Table of contents monday, September 9, 1: 30pm-4: 00pm modular Multi-Level Converters, hvdc, and dc grids I 3

Junction Temperature Sensing and Monitoring

Wednesday, September 21, 1:30PM-3:10PM, Room: 102A, Chair: Adam Skorek, Tanya Gachovska

1:30PM An IGBT Junction Temperature Measurement Method via Combined TSEPs For Eliminating Impact of Collector Current [#202]
Xiang Wang, Chong Zhu, Haoze Luo, Zhou Lu, Wuhua Li, Xiangning He, Jun Ma, Guodong Chen, Ye Tian and Enxing Yang, Zhejiang University, China; Aalborg University, China; Shanghai Electric, China

Junction temperature of insulated gate bipolar transistors (IGBTs) plays an important role in power semiconductor devices reliability. However, it is difficult to have direct access to the chip to obtain the junction temperature. This paper provides a new approach to extract the junction temperature by using combined thermo-sensitive electric parameters (TSEPs) during turn-off transient due to the parasitic inductor LeE. The turn-off delay time (tdoff) and voltage peak of LeE during turn of transient (VeEPeak) can both serve as TSEP for their temperature dependence. High linearity is inherited when the two TSEPs are combined to extract the junction temperature. The proposed method has the clear advantages of simple realization compared with individual TSEP to obtain junction temperature because it eliminates the influence of collector current (IC). Experiments have been implemented to evaluate the effectiveness of the proposed solution.

1:55PM DeltaTj Control of Switching Power Devices at Thermal Boundaries via Physics-Based Loss Manipulation [#857]
Timothy Polom, Boru Wang and Robert Lorenz, University of Wisconsin-Madison, United States

To optimize the lifetime of switching power semiconductors, this paper presents improvements in the performance and coordination of a control system to regulate power device junction temperature, Tj, and its change during power cycles, DeltaTj. This research proposes a DeltaTj control law which closes a control loop on the sample average rate of change state, Tj- dot-bar, and subsequently introduces active thermal capacitance and conductance to the closed-loop thermal system dynamics. Also proposed in this paper is a supervisory state machine which ensures thermal control interrupts nominal system-level control only when temperature bounds are exceeded, and coordinates smooth transitions as Tj(k) or DeltaTj(k) approach their respective boundaries. Experimental evaluation of the proposed control methods illustrates well damped Tj(k) and DeltaTj(k) responses and gradual adjustment of the manipulated inputs switching frequency and duty ratio. The paper also provides models to decouple the cross-coupling of manipulated inputs, allowing independent manipulation of conduction and switching losses.

2:20PM Online Junction Temperature Monitoring Using Turn-Off Delay Time for Silicon Carbide Power Devices [#1279]
Zheyu Zhang, Xuanlyu Wu, Fred Wang, Daniel Costinett, Leon Tolbert and Blalock Benjamin, the University of Tennessee, United States; Xi'an Jiaotong University, China

Junction temperature is a critical indicator for health condition monitoring of power devices. Concerning the reliability of emerging silicon carbide (SiC) power semiconductors due to immaturity of new material and packaging, junction temperature measurement becomes more significant and challenging, since SiC devices have low on-state resistance, fast switching speed, and high susceptibility to noise and parasitics in circuit implementations. This paper aims at developing a practical and cost-effective approach for online junction temperature monitoring of SiC devices using turn-off delay time as the thermo-sensitive electrical parameter (TSEP). The sensitivity is analyzed for fast switching SiC devices. A gate impedance regulation assist circuit is designed to improve the sensitivity by a factor of 60 and approach hundreds of ps per Celsius in the case study with little penalty of the power conversion performance. Also, an online monitoring system based on three gate assist circuits is developed to monitor the turn-off delay time in real time with the resolution within hundreds of ps. In the end, the micro-controller is capable of reading junction temperature during the converter operation with less than 0.5 Celsius measurement error. Two testing platforms for calibration and online junction temperature monitoring are constructed, and experimental results demonstrate the feasibility and accuracy of the proposed approach. Furthermore, the proposed gate assist circuits for sensitivity improvement and high resolution turn-off delay time measurement are transistor based and suitable for chip level integration.

2:45PM Simple Analog Detection of Turn-off Delay Time for IGBT Junction Temperature Estimation [#676]
Simon Weber, Michael Schlueter, Daniel Borowski and Axel Mertens, Leibniz University of Hanover, Germany

IGBT modules suffer from ageing due to thermal and power cycling. Bond wire lift-off or solder layer degradation are the known failure mechanisms. For condition monitoring, an estimation of the junction temperature during operation is necessary. For this purpose, an analog measurement board consisting of simple components is presented. The turn-off delay time is evaluated for temperature estimation. Moreover, a validation of the temperature estimation with an infrared camera is performed.

Wide Bandgap Applications: GaN

Wednesday, September 21, 1:30PM-3:10PM, Room: 202C, Chair: Filippo Chimento, Jean-Luc Schanen

1:30PM Design of a 10 kW GaN-based High Power Density Three Phase Inverter [#1059]
He Li, Xuan Zhang, Zhengda Zhang, Chengcheng Yao, Feng Qi, Boxue Hu, Liming Liu and Jin Wang, The Ohio State University, United States; ABB Corporate Research, United States

The medium power rating two-level three phase voltage source inverter is among the most popular power conversion systems. The typical switching frequency of the commercial medium power rating inverter, however, is limited to tens of kHz. By increasing the switching frequency and using emerging gallium-nitride devices, the size of the overall system can be greatly reduced. This paper begins by reviewing all commercially available GaN power transistors and their package technologies. The GS66516T device from GaN Systems is selected due to its suitable ratings and superior package performance. Then, a half-bridge structure is designed for this device to achieve low parasitic inductance and strong cooling capability at the same time. The dynamic characterization results of this 650V/60A Enhancement-mode GaN transistor are extracted with the proposed half-bridge structure. A gate drive circuit with comprehensive protection function is integrated. Based on the proposed phase-leg structure, a 10 kW three phase inverter prototype is built and the experimental waveform is shown at the end.

1:55PM High-frequency DC-DC Converter in Electric Vehicle Based on GaN Transistors [#1348]
Zhenjin Pang, Xiaoyong Ren, Junlin Xiang, Qianhong Chen, Xinbo Ruan and Wu Chen, Nanjing Univ. of Aeronautics and Astronautics, China; Southeast University, China

DC-DC converter in electric vehicle (EV) has higher demand for high efficiency and power density. Faster switching is beneficial to high power density. Gallium nitride (GaN) power transistors have the advantages of faster switching capability, smaller parasitic parameters and better electric parameters, which make them more suitable to achieve higher switching frequency and power density in DC-DC converter in EV. In this paper, the two-staged converter consisting of Buck and multi-phase interleaved half- bridge LLC series resonant converter (LLC-SRC) is chosen as the main topology. The current sharing problem caused by the mismatch between resonant components is analyzed and improved. Design consideration of driver of low-voltage GaN transistor and the efficiency improvement of high-voltage GaN transistor in LLC-SRC are analyzed. A 1MHz/2kW, 330V/12V DC-DC converter prototype is fabricated and efficiency comparison experiment with Si MOSFET is finished. Experimental results show that GaN power transistor has superior high frequency switching capability and efficiency advantage compared to Si MOSFET.

2:20PM A GaN-based Flying-Capacitor Multilevel Boost Converter for High Step-up Conversion [#1420]
Zitao Liao, Yutian Lei and Robert Pilawa-Podgurski, University of Illinois, United States

Compact high step-up DC/DC conversion to provide high DC voltage has numerous applications. A conventional boost converter has many limitations with regard to achieving high voltage gain with high power density and efficiency. The flying-capacitor multilevel (FCML) converter topology has many inherent advantages to overcome many of the limitations of the conventional boost converter. In this work, a GaN-based 7-level flying capacitor multilevel boost converter prototype has been implemented and achieved 100 V to 914 V conversion at 750 W output power with 92.7% peak efficiency, and 200 V to 966 V conversion at 900 W output power with 93.7% peak efficiency within the tested load range.

2:45PM A GaN based High Frequency Active-clamp Buck Converter for Automotive Applications [#1114]
Chenhao Nan, Raja Ayyanar and Youhao Xi, Arizona State University, United States; Texas Instruments Inc., United States

Automotive point-of-load (POL) dc-dc converters have wide input voltage range and strict EMI limitation requirements. In order to avoid the AM band interference, the switching frequency is desired to be above 2 MHz. The buck converter is widely used for this application but it has low efficiency at high switching frequency and poor EMI performance due to high dv/dt slew rate. A new active-clamp buck converter is employed in this paper, which features soft-switching for all switches, and thereby high efficiency and low EMI emission. The converter analysis, design considerations are presented. The converter is implemented with GaN FETs, due to its lower Figure-of-Merit (FOM), to further improve the efficiency. The implementation issues with GaN FETs are addressed. Then the experimental results with a 2.2 MHz prototype demonstrate superior performance of this converter.

Applications of Droop Control

Wednesday, September 21, 1:30PM-3:10PM, Room: 203AB, Chair: Tsorng-Juu Liang, Keyue Smedley

1:30PM Energy Storage Size and Fuel Consumption Reduction in a Microgrid Using Virtual Droop Control Framework [#1110]
Ashish Solanki and Adel Nasiri, SandC Electric, United States; UW-Milwaukee, United States

Microgrid can provide higher reliability, resiliency, sustainability, energy security and surety. Even though there are greater benefits, maintaining stability of microgrid is a major challenge especially with high penetration of renewables into microgrid. The existing approaches to address the problem of stability is to add large energy storage and increase generation capacity. In this paper, energy storage reduction and fuel minimization in a microgrid using Virtual Droop Control (VDC) has been proposed to deal with the problem of stability. The size selection of energy storage system for microgrid to ensure proper operation during microgrid transition has been discussed in this paper. The simulation results has been presented for 24 hours load, wind and solar PV radiation profile for the studied Fort Sill microgrid system using VDC controls. The energy profile for energy storage systems and natural gas generators and fuel consumption of natural gas generators have been presented for standard droop control method and compared with VDC controls.

1:55PM Seamless Black Start and Reconnection of LCL-filtered Solid State Transformer Based On Droop Control [#1397]
Yonghwan Cho, Yongsu Han, Richard Byron Beddingfield, Jung-Ik Ha and Subhashish Bhattacharya, North Carolina State University, United States; Seoul National University, Korea (South)

The solid state transformer (SST) is an emerging technology that can replace conventional passive transformers and actively manage renewable energy resources, energy storage devices, and loads. In this paper, a seamless black start control strategy is proposed for an SST-based smart grid system that has fault ride-through capability when it is islanded from the grid. Also, a method is developed to achieve smooth reconnection to the grid after a fault is cleared. The main component of the proposed control strategy is control of the high-voltage side converter of the SST (HV SST), which is based on a combination of droop control and an LCL filter. A single-loop controller for the capacitor voltage of the LCL filter is proposed, and simple criteria for setting compensator gains are provided. A low-voltage scaled SST system is introduced, and the controllers of the converters within the system are described. The proposed control strategy has been tested in simulation and experimentally on a low-voltage scaled testbed.

2:20PM A Circulating Current Suppression Method for Parallel Connected Voltage-Source-Inverters (VSI) with Common DC and AC Buses [#197]
Baoze Wei, Xiaoqiang Guo, Josep M. Guerrero and Juan C. Vasquez, Aalborg University, Denmark; Yanshan University, China

This paper describes a theoretical with experiment study on a control strategy for the parallel operation of three-phase voltage source inverters (VSI), to be applied to uninterruptible power systems (UPS). A circulating current suppression strategy for parallel VSIs is proposed in this paper based on circulating current control loops used to modify the reference currents by compensating the error currents among parallel inverters. Both of the cross and zero-sequence circulating currents are considered. The proposed method is coordinated together with droop and virtual impedance control. In this paper, droop control is used to generate the reference voltage of each inverter, and the virtual impedance is used to fix the output impedance of the inverters. In addition, a secondary control is used in order to recover the voltage deviation caused by the virtual impedance. And the auxiliary current control loop is added to acquire a better average current sharing performance among parallel VSIs, which can effectively suppress both of the cross and zero-sequence circulating currents. Experimental results are presented in order to verify the effectiveness of the proposed control strategy.

2:45PM Decentralized Method for Load Sharing and Power Management in a Hybrid Single/Three-Phase Islanded Microgrid Consisting of Hybrid Source PV/Battery Units [#1053]
Yaser Karimi, Josep M. Guerrero and Hashem Oraee, Sharif University of Technology, Iran; Aalborg University, Denmark

This paper proposes a new decentralized power management and load sharing method for a photovoltaic based, hybrid single/three-phase islanded microgrid consisting of various PV units, battery units and hybrid PV/battery units. The proposed method takes into account the available PV power and battery conditions of the units to share the load among them and power flow among different phases is performed automatically through three-phase units. Modified active power- frequency droop functions are used according to operating states of each unit and the frequency level is used as trigger for switching between the states. Efficacy of the proposed method in different load, PV generation and battery conditions is validated experimentally in a microgrid lab prototype consisted of one three- phase unit and two single-phase units.

DC Microgrids III

Wednesday, September 21, 1:30PM-3:10PM, Room: 203DE, Chair: Giovanna Oriti, Tsorng-Juu Liang

1:30PM A New Secondary Control Approach for Voltage Regulation in DC Microgrids [#790]
Saeed Peyghami-Akhuleh, Hossein Mokhtari, Pooya Davari, Poh Chiang Loh and Frede Blaabjerg, Sharif University of Technology, Iran; Aalborg University, Denmark

In this paper the effect of secondary controller on voltage regulation in dc Micro-Grids (MGs) is studied. Basically, centralized or decentralized secondary controller has been employed to regulate the voltage drop raised by the primary controller. However, in the case of high capacity MGs and long feeders with much voltage drop on the line resistances, the conventional methods may not guarantee the voltage regulation on the load busses. Therefore, in addition to compensate the voltage drop of the primary controller, it is necessary to regulate the voltage of critical loads. In this paper, a new voltage regulation strategy is proposed to regulate the voltage of MG by employing the average voltage of identified busses, which are determined by the proposed modal analysis. Numerical steady state analysis and preliminary simulation results validate effectiveness of the proposed scheme. Furthermore, experimental results with a scaled down laboratory prototype are performed to demonstrate the viability of the proposed approach.

1:55PM CERTS Microgrids with Photovoltaic Microsources and Feeder Flow Control [#1478]
Zhe Chen, Dinesh Pattabiraman, Robert H. Lasseter and Thomas M. Jahns, University of Wisconsin Madison, United States

A promising approach is proposed for extending the appealing features of the CERTS microgrid concept beyond its robust plug-and-play and autonomous control characteristics to include the integration of photovoltaic (PV) microsources with reduced volatility at the grid interface. The steady-state and transient characteristics of a proposed CERTS PV microgrid that incorporates a droop-controlled PV inverter source and a feeder flow-controlled microsource are explored under a variety of demanding conditions including time-variant changes in irradiance, microgrid islanding, and load change events. The proposed configuration exhibits well-behaved transient responses for a variety of events, as well as the ability to maximize the harvest of PV energy during all modes of operation. Simulations and experimental results from a CERTS microgrid testbed are provided to validate the proposed approach.

2:20PM Combined Optimization of SSCB Snubber and Freewheeling Path for Surgeless and Quick Bus Fault Interruption In Low-Voltage DC Microgrid [#365]
Wenjun Liu, Xiaoqi Xiong, Hua Yang, Kun Feng, Si Zhang and Fei Liu, Wuhan University, China

When a bus fault occurs in a low-voltage DC microgrid, solid-state circuit breakers (SSCB) on either end of the DC bus assume the responsibility of the isolation of the faulted section, while the freewheeling path attached between SSCB and the bus takes on the task of fault energy absorption and fault current damping. However, during fault interruption, the snubber attached in parallel with SSCB for overvoltage suppression will influence the fault energy absorption performance of freewheeling path, while the resistance of the freewheeling path will also influence the overvoltage suppression capability of the SSCB snubber. Hence, this paper analyzed the interaction between the two and proposed a combined design so as to optimize the fault interruption performance for minimum SSCB overvoltage, and shortest fault clearing time, with less cost and size. Preliminary experiments are conducted on a 400V/DC prototype, and the experimental results demonstrated the effectiveness of the design.

2:45PM Symmetric Droop Control for Improved Hybrid AC/DC Microgrid Transient Performance [#588]
Philip Hart, Robert Lasseter and Thomas Jahns, University of Wisconsin-Madison, United States

A droop-controlled, hybrid ac/dc microgrid represents a robust architecture that can coordinate the operation of multiple distributed sources while minimizing the power conversion stages. While the reduced-order nonlinear dynamics of grid-forming, droop-controlled inverter-based microgrids have been rigorously shown to be stable for a wide range of operating parameters, the problem of analyzing the transient stability characteristics of droop- controlled hybrid ac/dc architectures has not been adequately addressed. This work reviews the link between the Virtual Synchronous Machine control concept and grid-forming droop control, and introduces a new droop control strategy for ac/dc hybrid microgrids, termed Symmetric Droop Control (SDC). SDC better addresses the dynamic interactions between the ac and dc sub-grids of the hybrid microgrid, by ensuring that the interfacing inverter appropriately represents the dc sub-grid dynamics when interacting with other inverters. SDC can help to ensure that the reduced-order, nonlinear dynamics in both the ac and dc networks remain predictable and well-behaved during large disturbances in the ac network.

Wednesday, September 21, 3:30PM-5:10PM

Wind Energy Control and Operations

Wednesday, September 21, 3:30PM-5:10PM, Room: 203AB, Chair: Eduard Muljadi, Pedro Rodriguez

3:30PM Small Scale Reluctance Synchronous Generator Wind-Turbine System with DC Transmission Linked Inverters [#85]
Joshua Cole Mitchell, Maarten Jan Kamper and Christoph M. Hackl, Stellenbosch University, South Africa; Munich University, Germany

In this paper, an alternative layout for small scale back-to-back variable speed wind turbine systems is proposed. This novel layout is created by splitting the converters at their common DC bus and linking them through the wind turbine tower with a DC rated cable. The grid-tie voltage source converter (VSC) is placed at ground level and the generator VSC is placed in the nacelle. Not only does the DC transmission cable allow electrical power to be transferred with minimal copper losses, it allows the nacelle to be more compact and maintenance friendly. The control of a reluctance synchronous generator (RSG), variable speed, fixed pitch wind turbine and grid-connected system is detailed in this paper with specific attention to the control of the DC bus voltage with a DC-link cable. DC bus voltage dynamics are investigated with practical measurements on a 3 kW wind turbine emulator, RSG and LCL-filter grid-tie system. Power input steps at rated power are used to perturb the DC bus system for two different cable lengths. Two minutes of emulated wind speed using a 150 m cable length is also tested. The results show that the DC-link dynamics are controllable.

3:55PM Short-Term Forecasting of Inertial Response from a Wind Power Plant [#149]
Eduard Muljadi, Vahan Gevorgian and Anderson Hoke, National Renewable Energy Laboratory, United States

The total inertia stored in all rotating masses (synchronous generators, induction motors, etc.) connected to a power system grid is an essential force that keeps the system stable after disturbances. Power systems have been experiencing reduced inertia during the past few decades. This trend will continue as the level of renewable generation (e.g., wind and solar) increases. Wind power plants (WPPs) and other renewable power plants with power electronic interfaces are capable of delivering frequency response (both droop and/or inertial response) by a control action; thus, the reduction in available online inertia can be compensated by designing the plant control to include frequency response. The source of energy to be delivered as inertial response is determined by the type of generation (wind, photovoltaic, concentrating solar power, etc.) and the control strategy chosen. The importance of providing ancillary services to ensure frequency control within a power system is evidenced from many recent publications with different perspectives (manufacturer, system operator, regulator, etc.) This paper is intended to provide operators with a method for the real-time assessment of the available inertia of a WPP. This is critical to managing power system stability and the reserve margin. In many states, modern WPPs are required to provide ancillary services (e.g., frequency regulation via governor response and inertial response) to the grid. This paper describes the method of estimating the available inertia and the profile of the forecasted response from a WPP.