Dialogue Session IV: High-Power Converters and Systems

Monday, April 10, 2017


D4.1 Design of a SiC-Based Modular Multilevel Converter for Medium Voltage DC Distribution System

Jianghui Yu
Rolando Burgos
Niloofar Rashidi Mehrabadi
Dushan Boroyevich

Abstract
The Modular Multilevel Converter (MMC) is a promising converter choice for Medium Voltage DC (MVDC) distribution systems, where SiC devices can be readily adopted taking full advantage of this semiconductor’s properties. Specifically, 1.7 kV SiC MOSFETs are used to build 1kV modules operating at 100 kHz, thus eliminating the need to use multiple modules to generate a high equivalent switching frequency. Furthermore, a full-bridge configuration is used to take advantage of the additional control and energy storage capabilities that it offers when compared to conventional half-bridge modules. The full in-depth design, controls, and testing of the MMC prototype for MVDC distribution systems is presented in this paper, including among others: component selection, control algorithms, control hardware implementation, pre-charge and discharge circuits, and protection scheme. Experimental results are presented to demonstrate the proposed SiC-based converter.


D4.2. Busbar Design for SiC-Based H-Bridge PEBB using 1.7 kV, 400 A SiC MOSFETs Operating at 100 kHz

Niloofar Rashidi Mehrabadi
Igor Cvetkovic
Jun Wang
Rolando Burgos
Dushan Boroyevich

Abstract
This paper presents a systematic study of the busbar design and optimization for SiC-based H-bridge power electronics building block (PEBB) used in high-frequency and high-power applications. Step-by-step guidelines are presented in which the design considerations and analysis are given. This paper presents a double-sided busbar concept to create a compact PEBB design with improved thermal and switching performance, which result from having double-side cooling and symmetric minimized current commutation loop inductances, respectively. The proposed concept is verified experimentally by evaluating the high-speed switching performance of the PEBB up to 400 A.


D4.3. Analysis of D-Q Small-Signal Impedance of Grid-Tied Inverters

Bo Wen
Dushan Boroyevich
Rolando Burgos
Paolo Mattavelli
Zhiyu Shen


Please note: This paper will be presented by Zeng Liu (pictured), as Bo Wen has graduated.

Abstract
This paper analyzes the small-signal impedance of three-phase grid-tied inverters with feedback control and phaselocked loop (PLL) in the synchronous reference (d-q) frame. The result unveils an interesting and important feature of three-phase grid-tied inverters – namely, that its q–q channel impedance behaves as a negative incremental resistor. Moreover, this paper shows that this behavior is a consequence of grid synchronization, where the bandwidth of the PLL determines the frequency range of the resistor behavior, and the power rating of the inverter determines the magnitude of the resistor. Advanced PLL, current, and power control strategies do not change this feature. An example shows that under weak grid conditions, a change of the PLL bandwidth could lead the inverter system to unstable conditions as a result of this behavior. Harmonic resonance and instability issues can be analyzed using the proposed impedance model. Simulation and experimental measurements verify the analysis.


D4.4. A Transformerless Single-Phase Utility Interface Converter to Attenuate Common-Mode Voltage for DC Microgrid

Fang Chen
Rolando Burgos
Dushan Boroyevich

Abstract
DC power distribution gains popularity in future residential, renewable energy, transportation and data centers. Such dc system can be connected to ac system through bidirectional interface converter. Compared with isolated converters, transformerless solutions have the advantages of higher power density, efficiency, and lower cost. One concern about the non-isolated connection is the coupled common-mode voltage through ground and the resultant leakage current. In this paper, a two-stage cascaded single-phase ac-dc converter connecting 380 V bipolar dc system and 240 V split-phase ac system is used as an example to discuss the common-mode voltage attenuation. The cascaded converter uses full-bridges for both the ac-dc and the dc-dc stages to achieve decoupled common-mode voltage control. A common-mode equivalent circuit is established to analyze the origin of low-frequency common-mode voltage from the utility and high-frequency noise from the switching behavior of power converters. To attenuate high-frequency noise, a floating common-mode noise filter is connected across the ac and dc ports to contain the EMI noise within the converter. The low-frequency common-mode voltage is controlled by applying a common-mode duty cycle injection method to the full-bridge dc-dc stage. A 5 kW converter prototype is built to validate the attenuation method. The resultant dc bus voltage is symmetric to the ground and suitable for bipolar dc power distribution systems.


D4.5. A Frequency Domain Model for Beat Frequency Oscillation Analysis in Microgrid

Xiaolong Yue
Dushan Boroyevich
Rolando Burgos
Fang Zhuo


Please note: This paper will be presented by Chi Li (pictured), as Xiaolong Yue has graduated.

Abstract
Power electronic converters are typical nonlinear systems in frequency domain. With a perturbation frequency excitation, the current and voltage of power converters in steady state contain not only perturbation but also many sidebands. In a Microgrid containing a large amount of power converters, one converter's switching frequency ripples are other converters' perturbations and beat frequency components will be generated if switching frequencies of those power converters are different. As the control loops always take high gains in low frequency regions and the low pass filters almost have no effect on low-frequency disturbances, if the beat frequency component has certain values, they may be magnified and presented as oscillations. This paper develops a frequency domain model to describe the characteristics of power converter around switching frequency range and to analyze the high frequency interactions of power converters in Micorgrid. The voltage mode controlled boost converters are illustrated as demonstration. The proposed model indicates that switching frequency affects the output characteristics of power converters significantly. In a system consists of two boost converters in parallel, the beat frequency oscillation that traditional output impedance models fail to explain could be accurately predicted by the proposed model. In addition, based on this model, design guideline is proposed to avoid the potential beat frequency oscillation in parallel system. Simulation results validate the accuracy and effectiveness of the proposed method.


D4.6. Impact of PV Inverter Penetration on Voltage Profile and Power Loss in Medium Voltage Distribution Systems

Ye Tang
Rolando Burgos
Chi Li
Dushan Boroyevich

Abstract
With an increasing number of PV generators at the distribution system, the impact on the distribution system is no longer negligible. A comparison is done among by PV generators of different capacities, different injection locations and different reactive power control modes, for their impact on system voltage profile and power loss. Voltage sensitivity analysis is conducted to assess the impact of PV injection on system voltage profile. The accuracy of these sensitivity indices are proven by calculation results from MATLAB and simulation results from PSS/E. Guidelines are formed for utility to manage PV injection locations and regulate PV generator reactive power control to avoid voltage profile problem and reduce power loss. In comparing different PV injection locations, there’s tradeoff between voltage regulation and power loss minimization, and randomly distributed PV may increase or decrease system power loss, but optimally distributed PV generators decrease power loss compared to centralized PV.


D4.7. Capacitor Voltage Ripple Reduction with State Trajectory Analysis for Modular Multilevel Converter

Yadong Lyu
Chen Li
Yi-Hsun Hsieh
Fred C. Lee
Qiang Li
Rong Xu


Please note: This paper will be presented by Yi-Hsun Hsieh (pictured), as Yadong Lu has graduated.

Abstract
The modular multi-level converter (MMC) is the most prominent interface converter used between the HVDC grid and the HVAC grid. One of the important design challenges in MMC is to reduce the capacitor size. In the current practice, a rather large capacitor bank is required to store line-frequency related circulating energy, even though a number of control strategies have been introduced to reduce the capacitor voltage ripples. In the present paper, a novel control strategy is proposed by the means of harmonic injections in the conjunction with gain control to eliminate completely both the line frequency and the second-order harmonic of the capacitor voltage ripple. Ideally, the proposed method works with the full bridge topology. However, the concept also works with half bridge topology with significant reduction of line frequency related ripple. To gain a better understanding of nature of circulating energy and means of reducing it, the method of state plane analysis is employed to offer visual support. In addition, the design trade-off between full bridge MMC and half bridge MMC is presented and a novel control strategy for hybrid MMC is proposed. Finally, the work is supported with a scale down hardware demonstration.


D4.8. 10 kW Transformer and DC Stage Design for Electrical Vehicle Charging System

Xingye Liu
Rolando Burgos
Jun Wang
Nidhi Haryani

Abstract
This paper discusses the design procedure for the 10 kW transformer used in specific battery charging system. In order to fit the wide output voltage range, worst case consideration is used to determine some core design limitations and then simplified optimization method can be applied to finalize the core shape. As additional design considerations, a complementary design process about effective area and turns ratio is discussed separately. The complete design procedure covers flexible turns ratio case and non-flexible case. And the output power level brings additional changes to the design process. The procedure can be applied to any similar power level transformer design. The optimized transformer in this application gives 40W total loss at the worst operation point.


D4.9. Conceptual Design and Weight Optimization of Aircraft Power Systems with High-Peak Pulsed Power Loads

Qian Li
Balakrishnan Devarajan
Xuning Zhang
Rolando Burgos
Dushan Boroyevich
Pradeep Raj

Abstract
The more electric aircraft (MEA) concept has gained popularity in recent years. As the main building blocks of advanced aircraft power systems, multi-converter power electronic systems have advantages in reliability, efficiency and weight reduction. The pulsed power load has been increasingly adopted--especially in military applications--and has demonstrated highly nonlinear characteristics. Consequently, more design effort needs to be placed on power conversion units and energy storage systems dealing with this challenging mission profile: when the load is on, a large amount of power is fed from the power supply system, and this is followed by periods of low power consumption, during which time the energy storage devices get charged. Thus, in order to maintain the weight advantage of MEA and to keep the normal functionality of the aircraft power system in the presence of a high-peak pulsed power load, this paper proposes a novel multidisciplinary weight optimization technique. The presented weight optimization method mainly focuses on comparing and evaluating the weights among different power electronics system structures based on subsystem weight models, including the gearbox, synchronous generator, power electronic converters and supercapacitors. Finally, through a case study, it is shown that a system weight reduction of 11.9 % can be attained when applying the proposed optimization method to down-select alternative system configurations.


D4.10. Large Substrates Bonded by Silver Sintering and Their Thermal Performance

Shan Gao
Zhenwen Yang
Zhan Sun
Yansong Tan
Xin Li
Xu Chen
Guo-Quan Lu

Abstract
In a power device or module package, along the heat flow path from device junction to case, the interface between a large insulated-metal substrate and the heat spreader or heat-sink plate is usually the largest contributor to the junction-to-case thermal resistance. Conventional thermal interface materials, such as thermal grease, or phase change material, or solder, that are used to join large structures together have either poor thermal conductivity or low reliability. In this work, we extended a silver sintering technique, which has been proven to be an excellent die-attach solution, to bonding large substrates. We found that strong bonding strength could be achieved for bonding areas greater than 25 mm x 50 mm by sintering a nanosilver paste at temperatures below 270 °C under less than 5 MPa pressure. To characterize thermal performance of the large bonded interface, we mounted Si IGBT chips at different locations on the substrate. Using self-heating and temperature-sensitive threshold voltage of the IGBT device, we measured transient thermal responses at the various device locations. These measurements were then used to generate plots of the cumulative structural function, i.e. the cumulative thermal capacitance versus cumulative thermal resistance from the device junction through the bonded structure along the heat flow path. A two-dimensional map of the thermal resistance contributed by the bonded interface was obtained. We found that for a well-bonded structure, the average specific thermal resistance contributed by the sintered silver interface was 5.20 mm2K/W with a variation of 6%, across an area of 25 mm x 50 mm.


D4.11. Insulation design and evaluation via partial discharge test for power electronics application

Yue Xu
Shaoxi Yu
Rolando Burgos
Dushan Boroyevich

Abstract
Insulation design is important especially for medium voltage level, high power density power electronic devices in order to achieve high reliability and long lifetime, with a reasonable insulation size and weight. Most of the weak points inside a solid insulation is the void or defects especially those locates at the field crowding area. Via using a self-made laminated bus as an example, this paper will show how to model the real structure and do the field analysis, even with some voids. In order to reduce the stress for the insulation, several useful methods to decrease electric field crowding will be discussed. All these methods will be verified by FEA simulation. Then, a non-destructive partial discharge (PD) method to evaluate insulation design will be introduced. An experimental set-up will be built under lab condition and a single-void specimen will be made, modeled and tested first in order to verify the set-up. The self-made laminated bus PD character under line frequency excitation will be captured. The result shows that it should be good for 1kV application. Any kind of insulation design and possible improvement in the future will be tested under similar way. The same set-up can be used for PWM excitation as well. Deep understanding for how to evaluate and improve the insulation, especially for power electronics application, will be drawn in the future.


D4.12. DBC Switch Module for Management of Temperature and Noise in 220-W/in3 Power Assembly

Jong-Won Shin
Woochan Kim


Please note: This paper will be presented by Zichen Miao (pictured), as Jong-Won Shin has graduated.

Abstract
A switch module integrates semiconductor dies on a direct-bond-copper (DBC) substrate to achieve both noise robust- ness and low thermal resistance. Negative couplings between conductors as well as compact layout with 2.89-nH common-source inductance in the module eliminate self-turn-on from 420-A/μs di/dt. The low thermal resistance of the DBC substrate provides 2.35-°C/W thermal resistance from junction to heat sink and lim- its the temperature rise of semiconductors switches to 50 °C. The maximum rated power and system density of the DBC and PCB modules are provided for 400-V application. A 2-kW dc–dc boost converter switched between 400 kHz and 1 MHz achieved a power density of 220 W/in3 and an efficiency of 98.4% by employing the switch module.


D4.13. High Frequency Transformer Design for Modular Power Conversion from Medium Voltage AC to 400V DC

Shishuo Zhao
Qiang Li
Fred C. Lee


Please note: This paper will be presented by Bin Li (pictured), as Shishuo Zhao has graduated.

Abstract
The paper presents a high frequency modular medium voltage AC (4160 VAC and 13.8 VAC) to low voltage DC (400 VDC) power conditioning system block (PCSB) that are scalable so that they can be used for micro grids of different scale (several-hundred kW to multi-MW). The modular approach is intended to result in higher-volume, lower-cost, less-loss power electronics building blocks that service many applications, such as DC data center and electric vehicle charge station. In this paper, a 225kW, 500 kHz PCSB is demonstrated to direct converter 4160 VAC to 400V DC for a DC date center. WBG power devices and CLLC resonant converter are used to minimize switching related loss at high frequency. The high frequency transformer of CLLC resonant converter is one of the key elements for the proposed modular approach. This paper will focus on high frequency transformer design to realize high-voltage-isolation, high- efficiency and high-density at the same time. Based on a split winding transformer structure, transformer insulation material and dimension parameters are determined referring to insulation standard. Transformer magnetic loss model is reviewed based on which loss design trade-off is carefully analyzed. Finally a 500 kHz transformer prototype has been developed and demonstrated with 30kV isolation capability, whole CLLC resonant converter holds 98% peak efficiency and 48 W/in3 power density.


D4.14. A Novel PCB Winding Transformer with Controllable Leakage Integration for a 6.6kW 500kHz High Efficiency High Density Bi-Directional On-Board Charger

Bin Li
Qiang Li
Fred C. Lee

Abstract
The paper presents a high frequency modular medium voltage AC (4160 VAC and 13.8 VAC) to low voltage DC (400 VDC) power conditioning system block (PCSB) that are scalable so that they can be used for micro grids of different scale (several-hundred kW to multi-MW). The modular approach is intended to result in higher-volume, lower-cost, less-loss power electronics building blocks that service many applications, such as DC data center and electric vehicle charge station. In this paper, a 225kW, 500 kHz PCSB is demonstrated to direct converter 4160 VAC to 400V DC for a DC date center. WBG power devices and CLLC resonant converter are used to minimize switching related loss at high frequency. The high frequency transformer of CLLC resonant converter is one of the key elements for the proposed modular approach. This paper will focus on high frequency transformer design to realize high-voltage-isolation, high- efficiency and high-density at the same time. Based on a split winding transformer structure, transformer insulation material and dimension parameters are determined referring to insulation standard. Transformer magnetic loss model is reviewed based on which loss design trade-off is carefully analyzed. Finally a 500 kHz transformer prototype has been developed and demonstrated with 30kV isolation capability, whole CLLC resonant converter holds 98% peak efficiency and 48 W/in3 power density.


NP4.1. Design and Optimization of a Modular Inverter

Victor Turriate
Qiong Wang
Rolando Burgos
Dushan Boroyevich

There is no paper associated with this session number. Click below to view the poster.


NP4.2. WBG-Based 22kW Bidirectional On-Board Fast Charger

Rimon Gadelrab
Bin Li
Qiang Li
Fred C. Lee
Zhengyang Liu

There is no paper associated with this session number. Click below to view the poster.


NP4.3. Zero-CMV Buck Converter for UPS Applications

Paul Rankin
Sungjae Ohn
Rolando Burgos
Dushan Boroyevich

There is no paper associated with this session number. Click below to view the poster.


NP4.4. Design and Optimization of High-Frequency Transformer for Grid-Tied Energy Storage Systems

Alex Chu
Glenn Skutt
Rolando Burgos

There is no paper associated with this session number. Click below to view the poster.