Showing papers by "Huai Wang published in 2016"

A Review of the Condition Monitoring of Capacitors in Power Electronic Converters

Abstract: Capacitors are one type of reliability-critical components in power electronic systems. In the last two decades, many efforts in academic research have been devoted to the condition monitoring of capacitors to estimate their health status. Industry applications are demanding more reliable power electronics products with preventive maintenance. Nevertheless, most of the developed capacitor condition monitoring technologies are rarely adopted by industry due to the complexity, increased cost, and other relevant issues. An overview of the prior-art research in this area is therefore needed to justify the required resources and the corresponding performance of each key method. It serves to provide a guideline for industry to evaluate the available solutions by technology benchmarking, as well as to advance the academic research by discussing the history development and the future opportunities. Therefore, this paper first classifies the capacitor condition monitoring methods into three categories, then the respective technology evolution in the last two decades is summarized. Finally, the state-of-the-art research and the future opportunities targeting for industry applications are given.

Reliability of Capacitors for DC-Link Applications in Power Electronic Converters

Abstract: DC-link capacitors are an important part in the majority of power electronic converters which contribute to cost, size and failure rate on a considerable scale. From capacitor users' viewpoint, this paper presents a review on the improvement of reliability of dc link in power electronic converters from two aspects: 1) reliability-oriented dc-link design solutions; 2) conditioning monitoring of dc-link capacitors during operation. Failure mechanisms, failure modes and lifetime models of capacitors suitable for the applications are also discussed as a basis to understand the physics-of-failure. This review serves to provide a clear picture of the state-of-the-art research in this area and to identify the corresponding challenges and future research directions for capacitors and their dc-link applications.

Prediction of Bond Wire Fatigue of IGBTs in a PV Inverter Under a Long-Term Operation

Abstract: Bond wire fatigue is one of the dominant failure mechanisms in insulated-gate bipolar transistor (IGBT) modules under cyclic stresses. However, there are still major challenges ahead to achieve a realistic bond wire lifetime prediction in field operation. This paper proposes a Monte Carlo based analysis method to predict the lifetime consumption of bond wires of IGBT modules in a photovoltaic (PV) inverter. The variations in IGBT parameters (e.g., on-state collector–emitter voltage), lifetime models, and environmental and operational stresses are taken into account in the lifetime prediction. The distribution of the annual lifetime consumption is estimated based on a long-term annual stress profile of solar irradiance and ambient temperature. The proposed method enables a more realistic lifetime prediction with a specified confidence level compared to the state-of-the-art approaches. A study case of IGBT modules in a 10-kW three-phase PV inverter is given to demonstrate the procedure of the method. The obtained results of the lifetime distribution can be used to justify the selection of IGBTs for the PV inverter applications and the corresponding risk of unreliability.

Power control flexibilities for grid-connected multi-functional photovoltaic inverters

Abstract: This study explores the integration issues of next-generation high-penetration photovoltaic (PV) systems, where the grid is becoming more decentralised and vulnerable. In that case, the PV systems are expected to be more controllable with higher efficiency and reliability. Provision of ancillary and intelligent services, such as fault ride-through and reactive power compensation, is the key to attain higher utilisation of solar PV energy. Such functionalities for the future PV inverters can contribute to reduced cost of energy, and thus enable more cost-effective PV installations. To implement the advanced features, a flexible power controller is developed in this study, which can be configured in the PV inverter and flexibly change from one to another mode during operation. Based on the single-phase PQ theory, the control strategy offers the possibilities to generate appropriate references for the inner current control loop. The references depend on system conditions and also specific demands from both system operators and prosumers. Besides, this power control strategy can be implemented in commercial PV inverters as a standardised function, and also the operation modes can be achieved online in predesigned PV inverters. Case studies have verified the effectiveness and flexibilities of the proposal to realise the advanced features.

New Approaches to Reliability Assessment: Using physics-of-failure for prediction and design in power electronics systems

Abstract: Power electronics are facing continuous pressure to be cheaper and smaller, have a higher power density, and, in some cases, also operate at higher temperatures. At the same time, power electronics products are expected to have reduced failures because it is essential for reducing the cost of energy. New approaches for reliability assessment are being taken in the design phase of power electronics systems based on the physics-of-failure in components. In this approach, many new methods, such as multidisciplinary simulation tools, strength testing of components, translation of mission profiles, and statistical analysis, are involved to enable better prediction and design of reliability for products. This article gives an overview of the new design flow in the reliability engineering of power electronics from the system-level point of view and discusses some of the emerging needs for the technology in this field.

A Temperature-Dependent Thermal Model of IGBT Modules Suitable for Circuit-Level Simulations

Abstract: A basic challenge in the insulated gate bipolar transistor (IGBT) transient simulation study is to obtain the realistic junction temperature, which demands not only accurate electrical simulations but also precise thermal impedance. This paper proposed a transient thermal model for IGBT junction temperature simulations during short circuits or overloads. The updated Cauer thermal model with varying thermal parameters is obtained by means of finite-element method (FEM) thermal simulations with temperature-dependent physical parameters. The proposed method is applied to a case study of a 1700 V/1000 A IGBT module. Furthermore, a testing setup is built up to validate the simulation results, which is composed of a IGBT baseplate temperature control unit, an infrared camera with a maximum of 3 kHz sampling frequency, and a black-painted open IGBT module.

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 proposes three CPG strategies based on: 1) a power control (P-CPG), 2) a current limit method (I-CPG) and 3) 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 proposed 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&OCPG algorithm is the most suitable solution in terms of high robustness, but it presents poor dynamic performance.

System-level reliability assessment of power stage in fuel cell application

Abstract: High efficient and less pollutant fuel cell stacks are emerging and strong candidates of the power solution used for mobile base stations. In the application of the backup power, the availability and reliability hold the highest priority. This paper considers the reliability metrics from the component-level to the system-level for the power stage used in a fuel cell application. It starts with an estimation of the annual accumulated damage for the key power electronic components according to the real mission profile of the fuel cell system. Then, considering the parameter variations in both the lifetime model and the stress levels, the Weibull distribution of the power semiconductors lifetime can be obtained by using Monte Carlo analysis. Afterwards, the reliability block diagram can further be adopted to evaluate the reliability of the power stage based on the estimated power semiconductor reliability. In a case study of a 5 kW fuel cell power stage, the parameter variations of the lifetime model prove that the exponential factor of the junction temperature fluctuation is the most sensitive parameter. Besides, if a 5-out-of-6 redundancy is used, it is concluded both the B 10 and the Bi system-level lifetime can be remarkably increased compared to when no redundancy is used.

Capacitance estimation for dc-link capacitors in a back-to-back converter based on Artificial Neural Network algorithm

Abstract: The reliability of dc-link capacitors in power electronic converters is one of the critical aspects to be considered in modern power converter design. The observation of their ageing process and the estimation of their health status have been an attractive subject for the industrial field and hence for the academic research field as well. The existing condition monitoring methods suffer from shortcomings such as low estimation accuracy, extra hardware, and also increased cost. Therefore, the developed methods of condition monitoring that are based on software solutions and algorithms could be the way out of the aforementioned challenges and shortcomings. In this paper, a pure software condition monitoring method based on Artificial Neural Network (ANN) algorithm is proposed. The implemented ANN estimates the capacitance of the dc-link capacitor in a back-to-back converter. The error analysis of the estimated results is also studied. The developed ANN algorithm has been implemented in a Digital Signal Processor (DSP) in order to have a proof of concept of the proposed method.

Mission profile based sizing of IGBT chip area for PV inverter applications

Abstract: Maximizing the total energy generation is of importance for Photovoltaic (PV) plants. This paper proposes a method to optimize the IGBT chip area for PV inverters to minimize the annual energy loss of the active switches based on long-term operation conditions (i.e., mission profile). The design process is firstly introduced. Then the power loss, thermal characteristic and lifetime for IGBT modules with different chip areas are modeled. After that, the dependence of the annual energy loss and maximum junction temperature on the IGBT chip area and switching frequency is derived under a specific yearly mission profile. Simulation results are given to verify the thermal characteristics. Furthermore, a Monte Carlo based lifetime evaluation is presented to check the IGBT reliability. The proposed design method enables a reliability-oriented energy optimized sizing of active switches for PV inverter applications, which otherwise cannot be achieved by taking into account the power efficiency at rated condition or the weighted power efficiency at several loading levels only.

Lifetime estimation of electrolytic capacitors in a fuel cell power converter at various confidence levels

Abstract: DC capacitors in power electronic converters are a major constraint on improvement of the power density and the reliability. In this paper, according to the degradation data of tested capacitors, the lifetime model of the component is analyzed at various confidence levels. Then, the mission profile based lifetime expectancy of the individual capacitor and the capacitor bank is estimated in a fuel cell backup power converter operating in both standby mode and operation mode. The lifetime prediction of the capacitor banks at different confidence levels is also obtained.

Cost assessment of three power decoupling methods in a single-phase power converter with a reliability-oriented design procedure

Abstract: Electrolytic Capacitors (E-Cap) as the passive energy buffer in single-phase converter are often assumed to be the reliability bottleneck of power electronic system. Various Active Power Decoupling (APD) methods have been proposed intending to improve the reliability of the DC-link E-Caps qualitatively, making great effort to diverting the instantaneous pulsation power into extra reliable storage components. However, it is still an open question, which method is the most cost-effective one for a specific application with a given lifetime requirement. In this paper, two of the representative APD methods and the classical passive DC-link design method are evaluated from the reliability and cost perspective. The reliability-oriented design procedure is applied to size the chip area of active switching devices and the passive components to fulfill a specific lifetime target. Component cost models are applied to obtain the overall cost of each DC-link design methods. The cost comparisons are performed with a lifetime target of 10 years and 35 years. It reveals that different conclusions can be drawn with different lifetime targets in terms of cost-effectiveness.

A generic topology derivation method for single-phase converters with active capacitive DC-links

Abstract: Many efforts have been made to improve the single-phase power converters with active capacitive DC-link. The purpose is to reduce the overall DC-link energy storage and to achieve a reliable and cost-effective capacitive DC-link solution. A few review papers have already discussed the existing capacitive DC-link solutions, but important aspects of the topology assessment, such as the total energy storage, overall capacitive energy buffer ratio, cost, and reliability are still not available. This paper proposes a generic topology derivation method of single-phase power converters with capacitive DC-links, which derives all existing topologies to our best knowledge, and identify a few new topologies. A reliability-oriented design process is applied to compare the cost of different solutions with the lifetime target of 10 years and 35 years, respectively. It reveals that the most cost-effective solutions varies with the lifetime target.

Real mission profile based lifetime estimation of fuel-cell power converter

Abstract: Fuel cells are becoming a promising energy source for various applications due to its relatively high efficiency and reliability, and low pollution. They have been applied in backup power systems in telecom applications, where reliability and availability are high priority performance factors. This paper describes a lifetime prediction method for the power semiconductors used in the power conditioning of a fuel cell based backup system, considering both the long-term standby mode and active operation mode. The annual ambient temperature profile is taken into account to estimate its impact on the degradation of MOSFETs during the standby mode. At the presence of power outages, the backup system is activated into the operation mode and the MOSFETs withstand additional thermal stresses due to power losses. A study case of a 1 kW backup system is presented with two annual mission profiles in Denmark and India, respectively. The ambient temperature, occurrence frequency of power outages, active operation time and power levels are considered for the lifetime prediction of the applied MOSFETs. Comparisons of the accumulated lifetime consumptions are performed between standby mode and operation mode, and between Denmark and India, respectively.

Analytical model for LLC resonant converter with variable duty-cycle control

Abstract: In LLC resonant converters, the variable duty-cycle control is usually combined with a variable frequency control to widen the gain range, improve the light-load efficiency, or suppress the inrush current during start-up. However, a proper analytical model for the variable duty-cycle controlled LLC converter is still not available due to the complexity of operation modes and the nonlinearity of steady-state equations. This paper makes the efforts to develop an analytical model for the LLC converter with variable duty-cycle control. All possible operation models and critical operation characteristics are identified and discussed. The proposed model enables a better understanding of the operation characteristics and fast parameter design of the LLC converter, which otherwise cannot be achieved by the existing simulation based methods and numerical models. The results obtained from the proposed model are in well agreement with the simulations and the experimental verifications from a 500-W prototype.

A lifetime prediction method for LEDs considering mission profiles

Abstract: Light-Emitting Diodes (LEDs) has become a very promising alternative lighting source with the advantages of longer lifetime and higher efficiency than traditional ones. The lifetime prediction of LEDs is important to guide the LED system designers to fulfill the design specifications and to benchmark the cost-competitiveness of different lighting technologies. The existing lifetime data released by LED manufacturers or standard organizations are usually applicable only for specific temperature and current levels. Significant lifetime discrepancies may be observed in field operations due to the varying operational and environmental conditions during the entire service time (i.e., mission profiles). To overcome the challenge, this paper proposes an advanced lifetime prediction method, which takes into account the field operation mission profiles and the statistical properties of the life data available from accelerated degradation testing. It identifies also the key variables (e.g., heat sink parameters and lifetime-matching of LED drivers) that can be designed to achieve a specified lifetime and reliability level. Two case studies of an indoor residential lighting and an outdoor street lighting application are presented to demonstrate the prediction procedures and the impact of different mission profiles on the lifetime of LEDs.

Lifetime estimation of DC-link capacitors in a single-phase converter with an integrated active power decoupling module

Abstract: In single-phase inverters, DC-link capacitors are installed at the DC-link to buffer the ripple power between the AC side and DC side. Active decoupling methods introduce additional circuits at the DC side or AC side to partially or fully supply the ripple power. So that the demanded DC-link capacitor capacitance can be decreased. However, few research is about the effect of DC side and AC side decoupling on the DC-link capacitor reliability considering its electro-thermal stresses. This paper presents a quantitative analysis on the lifetime of capacitors with power decoupling circuits at the DC side and AC side, respectively. The ripple current spectrum of the capacitors is obtained by double Fourier analysis of a H-bridge inverter with natural sampling PWM modulation. A study case is demonstrated by a 2,000 W H-bridge inverter with 400 V DC-link voltage.

Reliablity assessment of fuel cell system — A framework for quantitative approach

Abstract: Hydrogen Fuel Cell (FC) technologies have been developed to overcome the operational and environmental challenges associated with using conventional power sources. Telecommunication industry, in particular, has implemented FC systems for the backup power function. The designers and manufacturers of such FC systems have great interest in verifying the performance and safety of their systems. Reliability assessment is designated to support decision-making about the optimal design and the operation strategies for FC systems to be commercial viable. This involves the properties of the system such as component failures, the system architecture, and operational strategies. This paper suggests an approach that includes Failure Modes and Effects Analysis (FMEA), Fault Tree Analysis (FTA), and Reliability Block Diagram (RBD). For a case study, and the service lifetime of a commercial 5 kW Proton Exchange Membrane Fuel Cell (PEMFC) system is estimated for backup power applications, in terms of the critical components, subsystems and the whole system.

Reliability evaluation of a single-phase H-bridge inverter with integrated active power decoupling

Abstract: Various power decoupling methods have been proposed recently to replace the DC-link Electrolytic Capacitors (E-caps) in single-phase conversion system, in order to extend the lifetime and improve the reliability of the DC-link. However, it is still an open question whether the converter level reliability becomes better or not, since additional components are introduced and the loading of the existing components may be changed. This paper aims to study the converter level reliability of a single-phase full-bridge inverter with two kinds of active power decoupling module and to compare it with the traditional passive DC-link solution. The converter level reliability is obtained by component level electro-thermal stress modeling, lifetime model, Weibull distribution, and Reliability Block Diagram (RBD) method. The results are demonstrated by a 2 kW single-phase inverter application.

Power loss analysis and comparision of DC and AC side decoupling module in a H-bridge inverter

Abstract: In single-phase inverters, a considerable amount of low-frequency ripples appear on the DC side due to the instantaneous power imbalance between the DC side and AC side. Recently, active methods using film capacitors or inductors as ripple power storage components introduced at the DC or AC side are applied to ease this issue. Nevertheless, there is lack of research regarding the selection of DC side or AC side power decoupling. This paper investigates the power losses of key components in both AC side and DC side active power decoupling solutions and compares their performance from an efficiency perspective. The analytical power loss models are derived based on the operation principles of the active power decoupling methods. A comparative study is performed based on a 500 W single-phase H-bridge inverter study case with 400 V DC-link voltage level. The results provide a guideline to justify whether or not to apply active power decoupling methods or which active power decoupling method to choose for a given application.

A new ZVS-PWM current-fed full-bridge converter with full soft-switching load range

Abstract: This paper presents a new soft-switched, current-driven full-bridge converter. A simple snubber is used in order to get ZVS for all the main switches. The soft-switching realization is independent of the load. The snubber is placed in parallel with the bridge. All the resonant circuit energy is recycled by the load in each half cycle. No additional current stresses appear on the main switches. A design-oriented steady-state analysis leads to the expressions of the dc voltage conversion ratio and ZVS analytical conditions, allowing for a trade-off design of the resonant inductor. A prototype has been built with a high energy transfer efficiency. The simulation and experiment results confirm the detailed theoretical analysis.

Approaching repetitive short circuit tests on MW-scale power modules by means of an automatic testing setup

Abstract: An automatic testing system to perform repetitive short-circuit tests on megawatt-scale IGBT power modules is presented and described in this paper, pointing out the advantages and features of such testing approach. The developed system is based on a non-destructive short-circuit tester, which has been integrated with an advanced software tool and a semiconductor device analyzer to perform stress monitoring on the considered device under test (DUT). A case-study is included in the paper concerning a 1.7 kV/ 1 kA IGBT module, which has been tested safely up to 30,000 repetitions with no significant damage. The developed system has been demonstrated to be very helpful in performing a large number of repetition tests as required by modern testing protocols for robustness and reliability assessment. The software algorithm and a demonstration video are available for download.

A component-reduced Zero-Voltage Switching three-level DC-DC converter

Abstract: The basic Zero-Voltage Switching (ZVS) three-level DC-DC converter has one clamping capacitor to realize the ZVS of the switches, and two clamping diodes to clamp the voltage of the clamping capacitor. In order to reduce the reverse recovery loss of the diode as well as its cost, this paper proposes to remove one of the clamping diodes in basic ZVS three-level DC-DC converter. With less components, the proposed converter can still have a stable clamping capacitor voltage, which is clamped at half of the dc link voltage. Moreover, the ZVS performance will be influenced by removing the clamping diode. But as long as the clamping capacitor is properly selected, the degradation of the ZVS performance can be neglected. The impact of the clamping capacitor on the ZVS performance is mathematically analyzed as well.