| 1. |
- Eslamian, Morteza, et al.
(författare)
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An Accurate Analytical Method for Leakage Inductance Calculation of Shell-type Transformers with Rectangular Windings
- 2021
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Ingår i: IEEE Access. - : Institute of Electrical and Electronics Engineers Inc.. - 2169-3536. ; 9, s. 72647-72660
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents an accurate analytical method for calculating the leakage inductance of shell-type E-core transformers with rectangular windings. For this purpose, first, an expression for calculating the leakage inductance per unit length inside the core window considering the core walls as the flux-normal boundary condition is derived. Then, a new accurate method for determining the Mean Length of Turns (MLT) based on the total stored energy is presented. The MLT is needed for the leakage inductance calculation using 2-D methods. By dividing the MLT into three partial lengths and calculating the corresponding leakage inductances using three different core window arrangements, the effect of core structure on the total leakage inductance is considered. The method is verified by 3-D FEM simulations as well as the leakage inductance measurements on two different fabricated transformer prototypes. The superiority of the method is also confirmed by comparisons with the previous analytical approaches. The proposed method enables the leakage inductance calculation with an error less than 1%, compared to the 3-D FEM results. Using the presented method, the leakage inductance calculations can be performed rapidly and accurately in the design stage without the need for time-consuming 3-D FEM simulations. CCBYNCND
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| 2. |
- Eslamian, Morteza, et al.
(författare)
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An Accurate Method for Leakage Inductance Calculation of Shell-Type Multi Core-Segment Transformers With Circular Windings
- 2021
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Ingår i: IEEE Access. - 2169-3536. ; 9, s. 111417-111431
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Tidskriftsartikel (refereegranskat)abstract
- The leakage field in shell-type transformers is strongly affected by the boundary conditions introduced by the core walls and thus the effect of the core should be considered properly in the leakage inductance calculation. In this paper, a new method for accurate calculation of the leakage inductance of shell-type multi core-segment transformers with circular windings is presented. For this purpose, first, the expressions for self and mutual inductances are derived in cylindrical coordinates considering the core walls as the flux-normal boundary condition. Then, a new approach is proposed for calculating the leakage inductance considering the number and dimensions of the used core segments. The method is developed at first for single and double core-segment transformers (known also as E-core and U-core transformers) and then adopted for shell-type segmented-core transformers. The method is verified by 3-D FEM simulations. The comparisons with the previous analytical methods demonstrate the superiority of the proposed method. A transformer prototype has been built and verification tests have been conducted. The comparisons show that the leakage inductance can be estimated with an error less than 1%, demonstrating a very high accuracy with the proposed method.
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| 3. |
- Eslamian, Morteza, 1984, et al.
(författare)
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Calculation of the leakage inductance of medium frequency transformers having rectangular-shaped windings using an accurate analytical method
- 2019
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Ingår i: 2019 21st European Conference on Power Electronics and Applications, EPE 2019 ECCE Europe.
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Konferensbidrag (refereegranskat)abstract
- Direct current (DC) can be used advantageously in transmission of power from offshore wind farms. A dual active bridge (DAB) converter is among those popular topologies which can be used for this high-power application. A DAB can benefit from the MFT’s leakage inductance (Ls) to transfer the power between the input and output bridges and this will result in higher power density. By controlling the switching on the two sides, the square wave voltage from the primary and the secondary converters will be adjusted to have a defined phase shift relative to each other. Such a transformer serves as an inductance, in addition to its natural duty of voltage adoption between the primary and secondary bridges. In other words, the design of the transformer should fulfil the criteria of having a determined leakage inductance. The voltage wave forms applied to the MFT in a DAB converter are not sinusoidal as in a conventional low frequency transformer. 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.
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| 4. |
- Kharezy, Mohammad, et al.
(författare)
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A Novel Oil-immersed Medium Frequency Transformer for Offshore HVDC Wind Farms
- 2021
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Ingår i: IEEE Transactions on Power Delivery. - 0885-8977 .- 1937-4208. ; 36:5, s. 3185-
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Tidskriftsartikel (refereegranskat)abstract
- Offshore wind farms pose the challenge of transmission of produced energy in an economic and effective way over long distances, justifying investments in High Voltage Direct Current (HVDC) systems. A solution to surmount the need for an expensive and large power transformer station is to connect each wind turbine to a DC/DC converter in which a Medium Frequency Transformer (MFT) is utilized to reduce the size of the station. Further, the DC outputs of the converters of the wind turbines can be connected in series to create a high DC voltage. In this solution, the MFTs are required to be insulated against a high DC voltage to ground in a limited compartment. This contribution presents a prototype of 125 kV 50 kW 5 kHz oil-paper insulated MFT, which has been manufactured and subjected to verification HV tests. The dielectric design of the prototype MFT based on finite element solution of non-linear Maxwell-Wagner equations is introduced and the acceptance criteria and practical design considerations are discussed.
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| 5. |
- Kharezy, Mohammad, et al.
(författare)
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Determination of Parasitic Capacitance of High-Power Medium Frequency Transformers : Case study of a high voltage DC biased transformer for wind power application
- 2023
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Ingår i: IEEE Transactions on Power Delivery. - : Institute of Electrical and Electronics Engineers (IEEE). - 0885-8977 .- 1937-4208. ; 38:6, s. 4263-
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Tidskriftsartikel (refereegranskat)abstract
- Determination of parasitic capacitances is a necessary step in the design process of transformers to be used in power electronic devices. At high switching frequencies, these capacitances together with inductive elements of the circuit cause unwanted oscillations during fast transients such as sharp fronts of square voltage waves appearing in a dual active bridge converter. In the paper, parasitic capacitances of high-power transformers supposed to operate at frequencies up to several kHz and high DC offset voltages (100 kV and higher) are analyzed. Two methods based on the equivalent circuit approach are suggested. To validate the methods, they are used to determine the parameters of an equivalent circuit of a prototype transformer developed earlier. The capacitances are calculated using the results of 3D FEM simulations performed based on the structure of the transformer as input. Furthermore, another set of data used for validation of the proposed calculation methods was obtained from the conducted direct measurements of frequency dependent impedance of the transformer. The accuracy of the proposed methods is discussed. It is concluded that the methods can be used for quick estimations of the parameters of transformers designed for power electronic applications.
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| 6. |
- Kharezy, Mohammad, et al.
(författare)
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Determination of Parasitic Capacitance of High-Power Medium Frequency Transformers: Case study of a high voltage DC biased transformer for wind power application
- 2023
-
Ingår i: IEEE Transactions on Power Delivery. - 0885-8977 .- 1937-4208. ; 38:6, s. 4263-4273
-
Tidskriftsartikel (refereegranskat)abstract
- Determination of parasitic capacitances is a necessary step in the design process of transformers to be used in power electronic devices. At high switching frequencies, these capacitances together with inductive elements of the circuit cause unwanted oscillations during fast transients such as sharp fronts of square voltage waves appearing in a dual active bridge converter. In the paper, parasitic capacitances of high-power transformers supposed to operate at frequencies up to several kHz and high DC offset voltages (100 kV and higher) are analyzed. Two methods based on the equivalent circuit approach are suggested. To validate the methods, they are used to determine the parameters of an equivalent circuit of a prototype transformer developed earlier. The capacitances are calculated using the results of 3D FEM simulations performed based on the structure of the transformer as input. Furthermore, another set of data used for validation of the proposed calculation methods was obtained from the conducted direct measurements of frequency dependent impedance of the transformer. The accuracy of the proposed methods is discussed. It is concluded that the methods can be used for quick estimations of the parameters of transformers designed for power electronic applications.
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| 7. |
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| 8. |
- Kharezy, Mohammad, et al.
(författare)
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Insulation Design of a Medium Frequency Power Transformer for a Cost-Effective Series High Voltage DC Collection Network of an Offshore Wind Farm
- 2019
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Ingår i: Proceedings of the 21st International Symposium on High Voltage Engineering. - Switzerland : Springer Publishing Company. - 9783030316792 ; 599, s. 1406-1417
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Konferensbidrag (refereegranskat)abstract
- 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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