High Voltage DC-biased Oil Type Medium Frequency Transformer; A Green Solution for Series DC Wind Park Concept
Doktorsavhandling, 2023

The electric energy generated by remote offshore wind parks is transported to the consumers using high voltage submarine cables. On the generation site, such transmissions are realized today by collecting the energy produced by several wind turbines in a bulky and expensive transformer placed on a dedicated platform. An alternative solution has been proposed recently, which allows to reduce the installation and maintenance costs by eliminating such a platform. It is suggested to equip each wind turbine in the wind park by an individual DC/DC converter and connect them in series to reach the DC voltage level required for an efficient HVDC energy transportation to the shore. The DC/DC converter is supposed to be a Dual Active Bridge (DAB) converter, which can be made reasonably small to be placed on the wind turbine tower or even in its nacelle. The key element of the converter defining its size and mass is a special transformer, which operates at voltages comprising a high (switching) frequency component superimposed on a high DC offset voltage. DC insulation design of such a transformer and investigation of the effects of a high DC insulation level on the other electromagnetic properties of the transformer is the subject of the present research.
In order to verify the concept a prototype of the transformer was built, and its evaluation presented. The unit has been manufactured for the rated power of 50 kW and rated voltages 0.4/5 kV including DC offset of 125 kV and square-shaped oscillations with the frequency of 5 kHz. The magnetic system was made of ferrite material and consisted of 10 shell-type core segments. The magnetic properties have been verified by measuring magnetization and losses at various frequencies in the range 1-10 kHz to cover the operational range of the DAB. The types and dimensions of the windings and their conductors were chosen to minimize the proximity and eddy current effects at higher frequencies. To reduce the size of the transformer and to allow for its efficient cooling, the active part was immersed in oil and cellulose-based materials (paper and pressboard) were used to build the high voltage insulation system.
The principles for dimensioning the insulation of the transformer are discussed. The criteria used for selecting insulating distances were based on the consideration of the electric field strength obtained from FEM simulations and using the non-linear Maxwell-Wagner model accounting for local variations of the electric field caused by accumulation of interfacial charges induced by DC stresses. The properties of the materials needed for the calculations were obtained by measuring their dielectric constants and electric conductivities. The methodology used for the measurements conducted for conventional mineral oil and eco-friendly biodegradable transformer oils and, respectively, for oil-impregnated paper/pressboard, is presented.
The methodologies used for obtaining parameters of the built transformer prototype needed for its integration in the power electric circuit of the DAB are introduced. A method developed for accurate calculations of the leakage inductance for the shell-type multi core transformers with circular windings is described. Two innovative methods for evaluations of parasitic capacitances based on high frequency equivalent circuits of the transformer are presented. The results of their verifications against performed Frequency Response Analysis measurements and FEM calculations as well as their accuracy are discussed.
Thermal performance of the developed transformer prototype is analysed based on the results of computer simulations of heat transfer in its active part under rated load. Identified hot spots and solutions for their elimination are presented.
Finally, the expected dimensions, weight, and efficiency of an actual DC/DC converter with the rated parameters corresponding to a 6 MW, 1.8 kV real wind turbine having a 250 kV offset DC voltage are estimated assuming that the developed transformer prototype is scalable. It is shown that the proposed solution allows for installing the full-scale converter having 2.2 Tons in weight and 1.8 m3 in volume on the bottom of the wind turbine’s tower.

High voltage DC

Insulation design

Offshore wind farm

Leakage inductance

High power isolated DC/DC converter

Dual active bridge

Medium-frequency power transformer

Parasitic capacitance

HVDC

Room EB, Hörsalsvägen 11, Göteborg
Opponent: Professor Andreas Küchler, Schweinfurt University of Applied Sciences, Germany

Författare

Mohammad Kharezy

Chalmers, Elektroteknik, Elkraftteknik

A Novel Oil-immersed Medium Frequency Transformer for Offshore HVDC Wind Farms

IEEE Transactions on Power Delivery,; Vol. 36(2021)p. 3185-3195

Artikel i vetenskaplig tidskrift

Performance of Insulation of DC/DC Converter Transformer for Offshore Wind Power Applications

Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP,; Vol. 2020-October(2020)p. 382-385

Paper i proceeding

Estimation of the winding losses of Medium Frequency Transformers with Litz wire using an equivalent permeability and conductivity method

2020 22nd European Conference on Power Electronics and Applications, EPE 2020 ECCE Europe,; (2020)

Paper i proceeding

Optimum Leakage Inductance Determination for a Q2L-Operating MMC-DAB with Different Transformer Winding Configurations

20th IEEE International Symposium on Power Electronics (Ee),; (2019)

Paper i proceeding

Calculation of the leakage inductance of medium frequency transformers having rectangular-shaped windings using an accurate analytical method

2019 21st European Conference on Power Electronics and Applications, EPE 2019 ECCE Europe,; (2019)

Paper i proceeding

Optimization and experimental validation of medium-frequency high power transformers in solid-state transformer applications

31st Annual IEEE Applied Power Electronics Conference and Exposition, APEC 2016; Long Beach Convention and Entertainment CenterLong Beach; United States; 20 March 2016 through 24 March 2016,; (2016)p. 3043 - 3050

Paper i proceeding

Design Methodology and Optimization of a Medium-Frequency Transformer for High-Power DC–DC Applications

IEEE Transactions on Industry Applications,; Vol. 52(2016)p. 4225-4233

Artikel i vetenskaplig tidskrift

Design methodology and optimization of a medium frequency transformer for high power DC-DC applications

2015 Thirtieth Annual IEEE Applied Power Electronics Conference and Exposition (APEC 2015), 30th Annual IEEE Applied Power Electronics Conference and Exposition (APEC), Charlotte, NC MAR 15-19, 2015,; (2015)p. 2532-2539

Paper i proceeding

M. Kharezy, M. Eslamian, T. Thiringer, Y. Serdyuk, and B. Khanzadeh, "Determination of Parasitic Capacitance of High- Power Medium Frequency Transformers: Case study of a high voltage DC biased transformer for wind power application"

M. Kharezy, M. Eslamian, and T. Thiringer, "Core Loss Evaluation of MFTs with Square-wave Voltage Excitation"

A green High voltage DC-biased oil type Medium Frequency Transformer
Electric energy generated by offshore wind parks is transported to the consumer with HV (High Voltage) submarine cables using AC (Alternating Current=växelspänning), the same as we have in our sockets at home. AC transmission is realized today by collecting the energy produced by several wind turbines in a bulky and expensive transformer, placed on a dedicated platform. Installation and maintenance costs could be reduced by eliminating such platforms using DC (Direct Current=likspänning), i.e., the same energy form that a battery gives out. Equipping each wind turbine with an individual DC/DC converter and connecting them in series to reach the required DC voltage level makes efficient HVDC energy transportation to the shore possible. The converter can be made sufficiently small to be placed on the wind turbine tower or even in its nacelle by implementing special MFTs (Medium Frequency Transformers), which operate at voltages comprising of a high-frequency component superimposed onto a high DC offset voltage. DC insulation design of such transformers and investigation of the effects of high DC insulation level on other electromagnetic properties of the transformer is the subject of the present research. A solution is presented, and a prototype is constructed. The unit has the rated power of 50 kW and rated voltages of 0.4/5 kV including DC offset of 125 kV and square-shaped oscillations with the frequency of 5 kHz. To reduce size and allow for efficient cooling, the transformer’s active part is immersed in an environmentally friendly oil, and cellulose-based materials is used for insulation. The approach is extended and the design of a full-scale DC/DC converter with rated parameters of 6 MW, 1.8 kV having a 250 kV offset DC voltage is presented, enabling installation of converters weighing 2.2 tons with volumes of 1.8 m3 at the bottom of the wind turbine towers.

Design of cost effective DC-based collection-network for inland-sea wind-farm using series high-frequency transformers

Energimyndigheten (44983-1), 2017-11-01 -- 2020-10-31.

Drivkrafter

Hållbar utveckling

Innovation och entreprenörskap

Styrkeområden

Energi

Fundament

Grundläggande vetenskaper

Ämneskategorier

Annan elektroteknik och elektronik

ISBN

978-91-7905-861-6

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5327

Utgivare

Chalmers

Room EB, Hörsalsvägen 11, Göteborg

Opponent: Professor Andreas Küchler, Schweinfurt University of Applied Sciences, Germany

Mer information

Senast uppdaterat

2023-08-30