Showing papers by "Huai Wang published in 2015"

Wide-Scale Adoption of Photovoltaic Energy: Grid Code Modifications Are Explored in the Distribution Grid

Abstract: Current grid standards largely require that low-power (e.g., several kilowatts) single-phase photovoltaic (PV) systems operate at unity power factor (PF) with maximum power point tracking (MPPT), and disconnect from the grid under grid faults by means of islanding detection. However, in the case of wide-scale penetration of single-phase PV systems in the distributed grid, disconnection under grid faults can contribute to 1) voltage flickers, 2) power outages, and 3) system instability. This article explores grid code modifications for a wide-scale adoption of PV systems in the distribution grid. In addition, based on the fact that Italy and Japan have recently undertaken a major review of standards for PV power conversion systems connected to low-voltage networks, the importance of low voltage ride-through (LVRT) for single-phase PV power systems under grid faults is considered, along with three reactive power injection strategies. Simulations are presented for a PV power system with a LVRT capability and ancillary services. An example of a full-bridge single-phase grid connected system is tested experimentally to demonstrate the potential benefits. Additionally, grid codes for advanced PV systems with the discussed features are summarized.

Frequency Adaptive Selective Harmonic Control for Grid-Connected Inverters

Abstract: In this paper, a frequency adaptive selective harmonic control (FA-SHC) scheme is proposed. The FA-SHC method is developed from a hybrid SHC scheme based on the internal model principle, which can be designed for grid-connected inverters to optimally mitigate feed-in current harmonics. The hybrid SHC scheme consists of multiple parallel recursive (nk ± m)-order (k = 0,1,2,. . ., and m ≤ n/2) harmonic control modules with independent control gains, which can be optimally weighted in accordance with the harmonic distribution. The hybrid SHC, thus, offers an optimal tradeoff among cost, complexity, and also performance in terms of high accuracy, fast response, easy implementation, and compatible design. The analysis and synthesis of the hybrid SHC are addressed. More important, in order to deal with the harmonics in the presence of grid frequency variations, the hybrid SHC is transformed into the FA-SHC, being the proposed fractional-order controller, when it is implemented with a fixed sampling rate. The FA-SHC is implemented by substituting the fractional-order elements with the Lagrange-polynomial-based interpolation filters. The proposed FA-SHC scheme provides fast on-line computation and frequency adaptability to compensate harmonics in grid-connected applications, where the grid frequency is usually varying within a certain range (e.g., 50 ± 0.5 Hz). Experimental tests have demonstrated the effectiveness of the proposed FA-SHC scheme in terms of accurate frequency adaptability and also fast transient response.

Prediction of bond wire fatigue of IGBTs in a PV inverter under long-term operation

Abstract: Bond wire fatigue is one of the dominant failure mechanisms in IGBT modules. However, the bond wire lifetime is not easily predictable and measurable to date due to several challenges. To overcome this challenge, this paper proposes a Monte Carlo based analysis method to predict the lifetime consumption of bond wires in a Photovoltaic (PV) inverter under long-term operation. The parameter distributions of IGBTs due to manufacturing variation and the annual stress profiles due to intermittent nature of solar irradiance and ambient temperature are taken into consideration. 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 on IGBT modules in a 10 kW three-phase PV inverter demonstrates the procedure and the results of the analysis. Finally, the lifetime distribution of bond wires permits to estimate the risk of unreliability of a single IGBT in a Photovoltaic (PV) inverter.

Reliability Oriented Design Tool For the New Generation of Grid Connected PV-Inverters

Abstract: This paper introduces a reliability-oriented design tool for a new generation of grid-connected photovoltaic (PV) inverters. The proposed design tool consists of a real field mission profile (RFMP) model (for two operating regions: USA and Denmark), a PV panel model, a grid-connected PV inverter model, an electrothermal model, and the lifetime model of the power semiconductor devices. An accurate long-term simulation model able to consider the one-year RFMP (solar irradiance and ambient temperature) is developed. Thus, the one-year estimation of the converter device thermal loading distribution is achieved and is further used as an input to the lifetime model. The proposed reliability-oriented design tool is used to study the impact of mission profile (MP) variation and device degradation (aging) in the PV inverter lifetime. The obtained results indicate that the MP of the field where the PV inverter is operating has an important impact (up to 70%) on the converter lifetime expectation, and it should be considered in the design stage to better optimize the converter design margin. In order to have correct lifetime estimation, it is crucial to consider also the device degradation feedback (in the simulation model), which has an impact of 20-30% on the precision of the lifetime estimation for the studied case.

Condition monitoring for DC-link capacitors based on artificial neural network algorithm

Abstract: In power electronic systems, capacitor is one of the reliability critical components. Recently, the condition monitoring of capacitors to estimate their health status have been attracted by the academic research. Industry applications require more reliable power electronics products with preventive maintenances. However, the existing capacitor condition monitoring methods suffer from either increased hardware cost or low estimation accuracy, being the challenges to be adopted in industry applications. New development in condition monitoring technology with software solutions without extra hardware will reduce the cost, and therefore could be more promising for industry applications. A condition monitoring method based on Artificial Neural Network (ANN) algorithm is therefore proposed in this paper. The implementation of the ANN to the DC-link capacitor condition monitoring in a back-to-back converter is presented. The error analysis of the capacitance estimation is also given. The presented method enables a pure software based approach with high parameter estimation accuracy.

Harmonics mitigation of dead time effects in PWM converters using a repetitive controller

Abstract: In order to prevent the power switching devices (e.g., the Insulated-Gate-Bipolar-Transistor, IGBT) from shoot-through in voltage source converter during a switching period, a dead time is added either in the hardware drivers of the IGBTs or implemented in the software Pulse-Width Modulation (PWM) scheme. Both methods will lead to a degradation of the injected current power quality. Thus, the harmonics induced by the dead time have to be compensated in order to achieve a satisfactory current as required by the standards. In this paper, a repetitive controller has been introduced to eliminate the dead-time effect in grid-connected PWM converters. The repetitive controller has been plugged into a proportional resonant based fundamental controller. Compared with the traditional dead-time compensation solutions, the repetitive controller can effectively compensate the dead-time harmonics as well as other low-order distortions, and also it is a simple method without hardware modifications. Experimental results are demonstrating the advantages of the proposed dead-time effect mitigation method compared to the resonant based harmonic compensator.

A humidity-dependent lifetime derating factor for DC film capacitors

Abstract: Film capacitors are widely assumed to have superior reliability performance than Aluminum electrolytic capacitors in DC-link design of power electronic converters. However, the assumption needs to be critically judged especially for applications under high humidity environments. This paper proposes a humidity-dependent lifetime derating factor for a type of plastic-boxed metallized DC film capacitors. It overcomes the limitation that the humidity impact is not considered in the state-of-the-art DC film capacitor lifetime models. The lifetime derating factor is obtained based on a total of 8,700 hours accelerated testing of film capacitors under different humidity conditions, enabling a more justified lifetime prediction of film capacitors for DC-link applications under specific climatic environments. The analysis of the testing results and the detailed discussion on the derating factor with different lifetime definitions and confidence levels are presented.

A review of the condition monitoring of capacitors in power electronic converters

Abstract: Capacitor is one of the reliability critical components in power electronic systems. In the last two decades, many efforts in the academic research have been devoted to the condition monitoring of capacitors to estimate their health status. Industry applications demand more reliable power electronics products with preventive maintenances. 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 of the 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 firstly classifies the capacitor condition monitoring methods into three categories, then the respective technology evolution from 1993 to 2015 is summarized. Remarks on the state-of-the-art research and the future opportunities targeting for practical industry applications are given.

Instantaneous thermal modeling of the DC-link capacitor in PhotoVoltaic systems

Abstract: Capacitors have been witnessed as one of the weak points in grid-connected PhotoVoltaic (PV) applications, and thus efforts have been devoted to the design of reliable DC-link capacitors in PV applications. Since the hot-spot temperature of the capacitor is one of the failure inducers, instantaneous thermal modeling approaches considering mission profiles for the DC-link capacitor in single-phase PV systems are explored in this paper. These thermal modelling approaches are based on: a) fast Fourier transform, b) look-up tables, and c) ripple current reconstruction. Moreover, the thermal modelling approaches for the DC-link capacitors take into account the instantaneous thermal characteristics, which are more challenging to the capacitor reliability during operation. Such instantaneous thermal modeling approaches enable a translation of instantaneous capacitor power losses to capacitor thermal loading from the operating conditions. As a consequence, it offers new insights into the temperature monitoring and reliability-oriented design of the DC-link capacitors, and thus a more reliable operation of single-phase grid-connected PV systems can be enhanced. Study results on a 3-kW single-phase grid-connected PV system have been adopted to demonstrate a look-up table based modelling approach, where real-field daily ambient conditions are considered.

Comprehensive investigation on current imbalance among parallel chips inside MW-scale IGBT power modules

Abstract: With the demands for increasing the power rating and improving reliability level of the high power IGBT modules, there are further needs of understanding how to achieve stable paralleling and identical current sharing between the chips. This paper investigates the stray parameters imbalance among parallel chips inside the 1.7 kV/1 kA high power IGBT modules at different frequencies by Ansys Q3D parastics extractor. The resulted current imbalance is further confirmed by experimental measurement.

Study on Oscillations During Short Circuit of MW-Scale IGBT Power Modules by Means of a 6-kA/1.1-kV Nondestructive Testing System

Abstract: This paper uses a 6-kA/1.1-kV nondestructive testing system for the analysis of the short-circuit behavior of insulated-gate bipolar transistor (IGBT) power modules. A field-programmable gate array enables the definition of control signals to an accuracy of 10 ns. Multiple 1.7-kV/1-kA IGBT power modules displayed severe divergent oscillations, which were subsequently characterized. Experimental tests indicate that nonnegligible circuit stray inductance plays an important role in the divergent oscillations. In addition, the temperature dependence of the transconductance is proposed as an important element in triggering for the oscillations.

Electro-thermal modeling of high power IGBT module short-circuits with experimental validation

Abstract: A novel Insulated Gate Bipolar Transistor (IGBT) electrothermal modeling approach involving PSpice and AN SYS/Icepak with both high accuracy and simulation speed has been presented to study short-circuit of a 1.7 kV/1 kA commercial IGBT module. The approach successfully predicts the current and temperature distribution inside the chip of power IGBT modules. The simulation result is further validated using a 6 kA/1.1 kV non-destructive tester. The experimental validation demonstrates the modeling approach's capability for reliable design of high power IGBT power modules given electrical/thermal behavior under severe conditions.

Reliability Assessment of Transformerless PV Inverters Considering Mission Profiles

Abstract: Due to the small volume and high efficiency, transformerless inverters have gained much popularity in grid-connected PV applications, where minimizing leakage current injection is mandatory. This can be achieved by either modifying the modulation schemes or adding extra power switching devices, resulting in an uneven distribution of the power losses on the switching devices. Consequently, the device thermal loading is redistributed and thus may alter the entire inverter reliability performance, especially under a long-term operation. In this consideration, this paper assesses the device reliability of three transformerless inverters under a yearly mission profile (i.e., solar irradiance and ambient temperature). The mission profile is translated to device thermal loading, which is used for lifetime prediction. Comparison results reveal the lifetime mismatches among the power switching devices operating under the same condition, which offers new thoughts for a robust design and a reliable operation of grid-connected transformerless PV inverters with high efficiency.

The feasibility study on thermal loading control of wind power converters with a flexible switching frequency

Abstract: Thermal loading of wind power converters is critical to their reliability performance. Especially for IGBT modules applied in a converter, both of the mean value and variation of the junction temperature have significant impact on the lifetime. Besides other strategies to reduce the thermal loading of the IGBT modules, the power losses and thereby the thermal stresses can be controlled by varying the PWM switching frequency according to power loading conditions. This paper investigates the feasibility to apply this flexible modulation strategy in a 3 MW wind power converter application to reduce the temperature fluctuations due to wind speed variations. The trade-off between the reduced amplitude of temperature fluctuations and the additional power losses that may be introduced is quantitatively studied.

Reliability engineering in power electronic converter systems

Abstract: Electrical energy conversion by power electronic systems can be classified into the following four categories: 1. Voltage conversion and power conversion for both direct current (DC) and alternate current (AC) 2. Frequency conversion 3. Wave-shape conversion 4. Poly-phase conversion.

Introducing state-trajectory control for the synchronous interleaved boost converter

Abstract: Synchronous interleaved boost converters (SIBCs) result in lower ripple currents and bidirectional power flow. The boost topology has a non-minimum phase characteristic, producing instability problems when a large bandwidth is required. Linear controllers inherently limit the boost controller bandwidth, resulting in a slow response. In this paper, state-trajectory control of the SIBC based on boundary control is proposed to provide an outstanding dynamic response during start-up and sudden load changes, close to the physical limit of the system. The proposed controller and derivation provides a rigorous framework that deals with four switching states, and three state equations, resulting in a simple control law with very fast dynamic response. The normalized trajectories for the SIBC are determined in the geometric domain along with the control law. The exact trajectories are used for fast transients, and approximate trajectories are employed for constant frequency in steady-state. Simulation and experimental results are provided to validate the proposed procedures.

A comprehensive investigation on the short circuit performance of MW-level IGBT power modules

Abstract: This paper investigates the short circuit performance of commercial 1.7 kV / 1 kA IGBT power modules by means of a 6 kA Non-Destructive-Tester. A mismatched current distribution among the parallel chips has been observed, which can reduce the short circuit capability of the IGBT power module under short circuit conditions. Further Spice simulations reveal that the stray parameters inside the module play an important role in contributing to such a phenomenon.

Waveform control method for mitigating harmonics of inverter systems with nonlinear load

Abstract: DC power systems connecting to single-phase DC/AC inverters with nonlinear loads will have their DC sources being injected with AC ripple currents containing a low-frequency component at twice the output voltage frequency of the inverter and also other current harmonics. Such a current may create instability in the DC power system, lower its efficiency, and shorten the lifetime of the DC source. This paper presents a general waveform control method that can mitigate the injection of the low-frequency ripple current by the single-phase DC/AC inverter into the DC source. It also discusses the inhibiting ability of the waveform control method on other coexisting harmonics, while the DC source delivers AC power to a nonlinear load. With the application of the waveform control, the average DC output power is supplied by the DC source, while the other harmonics pulsation power can be confined to the AC side (between the capacitors and the AC load). Theoretical analyses and experiment results are provided to explain the operation and the inhibiting ability, and to showcase the performance of the approach in tackling the harmonics. Results validate that the proposed solution can achieve significant mitigation of harmonic component at DC side with nonlinear load as well as a high quality output voltage without the need for extra large capacitive energy storage element implementation.

Artificial Neural Network Algorithm for Condition Monitoring of DC-link Capacitors Based on Capacitance Estimation

Abstract: In power electronic converters, reliability of DC-link capacitors is one of the critical issues. The estimation of their health status as an application of condition monitoring have been an attractive subject for industrial field and hence for the academic research filed as well. More reliable solutions are required to be adopted by the industry applications in which usage of extra hardware, increased cost, and low estimation accuracy are the main challenges. Therefore, development of new condition monitoring methods based on software solutions could be the new era that covers the aforementioned challenges. A capacitance estimation method based on Artificial Neural Network (ANN) algorithm is therefore proposed in this paper. The implemented ANN estimated the capacitance of the DC-link capacitor in a back-toback converter. Analysis of the error of the capacitance estimation is also given. The presented method enables a pure software based approach with high parameter estimation accuracy.

Reliability-oriented design of LC filter in buck DC-DC converter

Abstract: State-of-the-art LC filter design of buck DC-DC converter is based on the specifications of voltage and current ripples and constrains in power density and cost. Since lifetime is an important performance factor in reliability critical applications, this digest proposes a method to optimize the design of the LC filters from a reliability perspective, among other considerations. It investigates the design freedom between the values of inductor and capacitors, the physical formation of the LC network, and the corresponding electro-thermal stresses of the selected capacitors. The outcome enables an optimized LC filter design to fulfill the required lifetime. The theoretical analysis and simulation study are presented which are verified by the experimental results from a buck converter prototype.

Reliability of DC-link capacitors in power electronic converters

Abstract: DC-link capacitors are widely used in power electronic converters to balance the instantaneous power difference between the input source and output load and to minimize voltage variation in the DC-link. In some applications, they are also used to provide energy storage. Figure 3.1 shows some typical configurations of power electronic conversion systems with capacitive DC-links. Such configurations cover a wide range of power electronics applications, for example, power factor corrections, wind turbines, photovoltaic systems, motor drives, electric vehicles, and lighting systems. It should be noted that a capacitive DC-link discussed here does not necessarily consist of capacitors only. There could also be some inductive components (e.g., DC choke) in the DC bus in some of the above applications.