to enable buck-boost functionality and/or a wide input-output voltage operating range. However, a two-stage solution leads to a high number of switched bridge-legs and hence, results in high switching losses, if the degrees of freedom available for controlling the overall system are not utilised. If the DC/DC stage is used to vary the DC link voltage with six times the AC-side frequency, a pulse width modulation(PWM) of always only one phase of the DC/AC stage is sufficient to achieve three-phase sinusoidal output currents. The clamping of two phases(denoted as 1/3 PWM) leads to a drastic reduction of the DC/AC stage switching losses, which is further accentuated by a DC link voltage which is lower than for the conventional modulation schemes. This paper details the operating principle of a three-phase buck-boost converter system using 1/3 PWM and outlines an appropriate control system design. Subsequently, the switching losses and the voltage/current stresses on the converter components are analytically derived. There, a more than 66% reduction of the DC/ AC stage switching losses is calculated without any increase of the stress on the remaining converter components. The theoretical considerations are finally verified on a hardware demonstrator, where the proposed modulation strategy is experimentally compared against several conventional modulation techniques and its clear performance advantages are validated.

Medium-frequency transformers(MFTs) are one of the fundamental building blocks of modern power electronic converters. The usage of increased frequencies leads to improved characteristics, i.e., efficiency and power density(volumetric and gravimetric) but also to design challenges and constraints. This paper reviews the analytical modeling of MFTs. More particularly, the mapping between the design space and the performance space is analyzed. It is found that wide regions of the design space are mapped to a narrow region in the performance space, i.e., the optimum is flat and designs with very different parameters features similar performances(design space diversity). Scaling laws are derived for optimal MFTs operated at different power ratings and power densities, which provide a comprehensive and general insight on the achievable performances. In a next step, the results obtained with the analytical model are compared to numerical simulations. It is concluded that the derived scaling laws capture qualitatively and quantitatively the behavior of MFTs, but should be used with caution for accurate design processes.

This paper evaluates the thermal characterization of late generation SiC schottky diodes. 600 V/650 V SiC diodes from 3 well-known manufacturers are tested: Wolfspeed, Infineon and Rohm. A comprehensive study is performed for a wide temperature range from 20 °C(room temperature) up to 500 °C, aiming to find the absolute maximum parameters of SiC schottky diodes at extremely high temperature environments. Both static and dynamic characterizations are evaluated and explained. TCAD simulations are proposed to express the abnormal phenomenon occurred in test results, especially the mechanism of hole carrier transportation in extremely high temperature. This work exhibits the performance of SiC schottky diodes for high temperature application conditions.

In this paper, a novel quadratic boost converter is presented. The input current of the converter is non-pulsating and the currents of the inductors are relatively low. The operating principle and the mathematical model of the converter in continuous conduction mode(CCM) are given. The steady-state of the converter is calculated and the power loss of the converter is analyzed. Then the small-signal model of the converter is derived based on the state space averaging method. The inductor current and the voltage stress of the proposed converter arelow, which can improve the efficiency of the converter. The simulations and the circuit experiments are presented to verify the good performance of the converter.