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  • 1.
    Alikhanzadeh, Babak
    et al.
    Chalmers University of Technology, Sweden.
    Thiringer, Torbjörn
    Chalmers University of Technology, Sweden.
    Kharezy, Mohammad
    Optimum Leakage Inductance Determination for a Q2L-Operating MMC-DABwith Different Transformer Winding Configurations2019In: 2019 20th International Symposium on Power Electronics (Ee), 2019Conference paper (Refereed)
    Abstract [sv]

    This paper discusses the procedure for the determination of the optimum leakage inductance of a medium frequency transformer (MFT) for a quasi-two-level (Q2L) operating three-phase modular multilevel converter dual-active bridge (MMC-DAB) considering the effects of the MFT winding configuration. Three different winding configurations-namely, Y-Y, D-D, and Y-D-are considered for the connection of the MFT windings. The optimum leakage inductance requirement of the MFT, the currents total harmonic distortion (THD), and the transformer utilization factor (TUF) are compared for the three winding configurations. It is found that the Y-Y and the D-D configurations have similar optimum leakage inductance patterns, which are different from the Y-D configuration. Furthermore, it is established that an optimized leakage inductance value for a conventional DAB, can be utilized for the Q2L-operating MMC-DAB for a wide range of transition time with the Y-Y and the D-D winding configurations if less than a 5% error could be tolerated in its value. A comparison with respect to the TUF and the currents THD revealed that the Y-Y and the D-D configurations result in a higher TUF compared to the Y-D configuration; However, the Y-D configuration has approximately two times lower currents THD compared to the other two configurations

  • 2.
    Bahmani, Amin
    et al.
    Chalmers University of Technology, Sweden.
    Thiringer, Torbjörn
    Chalmers University of Technology, Sweden.
    Kharezy, Mohammad
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik, Elektricitet.
    Design Methodology and Optimization of a Medium Frequency Transformer for High Power DC-DC Applications2015In: Applied Power Electronics Conference APEC, 2015, , p. 8p. 2532-2539Conference paper (Other academic)
    Abstract [en]

    The high power medium frequency transformer (HPMFT) is one of the key elements of an isolated, bi-directional DC-DC converters in applications such as future all-DC offshore wind farms, traction and solid state transformers. This paper describes a design methodology taking into account the loss calculation, isolation requirements and thermal management. Incorporating this design methodology, an optimization process with a wide range of parameter variations is applied on a design example to find the highest power density while the efficiency, isolation, thermal and leakage inductance requirements are all met.

  • 3.
    Bahmani, Amin
    et al.
    Chalmers University of Technology, Sweden.
    Thiringer, Torbjörn
    Chalmers University of Technology, Sweden.
    Kharezy, Mohammad
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik, Elektricitet.
    Optimization and Experimental Validation of Medium-Frequency High Power Transformers in Solid-State Transformer Applications2016In: 2016 IEEE Applied Power Electronics Conference and Exposition (APEC), 2016, , p. 8p. 3043-3050Conference paper (Other academic)
    Abstract [en]

    High power isolated DC-DC converters are likely to provide solutions for many technical challenges associated with power density, efficiency and reliability in potential applications such as offshore wind farms, inter-connection of DC grids, MVDC in data centers and in future solid state transformer applications. The high power medium frequency transformer (HPMFT) is one of the key elements of such a converter to realize the voltage adaption, isolation requirements, as well as high power density. This paper describes a design and optimization methodology taking into account the loss calculation, isolation requirements and thermal management. Incorporating this design methodology, an optimization process with a wide range of parameter variations is applied on a 50 kW, 1 / 3 kV, 5 kHz transformer to find the highest power density while the efficiency, isolation, thermal and leakage inductance requirements are all met. The optimized transformers are then manufactured and will be presented in this paper.

  • 4.
    Elg, Alf Peter
    et al.
    RISE, SP – Sveriges Tekniska Forskningsinstitut.
    Bergman, Anders
    RISE, SP – Sveriges Tekniska Forskningsinstitut.
    Hällström, Jari
    MIKES Centre for Metrology and Accreditation, Finland.
    Kharezy, Mohammad
    RISE, SP – Sveriges Tekniska Forskningsinstitut.
    Nieminen, Tatu
    RISE, SP – Sveriges Tekniska Forskningsinstitut.
    Traceability and Characterization of a 1000 kV HVDC Reference Divider2015In: IEEE Transactions on Instrumentation and Measurement, ISSN 0018-9456, E-ISSN 1557-9662, Vol. 64, no 6, p. 1709-1715, article id 7100891Article in journal (Refereed)
    Abstract [en]

    This paper presents the characterization of a resistive high-voltage dc (HVDC) reference divider and methods to establish traceability. The divider is designed for use as a laboratory reference for calibration of HVDC measuring systems up to 1000 kV. Targeting a measurement uncertainty of 20 μV/V at full voltage has put a focus on the temperature coefficients of the resistors, elimination of humidity dependence, and control of leakage currents in the high-voltage arm. A scale factor calibration against a 50 kV divider at 10 kV leads to an expanded uncertainty of 15 μV/V.

  • 5.
    Eslamian, Morteza
    et al.
    UNIVERSITY OF ZANJAN, Iran.
    Kharezy, Mohammad
    RISE - Research Institutes of Sweden, Safety and Transport, Measurement Science and Technology.
    Thiringer, Torbjörn
    Chalmers University of Technology, Sweden.
    Calculation of the leakage inductance of medium frequency transformershaving rectangular-shaped windings using an accurate analytical method2019In: 2019 21st European Conference on Power Electronics and Applications (EPE '19 ECCE Europe), 2019Conference paper (Refereed)
    Abstract [en]

    To achieve the lowest loss by the Zero-Voltage Switching of a Dual Active Bridge converter, it is crucial to precisely calculate the embedded Leakage Inductance of the used Medium Frequency Transformer (MFT). An effective analytical method is proposed for calculation of the leakage inductance of the MFT with rectangular-shaped windings

  • 6.
    Kharezy, Mohammad
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik, Elektricitet.
    Reliability study of monitoring systems for power transformers in the smart grid2015Report (Refereed)
    Abstract [en]

    The project suggests appropriate methods for quality assurance of measurement processes and measuring equipment used for condition monitoring of power transformers, specifically for selection of on-line monitoring systems for: • Temperature • Partial discharge (PD) • Sound and vibration The developed methods can be developed further in subsequent projects to cover the quality of other types of measurement processes and measuring equipment as part of maintenance planning activities.

  • 7.
    Kharezy, Mohammad
    et al.
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik, Elektricitet.
    Lindgren, Mikael
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik, Kommunikation.
    Fiberoptic sensors for high-voltage applications1981Report (Refereed)
    Abstract [en]

    SP has experience in the field of fiberoptic communication and expertise in research in the field. Also, SP has expertise in the high-voltage field and there is currently a large interest in developing the high-voltage network in order to accommodate new small producers of electric power (wind mills, solar power, etc.) In this report we describe efforts to find areas of interest to develop fiberoptic sensors for offshore monitoring of high-voltage equipment. A large number of research papers and review articles have been collected and reviewed in order to investigate what possibilities are at hand for SP to provide support for development of new or existing fiberoptic sensor technologies.

  • 8.
    Kharezy, Mohammad
    et al.
    RISE - Research Institutes of Sweden, Safety and Transport, Measurement Science and Technology.
    Morteza, Eslamian
    University of Zanjan, Iran.
    Thiringer, Torbjörn
    Chalmers University of Technology, Sweden.
    Insulation Design of a Medium Frequency Power Transformer for a Cost-Effective Series High Voltage DC Collection Network of an Offshore Wind Farm2019In: Proceedings of the 21st International Symposium on High Voltage Engineering, Switzerland: Springer Publishing Company, 2019, p. 06-1417Conference paper (Refereed)
    Abstract [en]

    DC can be used advantageously in transmission of power from offshore wind farms, utilizing series connection of the individual turbines’ outputs. Employing individual wind turbine converters, operating at high frequency, the advantage of DC can be achieved without the need for a large central converter. This solution obviates the need of expensive, bulky and heavy 50 Hz transformer or centralized DC/DC converter platforms.

    Power electronic converters are among the enabling technologies to establish a series DC intertie system. The most important advantage of a DC/DC converter using a Medium Frequency Power Transformer (MFPT) is its compact size in comparison with a normal grid frequency transformer.

    The function of a MFPT is to provide voltage step up and isolation between generators and the HVDC link. The need for a compact structure dictates further design constraints. A MFPT should have a high-power transmission capability from a small volume as well as minimal internal insolation distances. Furthermore, in the series DC integration circuit, DC/DC converters can experience a very high offset DC voltage depending on the converter’s position with respect to earth (in the range of one or two hundred kilovolts), while the primary side (wind turbine generator side) is only exposed to a few kV.

    Due to its promising features, many research activities have been devoted to the design and optimization of MFPTs. However, only a few activities specifically addressed the insulation requirements for them.

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