| 1. |
- Abdulahovic, Tarik, 1976, et al.
(författare)
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Improvement of admittance measurements for modeling of an ungrounded reactor/transformer winding
- 2016
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Ingår i: International Journal of Modeling, Simulation, and Scientific Computing. - 1793-9615 .- 1793-9623. ; 7:3
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, a model of an ungrounded reactor winding is developed for transient voltage studies. The developed model is a black box model where the state-space model is developed using the directly measured admittance matrix of the reactor winding. Furthermore, an improved model is developed, where the accuracy of the admittance matrix measurement set in the low frequency band is obtained indirectly using an additional set of voltage ratio measurements. When the measured admittances are low, the accuracy of the diagonal admittance vectors in the low frequency band is low as well. However, the accuracy at low frequencies obtained using the indirect measurements is improved at nodes with the lowest admittances. In this paper, different approaches for indirect measurements are studied and the most accurate method is identified considering the physical structure of windings.
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| 2. |
- Abdulahovic, Tarik, 1976, et al.
(författare)
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Modeling of the energizing of a wind park radial
- 2007
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Ingår i: Nordic Wind Power Conference, Roskilde, Denmark.
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Konferensbidrag (refereegranskat)abstract
- This paper presents measurement and modeling ofthe energizing of a wind park radial. The measurements in this paper are taken from the Utgrunden wind park that consists of seven wind turbines each of 1.5 MW rated power. This park is connected to the Öland 55 kV grid at the Degerhamn station. The wind turbines in the Utgrunden wind park are of the pitchregulated doubly-fed induction generator (DFIG) type. The transient voltage and the energizing current graphs are presented and both active and reactive powers are analyzed as the radials to the wind park is connected. Furthermore, the model of the Utgrunden wind park radial is built using PSCAD/EMTDC software. For the cable model, the frequency-dependent (phase) model built in model is used. A built-in model with the saturation modeling is simulated for the purpose of the transients analysis and the results are compared with measurements. The results show that the measured resonance frequency agrees well with the measurements while the damping is underestimated in the simulations.
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| 3. |
- Abdulahovic, Tarik, 1976, et al.
(författare)
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Simulation accuracy of the built-in PSCAD and an owner-defined synchronous machine model
- 2010
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Ingår i: COMPEL - The International Journal for Computation and Mathematics in Electrical and Electronic Engineering. - : Emerald. - 0332-1649. ; 29:3, s. 840-855
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Tidskriftsartikel (refereegranskat)abstract
- Purpose - The purpose of this paper is to investigate the influence of time steps, integration methods, and saturation modeling on the accuracy of the synchronous machine model. This model is compared with the PSCAD built-in synchronous machine model in order to compare the accuracy of one of the most used synchronous machine models in a commercially available software versus a well-documented and widely accepted state-space synchronous machine model. Design/methodology/approach - In the paper, a synchronous condenser with the saturation phenomenon is modeled using state-space equations in the rotating dq-reference frame and is implemented both in Matlab/Simulink and PSCAD. Integration methods of up to the fifth order are implemented for increased accuracy. The saturation modeling includes modeling of the saturation in both d- and q-axis. A steady-state and dynamic performance comparison towards the built-in PSCAD synchronous machine model is performed. The saturation modeling does not include the saturation of the leakage fluxes. Findings - When the forward Euler method is used, in order to obtain less than 5 percent error, the time step should not exceed 5 its. The third-order Runge-Kutta method is the preferred choice and it provides desired accuracy when the time step is equal or smaller than 1,000 its. The built-in PSCAD model satisfies the error criteria for time steps smaller than 300 its. A small discrepancy of 2 percent is found during the steady-state test. Originality/value - The paper presents the performance of the higher order integration methods in an EMTP-type software environment where the trapezoidal integration method is most often used. It provides a good guide for building an owner-defined model. A comparison of a dynamic performance between the publicly documented state-space and a synchronous machine models commonly used for power system transient studies is presented.
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| 4. |
- Abdulahovic, Tarik, 1976, et al.
(författare)
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Transformers internal voltage stress during current interruption for different wind turbine layouts
- 2012
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Ingår i: EPE Joint Wind Energy and T&D Chapters. ; , s. 6-
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Konferensbidrag (refereegranskat)abstract
- The voltage transients generated during the breaker operations in cable systems, for instance, a wind park collection grid, can reach very low rise times. The rise times of these transients can be almost 50 times shorter than the rise time of a lightning pulse. Such transients can generate a very high voltage stress on the internal transformers insulation. In this paper, a test case is studied using verified models of different types of transformers and wind turbine layouts in order to account for typical wind turbine layouts found in modern wind farms. A critical switching scenario is chosen in order to provoke the highest possible voltage transients during a current interruption. Furthermore, internal overvoltages are estimated using model of a similarly sized winding. Simulations show that the magnitude of the voltage transients is higher than the basic lightning impulse insulation level (BIL) defined by present standards. Moreover, it is found that the rise time of the voltage surges is much shorter than the rise time of the lightning pulse. The shortest rise time of 40ns is obtained in a wind turbine layout where the wind turbine breaker is placed near the transformer. Due to very short rise times of the voltage transients, very high internal overvoltages are estimated in dry-type transformer windings. These internal overvoltages are much higher than overvoltages recorded at the basic lightning impulse level. For a wind turbine layout where a breaker is placed in the bottom of a tower and a dry-type transformer in a nacelle, the highest turn-to-turn voltage is estimated of about 1:5pu. This is almost 4 times higher turn-to-turn voltage compared to what is obtained during the BIL test. In a wind turbine layout where a breaker is placed close to the transformer, the amplitude of the turn-to-turn voltages reached 1:8pu due to lower stray capacitances and thus a shorter rise time of voltage strikes.
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| 5. |
- Abdulahovic, Tarik, 1976, et al.
(författare)
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Transformers internal voltage stress during current interruption for different wind turbine layouts
- 2013
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Ingår i: 2013 15th European Conference on Power Electronics and Applications, EPE 2013. - 9781479901166 ; , s. no. 6634759-
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Konferensbidrag (refereegranskat)abstract
- The voltage transients generated during the breaker operations in cable systems, for instance, a wind park collection grid, can reach very low rise times. The rise times of these transients can be almost 50 times shorter than the rise time of a lightning pulse. Such transients can generate a very high voltage stress on the internal transformers insulation. In this paper, a test case is studied using verified models of different types of transformers and wind turbine layouts in order to account for typical wind turbine layouts found in modern wind farms. A critical switching scenario is chosen in order to provoke the highest possible voltage transients during a current interruption. Furthermore, internal overvoltages are estimated using model of a similarly sized winding. Simulations show that the magnitude of the voltage transients is higher than the basic lightning impulse insulation level (BIL) defined by present standards. Moreover, it is found that the rise time of the voltage surges is much shorter than the rise time of the lightning pulse. The shortest rise time of 40ns is obtained in a wind turbine layout where the wind turbine breaker is placed near the transformer. Due to very short rise times of the transients, very high internal overvoltages are estimated in dry-type transformer windings. These internal overvoltages are much higher than overvoltages recorded at the basic lightning impulse level. For a wind turbine layout where a breaker is placed in the bottom of a tower and a dry-type transformer in a nacelle, the highest turn-to-turn voltage of about 1.5pu is estimated. This is almost 4 times higher turn-to-turn voltage then the voltage obtained during the BIL test. In a wind turbine layout where a breaker is placed close to the transformer, the amplitude of the turn-to-turn voltages reached 1.8pu due to lower stray capacitances and thus a shorter rise time of voltage strikes.
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| 6. |
- Abdulahovic, Tarik, 1976, et al.
(författare)
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Vacuum Circuit-Breaker Parameter Calculation and Modelling for Power System Transient Studies
- 2017
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Ingår i: IEEE Transactions on Power Delivery. - : Institute of Electrical and Electronics Engineers (IEEE). - 0885-8977 .- 1937-4208. ; 33:3, s. 1165-1172
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, a black-box vacuum circuit-breaker model is developed and its parameters are calculated. The developed model is deterministic and does not include the stochastic behavior of arc interruption. The phenomena, such as the current chopping, high-frequency current quenching capability, and rate of rise of dielectric strength are treated as deterministic and calculated using mean values. The vacuum breaker model is verified using a laboratory test setup, and simulation results show good agreement with the obtained measurements. The withstand voltage of the breaker during appearance of the voltage restrikes shows good agreement with the measurements. Moreover, the magnitude and repetitiveness of the high-frequency transients obtained in simulations are within a 10% margin when compared with measurements.
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| 7. |
- Abdulahovic, Tarik, 1976, et al.
(författare)
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Voltage Stress in a Transformer Winding During Very Fast Transients Caused by Breaker Closing Event
- 2014
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Ingår i: IEEE Transactions on Power Delivery. - 0885-8977 .- 1937-4208. ; 29:4, s. 1946-1954
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Tidskriftsartikel (refereegranskat)abstract
- Transformers connected at the medium-voltage level in cable grids, such as wind park collection grids and industrial grids, are exposed to the stress of very fast transients. These electric transients are mainly generated during breaker switching operations and the rise time of the transient voltage in such systems is much shorter compared to the rise times of transients generated in transmission systems at a high-voltage level. In this paper, the internal voltage stress is studied during very fast transients generated during transformer energization. Instead of using a breaker, the energizing tests are performed using a low-impedance pulse generator that can generate lightning impulse-shaped waveforms and voltage steps with rise times varying between 35 and 500 ns. Experiments show that during very fast transients with a 35-ns rise time and 1-p.u. magnitude, the interturn voltage exceeds the level obtained with a lightning impulse-shaped voltage waveform of 4.4 pu. Furthermore, during a specific switching scenario with delta-connected transformers, where the winding is excited from both ends, the same 1-p.u./35-ns voltage step generates an interturn voltage that exceeds the 1-p.u. level, which is more than 2.5 times higher voltage stress than during a lightning impulse test.
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| 8. |
- Acquaviva, Alessandro, 1987, et al.
(författare)
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Analytical Conduction Loss Calculation of a MOSFET Three-Phase Inverter Accounting for the Reverse Conduction and the Blanking Time
- 2021
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Ingår i: IEEE Transactions on Industrial Electronics. - 0278-0046 .- 1557-9948. ; 68:8, s. 6682-6691
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Tidskriftsartikel (refereegranskat)abstract
- The reverse conduction capability of MOSFETs is beneficial for the efficiency of a three-phase inverter. In this paper analytical expressions in closed form are presented which allow to quickly evaluate the conduction losses, considering the effect of the reverse conduction and blanking time for both sinusoidal PWM operation with and without third harmonic injection. The losses of a three-phase SiC MOSFET inverter suitable for traction applications are estimated with the proposed method and show good agreement of about 98.5 % with measurements, performed with a calorimetric setup.
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| 9. |
- Acquaviva, Alessandro, 1987, et al.
(författare)
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Computationally Efficient Modeling of Electrical Machines With Cooling Jacket
- 2019
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Ingår i: IEEE Transactions on Transportation Electrification. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 2332-7782. ; 5:3, s. 618-629
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Tidskriftsartikel (refereegranskat)abstract
- Modeling of electrical machines is a multiphysics problem. Depending on the phenomena of interest and the computational time constraint, this can be done at different levels of detail. In this article, the main approaches to model the thermal behavior of electrical machines with a liquid cooled casing around the stator (often referred to as cooling jacket) are analyzed and a novel approach is presented. The proposed method aims at creating computationally efficient 3-D multiphysics models of electrical machines with liquid cooled jacket. This model is based on the assumption of a fully developed flow in the cooling jacket which allows to scale the computational fluid dynamics (CFD) simulation to 1-D. The slot with a two layer concentrated winding and potting material is modeled using a composite material comprising of both the conductors and slot filler. Similarly, a unified material is used to model the end-windings. Experimental results on a traction machine for vehicle applications are presented showing good agreement with the simulations. Also, a comparison with a 3-D CFD is presented to verify the pressure drop in the pipe bend. Finally, the model is used to simulate a dynamic load cycle, which would be computationally extremely demanding with combined 3-D CFD and thermal FEA of the machine and its cooling.
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| 10. |
- Acquaviva, Alessandro, 1987, et al.
(författare)
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Design and Verification of In-slot Oil-Cooled Tooth Coil Winding PM Machine for Traction Application
- 2021
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Ingår i: IEEE Transactions on Industrial Electronics. - 0278-0046 .- 1557-9948. ; 68:5, s. 3719-3727
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Tidskriftsartikel (refereegranskat)abstract
- Tooth coil windings, in particular when using a double layer structure, present opportunities for in-slot liquid cooling. Since the windings are not overlapping, access to the slot from the end section for coolant liquids is enabled. In this paper, a solution for in-slot and instator direct oil cooling for a tooth coil winding machine is presented. The coils are pre-wound on bobbins and inserted on the stator teeth. The novelty of the design consists in the integration of the cooling, using a thermally conductive epoxy resin to create the channels within the slot as well as the positioning of the stator yoke cooling channels. A 50 kW machine for an automotive traction application is designed, manufactured and tested. Conjugate heat transfer simulations are used in the design process in combination with finite element analysis for the loss mapping. The thermal model is verified with measurements at 6 l/min oil flow and 17.5 A/mm2 continuous and 35 A/mm2 30 s peak. The thermal model is then used to establish a continuous operating point of 25 A/mm2 .
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