| 1. |
- Trilla, Lluis, 1981, et al.
(författare)
-
Wave farm collection grid analysis and design
- 2013
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- In this report, the collection grid of a wave farm is designed and analysed, the system under consideration goes from the connection point at each power converter (included in each wave energy unit) up to the point of common connection with the main grid. Time series that have been provided by Ocean Harvesting Technologies (OHT), being obtained from their simulation models for a scaled prototype and include data for mild, medium and strong sea states. The simulation models have been validated against small scale tank testing of a prime mover, a wave rider buoy, and scaled testing of the power take-off in a land based test-rig. Wave data is taken from EMEC, Scotland [7]. The design of the collection grid aims to optimize its size by minimizing its cost while maximizing the power output. For the economical assessment three layouts are taken into account and several scenarios for different energy prices and interest rates are presented. A life-cycle analysis is also done where the entire life span of the collection grid is considered showing that around 30Euro/MWh will be the share of the electricity price required to install and operate the collection grid. The power quality of the resultant layout and its compliance with the grid code of the UK are analysed as well. This analysis includes power factor and voltage fluctuation estimation, line-fault ride-through assessment and flicker contribution analysis. Given the pulsating nature of the wave power systems special emphasis in the study of flicker emissions is done concluding that the proposed wave park is within the grid code limitations and even a reduction of a 66% in the flicker coefficients is reported when considering the aggregation of several units in one model.
|
|
| 2. |
- Abdulahovic, Tarik, 1976, et al.
(författare)
-
Comparison of switching surges and basic lightning impulse surges at transformer in MV cable grids
- 2009
-
Ingår i: Nordic Wind Power Conference, Bornholm, Danmark.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The electric transients generated during the switching of a breaker placed near the transformer in a system with a substantial amount of cables can have a very short rise time and can reach very high magnitudes. In practice, the magnitude of the voltage surges is usually limited using the surge arresters while the rise time of the surge to which the apparatus is exposed, remains unchanged.In this paper, the characteristic of the switching transients generated in the MV cable grids are analyzed. For this purpose, cases with both dry-type and oil insulated transformers are studied. These cases are typical for a MV industrial grid and a wind park (WP) collection grid. Furthermore, the case with and without surge arrester protection is analyzed in order to observe the impact of the surge arresters on the voltage surges.Simulations show that the magnitude of the voltage surges exceeds the basic lightning impulse insulation level (BIL) defined by present standards. Furthermore, the rise time of the voltage surges is much shorter compared to the rise time of the lightning impulse especially in the case of the dry-type transformers and the WP collection grid. When surge arresters are used, the magnitude of the surges is limited to the BIL but the rise times of the surges can be twenty five times shorter compared to the BIL. Only when surge arresters are used with other fast transient mitigation equipment such as RC protection or surge capacitor protection, the magnitude of very fast voltage surges are brought down.
|
|
| 3. |
- Abdulahovic, Tarik, 1976, et al.
(författare)
-
Improvement of admittance measurements for modeling of an ungrounded reactor/transformer winding
- 2016
-
Ingår i: International Journal of Modeling, Simulation, and Scientific Computing. - 1793-9615 .- 1793-9623. ; 7:3
-
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.
|
|
| 4. |
- Abdulahovic, Tarik, 1976, et al.
(författare)
-
Modeling of the energizing of a wind park radial
- 2007
-
Ingår i: Nordic Wind Power Conference, Roskilde, Denmark.
-
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.
|
|
| 5. |
- Abdulahovic, Tarik, 1976, et al.
(författare)
-
Simulation accuracy of the built-in PSCAD and an owner-defined synchronous machine model
- 2010
-
Ingår i: COMPEL - The International Journal for Computation and Mathematics in Electrical and Electronic Engineering. - : Emerald. - 0332-1649. ; 29:3, s. 840-855
-
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.
|
|
| 6. |
- Abdulahovic, Tarik, 1976, et al.
(författare)
-
Transformers internal voltage stress during current interruption for different wind turbine layouts
- 2012
-
Ingår i: EPE Joint Wind Energy and T&D Chapters. ; , s. 6-
-
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.
|
|
| 7. |
- Abdulahovic, Tarik, 1976, et al.
(författare)
-
Transformers internal voltage stress during current interruption for different wind turbine layouts
- 2013
-
Ingår i: 2013 15th European Conference on Power Electronics and Applications, EPE 2013. - 9781479901166 ; , s. no. 6634759-
-
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.
|
|
| 8. |
- Abdulahovic, Tarik, 1976, et al.
(författare)
-
Vacuum Circuit-Breaker Parameter Calculation and Modelling for Power System Transient Studies
- 2017
-
Ingår i: IEEE Transactions on Power Delivery. - : Institute of Electrical and Electronics Engineers (IEEE). - 0885-8977 .- 1937-4208. ; 33:3, s. 1165-1172
-
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.
|
|
| 9. |
- Abdulahovic, Tarik, 1976, et al.
(författare)
-
Voltage Stress in a Transformer Winding During Very Fast Transients Caused by Breaker Closing Event
- 2014
-
Ingår i: IEEE Transactions on Power Delivery. - 0885-8977 .- 1937-4208. ; 29:4, s. 1946-1954
-
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.
|
|
| 10. |
- Acquaviva, Alessandro, 1987, et al.
(författare)
-
Analytical Conduction Loss Calculation of a MOSFET Three-Phase Inverter Accounting for the Reverse Conduction and the Blanking Time
- 2021
-
Ingår i: IEEE Transactions on Industrial Electronics. - 0278-0046 .- 1557-9948. ; 68:8, s. 6682-6691
-
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.
|
|
