Dehong Xu

A New Feedback Method for PR Current Control of LCL-Filter-Based Grid-Connected Inverter

For a grid-connected converter with an LCL filter, the harmonic compensators of a proportional-resonant (PR) controller are usually limited to several low-order current harmonics due to system instability when the compensated frequency is out of the bandwidth of the system control loop. In this paper, a new current feedback method for PR current control is proposed. The weighted average value of the currents flowing through the two inductors of the LCL filter is used as the feedback to the current PR regulator. Consequently, the control system with the LCL filter is degraded from a third-order function to a first-order one. A large proportional control-loop gain can be chosen to obtain a wide control-loop bandwidth, and the system can be optimized easily for minimum current harmonic distortions, as well as system stability. The inverter system with the proposed controller is investigated and compared with those using traditional control methods. Experimental results on a 5-kW fuel-cell inverter are provided, and the new current control strategy has been verified.

A PWM plus phase-shift control bidirectional DC-DC converter

A pulse-width modulation (PWM) plus phase-shift control bidirectional dc-dc converter is proposed. In this converter, PWM control and phase-shift control are combined to reduce current stress and conduction losses, and to expand ZVS range. The operation principle and analysis of the converter are explained, and ZVS condition is derived. A prototype of PWM plus phase-shift bidirectional dc-dc converter is built to verify the analysis.

Survey on Fault-Tolerant Techniques for Power Electronic Converters

With wide-spread application of power electronic converters in high power systems, there has been a growing interest in system reliability analysis and fault-tolerant capabilities. This paper presents a comprehensive review of conventional fault-tolerant techniques regarding power electronic converters in case of power semiconductor device failures. These techniques can be classified into four categories based on the type of hardware redundancy unit: switch-level, leg-level, module-level, and system-level. Also, various fault-tolerant methods are assessed according to cost, complexity, performance, etc. The intent of this review is to provide a detailed picture regarding the current landscape of research in power electronic fault-handling mechanisms.

Stator Current Harmonic Control With Resonant Controller for Doubly Fed Induction Generator

Voltage harmonics in the grid can introduce stator current harmonics in a doubly fed induction generator (DFIG) wind turbine system, which may potentially impact the generated power quality. Therefore, wind turbine current controllers need to be designed to eliminate the impact of grid voltage harmonics, especially low-order harmonics. This paper proposes a stator current harmonic suppression method using a sixth-order resonant controller to eliminate negative sequence fifth- and positive sequence seventh-order current harmonics. A stator current harmonic control loop is added to the conventional rotor current control loop for harmonic suppression. The overall control scheme is implemented in dq frame. Based on a mathematical model of the DFIG control system, the effects on system stability using the resonant controller, an analysis of the steady-state error, and the dynamic performance, are discussed in this paper. Taking these effects into account, the parameters of the resonant controller can be designed and effectively damp the influence from the grid voltage harmonics. As a result, the impacts of the negative sequence fifth- and positive sequence seventh-order voltage harmonics on the stator current as well as the electromagnetic torque are effectively removed. Simulations and experiments are presented to validate the theoretical analysis.

An improved control strategy for grid-connected voltage source inverters with a LCL filter

A novel current control strategy based on a new current feedback for grid-connected voltage source inverters with an LCL-filter is proposed in this paper. By splitting the capacitor of the LCL-filter into two parts, each with the proportional division of the capacitance, the current flowing between these two parts is measured and used as the feedback to a current regulator to stabilize and improve the system performances. Consequently, the V-1 transfer function of the grid-connected inverter system with the LCL-filter is degraded from a third-order function to a first-order one, therefore the closed-loop current feedback control system can be optimized easily for minimum steady-state error and current harmonic distortion, as well as the system stability. The characteristics of the inverter system with the proposed controller are investigated and compared with those using traditional control strategies. Experimental results are provided, and the new current control strategy has been verified on a 5 kW fuel cell inverter.

Seamless Transfer Control Strategy for Fuel Cell Uninterruptible Power Supply System

Due to the long cold start time and slow dynamics of proton exchange membrane (PEM) fuel cell (FC) stack, operating modes transfer control strategy for fuel cell uninterruptible power supply (FC-UPS) is different from the traditional uninterruptible power supply (UPS) system. In this paper, a seamless transfer control strategy, which is suitable for FC-UPS, is proposed. The power conversion architecture of FC-UPS is presented with the characteristic analysis of PEMFC and the requirements of UPS. Then, the scheme of the seamless transfer control strategy is investigated. The proposed seamless transfer control strategy is not only capable of guaranteeing the uninterruptible load voltage, but also protecting FC against the power demands beyond its allowable bandwidth during the transition for long lifespan and safety. Finally, the control scheme has been verified on a 10-kW FC-UPS prototype.

An Improved Islanding Detection Method for a Grid-Connected Inverter With Intermittent Bilateral Reactive Power Variation

An islanding detection method for a grid-connected inverter incorporating intermittent bilateral (IB) reactive power variation (RPV) is described. The inverter output with IB reactive power enables the detection of an islanding. The RPV range is derived in order to eliminate the nondetection zone. The design scheme of variation time in the method is obtained by estimating the transient response due to islanding occurrence. In addition, the effect of the proposed detection strategy on power quality is investigated. The proposed islanding detection method has been experimentally verified on a three-phase grid-connected 6-kW inverter.

DC-Voltage Fluctuation Elimination Through a DC-Capacitor Current Control for DFIG Converters Under Unbalanced Grid Voltage Conditions

Unbalanced grid voltage causes a large second-order harmonic current in the dc-link capacitors as well as dc-voltage fluctuation, which potentially will degrade the lifespan and reliability of the capacitors in voltage source converters. This paper proposes a novel dc-capacitor current control method for a grid-side converter (GSC) to eliminate the negative impact of unbalanced grid voltage on the dc-capacitors. In this method, a dc-capacitor current control loop, where a negative-sequence resonant controller is used to increase the loop gain, is added to the conventional GSC current control loop. The rejection capability to the unbalanced grid voltage and the stability of the proposed control system are discussed. The second-order harmonic current in the dc capacitor as well as dc-voltage fluctuation is very well eliminated. Hence, the dc capacitors will be more reliable under unbalanced grid voltage conditions. A modular implementation method of the proposed control strategy is developed for the DFIG controller. Finally, experiments are presented to validate the theoretical analysis.

1MHz-1kW LLC Resonant Converter with Integrated Magnetics

A design of IMHz-1kW LLC resonant converter is presented. The loss analysis based on theoretical derivation is performed to reveal the loss distribution. Integrated magnetic structure is adopted in the design to further increase the power density. A 1MHz 300V-400V input, 48V/20A output prototype is built to verify its advantage, and the efficiency at 400V input, 48V/20A output is 94% at the switching frequency of 875 KHz.

Design, Modeling, and Improvement of Integrated EMI Filter With Flexible Multilayer Foils

Integrated electromagnetic interference (EMI) filter design method with flexible multilayer (FML) foils is presented. An analysis model for integrated EMI filter with FML foils, distributed electromagnetic component (DEMC) model, is proposed. The DEMC model is verified by the experiment. In addition, method to improve the high-frequency performance of integrated EMI filter is investigated. Prototype of integrated EMI filter with FML foils is made for 1 kW switching power supply. In comparison with the traditional discrete EMI filter, it can reduce total volume by 45%.