Medium Voltage Power Conversion Research Articles

(Electronics and Photonics Division Award) Research Advances Wide- and Ultrawide-Bandgap Power Switching Devices

Wide bandgap semiconductors such as SiC and GaN (WBG) and emerging ultrawide-bandgap materials such as Ga2O3, AlGaN, and Diamond (UWBG) represent the next-generation materials for high performance medium voltage and high voltage power switch technology. Such devices have a wide range of immediate Naval applications, including high-power satellite communications and radar, unmanned underwater and aerial vehicles, ship drive components, and hybrid vehicle inverters. Vertical SiC power device technology has matured rapidly over the past two decades, owing to advances in substrates, a fundamental understanding of epitaxial growth to eliminate performance-limiting defects, as well as device design breakthroughs. This has enabled breakthroughs in highly integrated module design for medium voltage power conversion with switching frequency >100 kHz. In parallel, lateral GaN-based high electron mobility transistor (HEMT) technology has been highly successful for RF power amplifiers and is well positioned to supersede GaAs-based microwave circuits. Recently, GaN-based vertical and lateral power devices have attracted significant interest due to promising device results coupled with progress in native substrate, epitaxial growth, and processing technology developments. This seminar will present an overview of the WBG/UWBG power device efforts at NRL. This research has four primary focus areas – 1) thermal management in III-N power device structures by diamond thin film integration, 2) characterization of substrate materials and homoepitaxial layers used for drift regions of power devices identify appropriate specifications and benchmark performance, 3) process module development, particularly for selective area doping by ion implantation, and 4) pilot production manufacturing of 1.2kV-class PiN diodes including reliability assessments to identify and mitigate performance limiting defects.

Optimal design of high frequency magnetic links for power conversion used in grid connected solar and wind power plants

The high-frequency standard magnetic links were recently considered viable candidates for construction of the medium-voltage power converters, rather than link with the common dc specialized magnetic materials, like nano-crystalline and the amorphous materials. This provides a new route of portable and lightweight incorporation of renewable systems into the direct grid, stepping-up transformer. The main objective is to propose an ideal design approach for magnetic links and to validate their viability by the evaluation of a sample. This will discuss a specific approach to design, research, and implementation. The predicted result is that the proposed architecture strategy will be implemented effectively in the future with clean energy generation schemes and intelligent grids. The current approaches have some critical disadvantages, including voltage mismatch, minimum overall power extraction, and general mode problems because of several direct grid and common dc connections. With emphasis on implementing renewable energies, exploring different forms of configuring structures for efficiency and output is becoming increasingly relevant. Both technologies employ grid-based voltage converters, but conventional equipment poses several problems in terms of voltage balance and maximum power output. Magnetic connections are a viable solution to minimize the costs, reliability, and consistency of the network. Instead of several dc links, these can be solved. Thus, the plan will depend on its optimal design to monitor system costs while improving efficiency by adding a new magnet-powered voltage converter. Particularly the voltage mismatch and typical mode problems of converter systems are minimized. The electromagnetically designed common magnetic links are, however, a multi-physics challenge, affecting device performance and costs. The paper objectives are to propose an ideal design approach for magnetic links and to validate their viability by the evaluation of a sample.

10kV+ Rated SiC n-IGBTs: Novel Collector-Side Design Approach Breaking the Trade-Off between dV/dt and Device Efficiency

10kV+ rated 4H- Silicon Carbide (SiC) Insulated Gate Bipolar Transistors (IGBTs) have the potential to become the devices of choice in future Medium Voltage (MV) and High Voltage (HV) power converters. However, one significant performance concern of SiC IGBTs is the extremely fast collector voltage rise (dV/dt) observed during inductive turn-off. Studies on the physical mechanisms of high dV/dt in 4H-SiC IGBTs revealed the importance of collector-side design in controlling the phenomenon. In this paper we propose a novel collector-side design approach, which consists of four n-type layers with optimized doping densities and allows the control of dV/dt independently from the device performance. Further, we demonstrate a reduction of dV/dt by 87% without degrading the high switching frequency capability of the device, or the on-state performance, through the addition of two n-type epitaxial layers in the collector side, between the buffer and the drift regions.

Direct Arm Energy Control of the Modular Multilevel Matrix Converter

Modular multilevel matrix converter is a type of converter used in medium-voltage ac-ac power conversion. Controlling the energy content of the converter arms is a prerequisite for a proper converter operation, and at the same time a challenging control task. So far, there are several approaches presented in the literature that successfully deal with the challenge. However, almost all of them fail when it comes to the simplicity of implementation, and ease of understanding by control engineers and researchers entering the field. This paper extends the direct arm energy control concept, already introduced for the class of modular multilevel converters, to the modular multilevel matrix converter. Presented control approach is characterized by an intuitive and simple implementation, ability to control the arm energies to arbitrary values, and stable operation under all operating conditions, including grid imbalances. Validity of the control concept was demonstrated using an in-house-developed hardware-in-the-loop simulator of the modular multilevel matrix converter, with control algorithms being deployed to the industrial-grade controller.

A High-Efficiency 80-kW Split Planar Transformer for Medium-Voltage Modular Power Conversion

A high-efficiency medium-frequency split planar transformer (MFSPT) based on low-cost ferrite core (PC40) is proposed for medium-voltage modular power conversion. By adopting a split planar structure, high electrical insulation is achieved with high efficiency, high power density, as well as excellent heat dissipation performance at the same time. A finite-element analysis simulation is carried out to investigate the distribution of the magnetic field and electric field in the MFSPT. The designed MFSPT prototype achieves a power density of 21.1 kW/L and electrical insulation of 42 kV. The performance of the proposed MFSPT is experimentally evaluated on an 80 kW, 43 kHz <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLLC</i> converter and the peak efficiency of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLLC</i> converter is 99.33%. This letter shows for the first time that high power, high power density, high efficiency, and high electrical insulation can be achieved simultaneously by using the split planar structure. This letter is accompanied by a supplementary file demonstrating the applied voltage insulation test.

Hybrid Voltage Balancing Approach for Series-Connected SiC MOSFETs for DC–AC Medium-Voltage Power Conversion Applications

Using series-connected SiC <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> s to enable high-speed switching units is desired to meet the growing demand for high-density medium-voltage power converters. Due to its fast-switching speed, the voltage sharing of series-connected SiC <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> s is more sensitive to the surrounding parasitic components and gate-signal mismatch, which results in more challenges for the voltage balancing control. In this article, the gate turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> delay-time control is first used as an example to analyze the limitations of the existing active voltage balancing (AVB) control methods: 1) AVB control has a limitation of adjusting the delay time accurately under ac current conditions; and 2) the voltage imbalance of the body diodes cannot be solved by AVB control. To achieve voltage balancing control of series-connected SiC <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> s for ac current, this article proposes a new hybrid approach: 1) passive <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dv/dt</i> compensation: one small compensation capacitor is applied to balance the measured nonuniform distribution of parasitic capacitors <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">;</i> and 2) active gate signal turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> time adjustment: a digital closed-loop delay time control is applied to compensate the gate signal mismatch of <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> s. To verify the proposed balancing approach, a single-phase pump-back test is conducted to show the improvement of voltage sharing of both <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> s and body diodes.

Reducing Submodule Capacitance for Modular Multilevel Converter-Based Medium-Voltage Wind Power Converter

This study presents a strategy to reduce the capacitance of the submodule (SM) capacitor of a modular multilevel converter-based medium-voltage wind power converter. The design of the SM capacitor of the modular multilevel converter (MMC) should consider the actual operating conditions, especially the influence of wind speed, because wind speed will affect the voltage ripple and voltage amplitude of the SM capacitor. In the traditional method, the capacitance design of the SM capacitor will be based on relatively high wind speed and leave a certain safety margin. However, in most cases, the system operates at the highest-frequency wind speed (HFWS). For the MMC-based wind energy conversion system, a constant capacitor voltage ripple (CCVR) control method is proposed. Using this method, the SM capacitor voltage ripple can be significantly reduced by injecting the second harmonic component of circulating current. In this way, the SM capacitor with smaller capacitance can be used. Finally, the effectiveness of the proposed method is validated in RT-Lab Platform.

Technical Review of Traction Drive Systems for Light Railways

Due to environmental concerns, governments around the world are taking measures to decarbonise railway transport by replacing diesel traction systems with cleaner alternatives. While the electrification of railway systems is spreading rapidly, it is unlikely that all routes will be electrified as the volume of passengers will not justify the high infrastructure costs. Therefore, it is expected that, for several lines, a combination of hydrogen and electric traction will be used, with the latter partly provided by fixed infrastructure and partly by batteries. Railway traction drives will then need to change to accommodate these new types of power supply. A detailed review of the available traction motors and drives is provided with this review, given application to the new hybrid-electric systems. In particular, permanent magnet synchronous motors with multiphase windings are evaluated in comparison with traditional three-phase machines. Additionally, low and medium-voltage multisource power converters have been reviewed, taking into account the introduction of wide band-gap semiconductor devices.

For better density and efficiency: Technique of novel high-voltage SiC based power converters

Medium-voltage power converters have been widely used in diverse applications, such as electric grids, electric ship propulsion, and railway traction. Improving the efficiency and power density of such converters is crucial for cost and availability concerns.

Design of a 10 kV SiC MOSFET-based high-density, high-efficiency, modular medium-voltage power converter

Simultaneously imposed challenges of high-voltage insulation, high <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> , high-switching frequency, fast protection, and thermal management associated with the adoption of 10 kV SiC MOSFET, often pose nearly insurmountable barriers to potential users, undoubtedly hindering their penetration in medium-voltage (MV) power conversion. Key novel technologies such as enhanced gate-driver, auxiliary power supply network, PCB planar dc-bus, and high-density inductor are presented, enabling the SiC-based designs in modular MV converters, overcoming aforementioned challenges. However, purely substituting SiC design instead of Si-based ones in modular MV converters, would expectedly yield only limited gains. Therefore, to further elevate SiC-based designs, novel high-bandwidth control strategies such as switching-cycle control (SCC) and integrated capacitor-blocked transistor (ICBT), as well as high-performance/high-bandwidth communication network are developed. All these technologies combined, overcome barriers posed by state-of-the-art Si designs and unlock system level benefits such as very high power density, high-efficiency, fast dynamic response, unrestricted line frequency operation, and improved power quality, all demonstrated throughout this paper.

Modeling and test of a thermosyphon loop for the cooling of a megawatt-range power electronics converter

A thermosyphon loop, designed for the thermal management of a large Medium voltage power converter 5 MW overall, corresponding to a 2.4 kW thermal load per cooling unit) is presented. The device is mainly made of an evaporator, a condenser and a reservoir connected with plastic liquid and vapor lines. Novec 649 (3M) has been chosen as the working fluid due to environmental and electrical concerns. A model of the loop is described, and its predictions are compared with experiments. A first comparison yields a maximum mean deviation of 20 % between experimental results and numerical simulation at the maximum coolant temperature. The main sources of errors are identified, and improvements are proposed for better model accuracy. • Selection of an environment-friendly fluid (Novec 649). • Entire model of the loop thermo-syphon is presented. • Experimental validation on a full-scale protype (up to 2400 W thermal load) shows < 20% deviation.

A Multichannel High-Frequency Current Link Based Isolated Auxiliary Power Supply for Medium-Voltage Applications

The auxiliary power supply (APS) is one of the critical components inside medium-voltage (MV) power converters. Besides high insulation capability and small footprint, low common-mode (CM) coupling capacitance and multichannel output are the desired features of APS in the emerging silicon-carbide-based MV converters due to their fast switching speed. This article presents the design and optimization procedure of a high-density isolated APS using an LCCL-LC resonant topology with an operating frequency of 1 MHz. The proposed design procedure attains consistent soft-switching operation under a random number of output channels. The galvanic isolation is realized by a current-fed single-turn 1 MHz transformer that can achieve a breakdown voltage of over 20 kV while maintaining a small size. Design optimization on the insulation system of the current transformer is proposed to obtain both high partial-discharge inception voltage (PDIV) and low coupling capacitance. Finally, two versions of APSs are developed, using air and silicone as dielectric materials, which can reach PDIV of over 5 and 16 kV, respectively. The corresponding coupling capacitances are 1.86 pF and 3.6 pF. Both designs can provide a maximum power of 20 W on the receiving side, and 120 W on the sending side.

Review of Standards on Insulation Coordination for Medium Voltage Power Converters

The increasing viability of wide band gap power semiconductors, widespread use of distributed power generations, and rise in power levels of these applications have increased interest and need for medium voltage converters. Understanding the definitions of insulation coordination and their relationship to applications and methodologies used in the test environment allows system engineers to select the correct insulation materials for the design and to calculate the required distances between the conductive surfaces, accessible parts and ground accurately. Although, design guidelines are well established for low voltage systems, there are some deficiencies in understanding and meeting the insulation coordination requirements in medium voltage, medium frequency applications. In this study, an overview on standards for insulation coordination and safety requirements is presented to guide researchers in the development of medium voltage power electronic converters and systems. In addition, an insulation coordination study is performed as a case study for a medium frequency isolated DC/DC converter that provides conversion from a 13.8 kV AC system to a 4.16 kV AC system.

Analysis of Voltage Sharing of Series-Connected SiC MOSFETs and Body-Diodes

In recent years, SiC MOSFETs have gained strong attention in medium-voltage power conversion applications. To increase the blocking voltage level, series-connection of SiC MOSFETs is an attractive solution but may suffer a severe voltage unbalance issue. To gain insights into the voltage unbalance issue, this article presents a detailed study of the impact of parasitic capacitors on the voltage sharing of series-connected SiC MOSFETs and body-diodes. The parasitic capacitors are categorized into two groups for analysis: 1) parasitic capacitors from gate terminal; 2) parasitic capacitors from drain/source terminals. The study reveals that gate parasitic capacitors affect gate miller-plateau voltage and ultimately the dv/dt during the turn-off. The voltage sharing will be worse under higher turn-off current or larger gate resistor. The drain/source parasitic capacitors will introduce additional capacitance across device drain-source terminals which results in an unbalanced voltage sharing. The position of switching unit and the heatsink connection schemes will affect the distribution of drain/source parasitic capacitors to cause different voltage sharing results. The drain/source parasitic capacitors will also cause voltage unbalance of series-connected body-diodes under different conditions. To verify the analysis, the voltage sharing between two series-connected 10 kV SiC MOSFETs is tested under different parasitic capacitors conditions.

Partial Discharge Online Monitoring and Localization for Critical Air Gaps Among SiC-Based Medium-Voltage Converter Prototype

As a consequence of higher blocking voltage and power density by using SiC devices, detailed analysis and careful design of insulation are important for future medium-voltage power converters. Because of the insulation or mechanical coordination, air gaps inside the converters cannot be avoided. Moreover, some gaps are more critical than others, since much oversized design or electric shielding there cannot be acceptable. Admittedly, the final design should be partial discharge (PD) free under normal operation with a safety margin. However, due to possible change of air properties and insulation degradation, massive PD can be still initiated from high electric field regions along the gaps and may lead to the damage of devices, through the highly coupled power and driving circuitry. Therefore, with online PD monitoring and localization, insulation health condition of the critical gaps can be provided, which could have great values for converter operation, maintenance, and design. In this paper, after detailed acoustic signal propagation study, a fiber-optic acoustic sensor array has been implemented into a 6-kV H-bridge power electronics building block prototype, in order to realize online PD monitoring along the critical gaps. Finally, from experimental results, the proposed sensor array has good sensitivity, localization accuracy, and high immunity to the electromagnetic interference. It unveils a novel access to insulation online monitoring for future high voltage, high power density SiC-device-based converters.

Analysis of the Medium-Frequency Oscillation Issue in a Medium-Voltage High-Power Wind Energy Conversion System

One promising solution for future large wind turbine power conversion is to use medium-voltage power conversion systems for high efficiency and high power-density, e.g., by using a cascaded structure presented in this paper. However, a medium-frequency (MF) oscillation issue was observed while testing the experimental prototype of the above system. Given the complex multivariable coupling models of the multiwinding transformer and multiple inverters, conventional single-variable theories are not suitable. Therefore, this paper presents a new decoupling analysis method. Using a proposed linear transformation matrix, the output currents of multiple inverters can be transformed to a summation (grid) current and difference currents, where the former is the summation of inverter currents and the latter are the differences between currents of any two inverters. This transformation has enabled the use of single-variable theories, e.g., Bode diagram to analyze the summation current and difference current separately. It was found that the MF oscillation issue is caused by the equivalent impedance of the summation current being much larger than that of the difference current. Specifically, using small controller parameters cannot ensure the grid current quality and using large parameters may cause oscillation in the difference current. The proposed method can provide guidelines to design the parameters of current regulators and filters in multiple-inverter systems. Experimental results of a downscaled wind energy conversion system are presented to verify the proposed method.

Auxiliary Power Supply for Medium-Voltage Power Converters: Topology and Control

This paper presents an isolated auxiliary power supply for medium-voltage power electronics systems. The proposed converter comprises two stages: a non-isolated ac/dc stage that connects directly to the medium-voltage line, and an isolated dc/dc stage that provides 100-W output power at 24 V, with 10 kV isolation. The proposed modular ac/dc stage uses just one active semiconductor device per module, features an internal capacitor voltage balancing, and achieves power factor correction by employing predictive current control. High switching frequency operation of both converter stages enable a reduction in system size and weight when compared to traditional low-frequency transformer-based approach. The proposed converter is simulated and its operation is validated experimentally on a 100-W prototype.

A Reliable Medium-Voltage High-Power Conversion System for MWs Wind Turbines

With the increasing power ratings of large wind turbines, medium-voltage power conversion is drawing more and more attention because it generally offers higher power density and higher efficiency with reduced current levels. This paper evaluates the practical performance of a medium-voltage, fault-tolerant modular high-power conversion system based on cascaded modular converters. To avoid many large capacitor banks, the low-frequency voltage/power ripple in each cell has been attenuated through balancing the power flow between the H-bridge rectifier and the three-phase inverter by adding a frequency-adaptive quasi-resonant controller in the control loops. This can improve the system reliability and reduce the system cost and volume. In the experimental evaluation, an unexpected envelope oscillation issue was observed at the grid-side inverters. It was found that the above issue is caused by the discrepancy of the crystal oscillators on different digital signal processor boards. Then, a carrier-wave phase-locked loop has been proposed to eliminate the envelope oscillation issue. Experimental results of a downscaled three-phase five-level modular wind energy conversion system show that the dc-link voltage ripple can be eliminated without affecting the grid current quality.

Reliability evaluation of modular multilevel converter based on Markov model

The modular multilevel converter (MMC) is now the most attractive topology for medium and high voltage power conversion applications with several advantages over the traditional voltage source converter (VSC). However, due to a large number of sub-modules (SMs) in the MMC, system reliability is a big challenge in its practical application, where each SM may be considered as a potential point of failure. In this paper, a reliability evaluation based on the Markov model is proposed for the MMC. The failure rates of the power electronic devices and SMs are firstly analyzed. Then, the Markov model and the state transition equation of the system are built in detail. A general reliability evaluation function is established, in which the mean time to failure and reliability evaluation of the MMC with redundant SMs are also discussed. Finally, a practical direct current (DC) distribution example for reliability evaluation is analyzed, and the results verify that the reliability evaluation based on the Markov model could provide a useful reference for project design.

State-of-the-Art of the Medium-Voltage Power Converter Technologies for Grid Integration of Solar Photovoltaic Power Plants

More than 170 countries have already established renewable energy targets to meet their national increasing energy demand and also to keep their environment sustainable. Due to a number of features, the use of the multi-megawatt solar photovoltaic (PV) power plants is becoming the preferred choice for escalating and updating the power systems all over the world. Moreover, the solar PV power plant is also the first choice for meeting rapidly growing demands; as it can be installed relatively quickly, say in 6–12 months, compared to that of the fossil-fuel-based plants that may require more than 4–5 years. The traditional low-voltage (288–690 V) converter-based system requires a step-up transformer and a line filter to interconnect a solar PV power plant with medium-voltage grids. Recently, the use of medium-voltage converters without a step-up transformer and a line filter has become more attractive for direct medium-voltage grid integration of solar PV power plants. This paper aims to review the necessity and the technical challenges in developing medium-voltage power electronic converters, including the converter circuit topologies and control techniques used in the development of medium-voltage converters to interconnect solar PV power plants to medium-voltage grids directly. In this paper, a comprehensive review of the current research activities and the possible future directions of research to develop medium-voltage converter technologies to provide for a cost-effective grid integration of solar PV power plants are presented.