A Furtherance of High-Power Adjustable Speed Drive System

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ This is an Accepted Manuscript of an article published IEEE Industrial Electronics Magazine available at: 10.1109/MIE.2022.3218915 “© 2022 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other Works” Medium-Frequency AC Link Powered Machine Drive Systems ABSTRACT The high-power machine drive system is the power heart of the modern power industry, which has been used in a broad range of industry applications. Extensive research and development efforts have been carried out on the design and implementation of high-power machine drive systems. In this article, various converter topologies and research progress for high-power machine drive systems are reviewed. And a novel medium-frequency ac link powered machine drive system is analyzed and discussed for a furtherance of high-power adjustable speed drive systems. In addition, some prospects for future research are highlighted to provide a feasible reference for practitioners. I. INTRODUCTION Promoted by economies of scale and more cost-effective industrial processes, high-power machine drive systems are increasingly important for many power industry fields such as power generation, metallurgy, petrochemical, coal, rail transit, and electric ships. The pursuit of drive systems capacity often boosts the requirements for the voltage levels of the drive systems, which makes the traditional two-level low-voltage converters unusable [1]. Generally, higher voltage in high-power drive systems means lower rated current, smaller cables, smaller dc-link energy storage components and higher efficiency. The high-power machine drives have covered power ratings from 0.4 MW to more than A Furtherance of High-Power Adjustable Speed Drive Systems This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. 100 MW at the medium- or high-voltage level of 2.3 kV to 13.8 kV [2]-[4]. In order to meet the corresponding industrial demand and energy constraints, the utilization of optimized or advanced highperformance power electronic converters has been the primary consideration. In high-power machine drive systems, the performance of high-power power electronic converters often significantly determines the operating effects of the entire system [5]. In the face of rapidly growing and renewing industrial demands, one of the main application barriers for the development of medium- and high-voltage high-power machine drive technology is to find more suitable high-performance drive solutions for many demanding high-power industrial processes. So far, with the rapid developments of power electronics technology and the introduction of highpower semiconductors, this highly active field has developed a variety of different converter topologies that satisfies the needs of some drive applications [6]-[10]. But they still cannot provide approving performance for specific occasions with stricter requirements, such as high-voltage, high-power and variable-speed drive scenarios. The general requirements for the high-power drive systems include high efficiency, high torque, low manufacturing cost, small physical size, high reliability, effective fault protection, and friendly grid-side connection. Some of the application-specific requirements include high dynamic performance, regenerative braking capability and wide speed range [11]-[13]. It is worth mentioning that there are often contradictions between these requirements. Consequently, it is necessary to consider relatively optimal topology configuration and control strategy for specific applications and actual conditions [14], which is also the main challenge of high-power drive systems in applications. The succeeding sections introduce the technical overview of commercial high-power drive topologies and present the industrial processes they support. Among them, these industrial processes This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. include a wide range of high-power variable-speed drive scenarios (e.g., pumps and fans) and some specific applications with unconventional requirements (e.g., high-performance magnetic levitation traction drives and ship propulsion drives). At the same time, this article discusses the recent new converter modular multilevel converter and its variations for medium- or high-voltage, high-power drivers. Moreover, a potential drive configuration based on the medium-frequency ac link (MFACL) is specifically mentioned, providing a new solution for future research and applications. II. THE STATE OF THE ART OF HIGH-POWER MACHINE DRIVE SYSTEMS The development process of high-power drive converters is shown in Figure 1, and in fact this process has always been driven by the development of power semiconductor devices. Fully controlled and integrated power semiconductor devices make converters with higher modularity and better control performance gradually become the protagonists in high-power drives. Several high-power, medium- and high-voltage drive topologies shown in Figure 1 are widely favored in both literature research and industry applications. According to different structures, they can be divided into direct converters without a dc stage and indirect converters with a dc stage. Furthermore, indirect converters can be divided into current source converters (CSC) with an inductor dc stage and voltage source converters (VSC) with a capacitor dc stage. A general block diagram of high-power machine drive is shown in Figure 2. The grid- and machine-side filters are optional in connection with the system requirements and the type of converter employed. Phase-shifting transformers with multiple secondary windings are usually mainly used to reduce line current distortion. In the drive system, the power converters are quite significant to control the machine so to ensure the performance of the machine, and therefore they are the core component This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. of the drive system. The specific topologies of the above-mentioned high-power converters described above will be briefly reviewed in the following sections. In addition, as analysis criteria, their operational and performance characteristics will also be taken into account. 1940s1970s Invention of fully-controlled semiconductor devices Power Diode、 Thyristor GTO、GTR、 Power MOSFET 1980s Invention of IGBT IGBT、IGCT After 1990s Power integrate circuits and new devices to the market Power integrate circuit、 SiC Device、GaN Device PHASE Ⅰ PHASE Ⅱ PHASE Ⅲ PHASE Ⅳ Cycloconverter Load Commutated Inverter PWM Current Source Converter Cascaded H-Bridge Converter Neutral-Point-Clamped Converter Modular Multilevel Converter Direct Converters （No DC Stage）Current Source Type （Inductor DC Stage）Voltage Source Type （Capacitor DC Stage） Flying Capacitor Converter FIGURE 1 - Development process of high-power drive converters. L C Supply Grid-side Filter Transformer Rectifier DC Filter Inverter Machine-side Filter Machine 1 2 3 4 5 6 7 8 Direct Converter 9 + - Ld Cd - + L C Options 1 Options 2 123456 7 8 9 Direct Converters Indirect Converters M FIGURE 2 – A general block diagram of high-power machine drive systems. This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. Cycloconverter Fed Drives Cycloconverters (CCVs) are mature direct ac-ac converters based on anti-parallel connection of thyristor converters, which use natural commutation control technology and have no dc link [15]-[17]. Table I shows the basic configuration of a 6-pulse CCV, whose operation principle is that the input voltage waveforms of different phases are successively spliced into a variable amplitude and low frequency ac voltage to realize low-frequency variable speed operation. The main features of CCV are as follows. ⚫ It has a compact structure, with low switching loss and high efficiency. ⚫ Its control scheme is mature and it allows the energy flow in both directions. ⚫ Its device has high reliability and its maintenance is convenient [15]. ⚫ Its power factor changes with the modulation ratio, and its harmonic pollution is high, which requires to configure a reactive power compensation device or a harmonic control device with a considerable capacity. ⚫ Its maximum output frequency is usually limited to less than by 1/3 or 1/2 of its grid-side frequency, which affects the range of machine speed regulation. These features make CCV suitable for drive systems that behave as low speed and high torque, including steel rolling mills, ore mills, mine winders, and ship drives. In actual industrial applications, 12-pulse and 24-pulse CCVs are also common choices for reducing the harmonic interaction at the grid and at the machine side. In general, although the technology is relatively mature, changing power factor and inter-harmonic performance are still trouble to be solved urgently. Load Commutated Inverter Fed Drives Load Commutated Inverter (LCI) is one of the earliest inverters developed for high-power variable This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. M 6-Pulse CCV Direct Converter M L d id Cf 12-Pulse LCI PWM-CSC Ld id1M Ld id2 Current Source Converter TABLE I. BASIC CONFIGURATION AND MAIN CHARACTERISTICS OF HIGH-POWER MACHINE DRIVE SYSTEM (PART 1) CCV LCI PWM-CSC Converter Type Typical System Diagram Control Scheme FOC FOC FOC Modulation Scheme Cosine Wave Crossing Technology Phase controlled voltage at rectifier side Line commutation at inverter side Trapezoida PWM Current SHE-PWM Current SVM Notes: 1) FOC refers to the field-oriented control. 2) SHE-PWM refers to the selective harmonic eliminated PWM. 3) SVM refers to the space vector modulation. 4) AFE refers to the active front end. 5) SCR refers to the silicon controlled rectifier. Speed-Torque Quadrant n T n T n T With AFE n TWith SCR Front End Output Waveforms Output Voltage Output Current Output Voltage Output Current Output Voltage Output Current speed drives [18]. The basic configuration of a 12-pulse LCI fed machine drive is shown in Table I, which mainly consists of a phase-controlled rectifier and an inverter based on silicon-controlled rectifier (SCR). The adjustable dc current id is provided by the phase-controlled rectifier and smoothed by the dc inductor Ld, and then the machine is driven by the LCI. Since the SCR device has no selfturn-off capability, the commutation of the LCI needs to be assisted by the load with a leading power factor. Hence, the ideal load of the LCI is the high-power synchronous machines that can easily adjust the power factor status. Due to low manufacturing cost (inexpensive SCR devices), high efficiency (up to 99%), reliable operation and inherent regenerative braking capability, LCI is often suitable for ultra-large drive applications that are sensitive to equipment cost and efficiency, such as rail traction locomotives, gas turbines and large wind tunnel test. The rated power of them can even be as high as 100MW (a typical example is a 100MW synchronous machine drive installed in a NASA’s wind tunnel test facility [19]). This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. However, the use of SCR devices and lack of Pulse Width Modulation (PWM) operation significantly affect the control performance of the machine side and the harmonic characteristics of the grid side, including high torque ripple, slow transient response and variable input power factor. This feature makes harmonic filters a necessary component for LCI drives to reduce the line current THD. PWM Current Source Converter Fed Drives A PWM current source converter (CSC) replaces the SCRs by symmetrical gate turn-off (GTO) or integrated gate-commutated thyristor (IGCT) with self-extinguishing capability, applying to applications where better harmonic characteristics and reliability are desired. At the same time, a variety of PWM schemes including selective harmonic elimination (SHE), trapezoidal pulse-width modulation (TPWM) and space vector modulation (SVM) are allowed to be used in PWM-CSC, which gratifyingly improves the control performance of the drive systems [8],[20]. Table I shows the typical configuration of a PWM-CSC drive system, which adopt an inductor choke as a dc link and use a three-phase capacitor Cf at its output to assist the commutation of the switching device. The main advantages of this topology are the absence of dv/dt, simple converter structure and more excellent harmonic performance. In addition, two or more PWM-CSCs can be operated in parallel to meet the higher rated power of the drive system (>10MW) [21]. Its main disadvantage is the lower dynamic performance because of the large dc inductor, which restricts this topology to be only suited for low dynamic requirements applications, such as pumps, fans, and compressors. Neutral Point Clamped Inverter Fed Drives The neutral point clamped (NPC) converter is also a widely used multi-level VSC. The classic configuration of IGBT-based three-level NPC inverter drive is shown in Table I. The grid-side This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. Voltage Source Converter TABLE I. BASIC CONFIGURATION AND MAIN CHARACTERISTICS OF HIGH-POWER MACHINE DRIVE SYSTEM (PART 2) NPC FC CHB Converter Type Typical System Diagram Control Scheme FOC、V/F、DTC Notes: 1) FOC refers to the field-oriented control. 2) V/F refers to the voltage-frequency ratio control. 3) DTC refers to the direct torque control. 4) SHE-PWM refers to the selective harmonic eliminated PWM. 5) SVM refers to the space vector modulation. 6) AFE refers to the active front end. 7) DFE refers to the diode front end. FOC、V/F FOC、V/F Modulation Scheme Speed-Torque Quadrant n T With AFE n T With DFE Output Waveforms M O 3L-NPC M O 5L-ANPC ... ... M CHB ... L LM ... SM + Udc - iua ila 5L-FC MMC MMC Level-Shifted PWM SHE-PWM、SVM Phase-Shifted PWM Level-Shifted PWM Phase-Shifted PWM Level-Shifted PWM Phase-Shifted PWM Level-Shifted PWM FOC、V/F n T With AFE n T With DFE n T Generally With DFE n T With AFE n T With DFE M Output Voltage Output Current Output Voltage Output Current Output Voltage Output Current Output Voltage Output Current Output Voltage Output Current (3L-NPC) (5L-ANPC) (5L-FC)(4L-CHB) (5L-MMC) converter can be a 12 or 24 pulse diode rectifier while the machine side inverter uses IGBTs and antiparallel diodes as a power unit for series connection. Meanwhile the dc-link adopts capacitors to isolates the neutral point that clamps through diodes. The main applications are pumps, fans and conveyors. In addition, this topology provides excellent speed and torque adjustment performance for the magnetic levitation train, paper mills and other applications that require high control performance. In addition, the three-level NPC inverter shown in Table I can only be used in driving situations of 4.16kV and below. Due to voltage restriction of power electronics module, power converters need more voltage levels in series to obtain higher machine voltage and power capacity. But in fact, due to the immaturity of semiconductor series technology and the limitation of neutral point balancing [22], it is difficult to expand the voltage levels. Therefore, the voltage and power capacity of the NPC This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. aforementioned MMC-based high-power, low-speed drive system while still taking full advantage of its benefits is to revisit a possible new configuration based on the MMC [44]. As shown in Figure 4, a novel medium-frequency ac link (MFACL) powered machine drive system is proposed [45], which can inherit the advantages of high voltage and modularity of the MMC and guarantee the high torque demand of the machine at low speed. In this Section, the configuration, control and performance of this novel drive system will be introduced and analyzed. MFACL Powered Machine Drive System Configuration SM idc + - Ud c iu1 il1 SM SM ... SM SM ... SM SM iu2 il2 SM SM ... SM SM ... SM SM iuk ilk SM SM ... SM SM ... SM i1i2ik Phase U Phase V Phase W Medium- Frequency AC Link fg M ima imb imc Low- Frequency fm MMC CCV Phase 1Phase 2Phase k ... ... 1... 2k Ls Ls Ls Ls Ls Ls Lf Half-Bridge SM Full-Bridge SM ... Ccom ... u1,u2uk ... io1 io2 iok ... 1 2 k CCV Half-Wave Modules ... ... dq to abc imd - +PI ωmLmq imd* - +PI ωm* ωm- +PI imq + - +- Gate Signals B. Control Scheme of CCV Output Voltage Control iuk ilk Udc /2 Udc /2 ++ + + - + uk_ref uk uuk_ref ulk_ref uk_com Gate Signals A. Control Scheme of MMC Circulating Current Suppressing Modulation & Capacitor Voltage Balancing ikiok +_ + _ _ + + + +_ + _ _ + + + ud* ud uq id iq id* iq* ud uq udiff_d + + PI PI ωgL ωgL PI PI udiff_q uk_ diff ωgt ωgCcom ωgCcom dq to abc iod + ioq + uq* (a) (b) (c) (e) +_ +_+ icir_d ucom_d PI PI uk_ com dq to abc icir_q abc to dq 2ωgt 1 2 0 0 iuk ilk ++ 2ωgt uk_ diff ucom_q (d) imd,q imj ud,q* ud,q id,q iod,q abc to dq uk_ref uk ik iok _ _ _ ωmLmd 2ωgLs 2ωgLs Cosine Wave Crossing FIGURE 4 – A novel MFACL powered machine drive system configuration and its operation control strategy. (a) The MFACL powered machine drive system configuration. (b) Overall control diagram of the MMC. (c) Output voltage control diagram of the MMC. (d) Circulating current suppression control diagram of the MMC. (e) The control diagram of the CCV. This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. ⚫ Topology and Control Scheme of MFACL Powered Machine Drive System Figure 4(a) shows a novel MFACL powered converter configuration suitable for high-torque and variable-speed drive systems. The detailed parameter design of the MFACL can refer [45]. Uncontrolled rectifiers or active rectifiers can be configured on the grid side to transform the ac grid into the stable dc link. This MFACL powered machine drive system is proposed to make up for the shortcomings of the MMC-based direct drive systems. In the novel MFACL powered machine drive system, the MMC operates under the medium frequency and produces MFACL, and accordingly the SM capacitor voltage ripple can be reduced to a small value no matter what the operation speed of the machine. The CCV convert the MFACL power to the adjustable low-frequency ac power required for the operation of the machine. The advantage of adopting MFACL is that it not only inherits the strengths of the MMC in voltage expansion and control performance, but also greatly compensate for the limitation of the CCV’s output frequency range, ensuring the quality of machine current waveform. In addition, a voltage regulator and filter circuit is adopted to support the voltage of the MFACL and ensure the stable operation of the drive system. By designing its filtering parameters and adopting appropriate voltage control algorithms, the problem of harmonic pollution of CCV can also be solved well. Due to the modular structure, high performance and outstanding stability of the MFACL powered machine drive system, this novel drive system has high competitiveness. Figure 4(b)~(e) show the control scheme block diagram of the new drive system. Subscript k represents the corresponding electrical variable of phase k, and subscript ωm represents the angular frequency of the machine. As shown in Figure 4(b), the overall control diagram of the MMC includes output voltage control, circulating current suppression control, and SMs’ capacitor voltage balancing This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. control, which provides a double closed loop system to control the amplitude and frequency of its output voltage. uk _ref, uk are respectively the reference value and actual value of the MMC output voltage. ik, iok are respectively the MMC output current and the CCV input current. uuk_ref and ulk_ref are reference voltages of the upper and lower arms of phase k in the MMC, which can be sent to the modulation and capacitor voltage balancing control block to generate the gated signals for the MMC. Figure 4(c) shows the output voltage control in detail, which adopts double-loop vector control to ensure a constant voltage of the MFACL. ud*, uq* are the dq-axis components of reference voltages uk_ref. ud, uq and id, iq are the dq-axis components of output voltages uk and output currents ik, respectively. iod, ioq are the dq-axis components of the CCV’s input currents iok. ωg is the angular frequency of output voltage and ωg=2πfg, where fg is the MMC’s output frequency. According to the voltage commands ud* and uq*, where ud* is equal to the amplitude of MMC’s phase voltage and uq* is 0, the outer loop generates the current commands id* and iq*. The inner loop produces the dq-axis reference voltages udiff_d and udiff_q. And then, the reference voltage uk_diff for phase k is derived by the inverse dq transformation. Figure 4(d) shows the circulating current suppression control in detail. iuk and ilk are the upper and lower arm currents in phase k, respectively. The command values of circulating current component icir_d and icir_q are set to 0, and the uk_com generated by the closed loop is used as the circulating current regulation voltage. The control scheme of the CCV shown in Figure 4(e) mainly includes the field oriented control (FOC) and cosine wave crossing control, which mainly ensures a desired speed ɷm* of the machine. imd, imq and Lmd, Lmq are the dq-axis components of machine currents and inductances, respectively, and the d-axis machine current command imd* is often set to 0. The gating signal for the CCV is generated by cosine wave crossing control. It is worth mentioning that CCVs have a relatively mature control scheme, which makes the control of machine speed and reactive power easy and trustworthy [15]. This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. ⚫ Performance Comparison This Section mainly compares the performances between the novel MFACL powered machine drive system and classic large-capacity drive systems. Figure 5 is a simulation comparison between the novel MFACL powered machine drive system and the original MMC direct drive system. The parameters of the simulation system are shown in Table II. It can be seen that the SM capacitor voltage ripple is significantly reduced under low-speed operation, which makes the traditional MMC direct drive problems well resolved. This means that MFACL powered machine drive system can employ much smaller SM capacitors, thus taking advantage of hardware cost and size. - + Ud c MMC L C Medium- Frequency AC Link CCV M 1 2 3 4 5 SM Capacitor Voltage of MMC 1 0.48 1.0 1.52 1.1 (a) Csm voltage [p.u.] Time (s) 1.466 0.501 Tranditional MMC direct drive system 1 0.88 1.0 1.12 1.1 (b) Time (s) 1.042 0.947 MFACL powered drive system Csm voltage [p.u.] VS Machine Current 1 1.2 -1 0 1 (f) Machine Current (kA) Time (s) Machine Voltage 1 1.2 (e) Machine Voltage (30kV/div) Time (s) MFACL Current 1 1.05 -1 0 1 (d) MFACL Current (kA) Time (s) MFACL Voltage 1 -15 0 15 (c) MFACL Voltage (kV) Time (s) 1.05 TABLE II. SIMULATION PARAMETER OF PROPOSED MACHINE DRIVE SYSTEM Parameter PMSM MMC CCV Value Commutation Inductance (mH) Rated Frequency (Hz) DC-Side Voltage (kV) SM Capacitance (mF) Rated Power (MW) Rated Stator Line Voltage (kV) Rated Current (kA) Arm SM Number n 5 6.6 0.657 20 100 12 1.38 0.63 Rated Frequency (Hz) 10 Rated Displacement Factor 0.95 Arm Inductance (mH) 3 ΔUCsm= 0.965 p.u. ΔUCsm= 0.095 p.u. Number of Phases k3 umab umbc umca FIGURE 5 – The simulation results. (a) The capacitor voltage of SMs in the MMC direct drive system. (b) The capacitor voltage of submodules in the new drive system. (c) MFACL voltage. (d) MFACL current. (e) Machine voltage. (f) Machine current. This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. A prototype of MFACL powered machine drive system is built in the lab, as shown in the following Figure 6. A dc power supply is used to support the dc-link voltage. The control algorithm is implemented by digital signal processor (DSP) and the drive signals are transferred to driving circuit of each device by optical fibers. Table III lists the parameters of the experimental system. Experimental platform waveforms correspond to the waveforms in the simulation system one by one, which conforms the simulation result. Hence, the effectiveness of MFACL powered machine drive system is verified by the simulation and experiment. SM Capacitor Voltage of MMC VS Csm voltage (1 V/div) (b) ΔUCsm= 0.049 p.u. MFACL powered drive system Machine Voltage (100 V/div) umab umbc umca 1 2 3 Time(0.04 s/div) Machine Voltage TABLE III. EXPERIMENT PARAMETER OF PROPOSED MACHINE DRIVE SYSTEM Parameter PMSM MMC CCV Commutation Inductance (mH) Rated Frequency (Hz) DC-Side Voltage (V) SM Capacitance (mF) Rated Power (W) Rated Stator Line Voltage (V) Rated Current (A) Arm SM Number n Rated Frequency (Hz) Rated Displacement Factor Arm Inductance (mH) Number of Phases k Value 400 44 8 140 100 4 2.7 1 10 0.95 0.5 3 MFACL Voltage (50 V/div) Time (0.01 s/div) 1 MFACL Voltage Machine Current (5 A/div) 1 Time (0.04 s/div) Machine Current MFACL Current (5 A/div) Time (0.01 s/div) 1 MFACL Current 35.9V (1.026 p.u.) 34.2V (0.977 p.u.) Time (0.01 s/div) 3 Csm voltage (5 V/div) ΔUCsm= 0.409 p.u. Tranditional MMC direct drive system 41.5V (1.186 p.u.) 27.2V (0.777 p.u.) Time (0.01 s/div) 3 ucua ucla ucua ucla MMC CCV MFACL PMSM (a) (c) (d) (e) (f) FIGURE 6 – The experiment results. (a) The capacitor voltage of SMs in the MMC direct drive system. (b) The capacitor voltage of submodules in the new drive system. (c) MFACL voltage. (d) MFACL current. (e) Machine voltage. (f) Machine current. This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. Figure 7 compares the performance of the classic drive system for high-power applications and the novel MFACL powered machine drive system. Figure 7(a) summarizes their power-voltage diagrams. The voltage and power range of each classic topology are mainly derived from specific application cases and existing engineering product manuals [46]-[49]. Since the novel MFACL powered machine drive system has not yet been commercialized, the parameters are mainly based on the analysis of existing commercial power devices and inverter technologies. It can be seen that the voltage and power range of the MFACL powered machine drive system is undoubtedly wide, which has its respective pros and cons compared with the ultra-large capacity converter topology LCI and CHB fed drive system. Due to the high modularity and scalability, the novel MFACL powered machine drive system is expected to develop further in the future. Figure 7(b) qualitatively compares their main characteristics from three aspects including power quality, control performance and hardware feature. Among them, the power quality is mainly related to the friendliness of the system to the grid, and the hardware features determine the reliability, cost and size of the system. Figure 7(c) lists more detailed performance and functional summaries of each topology, where some indicators that the machine drive system pays special attention to are selected for comparison. For the rationality of comparison, in the Figure 7(c), all voltage source converters adopt the multi-pulse DFE as the front end, and all current source converters adopt multi-pulse SCR as the front end. On the whole, the novel topology based on the MFACL is a respectively ideal drive choice. From multiple perspectives, the overall performance of the CHB inverter fed drive system is closer to it. But the CHB inverter’s complex and space-occupying phase-shifting transformers significantly also reduce its power density and practical applicability while the particularity of the cable connection capacity limits its scope of use. Hence, in high-power, medium-, high-voltage and This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. variable-speed drive applications, the newly proposed MFACL powered machine drive system has strong competitiveness and attractiveness, which has achieved a good balance in all aspects of actual drive system requirements. Performance Attributes Power Quality Controllability Hardware Feature LCI PWM-CSC CCV NPC/FC CHB MMC MFACL-MDS Structure Complexity Reliability or Redundancy Semiconductor Part Count Dynamic Performance PWM Technique Power Density Line-side Harmonics Output Current THD Power Factor Performance at Low Speed & High Torque (c) 4.16 6.6 7.2 10.3 11.8 [kV] 10 25 50 75 100 [MW] 2.3 LCI PWM-CSC CHB CCV NPC/FC MMC & MFACL Powered Machine Drive System (MFACL-MDS) (a) PWM CSI Superior Intermediate Inferior Hardware Feature Power Quality Controllability NPC /FC LCI CCV CHB MMC MFACLMDS (b) PWM-CSC FIGURE 7 Performance Comparison of different drive systems for high-power drive applications. ⚫ Case Study In the practical application of the drive system, cost, physical size and weight are often taken into important consideration to restrict the drive system to achieve the desired demand. Table IV gives a case study to provide a more comprehensive reference for several advanced solutions described above. In Table IV, all converters draw power from the ac grid with power frequency and all of them except This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. the BTB-MMC adopt the unified multi-pulse DFE as the front end. The BTB-MMC system adopts the MMC rectifier and inverter with full-bridge (FB) SMs to form a machine drive system, so as to reduce the DC-link voltage for reducing SM capacitor voltage variation at a low machine speed [50]. The MFACL powered machine drive system and other related drive configurations are based on the 6.6 kV/5 MW system in Table II. The data of power switches, passive components and transformers are all from the mainstream supplier such as Infineon, TDK, etc. The SM capacitance is selected to ensure that the voltage fluctuation is less than ±5% in the whole speed range. The system efficiency is simulated at rated situation with the professional tool PLECS, where the method of loss calculation comes from [51], [52]. As shown in the Table IV, all solutions contain the same stages, and the MFACL powered machine drive system has obvious advantages in terms of components weight and size. Although slightly inferior to CHB inverter fed drive system in terms of price, the MFACL powered machine drive system still remains competitive in the class of solutions considering its effective improvement in size and weight. TABLE IV. A CASE STUDY BASED ON THE 6.6kV/5MW CASCAD DRIVE SYSTEMS Variable DC Voltage Method [39] Power Channel Method [40] Items Semiconductor Componets Switch Device Voltage Stress 3.3 kV IGBT × 156 6.8 kV Diode × 36 High-Frequency Injection Method [41] BTB-MMC [50] CHB [28] MFACL-MDS Passive Componets SM Capacitor 30 mF × 72 Rectifier-Transformer × 3 3.3 kV IGBT × 288 6.8 kV Diode × 36 Switch Device Current Stress Estimated Price of Main Components Estimated Volume of Main Components Estimated Weight of Main Components IGBT 1667 V SM Capacitor 2.76mF × 72 AC Capacitor 0.1mF × 3 Rectifier-Transformer × 3 IGBT 1667V SCR 4194V IGBT 514A SCR 653A IGBT 410 A 1083, 410 USD 22,225 L 21,414 kg SM Capacitor 2 mF × 72 Rectifier-Transformer × 3 3-windingTransformer × 12 IGBT 1667 V IGBT 410 A 592,032 USD 6,373 L 5,559 kg 3.3 kV IGBT × 144 6.8 kV Diode × 36 SM Capacitor 13.8 mF × 72 Rectifier-Transformer × 3 IGBT 1667 V IGBT 1124 A 912,000 USD 10,814 L 10,302 kg 3.3 kV IGBT × 576 SM Capacitor 4.4 mF × 72 SM Capacitor 13.8 mF × 72 IGBT 1667 V IGBT 410 A 1227,456 USD 11,014 L 10,901 kg 3.3 kV IGBT × 60 3.3 kV Diode × 90 SM Capacitor 10 mF × 15 Rectifier-Transformer × 15 IGBT 1667 V IGBT 653 A 540,120 USD 14,814 L 12,273 kg 3.3 kV IGBT × 144 8 kV SCR × 36 6.8 kV Diode × 36 584,748 USD 5,285 L 4,868 kg System Efficiency ＞ 96% ＞ 95% ＜ 95% ＞ 96% ＞ 96% ＞ 96% This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. In general, the MFACL powered machine drive system overcomes the low-frequency operation problem of the MMC, which is well suited to the application scenarios of high voltage, high power and low speed. In addition, the MFACL powered machine drive system has a relatively flexible configuration scheme, including the ability to be compatible with the dc bus and the ability to drive multi-phase machines. At the same time, the quantitative comparison also proves that the adoption of the MFACL makes the drive system more compact and competitive in terms of volume and weight. These advantages provide the MFACL powered machine drive system a unique superiority in gearless direct drive systems for hoists, grinders, ship propulsion, etc. IV. CONCLUSION AND FUTURE RESEARCH In this article, the basic configuration and research status of various practical topologies under highpower drive scenarios are presented. Meanwhile, a novel MFACL powered machine drive system is analyzed for the current industrial needs of high voltage, high power and variable speed. The article reviews the topological characteristics and operational performance of these current drive solutions, where common industry and application requirements are all used as comparison criteria. In addition, some prospects for future research are summarized to provide a feasible reference for practitioners. In the future, further solving the conflicts between capacity requirements, control performance, power quality and hardware limitations will still be the main topics in the research and development of industrial drive systems. Modular drive system design and integration will become a trend due to its high reliability and easy scalability. In addition, multifunctional drive systems with excellent flexibility design and grid-side adaptability will also receive attention. Finally, a dedicated drive system for specific industrial scenarios is worth exploring, on account of the different requirements This article has been accepted for publication in IEEE Industrial Electronics Magazine. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/MIE.2022.3218915 © 2022 IEEE. 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