Environmental aspects of fuel cells: A review.

The Neutral Point Clamped (NPC) Multi-Level Inverters (MLI) have been ruling the power electronics industries for the past two decades. The Multi-Carrier Pulse Width Modulation (MCPWM) is common PWM techniques which are widely used in NPC-MLI applications. However, MCPWM is not having a good impact on the balancing of DC-link voltages, Common Mode Voltage (CMV) and limiting the Total Harmonics Distortion (THD). The Selective Harmonic Elimination (SHE) technique is introduced for reducing the THD, however all the switching angles should be maintained less than $\pi $ /2 to keep the eliminated harmonics at constant level which narrows down the modulation index range. Hence, in recent days Space Vector Modulation (SVM) technique is widely used in NPC-MLI, which gives better DC-link voltage balancing, self-neutral point balancing, near-zero CMV reduction, better-quality harmonics profile and switching loss minimization. Hence, it is a preferred solution for the majority of electrical conversion applications such as electric traction, high power industrial drives, renewable power generation, and grid-connected inverters, etc. The paper gives a comprehensive review of the SVM for NPC-MLI. First, this paper deliberates the state of art for two-level SVM and extends it to three-level (3L) SVM. Also compares the 3L SVM performance with other MCPWM techniques. Followed by the various modified MLI SVM techniques in terms of their implementations, DC-link capacitor balancing, and reduction of CMV. Further, the review of MLI SVM is widened to open-end winding Inverters and multiphase MLIs. The final part of this paper discussed the future trends and research directions on MLI SVM techniques and its applications.

/pdf/a-comprehensive-review-on-space-vector-modulation-techniques-4knkky1lpu.pdf

A Comprehensive Review on Space Vector Modulation Techniques for Neutral Point Clamped Multi-Level Inverters

For the two-stage single-phase power factor correction (PFC) converter, its instantaneous input power pulsates at twice the line frequency, generating the second harmonic current (SHC) at the dc-bus port. The dc–dc stage is used to regulate the output voltage or the dc bus voltage in different applications, and the SHC reduction schemes for the dc–dc stage with different control objectives are discussed in this article. When the dc–dc stage regulates the output voltage, it is pointed out that the control bandwidth of the dc–dc stage is required to be high enough and the dc bus capacitor should be large enough to suppress the SHC, and the design of the dc bus capacitor is given. When the dc–dc stage regulates the dc bus voltage, a virtual impedance is added in series with the input/output of the dc–dc stage to suppress the SHC, and a virtual impedance is added in parallel with the dc bus to improve the dynamic performance. The selection and design of the virtual impedances are also given. Based on that, three specific control schemes are proposed for the dc–dc stage to suppress the SHC, and the parameters design method is also presented. Finally, a 3.3-kW two-stage single-phase PFC converter is fabricated and tested in the lab to verify the effectiveness of the proposed SHC reduction schemes. 

Second Harmonic Current Reduction Schemes for DC–DC Converter in Two-Stage PFC Converters

Series-connected voltage sources converters (SVSCs), (i.e., dynamic voltage restorer, fault current limiter, electrical spring, etc.), have been widely used in the electricity grid to improve the electricity grid performances in terms of voltage fluctuation compensation, flexible power flow regulation, fault overcurrent limitation, flexible access of renewable energy, etc. However, SVSCs have a relatively short effective operating time and a low utilization rate, because they are generally operated during abnormal grid conditions. The new trend is to increase the utilization by extending the functions of SVSC, e.g., integrating the voltage compensation and overcurrent limiting in a single SVSC. On the other hand, the series connection architecture results in the SVSCs experiencing overvoltage and overcurrent from the main grid, and consequently the reliability of SVSC is challenged. Based on these details, this paper systematically studies the state of the art multifunctional techniques and the corresponding strategies of SVSCs, which can be mainly divided into three categories as follows: function integration between different SVSC; function coordination between SVSC and parallel-connected VSC(PVSC); function integration between SVSC and PVSC. The most representative topology and control optimization method in each category is elaborated, and a brief comparison of them is conducted as well. On this basis, the key problems and challenges in the research and engineering application of multifunctional SVSC system are summarized. • The state of the art multifunctional techniques of SVSCs is systematically studied. • The function coordination between SVSC and parallel-connected devices improves the output performance of the parallel part. • The multifunctional SVSC with fault current limiting capability can protect itself and the power grid actively. • The function integration between SVSC and PVSC open a new way to improve the utilization rate of power electronic devices. • The multifunctional SVSC with minimized additional physical switches and topology complexity needs further studied. 

An overview of series-connected power electronic converter with function extension strategies in the context of high-penetration of power electronics and renewables

A magnetic linked multiport fractional converter (MLMFC) is proposed in this article for application to doubly fed induction generator-based variable speed wind power generating systems. With the proposed MLMFC topology, no back-to-back converter and heavy step-up line-frequency transformer are needed.Moreover, the proposed MLMFC ensures isolation between the rotor-side and grid-side of the system. An improved current sensorless dual-loop individual voltage balance controller (DIVBC) is proposed at the grid-side for individual dc-link voltage balancing for the MLMFC with an interstage controller decoupling technique. The proposed DIVBC shows improved dc-link voltage tracking and dc-link disturbance rejection capabilities. Moreover, an improved current sensorless dual-loop proportional-integral resonant voltage controller (DPIRVC) is proposed at the rotor-side for the strict regulation of the dc-link voltage, the suppression of the dc-link voltage ripples, and the achievement of low controller sensitivity to the load disturbances. Hence, with the combination of the proposed DIVBC with the DPIRVC, the controller architecture requires a low dc-link capacitance without the need to utilize any dc-link current sensors to feedforward any disturbances to the controller. Moreover, the proposed controller architecture shows high stability and robustness under capacitance mismatches, load disturbances, voltage sag and swell, grid fault, and different wind speeds. The superior performance of the proposed control strategy is demonstrated through simulation and laboratory experiments.

A Magnetic Linked Multiport Fractional Converter for Application to Variable Speed Wind Power Generating Systems

The single-phase unified power quality conditioner (UPQC) is widely used to improve power quality for sensitive end-users. However, the inherent instantaneous power difference between parallel and series converter will generate significant low-frequency dc-link voltage ripple and degrade the compensation performance of UPQC. The mechanism of dc-link voltage ripple generation and its influences on compensation voltage and current are analyzed in this article. To suppress the influence, a control strategy is proposed for the single-phase UPQC. For parallel converter, a notch filter is introduced in the outer voltage loop to prevent the voltage ripple entering the control loop, the specific order harmonics compensation is proposed in the inner-current loop to reduce the grid current harmonics. For the series converter, the dc-link voltage feedback is adopted to suppress the influence of voltage ripple on the compensation performance. Simulation and experimental results show that the grid current total harmonic distortion (THD) can be reduced effectively with the fixed dc-link capacitance compared with the traditional control strategy. More than half of the load voltage THD are also decreased by the dc-link voltage feedback control. Moreover, with the smaller dc-link capacitance, single-phase UPQC can maintain the better performance under the proposed control strategy.

Control Strategy of Single-Phase UPQC for Suppressing the Influences of Low-Frequency DC-Link Voltage Ripple

A single-phase isolated ac–dc–dc converter with low total harmonic distortion and high power factor usually suffers from a significant ripple power at double line frequency. The inherent ripple power brings a significant low-frequency voltage ripple on the dc-link capacitor. To reduce the voltage ripple, bulky electrolytic capacitors and hardware low-pass filter are widely adopted. However, the converter becomes bulky because of large capacitor. In order to reduce the dc-link capacitance, optimal control strategies are proposed for ac–dc and dc–dc converter, respectively, in this article. First, the notch filter with current disturbance feedforward strategy (NF-CDFS) are proposed to minimize input harmonic current of ac–dc converter. Second, the feedback linearization with voltage disturbance feedforward strategy (FLVDFS) is designed to suppress the output voltage ripple of dc–dc converter based on generalized state space averaging model. With the proposed NF-CDFS plus FLVDFS control strategy, low input harmonic current and low output voltage ripple are achieved. In addition, more than about 85% of dc-link capacitance is reduced compared to conventional methods.

Optimal Input and Output Power Quality Control of Single-Phase AC–DC–DC Converter With Significant DC-Link Voltage Ripple

A fast and flexible nearest-level-equivalent space vector modulation algorithm for three-phase multilevel converters

High speed and heavy loads have become more prevalent in the traction power supply system recently. To ensure system operating stability, better power quality, and sufficient power capacity, improvements are needed over the conventional traction system. Inspired by the concept of a microgrid (MG), an AC co-phase traction MG system was proposed. Substations were connected to the traction grid as distributed generators operate in islanded mode. Droop control was adopted as the primary control to stabilize the system’s operating frequency and voltage. Considering the operating features of the substation and locomotive load, a de-centralized secondary control strategy was proposed for AC co-phase traction MG system operation with enhanced resiliency. The proposed control strategy could increase system stability and prevent circulation currents between substations. Moreover, the proposed de-centralized coordination between substations does not rely on communication, which promotes the system’s “plug-and-play” functionality. Stability analysis was undertaken and the proposed controller was proved to be exponentially stable. The dynamic response of the proposed controller was validated using comprehensive case studies in MATLAB/Simulink.

https://www.mdpi.com/1996-1073/14/1/7/pdf

Decentralized Control Strategy for an AC Co-Phase Traction Microgrid

The dual active bridge (DAB) converter is widely used in renewable energy power generation systems with wide input voltage characteristics. An inappropriate duty cycle will lead to a larger inductor root-mean-square (RMS) current and low efficiency. In this article, an efficiency-oriented optimized triple-phase-shift (OTPS) scheme is proposed for the DAB converter that can reduce the inductor rms current for a wide input voltage and realize zero-voltage switching (ZVS). The ZVS condition contains the direction and amplitude of the inductor current to make the ZVS area more accurate. In addition, the OTPS scheme also has the capability of voltage balancing without additional voltage equalizers under unbalanced load. The influence of deadtime on voltage balance is analyzed and a voltage balancing scheme with compensation of the duty cycle is proposed. To reduce resource occupation and operation time, an online implementation scheme of variable parameter control based on field-programable gate array (FPGA) is proposed. Without a look-up table, the sum of the operation time of the control module and modulation module is only 0.66 $\mu$s, and the required memory bits are only 459 k. Both operation time and memory bits are reduced by more than 90% compared to the existing literature. Finally, the whole proposed process is verified by the experimental results.

Lan Ma

Papers

Control Strategy of Single-Phase UPQC for Suppressing the Influences of Low-Frequency DC-Link Voltage Ripple

Optimal Input and Output Power Quality Control of Single-Phase AC–DC–DC Converter With Significant DC-Link Voltage Ripple

A fast and flexible nearest-level-equivalent space vector modulation algorithm for three-phase multilevel converters

Decentralized Control Strategy for an AC Co-Phase Traction Microgrid

Online Digital Implementation of Wide Voltage Range RMS-Current-Optimized Control With Voltage Balancing Capability for DAB Converter