Performance Improvement of Medium Voltage Modular Multilevel Converter based Motor Drive using SiC MOSFETs

The auxiliary submodule power supply is a vital component of a modular multilevel converter submodule or any multi-submodule converter. Considering the high isolation requirements and difficulties to provide power from the ground potential, the auxiliary submodule power supply must be simple, work reliably and not compromise the submodule's reliability. A flyback supply from the submodule's dc link with multiple sets of isolated secondary windings solves at once the low-voltage generation and the required voltage isolation for semiconductor gate circuits and protection without need for an externally supplied low-voltage input to the submodule (high-isolation connection). This paper presents an isolated, flyback-based auxiliary submodule power supply with planar magnetic, printed circuit board integrated windings and multiple isolated outputs for a medium voltage modular multilevel converter as well as detailed electrical and magnetic mathematical modelling, technological integration challenges description and proper experimental test considerations.

Dual three-phase (asymmetrical six-phase) induction motors have received significant attention in high power medium voltage drive applications. Typically, two three-phase voltage source converters are needed to feed such a drive system. In medium voltage applications, multilevel inverters are preferred as they provide a stepped output voltage waveform that reduces dv/dt stresses and provides a better output current waveform, which reflects positively on the machine developed torque and overall drive performance. Among different multilevel inverters, the Modular Multilevel Converter (MMC) has recently shown promise in medium and high voltage applications. For a dual three-phase machine, two MMCs with 6 arms each, are needed. This paper proposes a nine-arm MMC (9A-MMC) to drive such a motor. The proposed approach reduces the number of involved arms by 25%, i.e. reduces the number of required dc capacitors, semiconductor devices and their gate drive circuits, which reduces system complexity and cost. A complete analysis of the proposed system is presented and a simulation model is used to verify the proposed concept.

In this paper, a switching strategy is proposed for the extended phase shift (EPS) controlled dual active bridge (DAB) converter which using the hybrid switch (HyS) configured by paralleling SiC MOSFET and Si IGBT, considering the special soft switching characteristic of DAB. For most of the existing literature, the common switching strategy of HyS is to switch on the SiC MOSFET before Si IGBT and switch off the SiC MOSFET after Si IGBT. However, if only using this switching strategy, the SiC MOSFET may be damaged by long-time operating at the over rated current condition. The proposed switching strategy for the HyS is that the SiC MOSFET is only conducted for a short while during the process of switching Si IGBT on or off. The proposed switching strategy can shorten the conduction time of SiC MOSFET, decrease the losses of HyS and extend the safe operation area of HyS. Simulation results on a 30 kW HyS DAB converter verify the availability of the proposed switching strategy.

During the operation of a three line ac link in parallel with a ac-dc-ac link system that employs Back-to-Back (B2B)Medium Voltage Modular Multilevel Converters (MMC), a path for the low frequency zero sequence currents is shown to exist. Apart from causing additional losses, these currents can result in limitations on power capacity. A detailed analysis on the influence of cell capacitance, arm inductance and link conductor length on the circulating zero sequence currents and the means to mitigate them through a simulated control strategy is investigated. The simulations show that the dc link employing B2B-MMC can successfully operate in parallel with ac link without transformer isolation by controlling the zero sequence currents.

The high breakdown voltage, good thermal properties, and (in case of GaN) a higher field effect mobility compared to Si make the expected performance of GaN and SiC based power devices to be potentially vastly superior to that of the today leading power device technology - Si Insulated Gate Bipolar Power Transistors (Si IGBTs). After many year of development, commercial SiC power diodes and MOSFETs now outperform recent Si IGBTs and are poised to compete in the renewable energy and electrical vehicle markets. The recent generation of the hybrid SiC-Si IGBT modules combine the best features of both competing technologies.. These device hybrid modules handle powers up to 350 kW range at voltages well over 1 kV and have several times higher switching speed than the recent generations of Si IGBTs. SiC power technology is more mature than GaN power technology. However, the GaN enhancement mode High Electron Mobility Transistor (E-HEMT) on Si technology reached power ratings over 10 kW operating at 200 kHz with lower switching losses than Si MOSFETs and SiC MOSFETs. GaN devices already take about 20% of the RF power market with expectations of reaching 40% by 2025. Still, the existing device designs are not fully taking advantages of the superior materials properties of GaN. The lateral-vertical designs for a higher breakdown voltage; quantum well channel designs limiting high electron injection into the buffer layers; a low conducting passivation making the electric field distribution in the gate-to-drain spacing more uniform and therefore increasing the breakdown voltage, improving reliability, and increasing lifetime; perforated channel designs for decreasing the effect of the parasitic resistances and increasing the switching speeds are the innovative design features poised to improve the competitive edge of SiC and GaN technologies. For ultimate performance, the Si substrate in the GaN on Si technology should be replaced by SiC or even diamond, even though in a short run it would be hard for such technologies to be cost competitive. In a more distant feature, even diamond by itself might emerge as superior high power material. In a more near future, AlInN/AlN/GaN on Si technology could become a leading technology for high power applications.

The high switching frequency (> 20 kHz) of SiC MOSFET module makes it as an attractive alternative of Si IGBT module for high power density applications. Since SiC MOSFET and Si IGBT have the similar MOS-gated structure, it is normally regarded that the gate driver of SiC MOSFET can directly inherit from that of Si IGBT. However, considering the different device physics properties, some special design considerations need to be taken. In addition, the higher dv/dt and di/dt in SiC MOSFET module leads to more serious EMI issues, which will comprise the switching speed in return. In this work, the high-speed gate drivers for Si IGBT and SiC MOSFET modules with similar ratings are designed and optimized. Based on the experiment of switching characterization, the considerations of gate driver design to migrate from Si IGBT module to SiC MOSFET module are presented. And the EMI issue is the major challenge in the gate driver design for SiC MOSFET module.

Emerging applications, e.g., traction systems and utility scale renewable energy systems, demand for Medium Voltage (MV)power electronic switches with blocking capabilities above 20kV, which cannot be provided with today's 10kV or 15kV SiC power semiconductors. Therefore, this work investigates different concepts for the realization of a MV half-bridge, i.e., a series connection of 10kV SiC MOSFETs, a JFET Super Cascode arrangement, and Modular Multilevel Converter (MMCs)and Flying Capacitor Converter (FCCs)topologies. The FCC topology features several advantages such as reduced switching losses, reduced chip area, lower $\mathrm{d}v/\mathrm{d}t$ of the switching transitions, and robust voltage balancing. Moreover, the volume of the flying capacitors can be reduced with Quasi-2-Level (Q2L) operation of the half-bridge, where the intermediate voltage levels are only used during very short time intervals, i.e., during the switching transitions. This paper analyzes the design, the switching behavior, and the voltage balancing of the Q2L-FCC bridge-leg and confirms its suitability as a versatile MV switch. Finally, the integration of a complete bridge-leg, including gate drivers, isolated cooling interfaces, measurements, and Q2L control into a 300 kVA/40 kV SiC Super-Switch Intelligent Power Module (SiC-SS-IPM)is presented.

SiC MOSFET being fast switching, high voltage blocking, high junction temperature withstanding, and low loss device, has the edge over Si IGBT. Therefore, SiC MOSFET's applications are in high power density matrix converter, battery charger in electric vehicles, and many other. The matrix converter utilises four-quadrant switches, which allow both positive and negative current and block both positive and negative voltage. Since SiC MOSFETs being much better than Si IGBTs, the four-quadrant switches are realised using them. The conducting and blocking devices for the four-quadrant switch vary with the change in quadrant of operation, and the switching involves the turn ON and turn OFF of two devices. Therefore, the characteristics and power loss are bound to vary with the quadrant of operation. Very limited work is available on SiC based four-quadrant switches in the literature. Hence, a thorough investigation of the four-quadrant switch as a single entity along with power loss analysis is done in this paper. Further, this paper presents approximate conduction and switching power loss analysis of the four-quadrant switch. The research is performed by developing approximate mathematical equations for power loss and then testing the SiC MOSFET based four-quadrant switch in all four quadrants.

Due to the high dc voltage utilization and capability of dc fault blocking, the full-bridge submodule-based modular multilevel converter (FBSM-MMC) becomes a prevalent topology for medium voltage high power applications. However, using the Si IGBT for each FBSM can induce high switching loss while the SiC MOSFET suffers from high cost because of limitations of fabrication technologies. To address this issue, a hybrid FBSM consisting of both SiC MOSFET and Si IGBT is studied in this article. In addition, an optimal control scheme is proposed, which adjusts the current conducting path of the FBSM and concentrates the high-frequency switching actions into the SiC module. The proposed control scheme only modifies the submodule (SM) level switching and has good compatibility with the existing MMC modulation schemes. Compared with the MMC solely formed by SiC MOSFET, the hybrid SM has similar switching loss and better performance in terms of conduction loss and cost. The experimental results prove the validity of the proposed topology and control scheme, and the further loss and cost analysis verifies that the Si/SiC hybrid FBSM can reduce the loss by 16.3% and save the device cost by 41.0% compared with the SiC FBSM.

1200V SiC MOSFETs have superior characteristics compared to silicon (Si) IGBTs. However due to parasitic inductances in the converter layout, one to one replacement of Si IGBT with SiC MOSFET may result in higher overshoot in the device voltage and current. Active gate driving mechanism can control these unwanted stresses on the device without compromising its switching speed. The limited bandwidth of OP-AMP used in a conventional closed loop active gate driver poses a major hindrance to drive fast switching SiC MOSFET. To address this issue a new current controlled Active Gate Driver mechanism is proposed. The proposed AGD can be realized with cheaper electronic components with sufficient bandwidth to drive a SiC MOSFET. The proposed technique ensures better controllability on the device by directly controlling the injected gate current. The proposed AGD has been experimentally verified in a double pulse test set up with a 1200 V, 35 A SiC MOSFET (ROHM make: SCH2080KE).

In this paper, a one-step Model Predictive Control system is executed in the Multilevel modular converter (MMC) to control the speed of an Interior Permanent Magnet (IPM) machine. Maximum Torque per Ampere (MTPA) and Field Weakening (FW) control technique is utilized for the greatest yield torque. The proposed Control strategy tracks the reference produced by MTPA and FW calculation by freely directing voltages from the MMC modules. The proposed method also studies the relation between the switching losses of MOSFETs and sampling frequency, cost function and a number of modules in an MMC. Finally, simulation results are provided to demonstrate the appropriateness of this control procedure.

For a trade-off between turn-off and on-state of power switches in medium and high power applications, it is crucial to consider the switching speed restrictions due to turn-off peak voltage. By varying the p-emitter efficiency and the front side plasma level of an IGBT, an <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$E_{\text{off}}-V_{\text{ce},\text{sat}}$</tex> trade-off curve for a fixed turn-off peak voltage can be obtained. It is shown that a turn-off behavior similar to that of a SiC MOSFET can be achieved with an IGBT with low carrier confinement, but with the drawback of a higher on-state voltage drop. At low current operations, the losses dissipated by the SiC MOSET are lower, not only in the on-state, but also during turn-off. The charge carrier plasma in the IGBT reduces both <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{d}v/\mathrm{d}t$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{d}i/\mathrm{d}t$</tex> for lower current densities. Switching curves at the rim of the RBSOA and beyond show that there is no dynamic avalanche in the SiC MOSFET. However, a quasi-static clamping mode can be observed for both types of devices.

Increasing concern about the environmental impact of ships has made Shore-to-Ship (STS) power an attractive solution for ship owners and port authorities worldwide in reducing emissions at ports. Existing shore-to-ship solutions for 0.1 MVA to 5 MVA applications employ silicon (Si) IGBT based static frequency converters. Recent developments in high voltage silicon carbide (SiC) devices have facilitated improvement in efficiency and power density of medium voltage (MV) converters in various applications. This paper proposes an MV STS system enabled by series connection of three 3.3 kV SiC MOSFETs, which shows the potential for improved power density and efficiency compared to existing Si IGBT based solutions. A 100 kVA 3-phase two-level voltage source converter (VSC) with series connected 3.3 kV SiC MOSFETs is designed and demonstrated. Experimental results for the series connected 3.3kV SiC MOSFET based converter is shown at 6 kV dc link voltage to validate the design and operation of such a system. Successful demonstration of a MV converter system enabled by series connection of high voltage SiC MOSFETs can open up opportunities to replace conventional Si IGBT based converters with SiC MOSFET based converters in applications interfacing with medium voltage grid.

The recent advancement in the technology of SiC MOSFETs has spurred interest in designing compact and high switching frequency (10–20 kHz) power converters. However, grid-integration of these power converters at medium voltage (MV) scale would require a conventional transformer. With the development of new high voltage (HV) 10 kV and 15 kV SiC MOSFETs, these converters can directly interface with medium voltage (MV) grids without the need for line-frequency transformers, using simple two-level and three-level topologies. The application of these devices is currently being explored in all MV Applications (8 kV to 30 kV) like Solid State Transformer, MV Drives, Power Conditioning Systems, and MVDC isolators. This paper discusses application of 10 kV SiC MOSFETs and JBS Diodes for enabling Asynchronous Microgrid Power Conditioning System (AMPCS). This medium voltage power converter is enabled by series-connection of devices, in a Three-Level Neutral Point Clamped (3L-NPC) configuration. The voltage balancing of these series-connected devices is achieved by using R C-snubbers. This paper addresses the different conduction modes and switching sequences of a 3L-NPC pole, which is used as building block for the three-phase converter. The switching loss analysis, for various snubber values, is presented for the MOSFETs and the clamping diodes along with experimental results. This research helps in providing an overview of switching losses that are disspated through the device (and heatsink) and through the snubber resistor in a 3L-NPC convertor pole.

The recent growth in power generation using renewable energy sources has led to extensive research and development of robust and resilient power converters, which can integrate them with the medium voltage (MV) grids (13.8 kV,60Hz). Conventional power converters need a line frequency transformer for their integration to the MV grid, which increases the overall footprint and installation cost of the system. Therefore, a compact and lightweight alternative are required for largescale integration of the renewable energy source to the MV grid. With the advent of high voltage SiC MOSFETs, the operating frequency of grid converter can be increased up to 10-20 kHz, thus significantly reducing the size of filter inductors. The use of these devices in multi-level configurations with series-connected devices facilitates the design of power converters that can interface directly with MV grid, eliminating the need for line frequency transformers. The converter presented in this paper is designed to interface a 13.8 kV three-phase grid to a dc link of 24 kV. A three-level neutral point clamped (3L-NPC) topology enabled by series-connected 10 kV 15 A SiC MOSFETs and 10 kV 15 A SiC JBS diodes is presented. This paper focuses on the advantages, design considerations, and challenges associated with a medium voltage 3L-NPC converter. Experimental results show the successful operation of series-connected 10 kV 15 A SiC MOSFETs and JBS Diodes at medium voltage levels and highlights the series connection that is realized with snubber circuits for voltage balancing.