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

Battery Management for Transportation Electrification I

Thursday, September 22, 8:30AM-10:10AM, Room: 102D, Chair: Yilmaz Sozer, Berker Bilgin

8:30AM A Pack-to-Cell-to-Pack Battery Equalizer with Soft-Switching Based on Buck-Boost and Bidirectional LC Resonant Converters [#680]
Zeyuan Li, Yunlong Shang, Bin Duan and Chenghui Zhang, Shandong University, China

A pack-to-cell-to-pack battery equalizer with soft-switching based on buck-boost converter and bidirectional LC resonant converter is proposed to achieve a fast and high-efficiency battery equalization. The buck-boost converter is employed to transmit energy from a high tension supply to a low one in the buck mode or from a low one to a high one in the boost mode with zero-voltage switching (ZVS). The bidirectional LC resonant converter is employed to automatically transfer energy with zero-current switching (ZCS) with electrical isolation. A prototype with four lithium-ion battery cells is implemented. Experimental results show the proposed scheme exhibits outstanding balancing performance, and the measured peak conversion efficiency is about 93% in the cell-to-pack mode and 72.5% in the pack-to-cell mode.

8:55AM A New Perspective on Battery Cell Balancing: Thermal Balancing and Relative Temperature Control [#689]
Ye Li and Yehui Han, University of Wisconsin-Madison, United States

This paper introduces a new perspective on cell balancing (equalization) in a lithium-ion battery pack by proposing the thermal balancing concept of battery cells. Thermal balancing can be achieved by relative temperature control based on either conventional active cell balancing circuits or module-integrated systems. Compared to voltage/state-of-charge (SoC) balancing, relative temperature can be used as the balancing target with several advantages. First, absolute temperature directly affects the lifetime and usable capacity of battery cell. Second, the relative temperature between cells degrades the uniformity of the cells and increase the imbalance in various parameters. Third, temperature usually has a slow dynamic which makes it an ideal control target for low balancing power compared to the high charging/discharging power. Last, thermal balancing control can take over the job of the thermal management system to a certain degree. Therefore the balancing function could be potentially built into the thermal management system. In this paper, two algorithms are developed to achieve thermal balancing: direct relative temperature control and virtual heat sink temperature control. A simple battery system thermal model is developed and the proposed control algorithms are validated.

9:20AM Advanced Cell-level Control for Extending Electric Vehicle Battery Pack Lifetime [#1451]
Muhammad Muneeb Ur Rehman, Fan Zhang, Michael Evzelman, Regan Zane, Kandler Smith and Dragan Maksimovic, Utah State University, United States; University of Colorado, United States; National Renewable Energy Laboratory, United States

A cell-level control approach for electric vehicle battery packs is presented that enhances traditional battery balancing goals to not only provide cell balancing but also achieve significant pack lifetime extension. These goals are achieved by applying a new life-prognostic based control algorithm that biases individual cells differently based on their state of charge, capacity and internal resistance. The proposed life control approach reduces growth in capacity mismatch typically seen in large battery packs over life while optimizing usable energy of the pack. The result is a longer lifetime of the overall pack and a more homogeneous distribution of cell capacities at the end of the first life for vehicle applications. Active cell balancing circuits and associated algorithms are used to accomplish the cell-level life extension objectives. This paper presents details of the cell-level control approach, selection and design of the active balancing system, and low-complexity state-of-charge, capacity, and series resistance estimation algorithms. A laboratory prototype is used to demonstrate the proposed control approach. The prototype consists of twenty-one 25 Ah Panasonic lithium-Ion NMC battery cells from a commercial electric vehicle and an integrated BMS/DC-DC system that provides 750 W to the vehicle low voltage auxiliary loads.

9:45AM A Battery Cell Balancing Control Scheme with Minimum Charge Transfer [#296]
Zhiyuan Shen, Handong Gui and Leon Tolbert, Silergy Corp., China; The University of Tennessee, Knoxville, United States

According to the analysis of the imbalance in series-connected lithium-ion batteries, the state-of-health (SOH) and the energy utilization is mainly influenced when the cells are fully charged or discharged. Therefore, balancing is not necessarily required all the time. In addition, the energy loss of the balancing system is closely related to the charge transfer among different cells. This paper proposes an active balancing control scheme that utilizes the required charge of each cell as the balancing reference. With the scheme, the charge transfer during the balancing process can be minimized so that the energy loss can be reduced and the balance can be achieved when the cells are fully charged or discharged, which not only improves the battery lifetime but also increases the available energy. The proposed control is applied in a balancing system based on the direct cell-to-cell architecture and bi-directional buck/boost converter. Experimental results have verified the effectiveness of the proposed control.

Grid Connected Single-Phase Inverters

Thursday, September 22, 8:30AM-10:10AM, Room: 202A, Chair: Mahshid Amirabadi, Fernando Briz

8:30AM Double Line Frequency Ripple Cancelling for Single-Phase Quasi-Z-Source Inverter [#842]
Yuan Li, Wenqiang Gao, Jiayi Li, Rui Zhang and Fan Fang, Sichuan University, China

Double line frequency ripple power inherently exists in single phase dc-ac or ac-dc pulse width modulation converters because of instantaneous power unbalance between the dc and ac side. In order to achieve stable operation and maintain a high maximum power point tracking (MPPT) efficiency, traditional single-phase quasi-Z-source inverter (QZSI) for photovoltaic application uses large electrolytic dc capacitors in the impedance network to suppress this double line frequency ripple. This paper proposed an active power decoupling method to cancel the ripple in both the input and impedance network. By adding another phase leg, an ac capacitor and a high frequency inductor in series, the second ac power path is developed to absorb the power pulsating at twice the line frequency. With selected capacitance and closed- loop control for the voltage on the added ac capacitor, the ripple would be eliminated with the minimum costs. The other ac output of the QZSI would not be influenced because the original two phase legs (the original H-bridge) maintains the same operation. The merits of the presented method includes 1) reduced device size by replacing bulky dc capacitor to small ac capacitor; 2) ripple free input to achieve high MPPT efficiency. Theoretical analysis, control strategy, simulation and experimental results are provided to demonstrate and verify the method.

8:55AM Hybrid control scheme for the current loop of a grid connected inverter operating with highly distorted grid voltage [#992]
Julio Cesar Viola, Jose Restrepo, Jose Manuel Aller and Flavio Quizhpi, Universidad Politecnica Salesiana/Prometeo Proj., Ecuador; Universidad Simon Bolivar, Venezuela; Universidad Simon Bolivar/Prometeo Project, Venezuela; Universidad Politecnica Salesiana, Ecuador

The complete design of the current control loop for single-phase inverters connected to weak grids is presented. The problems associated to the design of current control loop when highly distorted grid voltages are present and LCL filters are used to couple the inverters to the grid, are analyzed. Fourier coefficients decomposition is used to obtain an ahead-of-time version for the nonideal voltage signal which allows the compensation of the current control loop delay. Also the relevance of nonideal characteristic of the inverter as the deadtime effect on the shape of the controlled current is analyzed and compensated. Simulations and experimental results are included for the proposed methods.

9:20AM Single-Phase LLCL-Filter-based Grid-Tied Inverter with Low-Pass Filter Based Capacitor Current Feedback Active damper [#1343]
Liu Yuan, Wu Weimin, He Yuanbin, Chung Shu-Hung and Blaabjerg Frede, Shanghai Maritime Univ., China; City Univ. of Hong Kong, Hong Kong; Aalborg Univ., Denmark

The capacitor-current-feedback active damping method is attractive for high- order-filter-based high power grid-tied inverter when the grid impedance varies within a wide range. In order to improve the system control bandwidth and attenuate the high order grid background harmonics by using the quasi-PR control method, the digital delay is preferred to be reduced as much as possible. However, when the time between the sampling point and the updating point is reduced, the disturbance caused by the switching noises will be easily introduced into the sampling signals, resulting in possible disability. In this paper, a low pass filter is proposed to be inserted in the capacitor current feedback loop of LLCL-filter based grid-tied inverter together with a digital proportional and differential compensator. The detailed theoretical analysis is given. For verification, simulations on a 2kW/220V/10kHz LLCL-filter-based grid- tied inverter have been presented.

9:45AM A single-phase tri-state integrated Buck-Boost inverter suitable to operate in grid-connected and island modes [#1076]
Jose Carlos Pena, Cindy Paola Guzman and Carlos Alberto Canesin, Universidade Estadual Paulista, Peru; Universidade Estadual Paulista, Colombia; Universidade Estadual Paulista, Brazil

This paper introduces a methodology to model and control a single-phase tri state integrated Buck-Boost inverter suitable for low power distributed generation with the capability to operate in grid connected and island modes. The tri-state modulation is adopted in order to achieve power decoupling by means of average inductor current. This way it is possible to control the DC/AC conversion independently of the input current. The proposed strategy considers a multiple loop control scheme, where controllers are tuned based on a linearized models of the system. Main experimental results from a 300W prototype are included and discussed.

Modular Multilevel Converters (MMC) I

Thursday, September 22, 8:30AM-10:10AM, Room: 202B, Chair: Jiangchao Qin, Wim van der Merwe

8:30AM DC Fault Ride Through of Multilevel Converters [#1019]
Geraint Chaffey, Paul Judge, Michael Merlin, Philip Clemow and Tim Green, Imperial College London, United Kingdom

Modular Multilevel Converters (MMC) can provide significant advantages for power transmission applications, however there are remaining challenges trading off DC fault response, losses and controllability. Alternative multilevel converter topologies using combinations of full bridge and half bridge submodules or series switches allow for competitive efficiency whilst retaining control over the DC fault current. Several possible converter fault responses are analysed to evaluate appropriate converter control actions. Experimental results from a 60 submodule 15 kW demonstrator are presented to validate the DC fault performance of the full bridge MMC, the mixed stack MMC and the alternate arm converter. It is shown that each can control the current into a low impedance DC fault, and there no requirement to block the semiconductor devices.

8:55AM Reverse Blocking Sub-Module Based Modular Multilevel Converter with DC Fault Ride-Through Capability [#118]
Xiaofeng Yang, Yao Xue, Bowei Chen, Zhiqin Lin, Yajie Mu, Trillion Q. Zheng and Seiki Igarshi, Beijing Jiaotong University, China; Fuji Electric Co., Ltd., Japan

Modular multilevel converter (MMC) is one of the promising voltage source converter topologies in the field of high voltage direct current (HVDC) transmission system. Based on analysis of the existing sub-module topologies, an improved half-bridge sub-module topology based on reverse blocking IGBTs (RB-HBSM) was proposed for solving the fault ride through issues. In this paper, the fault current blocking mechanism and the electrical stress of the main power switches are studied firstly in detail. Then the fault control flow chart is presented. Feasibility of the proposed sub-module topology and the fault protection theory are verified by simulation. The proposed RB-HBSM-MMC topology is able to block the fault current without changing the original control and modulation strategies compared with the conventional MMC, thus it further reduces the complicity of industry design.

9:20AM Closed-loop Control of the DC-DC Modular Multilevel Converter [#671]
Heng Yang and Maryam Saeedifard, Georgia Institute of Technology, United States

The DC-DC Modular Multilevel Converter (MMC), which has originated from the AC- DC MMC circuit topology, is an attractive converter topology for interconnection of medium-/high-voltage DC grids. Proper operation of the DC-DC MMC necessitates injection of an AC circulating current to maintain its submodule (SM) capacitor voltages balanced. The AC circulating current, however, needs to be minimized for efficiency improvement. In addition, a unique type of imbalance amongst the SM capacitor voltages that is caused by DC power flow needs to be mitigated. This paper proposes a closed-loop control strategy for the DC-DC MMC to simultaneously regulate the DC-link currents, maintain the SM capacitor voltages balanced and minimize the AC circulating current. Performance and effectiveness of the proposed control strategy are evaluated based on simulation studies in the Matlab Simulink and experimentally verified on a laboratory prototype.

9:45AM New MMC Capacitor Voltage Balancing using Sorting-less Strategy in Nearest Level Control [#996]
Mattia Ricco, Laszlo Mathe and Remus Teodorescu, Aalborg University, Denmark

This paper proposes a new strategy for balancing the Capacitor Voltages (CVs) for Modular Multilevel Converters (MMCs). The balancing is one of the main challenges in MMC applications and it is usually solved by adopting a global arm control approach. For performing such an approach, a sorted list of the SubModules (SMs) according to their capacitor voltages is required. A common way to accomplish this task is to implement a sorting algorithm in the same controller used for the modulation technique. However, the execution time and the computational efforts of these kinds of algorithms increase very rapidly when the number of SMs grows. A novel idea is presented in this paper by using a mapping strategy that directly stores in a ranked list the SMs according to the measured CVs. Avoiding the use of sorting algorithms leads to a considerable reduction of the execution time as well as the computational efforts.

DC-DC Isolated: LLC

Thursday, September 22, 8:30AM-10:10AM, Room: 102C, Chair: Jason Stauth, Vladimir Blasko

8:30AM A New Tightly Regulated Dual Output LLC Resonant Converter with PFM plus Phase-shift Control [#216]
Xun Gao, Hongfei Wu, Yan Xing, Haibing Hu and Yu Zhang, Nanjing Univ. of Aeronautics and Astronautics, China; Shanghai Institute of Space Power-sources, China

A new dual-output LLC resonant converter is presented for high power density and high efficiency dual-output application. A new control freedom is provided by replacing two diodes in the secondary-side rectifier with two MOSFETs. Variable frequency modulation plus secondary side phase-shift modulation scheme is adopted to realize precise regulation of both the two outputs simultaneously. Meanwhile, zero-voltage switching and zero-current switching can be ensured for all the MOSFETs and diodes, respectively, to reduce switching losses. The operational principles, characteristics, control strategies, and design considerations are analyzed in detail. Experimental results with a 420W-rated dual-output prototype are provided to verify the effectiveness and advantages of the presented converter.

8:55AM Analytical Model for LLC Resonant Converter With Variable Duty-Cycle Control [#779]
Yanfeng Shen, Huai Wang, Frede Blaabjerg, Xiaofeng Sun and Xiaohua Li, Aalborg University, Denmark; Yanshan University, China

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.

9:20AM Three-Phase LLC Resonant Converter with Integrated Magnetics [#1473]
Wilmar Martinez, Noah Mostafa, Yuki Itoh, Masayoshi Yamamoto, Jun Imaoka, Kazuhiro Umetani, Kimura Shota, Nanamori Kimihiro and Endo Shun, Shimane University, Japan; Kyushu University, Japan; Okayama University, Japan

Recently, Electric Vehicles (EVs) have required high power density and high efficiency systems in order to save energy and costs. Specifically, in the DC-DC converter that feeds the non-propulsive loads in these vehicles, where the output voltage is much lower than the one of the energy storage unit. Therefore, the output current becomes quite high, and the efficiency and power density are reduced due to the high current ratings. Furthermore, magnetic components usually are the biggest contributors to the mass and volume in these converters. This paper proposes a Three-phase LLC resonant converter with one integrated transformer where all the windings of the three independent transformers are installed into only one core. Using this technique, a high reduction in the core size and thereby an increment in the power density and a reduction of the production cost are obtained. In addition, this integrated transformer is intended to be applied in the novel Three-phase LLC resonant converter with Star connection that is expected to offer reduction of the imbalanced output current, which is produced by tolerances between the phase components. Finally, the proposed converter with the novel integrated transformer is discussed and evaluated from the experimental point of view. As a result, a 70% reduction in the mass of the magnetic cores was achieved.

9:45AM Accurate ZVS Boundary in High Switching Frequency LLC Converter [#1546]
Ren Ren, Liu Bo, Jones Edward Andrew, Wang Fred, Costinett Daniel Jes and Zhang Zheyu, the University of Tennessee, United States

Due to the realization of zero voltage switching (ZVS) under the full load range, LLC resonant converter is widely adopted in the telecom, battery charger and several applications, characterized with high efficiency, high frequency and high power density, to realize DC conversion. Recently, by using Gallium Nitride (GaN) HFETS, switching frequency of LLC converters is further increased. However, ZVS failure cannot be predicted accurately in the high switching frequency condition by only considering traditional constraints generally applied in the low frequency design. The traditional constraints result in a too optimistic estimation of the dead time to obtain ZVS without considering the reverse resonance under the dead time and the design of resonant parameters at high resonant frequency and high load condition. The experiment shows the LLC converter loses ZVS even through the converter satisfies the ZVS constraints proposed by previous paper. In this paper, the failure mode will be investigated in detail and an accurate ZVS boundary is proposed for high frequency LLC converter design. The proposed theory was verified on a 1 MHz, 1500 W LLC prototype.

Modeling and Control of Grid Connected Converter I

Thursday, September 22, 8:30AM-10:10AM, Room: 202D, Chair: Sung Yeul Park, Fernando Briz

8:30AM A Unified Control of Back-to-Back Converter [#481]
Alberto Rodriguez-Cabero, Francisco Huerta Sanchez and Milan Prodanovic, IMDEA Energy Institute, Spain

In this paper a novel unified modelling and control procedure for Back-to- Back converters is introduced. The proposed procedure captures in a single state-space model detailed converter dynamics of both LCL filters and DC- capacitor. A linear multivariable approach has been identified as the most suitable control strategy for taking full advantage of the unified model. A state-feedback controller has been designed for controlling both inverter output currents and DC-bus voltage. It has been shown the main advantage of the proposed modelling procedure is in its detailed knowledge of the existing coupling between the AC currents and the DC-capacitor voltage. As a result, the proposed unified controller takes into account the whole system dynamics in order to maintain the DC-bus voltage nearly constant. The proposed control strategy was experimentally validated by using two 15kVA VSCs (Voltage Sourced Converters) in Back-to-Back configuration. The performance of the unified controller was compared with the performance of the conventional strategy based on the cascade control structure. It was experimentally demonstrated the proposed unified control significantly outperforms the conventional control of DC-bus voltage.

8:55AM Control of an Islanded Power-Electronic Converter as an Oscillator [#688]
Ricardo Perez, Cesar Silva and Amirnaser Yazdani, Universidad Tecnica Federico Santa Maria, Chile; Ryerson Universiy, Canada

This paper proposes the use of the feedback-linearization control technique for the regulation of the voltage amplitude and frequency of power-electronic converters. This control strategy linearize the trajectory of a nonlinear system between any two equilibrium points, and thus allowing to use linear control technique. The reference tracking robustness of the proposed control technique is assessed under linear and nonlinear loads, in addition to bidirectional power flow, through simulations.

9:20AM Power control for Grid-connected Converter to Comply with Safety Operation Limits during Grid Faults [#932]
Shida Gu, Xiong Du, Ying Shi, Yue Wu, Pengju Sun and Heng-Ming Tai, Chongqing University, China; University of Tulsa, United States

This paper presents a new power control strategy of grid-connected converter under grid fault conditions. Positive- and negative-sequence reactive power are controlled separately in order to effectively support unbalanced grid voltage. Both maximum phase current limit and maximum active power oscillation limit are taken into consideration which makes the converter operate safely under grid faults. Spare capacity are fully used to generate active power. Reactive power is generated immediately and active power is generate gradually during calculation procedure which makes it possible for online application. Also, generating active power gradually lighten the burden of crowbar or other energy transfer devices and bring less impacts to grid. Experiment results prove that the proposed method is effective.