All SiC PWM rectifier‐based off‐board ultrafast charger for heavy electric vehicles
2019; Institution of Engineering and Technology; Volume: 13; Issue: 3 Linguagem: Inglês
10.1049/iet-pel.2019.0583
ISSN1755-4543
AutoresDoğan Yıldırım, Serkan Öztürk, I. Çadırcı, Muammer Ermiş,
Tópico(s)Multilevel Inverters and Converters
ResumoIET Power ElectronicsVolume 13, Issue 3 p. 483-494 Special Issue: WBG Semiconductor Power Electronics for Industrial and Automative ApplicationsFree Access All SiC PWM rectifier-based off-board ultrafast charger for heavy electric vehicles Doğan Yildirim, Doğan Yildirim Department of Electrical and Electronics Engineering, Middle East Technical University, METU Campus, Ankara, TurkeySearch for more papers by this authorSerkan Öztürk, Serkan Öztürk Department of Electrical and Electronics Engineering, Hacettepe University, Beytepe Campus, Ankara, TurkeySearch for more papers by this authorIşık Çadirci, Işık Çadirci Department of Electrical and Electronics Engineering, Hacettepe University, Beytepe Campus, Ankara, TurkeySearch for more papers by this authorMuammer Ermiş, Corresponding Author Muammer Ermiş ermis@metu.edu.tr Department of Electrical and Electronics Engineering, Middle East Technical University, METU Campus, Ankara, TurkeySearch for more papers by this author Doğan Yildirim, Doğan Yildirim Department of Electrical and Electronics Engineering, Middle East Technical University, METU Campus, Ankara, TurkeySearch for more papers by this authorSerkan Öztürk, Serkan Öztürk Department of Electrical and Electronics Engineering, Hacettepe University, Beytepe Campus, Ankara, TurkeySearch for more papers by this authorIşık Çadirci, Işık Çadirci Department of Electrical and Electronics Engineering, Hacettepe University, Beytepe Campus, Ankara, TurkeySearch for more papers by this authorMuammer Ermiş, Corresponding Author Muammer Ermiş ermis@metu.edu.tr Department of Electrical and Electronics Engineering, Middle East Technical University, METU Campus, Ankara, TurkeySearch for more papers by this author First published: 01 February 2020 https://doi.org/10.1049/iet-pel.2019.0583Citations: 2AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract This study deals with the design, implementation and experimental performance of all silicon carbide pulse width modulation (SiC PWM) rectifier-based off-board ultrafast chargers (UFCs) for lithium titanate batteries of heavy electric vehicles. Different UFC configurations are proposed, depending upon the nominal battery voltage (400–1000 V DC) and charge capacity. Operating principles and control of UFC at unity and leading power factors are assessed and corresponding operating modes of PWM rectifier are discussed. The combined effect of reverse conduction characteristic of SiC power metal oxide semiconductor field effect transistor (MOSFET) and built-in SiC Schottky diode is taken into account in all the analyses carried out. The operating performance of the developed UFC, such as the switching characteristics of SiC power MOSFET modules, efficiency, input current total demand distortion (ITDD), and thermal limitations of the SiC PWM rectifier have been assessed for various charge voltages and charge capacities, both by computer simulations and laboratory tests. Power circuit layout considerations of the proposed system are also given in this study. Excellent performance results for 10 kHz switching frequency are obtained from the developed 200 kW UFC, with operating efficiencies higher than 98.5% for all charging rates up to five times the battery capacity, and ITDDs <2.2% for the whole operating range. 1 Introduction Compared to classical silicon (Si) devices, the silicon carbide (SiC) power metal oxide semiconductor field effect transistor (MOSFET), as a wide bandgap device, enables more efficient and compact converters to save electric energy [1]. SiC power MOSFETs have been developed towards high-performance traction inverters over the past 20 years [1]. A traction system has been developed for high-speed trains by applying SiC devices for weight reduction and compactness [2]. The market of the SiC power components is forecast to be worth 1.4 billion USD in 2023, and adoption in automotive is a key trend for SiC over the next few years [3]. More than 20 automobile companies are now using the SiC components in main power inverters, as well as charging infrastructure [4]. A comprehensive review of on-board and off-board battery charger topologies, charging power levels, and infrastructure for plug-in electric and hybrid vehicles is given in [5]. The state-of-the-art and future trends for high-power on-board chargers for electric vehicles (EVs) are presented in [6]. Various Si power semiconductor-based, a few kilowatt (kW) on-board battery chargers are recommended in [7-9]. The experimental validation of an off-board, three-phase fast battery charger for EVs-based on a dual-stage power converter with Si insulated gate bipolar transistors (IGBTs) is presented in [10]. A 48 V/11 kW on-board battery charger made of a buck type SWISS rectifier and a dual active bridge DC/DC converter was realised with SiC power MOSFETs for the DC bus switches and Si IGBTs for the grid-side switches, aiming at high efficiency, high power density, and low cost [11]. The conventional half-bridge LLC topology is adopted in [12] to realise a 10-kW all SiC bidirectional charger used in EVs. An ultrafast charging station power architecture has been proposed in [13] for electric-driven buses. The ultrafast charging stations for EVs can provide a charging power up to 350 kW and charge 100 kWh batteries in 12° in the whole operating range. A sample switching pattern applied to high-side SiC power MOSFETs is given in Fig. 5a. Corresponding switching pattern applied to low-side SiC power MOSFETs is its complimentary. Sample gate signals are generated from the supply voltage waveforms, and during the operation at a switching frequency of fsw = 10 kHz pulse widths are continuously changing from one computation step to the next to create a purely sinusoidal fundamental current component in the steady-state. Fig. 5Open in figure viewerPowerPoint Current conduction paths of all SiC PWM Rectifier for 1.0 pf operation (a) Sample switching pattern applied to high-side SiC power MOSFETs, (b) Simulation instant marked on supply voltage and current waveforms, (c), (f) Charging, (d), (e) Rectification The simulation results obtained from PSIM Electronic Simulation Software for the time instant marked on Fig. 5b are given in Figs. 5c–f for 1.0 pf operation. Polarities of VR(t),VS(t),VT(t) are positive, positive and negative as marked on these figures, respectively. There are six possible consecutive cases for the polarities of the supply voltages and hence currents for 1.0 pf operation the duration of each is 60° over a full cycle. In each of these consecutive cases there are two charging and two rectification states. In the charging state boost reactors are charged from the AC supply (Figs. 5c and f), while in the rectification state active power is transferred from the AC supply to the DC link (Figs. 5d and e). In charging periods LTO battery pack is supplied only from the DC link capacitor (Figs. 5c and f as marked by red coloured current path). Sample simulation results for operation at 0.8 pf leading are as given in Fig. 6. The basic difference between leading pf operation and 1.0 pf or lagging pf operation is the presence of very short periods in which power is regenerated from the DC link capacitor to the AC supply. This operation state is given in Fig. 6d and the corresponding current path is marked by blue colour. Fig. 6Open in figure viewerPowerPoint Current conduction paths of all SiC PWM Rectifier for 0.8 pf leading operation (a) Sample switching pattern applied to high-side SiC powerMOSFETs, (b) Simulation instant markedon supply voltage and current waveforms, (c),(f) Charging,(d) Regeneration,(e) Rectification 3 Design and implementation 3.1 Design of all SiC PWM rectifier as a UFC The maximum operating virtual junction temperature, Tvj, of the chosen half-bridge modules is specified to be 150°C by the manufacturer. However, in the design of the all SiC PWM rectifier as a UFC, max Tvj is limited to 120°C by considering weaknesses of bonding within the chip, and for longer life expectancy. To illustrate the effects of max Tvj on the output current capacity Idc(max), of the PWM rectifier in Fig. 1a, the variations in Idc(max) are calculated as a function of output voltage Vdc at fsw = 10 kHz by using the Wolfspeed SpeedFit simulation software and given in Fig. 7. The safe operating area of the PWM rectifier is also marked on Fig. 7. The corresponding variations in input power, Pin, and output power, Pout, are also plotted as a function of Vdc, as given in Fig. 8. Following conclusions can be drawn from Figs. 7 and 8. The maximum power that can be delivered by the PWM rectifier depends on nominal voltage of LTO battery pack and hence on Vdc. Fig. 7Open in figure viewerPowerPoint Output current versus output voltage of all SiC PWM rectifier at Tvj = 120°C (green curve) and Tvj = 150°C (red curve) Fig. 8Open in figure viewerPowerPoint Output current (red), input power (green) and output power (blue) versus output voltage of all SiC PWM rectifier at Tvj = 120°C Since switching loss increases significantly with higher drain-to-source voltages, VDS, of SiC power MOSFETs, output power capacity decreases with increasing Vdc. Since Pout is very close to Pin, PWM rectifier operates at very high-efficiency values for various battery pack configurations. All SiC power MOSFET modules can be operated at switching frequencies higher than the chosen fsw of 10 kHz. To obtain the effects of fsw on the output power of the PWM rectifier, and corresponding efficiencies against nominal voltage of the LTO battery pack, characteristics in Figs. 9 and 10 are obtained from Wolfspeed SpeedFit simulation software. As can be observed from these characteristics, fsw = 10 kHz gives higher output power capacity and efficiency. On the other hand, the use of SiC power MOSFET modules and switching them at relatively high frequencies to process high electrical powers cause smaller size and weight power converters in comparison with all Si- or Hybrid-IGBT based converters. Fig. 9Open in figure viewerPowerPoint Output power versus output voltage for different switching frequencies Fig. 10Open in figure viewerPowerPoint Efficiency versus output voltage for different switching frequencies 3.2 Implementation The self-designed and -constructed SiC PWM rectifier and its power stage are as shown in Figs. 11 and 12, respectively. Major components of the power stage are top-to-top mounted as illustrated in Fig. 12. All system elements are placed in a steel cabinet for laboratory tests (Fig. 11). As can be observed from Fig. 12, increasing fsw from 10 to 20 kHz makes a marginal contribution in reducing the size and weight for the same SiC power modules at the expense of lower output power capacity and efficiency with a substantial increase in the capacity of cooling fan. Therefore in this research work, 10 kHz is chosen as the optimum value of fsw. Fig. 11Open in figure viewerPowerPoint General view of the developed all SiC PWM Rectifier Fig. 12Open in figure viewerPowerPoint Power stage of PWM converter (a) Top layer, (b) 3rd layer, (c) 2nd layer, (d) Bottom layer of PWM rectifier 3.3 Controller In this research work, a MATLAB-based master control system is recommended as illustrated in Fig. 13. The master control unit (MCU) communicates between the LTO battery pack and PWM rectifier via USB/CAN converter. Communication between LTO battery pack and UFC is bidirectional. In a practical application, master controller can be implemented on an industrial PC. Fig. 13Open in figure viewerPowerPoint Block diagram of communication system and MCU The battery current, voltage, and state-of-charge (SOC) are internally measured and transferred to the MCU via CAN communication interface by means of the embedded battery management system. According to the set values coming from MCU, battery charge voltage, and current patterns are generated. All measurement data obtained from transducers are then transferred to the MCU via CANopen communication protocol. By communicating with the LTO battery pack, it controls the operation of main line contactor, soft-start circuitry, and auxiliary equipment. The DSP-based control system of SiC PWM rectifier is given in Fig. 14, where SVPWM modulation technique is employed to synthesise the three-phase input current waveforms synchronised to the supply frequency, with minimised current total demand distortion (ITDD). It is also suitable for SPWM technique. In PWM rectifier implementation, Id controls the phase angle between the voltage and the current, and therefore the reactive power, while Iq, the active power. To operate the PWM rectifier at unity power factor, Id should be set to zero. The control system is composed of two cascaded control loops, the outer loop generates the set value of battery charge voltage, Vdc(ref) according to the Idc(ref) coming from MCU and measured battery voltage, Vbat. The inner loop, however, generates Iq(ref) for active power control, while Id(ref) value is set by the MCU according to the reactive power demand. Fig. 14Open in figure viewerPowerPoint Block diagram of DSP-based controller for SiC PWM rectifier 4 Experimental results The performance of the PWM rectifier in Figs. 1a, 11 and 12 as an UFC are obtained by laboratory tests for a 46 Ah LTO battery pack having a nominal voltage in the range from 600 to 1000 V. From these results, agreed power between the Electricity Distribution System Operator (DSO) and the owner of UFC station (containing only one PWM rectifier), efficiency, ITDD, and some technical operating characteristics can be determined. 4.1 Connection agreement between DSO and UFC station owner To determine the agreed power to be specified in the Connection Agreement executed between DSO and UFC station owner the variations in input power, Pin and input current, Iin characteristics against the nominal value of LTO battery pack voltage are obtained as shown in Fig. 15. These results correspond to calculated Tvj in the range of 100–110°C above Ta = 45°C. The variations in output power, Pout and output current, Iout against output voltage, Vdc are also obtained as given in Fig. 16 for the same test conditions to calculate efficiency variations of the LTO charger. Fig. 15Open in figure viewerPowerPoint Input current and input power against output voltage (at Tvj = 110°C for a 400 V, 50 Hz grid) Fig. 16Open in figure viewerPowerPoint Output current and output power against output voltage (at Tvj = 110°C for a 400 V, 50 Hz grid) Fig. 15 shows that maximum power demand of the UFC from LV bus of the distribution system at 1.0 pf varies in the range nearly from 200 to 150 kW. Therefore, the agreed power between DSO and UFC station can be chosen to be 200 kW at 1.0 pf which corresponds to 200 kVA and maximum input current of Iin = 291 A rms. An ultrafast charging station with multiple chargers should be supplied from an MV bus of the distribution system, e.g. 34.5 kV, 50 Hz. 4.2 Efficiency The percentage efficiency variations against output voltage for fsw = 10 kHz are determined from the experimental data shown in Figs. 15 and 16. Input and output powers are calculated from the voltage and current measurements carried out on the supply and DC link sides. Rogowski CWTUM/3/B current probe and Tektronix P5205A high voltage differential probe are used on the AC side. Tektronix TCP404XL current probe with Tektronix TCPA300 current probe amplifier and Tektronix P5205A high voltage differential probe are used on the DC side. Input power consumed from the supply excludes only the power consumption of the axial cooling fan in efficiency calculations. These values are given in Table 2, in comparison with theoretical efficiency values obtained from Wolfspeed SpeedFit simulation software. It can then be concluded that experimental efficiencies are slightly lower than but very close to theoretical values. Table 2. Efficiency values for the operating points in Figs. 15 and 16 Efficiency, % Output voltage, Vdc 600 750 800 900 1000 experimental 99.02 98.90 98.87 98.76 98.68 theoretical 99.22 99.08 99.05 99.03 98.99 The operating points in Figs. 15 and 16 and hence in Table 2 correspond to charging rates varying in the range from 7 to 3 C for a 46 Ah LTO battery pack. However, in practice, it is not recommended to charge the 46 Ah LTO battery pack at a rate higher than 5 C for a longer life expectancy. On
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