Showing papers by "Huai Wang published in 2018"

Mission Profile Based System-Level Reliability Analysis of DC/DC Converters for a Backup Power Application

Abstract: Reliability analysis is an important tool for assisting the design phase of a power electronic converter to fulfill its life-cycle specifications. Existing converter-level reliability analysis methods have two major limitations: 1) being based on constant failure rate models; and 2) lack of consideration of long-term operation conditions (i.e., mission profile). Although various studies have been presented on power electronic component-level lifetime prediction based on wear-out failure mechanisms and mission profile, it is still a challenge to apply the same method to the reliability analysis of converters with multiple components. Component lifetime prediction based on associated models provides only a $B_{X}$ lifetime information (i.e., the time when X % items fail), but the time-dependent reliability curve is still not available. In this paper, a converter-level reliability analysis approach is proposed based on time-dependent failure rate models and long-term mission profiles. Two different methods to obtain the component-level time-to-failure are illustrated by a case study of dc/dc converters for a 5 kW fuel cell-based backup power system. The reliability analysis of the converters with and without redundancy is also performed to assist the decision making in the design phase of the fuel cell power conditioning stage.

A Dual Active Bridge Converter With an Extended High-Efficiency Range by DC Blocking Capacitor Voltage Control

Abstract: A Dual active bridge (DAB) converter can achieve a wide high-efficiency range when its input and output voltages are equal, assuming a 1:1 turns ratio for its isolation transformer. If its input or output voltage is doubled, efficiency of the DAB will drop significantly, because of the introduction of the hard switching and high circulating power. Thus, a new modulation scheme has been proposed, whose main idea is to introduce a voltage offset across the dc blocking capacitor connected in series with the transformer. Operational principle of the proposed modulation has been introduced, before analyzing its soft-switching area and circulating power mathematically. The final modulation scheme is not difficult to implement, but can help the DAB achieve soft switching, low circulating power, and thereby high efficiency, even with its input or output voltage doubled. These features have been verified by experimental results obtained with a 1.2-kW prototype.

Analysis and Mitigation of Dead-Time Harmonics in the Single-Phase Full-Bridge PWM Converter With Repetitive Controllers

Abstract: In order to prevent the power switching devices (e.g., an insulated-gate bipolar transistor, IGBT) from shoot-through in voltage-source converters during a switching period, the dead time is added either in the hardware driver circuits of the IGBTs or implemented in software in pulse width modulation (PWM) schemes. Both solutions will contribute to a degradation of the injected current quality. As a consequence, the harmonics induced by the dead time (referred to as “dead-time harmonics” hereafter) have to be compensated in order to achieve a satisfactory current quality, as required by standards. In this paper, the emission mechanism of dead-time harmonics in single-phase PWM inverters is, thus, presented considering the modulation schemes in detail. More importantly, a repetitive controller has been adopted to eliminate the dead-time effect in single-phase grid-connected PWM converters. The repetitive controller has been plugged into a proportional-resonant-based fundamental-frequency current controller so as to mitigate the dead-time harmonics and also to maintain the control of the fundamental-frequency grid current in terms of dynamics. Simulations and experiments are provided, which confirm that the repetitive controller can effectively compensate the dead-time harmonics and other low-order distortions, and also, it is a simple method without hardware modifications.

Benchmarking of Constant Power Generation Strategies for Single-Phase Grid-Connected Photovoltaic Systems

Abstract: With a still increase of grid-connected photovoltaic (PV) systems, challenges have been imposed on the grid due to the continuous injection of a large amount of fluctuating PV power, like overloading the grid infrastructure (e.g., transformers) during peak power production periods. Hence, advanced active power control methods are required. As a cost-effective solution to avoid overloading, a constant power generation (CPG) control scheme by limiting the feed-in power has been introduced into the currently active grid regulations. In order to achieve a CPG operation, this paper presents three CPG strategies based on a power control method (P-CPG), a current limit method (I-CPG), and the perturb and observe algorithm (P&O-CPG). However, the operational mode changes (e.g., from the maximum power point tracking to a CPG operation) will affect the entire system performance. Thus, a benchmarking of the presented CPG strategies is also conducted on a 3-kW single-phase grid-connected PV system. Comparisons reveal that either the P-CPG or I-CPG strategies can achieve fast dynamics and satisfactory steady-state performance. In contrast, the P&O-CPG algorithm is the most suitable solution in terms of high robustness, but it presents poor dynamic performance.

A Bidirectional Resonant DC–DC Converter Suitable for Wide Voltage Gain Range

Abstract: This paper proposes a new bidirectional resonant dc–dc converter suitable for wide voltage gain range applications (e.g., energy storage systems). The proposed converter overcomes the narrow voltage gain range of conventional resonant dc–dc converters, and meanwhile achieves high efficiency throughout the wide range of operation voltage. It is achieved by configuring a full-bridge mode and a half-bridge mode operation during each switching cycle. A fixed-frequency phase-shift control scheme is proposed and the normalized voltage gain can be always from 0.5 to 1, regardless of the load. The transformer root-mean-square (rms) currents in both the forward and reverse power flow directions have a small variation with respect to the voltage gain, which is beneficial to the conduction losses reduction throughout a wide voltage range. Moreover, the power devices are soft-switched for minimum switching losses. The operation principles and characteristics of the proposed converter are firstly analyzed in this paper. Then the analytical solutions for the voltage gain, soft-switching, and rms currents are derived, which facilitates the parameters design and optimization. Finally, the proposed topology and analysis are verified with experimental results obtained from a 1-kW converter prototype.

The Impact of Topology and Mission Profile on the Reliability of Boost-type Converters in PV Applications

Abstract: This paper investigates the impact of different converter topologies and mission profiles on the reliability of dc/dc boost-type PV converters The reliability of three boost-type converters with the same input/output specifications is modeled employing a mission profile-based reliability evaluation method considering non-constant failure rate for the electrical components This study identifies the contribution of active and passive components on the converter reliability for identifying the most failure prone components Furthermore, the applicability of converter structures for different climate conditions is demonstrated seen from the reliability point of view

Protection Scheme for Modular Multilevel Converters Under Diode Open-Circuit Faults

Abstract: The modular multilevel converter (MMC) is attractive for medium- or high-power applications because of the advantages of its high modularity, availability, and high power quality. Reliability is one of the most important challenges for the MMC consisting of a large number of power electronic devices. The diode open-circuit fault in the submodule (SM) is an important issue for the MMC, which would affect the performance of the MMC and disrupt the operation of the MMC. This paper analyzes the impact of diode open-circuit failures in the SMs on the performance of the MMC and proposes a protection scheme for the MMC under diode open-circuit faults. The proposed protection scheme not only can effectively eliminate the possible caused high voltage due to the diode open-circuit fault but also can quickly detect the faulty SMs, which effectively avoids the destruction and protects the MMC. The proposed protection scheme is verified with a downscale MMC prototype in the laboratory. The results confirm the effectiveness of the proposed protection scheme for the MMC under diode open-circuit faults.

A Novel Type-2 Fuzzy Logic for Improved Risk Analysis of Proton Exchange Membrane Fuel Cells in Marine Power Systems Application

Abstract: A marine energy system, which is fundamentally not paired with electric grids, should work for an extended period with high reliability. To put it in another way, by employing electrical utilities on a ship, the electrical power demand has been increasing in recent years. Besides, fuel cells in marine power generation may reduce the loss of energy and weight in long cables and provide a platform such that each piece of marine equipment is supplied with its own isolated wire connection. Hence, fuel cells can be promising power generation equipment in the marine industry. Besides, failure modes and effects analysis (FMEA) is widely accepted throughout the industry as a valuable tool for identifying, ranking, and mitigating risks. The FMEA process can help to design safe hydrogen fueling stations. In this paper, a robust FMEA has been developed to identify the potentially hazardous conditions of the marine propulsion system by considering a general type-2 fuzzy logic set. The general type-2 fuzzy system is decomposed of several interval type-2 fuzzy logic systems to reduce the inherent highly computational burden of the general type-2 fuzzy systems. Linguistic rules are directly incorporated into the fuzzy system. Finally, the results demonstrate the success and effectiveness of the proposed approach in computing the risk priority number as compared to state-of-the-art methods.

Modeling framework of voltage-source converters based on equivalence with synchronous generator

Abstract: Along with the increasing penetration of distributed generation with voltage-source converters (VSCs), there are extensive concerns over the potential virtual rotor angle stability, which is characterized by oscillations of power and frequency during the dynamic process of synchronization in the grid. Several control strategies have been developed for VSCs to emulate rotating inertia as well as damping of oscillations. This paper classifies these strategies and provides a small-signal modeling framework including all kinds of VSCs in different applications for virtual rotor angle stability. A unified perspective based on the famous Phillips–Heffron model is established for various VSCs. Thus, the concepts of equivalent inertia and the synchronizing and damping coefficients in different VSCs are highlighted, based on the similarities with the synchronous generator (SG) system in both physical mechanisms and mathematical models. It revealed the potentiality of various VSCs to achieve equivalence with the SG. This study helps promote the unity of VSCs and traditional SGs in both theories and methods for analyzing the dynamic behavior and enhancing the stability. Finally, future research needs and new perspectives are addressed.

Design for reliability and robustness tool platform for power electronic systems — Study case on motor drive applications

Abstract: Because of the high cost of failure, the reliability performance of power semiconductor devices is becoming a more and more important and stringent factor in many energy conversion applications. Thus, the need for appropriate reliability analysis of the power electronics emerges. Due to its conventional approach, mainly based on failure statistics from the field, the reliability evaluation of the power devices is still a challenging task. In order to address the given problem, a MATLAB based reliability assessment tool has been developed. The Design for Reliability and Robustness (DfR2) tool allows the user to easily investigate the reliability performance of the power electronic components (or sub-systems) under given input mission profiles and operating conditions. The main concept of the tool and its framework are introduced, highlighting the reliability assessment procedure for power semiconductor devices. Finally, a motor drive application is implemented and the reliability performance of the power devices is investigated with the help of the DfR2 tool, and the resulting reliability metrics are presented.

Efficiency Enhancement of Bridgeless Buck-Boost PFC Converter with Unity PF and DC Split to Reduce Voltage Stresses

Abstract: This paper proposes a unity power factor (PF) bridgeless buck-boost power factor correction (PFC) converter to minimize conduction losses caused by the diode bridge. Moreover, compared to the conventional buck-boost PFC converter, the proposed converter has lower voltage stresses across switches, which further improves the overall efficiency of the proposed converter, especially when it operates in the light load conditions. Operation modes with inductors working in the discontinuous conduction mode (DCM) are given in detail, and comparative analysis is conducted to show the proposed converter performance regarding the PF, component stresses, and output ripple. The proposed and conventional converter prototypes with load range of 20∼100 W are built and tested. The obtained results verify merits and theoretical predictions of the proposed.

Mission Profile Based Power Converter Reliability Analysis in a DC Power Electronic Based Power System

Abstract: This paper investigates the reliability of power electronic converters employed for different applications in a dc power system employing a mission profile based reliability approach. The power electronic converter reliability depends on its loading profile, which induces thermal stresses on the failure-prone components, hence, limiting the converter lifetime. The loading profile of different converters in a power system is relevant to the mission profiles (including environmental and operational conditions), which can be controlled by a power management strategy. Moreover, the battery converter sizing in a renewable based power system may affect the power flow through the converters, and hence, its loading profiles. Therefore, this paper studies the impact of mission profile, power management strategy, and battery converter capacity on the reliability of different converters. The obtained results are presented through numerical analysis.

Modeling and Control of DC-DC Converters

Abstract: This chapter discusses the modeling and control methods for basic DC-DC converters by three specific case studies. It covers both the frequency-domain control methods and time-domain control methods. Specifically, the voltage mode control of a Buck converter, current mode control of a Boost converter, and a unified second-order boundary control of a Buck-Boost converter are presented. The analysis methods and control schemes can be extended to other DC-DC converter applications.

Impact of Long-Term Mission Profile Sampling Rate on the Reliability Evaluation of Power Electronics in Photovoltaic Applications

Abstract: Due to the increased cost, and time-demanding approach of conventional reliability improvement procedures, the transition towards model-based reliability assessment of power electronics becomes more and more crucial. Although important steps have been taken in this direction, the resulting lifetime prediction is still subject to different assumptions and uncertainties (e.g. mission profile data, modeling errors, lifetime models, etc.), Thus, this paper aims at investigating and quantifying the impact of the mission profile resolution on the reliability estimation of power IGBT modules and DC-link capacitors. For a 10 kW PV application case study, three mission profile sampling rates (1 minute/data, 30 minutes/data, and 60 minutes/data) are considered and benchmarked, with respect to the predicted lifetime of the power electronic components/system. Finally, an uncertainty analysis is performed for the resulting reliability metrics, and some initial guidelines for mission profile resolution selection are provided.

Lifetime benchmarking of two DC-link passive filtering configurations in adjustable speed drives

Abstract: Electrolytic capacitors with a DC-side inductor, and the slim DC-link capacitor are two typical filtering configurations in Adjustable Speed Drives (ASDs). The reliability performance of these capacitive DC-link solutions is an essential aspect to be considered, which depends on both component inherent capability and the operational conditions (e.g., electro-thermal stresses) in the field operation. This paper studies the reliability performance of the LC filter and slim capacitor based filter in a standard ASD system. Considering the variations of the capacitor parameters, environment stresses, and lifetime model, the lifetime of the two configurations are compared. Moreover, scalability analysis is presented in terms of power rating, amplitude and phase angle unbalance levels at the grid side. The results serve as a guideline for reliability aspect performance benchmarking of different DC-link solutions in ASD applications.

Balanced Conduction Loss Distribution among SMs in Modular Multilevel Converters

Abstract: Due to the parameter mismatch, the unbalanced power loss distribution among SMs in the modular multilevel converter (MMC) can be introduced and further deteriorated by the low-frequency asynchronous switching transients related to no-carrier modulation techniques. The unbalanced thermal stress can reduce the reliability of the MMC and increase the complexity of cooling system design. Nevertheless, an internal balance mechanism exists in the MMC thanks to the capacitor voltage balancing. It contributes to an even conduction loss dissipation among SMs, which is studied and revealed in this paper. Moreover, a computationally light conduction loss estimation method is proposed correspondingly relying on the characteristics of semiconductors and the arm current only. Simulations and experiments are conducted to verify the effectiveness the proposed method.

Submodule Level Power Loss Balancing Control for Modular Multilevel Converters

Abstract: This paper investigates the power loss imbalance in Modular Multilevel Converters (MMCs) with Nearest Level Modulation (NLM) resulting from the low switching frequency operation and the parameter mismatch. The imbalance might pose a challenge to the cooling system design as well as the reliability of the MMC. To address this problem, a submodule-level power loss balancing control (PLBC) is proposed. Compared with the normal control strategy without the thermal balancing, this method is able to decrease the degree of power loss imbalance among submodules (SMs) to at least half without deteriorating the performance of the converter efficiency and the capacitor voltage ripple. The effectiveness of the proposed control is validated by simulations.

On Power Electronized Power Systems: Challenges and Solutions

Abstract: With the revolution of renewable energy, the power system is being more complicated and integrated with more and more power electronics, which is referred to as the “power electronized” power system. In this case, the analysis, control, and operation of the entire power system should tone with the energy-paradigm transition pace. This paper thus explores challenges in power electronized power systems and highlights the stability issue. It serves to spark inspiration on potential solutions to these issues. As one of the solutions to cope with the interconnection of power converters with multi-synchronous generator systems, the virtual inertia concept is discussed in this paper. The impact of highly aggregated power electronic-based power systems is presented. More importantly, prospective solutions considering multiple timescales for the enhancement of power electronized power systems are briefed. A case study is exemplified in this paper to further highlight the analysis and discussion.

System-Level Lifetime Prediction for LED Lighting Applications Considering Thermal Coupling Between LED Sources and Drivers

Abstract: An accurate lifetime prediction method of light-emitting diode (LED) lighting systems is important to guide designers to fulfill design specifications and to benchmark cost-competitiveness of different lighting technologies. Currently, the lifetime of an LED system is typically predicted from the source and driver separately, and then the thermal design is also optimized independently. In practice, LED sources and drivers are usually compacted in a single fixture. The heat dissipated from LED sources and drivers are coupled and further affect heat transfer performances, which may degrade the whole system and accelerate the failure. In this paper, a thermal model concerning thermal coupling is proposed with the finite-element method (FEM) simulation to acquire key parameters in the thermal path. The proposed model shows a better estimation of thermal stresses of key components in the LED system, and therefore an improved lifetime prediction method for the LED system is proposed. Moreover, with a given lifetime requirement, this method can be used to guide the thermal design of LED systems. A case study of an outdoor lighting application is demonstrated by both FEM simulation and experimental verification.

Reliability analysis of grid-interfaced filter capacitors

Abstract: Growing with the increased adoption of renewable energy for the power generation, the reliable and cost-effective operation of grid-connected inverters is of more and more importance. A filter is interfaced between an inverter and the utility grid to reduce the switching harmonics. According to the modulation scheme and the LCL filter impedance, the electrical stresses of the filter capacitor can be thoroughly investigated. With the help of the electro-thermal model, its long-term thermal stress can be obtained based on the mission profile like wind speed, ambient temperature. The reliability of the filter capacitor bank is obtained based on its individual capacitor reliability curves and reliability block diagram method. A case study on a 2MW wind turbine system demonstrates the relationship between the lifetime of the capacitor bank and the single capacitor. Moreover, the severe voltage and current stresses of the filter capacitors are analyzed during abnormal operations (e.g., fault ride-through) with asymmetrical parasitic parameters.

Winding design of series AC inductor for dual active bridge converters

Abstract: The ac resistance and parasitic capacitance of the inductor are the primary considerations in the winding design for the dual-active bridge converter (DAB). They are dependent of up to four independent structure variables. The interactive restrictions between those variables makes the design difficult. In this paper, the core-related capacitances between the central limb, side limb, yoke and winding are derived, and a local optimization for it is proposed. Moreover, a total winding capacitance design method is proposed by mapping the four dimensional problem into two dimensions. The analysis and design are verified by finite element method simulations and experimental results on a 100 kHz prototype are performed.

A DC-link Capacitor Voltage Oscillation Reduction Method for a Modular Multilevel Cascade Converter with Single Delta Bridge Cells (MMCC-SDBC)

Abstract: This paper proposes a capacitor voltage oscillation reduction method by using third harmonic zero-sequence current for Modular Multilevel Cascade Converter (MMCC) with Single Delta Bridge Cells (SDBC). A practical case study on an 80 MVar/ 33 kV MMCC-SDBC based STATCOM is used to demonstrate the method. The impact of the third harmonic zero-sequence current level of the capacitor oscillation reduction and the electro-thermal stresses on IGBT modules is investigated. An optimal parameter of the current level is obtained by compromising the above two performance factors. The capacitor bank volume is reduced by 23 % by applying the proposed method.

Thermal Coupling and Network Modeling for Planar Transformers

Abstract: With the application of wide band gap devices in power electronics, planar transformers are preferred to further reduce the profile and manufacture cost. As a critical factor affecting the reliability, the temperature of the planar transformer is unevenly distributed and should be considered in the design phase. In this regard, a detailed thermal model enabling the temperature prediction for both the core and windings is imperative. In this paper, a thermal impedance network of the planar core is proposed considering the thermal coupling of the ambient, the core and the printed circuit board (PCB) windings. With the time-efficient computational fluid dynamics (CFD) simulation, the parameters of the network are extracted and the matrix-based model is obtained. Moreover, the model can be well coupled with the electric circuit model to perform the multi-physics optimization and simulation of the planar transformer. Finally, the proposed model is verified by both simulation and experiment results on a MHz prototype.

A Temperature-dependent Thermal Model of Silicon Carbide MOSFET Module for Long-term Reliability Assessment

Abstract: The silicon carbide (SiC) device is by far the most promising technology for the next-generation power electronic systems. However, the wide application of SiC device is inhibited by its reliability uncertainties, and a comprehensive SiC thermal model, which considers the temperature-dependency, is still missing for long-term reliability assessment. Thus, this paper proposes a temperature-dependent thermal model of SiC MOSFET module, which is composed of RC lumped elements and it is suitable for long-term reliability analysis. To begin with, the temperature-dependent thermal properties of the packaging materials (including SiC) are fully investigated. Then, the finite element method (FEM) based analysis containing temperature-dependency is utilized to extract both the self-heating and cross-coupling thermal impedances. Finally, a diagram of the RC lumped temperature-dependent thermal model is proposed, which is verified using a 3-level active neutral-point clamped (3-L ANPC) study case by performing its PLECS simulation.