| 1. |
- Abbas, Muhammad, et al.
(författare)
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Comparison of Multiplierless Implementation of Nonlinear-Phase Versus Linear-Phase FIR filters
- 2008
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Konferensbidrag (refereegranskat)abstract
- FIR filters are often used because of their linear-phase response. However, there are certain applications where the linear-phase property is not required, such as signal energy estimation, but IIR filters can not be used due to the limitation of sample rate imposed by the recursive algorithm. In this work, we discuss multiplierless implementation of minimum order, and therefore nonlinear-phase, FIR filters and compare it to the linear-phase counterpart.
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| 2. |
- Carlson, Ola, 1955, et al.
(författare)
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Power System Voltage Stability Related to Wind Power Generation
- 2006
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Ingår i: 2006 European Wind Energy Conference and Exhibition, Athens, Greece.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- As is well known the amount of installed wind power increases every day. In the past single wind turbines were installed separately, today wind turbines often are connected to the grid as a group of many turbines, i.e. wind parks. Grid integration considerations have moved from flicker phenomena in the low voltage grid to voltage stability analyses in the transmission grid. This paper starts with an overview of the voltage stability phenomena in a transmission system. Suitable models of wind power generation for power system stability studies - both long and short-term stability - are presented. Different levels of detail of the wind generation models are investigated carefully considering the type of stability study, result accuracy and simulation efficiency. Important aspects such as protection, fault ride-through capability and the capability of producing reactive power are taken into account. The models are then validated against measurements.Three types of wind turbines are considered in this study; fixed speed with induction generator, variable speed operation with doubly fed induction generator and variable speed turbine with full power converter.The main phenomenon that determines the behaviour of a system where fixed speed wind turbines are connected is the large reactive power consumed by the induction machines immediately after the fault has been cleared. This mechanism leads to a voltage stability problem with a risk of voltage collapse as a final consequence. Investigations in short and long term voltage stability have examined the factors affecting the stability limit such as: generated power, fault types, fault duration, fault clearing time, fault voltage and remaining voltage. Variable speed wind turbines with power electronic converter are capable of producing active power at unity power factor. Possibility lies to control reactive power during grid disturbance with the help of an over-rated grid side converter. In this paper the long- and short-term voltage stability aspects of variable speed wind turbines with power electronic converters are illustrated by case studies. Large scale wind power integration at the transmission level has the possibility to improve long term voltage stability of the grid by injecting reactive power. The possibility to produce reactive power increases with a decreased active power production. As well small scale integration at the distribution level can increase short-term voltage stability of the local grid by utilizing its fast reactive power control capability.Finally, several issues related to the modelling and the compatibility of wind power generation in typical power system simulation tools is discussed.
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| 3. |
- Hore, Samar Kumar, et al.
(författare)
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Detecting arsenic-related skin lesions : Experiences from a large community-based survey in Bangladesh
- 2007
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Ingår i: International Journal of Environmental Health Research. - : Informa UK Limited. - 0960-3123 .- 1369-1619. ; 17:2, s. 141-149
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Tidskriftsartikel (refereegranskat)abstract
- A cross-sectional survey was conducted in Matlab, Bangladesh, to determine the prevalence of skin lesions (a three-step procedure) associated with arsenic exposure and discuss validity and feasibility in relation to recommended screening algorithms. Cases with skin lesions were identified by screening above 4 years of age (n = 166,934). Trained field teams conducted a careful house-to-house screening and identified 1682 individuals with skin lesions, who were referred to physicians for confirmation. Physicians diagnosed 579 cases as probable and documented all these with digital photographs. Two experts inspected all photographs for consensus agreement that was reached for 504 cases. Using the experts' opinions as reference, the positive predictive value of the physicians' diagnosis was 87% (male = 82% vs. female = 94%; p < 0.01). The physicians had difficulties in separating arsenic-induced keratosis from differential diagnoses, while probability for correct diagnosis was high for arsenic-related pigmentation changes. Including information on current arsenic concentration in drinking water (which was masked at time of skin examination) or urine in the diagnostic algorithm should have increased the number of false negative cases. In the present transition of drinking water sources these markers of current exposure levels provide no information on past exposure. A 2 - 3 step procedure with house-to-house screening and clinic-based confirmation of arsenic-induced skin lesions is a feasible approach. Information on arsenic concentration in current water sources or in urine should not have improved the precision in the diagnosis. These results may have policy implications for community screening of arsenic-related skin lesions in Bangladesh and elsewhere.
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| 4. |
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| 5. |
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| 6. |
- Kashif, Ahsan-Ullah, 1974-, et al.
(författare)
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Flexible power amplifier designing form device to circuit level by computational load-pull simulation technique
- 2008
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Ingår i: Microelectonics Technology and Devices - SBMicro 2008, Vol. 14, issue 1. - Pennington, New Jersey : Electrochemical Society. - 9781566776462 ; , s. 233-239
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Konferensbidrag (refereegranskat)abstract
- Matchingnetwork is major issue in broadband power amplifiers due tothe fact that the transistor impedances are varying both withfrequency and signal level. Thus it is difficult to matchthese impedances both at the input and output stages. Thetunable matching networks are very demanding and desired for buildingflexible systems, but their accuracy depends on the transistor performanceunder the large signal operation. Computational load pull (CLP) simulationtechnique is a unique way to extract the impedances ofpower transistor at desired frequencies which make the design ofmatching network much easier for multiple bands power amplifiers. AnLDMOS transistor is studied and its optimum impedances are extractedat 1, 2 and 2.5 GHz. Through optimum impedance, thetunable matching networks can be easily design for broadband amplifiers.
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| 7. |
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| 8. |
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| 9. |
- Kashif, Ahsan-Ullah, et al.
(författare)
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Influence of interface state charges on RF performance of LDMOS transistor
- 2008
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Ingår i: Solid-State Electronics. - : Elsevier. - 0038-1101 .- 1879-2405. ; 52:7, s. 1099-1105
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Tidskriftsartikel (refereegranskat)abstract
- Si-LDMOS transistor is studied by TCAD simulation for improved RF performance. In LDMOS structure, a low-doped reduced surface field (RESURF) region is used to obtain high breakdown voltage, but it reduces the transistor RF performance due to high on-resistance. The interface charges between oxide and the RESURF region are studied and found to have a strong impact on the transistor performance both in DC and RF. The presence of excess interface state charges at the RESURF region results not only higher DC drain current but also improved RF performance in terms of power, gain and efficiency. The most important achievement is the enhancement of operating frequency and RF output power is obtained well above 1 W/mm up to 4 GHz.
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| 10. |
- Kashif, Ahsan-Ullah, et al.
(författare)
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Reduction in on-resistance of LDMOS transistor for improved RF performance
- 2009
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Ingår i: Microelectronics Technology and Devices - SBMicro 2009, Vol. 23, issue 1. - Pennington, New Jersey : The Electrochemical Society. - 9781566777377 ; , s. 413-420
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Konferensbidrag (refereegranskat)abstract
- Inan LDMOS transistor, a low doped drift (LDD) region atthe drain side is created to enhance the breakdown voltage(BVDS), but this increases on-resistance (Ron) which degrades the transistorRF performance. In this paper, the LDD region of LDMOStransistor is optimized using two different techniques, (i) a dualimplanted-layer p- and n-region in LDD and (ii) an excessinterface charge at the RESURF of LDD. Both techniques areused to enhance the carrier density for lower Ron. Thecomparison revealed that excess interface charge provides 43 % reductionin Ron with BVDS of 70 V, while the dual-implantedregion provides 26 % reduction in Ron together with BVDSof 64 - 68 V.
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| 11. |
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| 12. |
- Shah, Faiz Ullah
(författare)
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Boron compounds as additives to lubricants : synthesis, characterization and tribological optimization
- 2009
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Developing new technological solutions, such as use of lightweight materials, less harmful fuels, controlled fuel combustion processes or more efficient exhaust gas after-treatment, are possible ways to reduce the environmental impact of machines. Both the reduction of wear and the friction control are key issues for decreasing of energy losses, improving efficiency and increasing of the life-span of an engine. Dialkyldithiophosphates (DTPs) of different metals have been extensively used as multifunctional additives in lubricants to control friction and reduce wear in mechanical systems. Among these DTP-compounds, zinc dialkyldithiophosphates (ZnDTPs) are the most common additives used for more than 60 years. These additives form protective films on steel surfaces and, thus, control friction and reduce wear. However, ZnDTPs contain zinc and large amounts of phosphorus and sulphur, which are human health hazards and cause environmental pollution by degrading catalytic converters in automobiles. Therefore, replacement of ZnDTPs byzinc free compounds with reduced amounts of sulphur and phosphorus are urgently needed.Boron-containing compounds are known as corrosion inhibitors, antioxidants, friction modifiers and effective anti-wear additives either dissolved in oil or as an insoluble and inorganic borate salts dispersed in oil in the nanoparticulate form. These compounds are emerging as attractive replacements for the compounds already used as additives in lubricants. The focus of the present work is on the development and tribological investigations of new boron compounds as highperformance additives in lubricants. These should be ashless with significantly reduced amounts sulphur and phosphorus, which is favourable for the environment protection. The work was carried out in the following steps: (1) Synthesis of new additives; (2) Characterization of the additives with FTIR, (1H, 13C, 31P, 11B) NMR spectroscopy, elemental analysis and thermal analysis (TG/DTG, DTA and QMS); (3) Tribological evaluation using four ball Tribometer; (4) Surface analysis using an optical profiler and Scanning Electron Microscopy coupled with X-ray Energy Dispersive Spectroscopy (SEM/EDS). It was found that the novel boroncompounds have considerably better antiwear performance and higher stability of the coefficient of friction with time as compared with ZnDTP. These novel compounds are ashless with reduced amounts of sulphur and phosphorus. They also have excellent tribological properties, high thermal stability, good miscibility with oils and positive environmental issues which make them an attractive alternative to ZnDTP.
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| 13. |
- Shah, Faiz Ullah, et al.
(författare)
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Synthesis, physicochemical, and tribological characterization of S-Di-n-octoxyboron-O,O'-di-n-octyldithiophosphate
- 2009
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 1:12, s. 2835-2842
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Tidskriftsartikel (refereegranskat)abstract
- Dialkyldithiophosphates (DTPs) of zinc(II), copper(II), and other metals have been extensively used as multifunctional additives in lubricants to control friction and reduce wear in mechanical systems. Among these DTP compounds, zinc dialkyldithiophosphates (ZnDTPs) are the most common additives extensively used for more than 60 years. These additives form a protective film on steel surfaces and, thus, control friction and reduce wear. However, ZnDTPs contain zinc and large amounts of phosphorus and sulfur, which impair the environment, both directly and indirectly, by adversely affecting the performance of catalytic converters of various automobiles. For this reason, environmental legislation imposes limitations on concentrations of phosphorus, sulfur, and zinc in the lubricants. In this work, we report on zinc-free S-di-n-octoxyboron-O,O′-di-n-octyldithiophosphate (DOB-DTP) lubricant additive with amount of phosphorus and sulfur reduced by half in a molecule as compared with ZnDTPs. DOB-DTP was synthesized by a reaction in two steps under inert nitrogen atmosphere. The final product, a viscous liquid, was characterized by the elemental analysis, FT-IR, multinuclear 1H, 13C, 31P, and 11B NMR spectroscopy and thermal analyses. Tribological performance of a mineral oil with this new additive was evaluated in comparison with O,O′-di-n-butyl-dithiophosphato-zinc(II) (ZnDTP) using a four-ball tribometer. The surface morphology and the elemental composition of the tribofilms were characterized using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS). The results show that DOB-DTP has a considerably better antiwear performance and higher stability of the coefficient of friction with time as compared with ZnDTP. Both phosphorus and sulfur were detected by the EDS on the worn steel surfaces at all concentrations of additives in the base oil.
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| 14. |
- Shah, Faiz Ullah, et al.
(författare)
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Synthesis, structure and characterization of some Schiff bases bearing phenylferrocene
- 2007
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Ingår i: Applied organometallic chemistry. - : Wiley. - 0268-2605 .- 1099-0739. ; 21:9, s. 758-762
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Tidskriftsartikel (refereegranskat)abstract
- Some novel Schiff bases bearing phenylferrocene were synthesized by condensation reaction of 4-ferrocenylaniline with different aromatic aldehydes. The compounds prepared were characterized by spectroscopic methods (IR, UV-visible, 1H and 13C NMR) and elemental analysis. The single crystal analysis of compound F1 [monoclinic, space group, P21/c (no. 14), a = 19.858(2), b = 7.416(2), c = 12.095(5) Å, β = 106.257(14) ] indicates a trans imine bond with a bond length of 1.270(2) Å, typical of a carbon-nitrogen double bond.
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| 15. |
- Sheikh, Zaka Ullah, et al.
(författare)
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Wideband Linear-Phase FIR Differentiators Utilizing Multirate and Frequency-Response Masking Techniques
- 2009
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Ingår i: Proceedings of IEEE International Symposium on Circuits and Systems, Taipei, Taiwan, May 24–27, 2009.. - : IEEE. ; , s. 293-296
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Konferensbidrag (refereegranskat)abstract
- This paper introduces a structure for the realization of wide-hand linear-phase finite-length impulse response (FIR) differentiators. It is based on multirate and frequency-response masking techniques. Examples show that this structure achieves computational savings between some 10% and 65% for bandwidths in the range between 0.9 pi and 0.98 pi in comparison with conventional linear-phase differentiators.
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| 16. |
- Ullah, Faiz, et al.
(författare)
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Determination of heterocyclic aromatic amines in human urine by using hollow-fibre supported liquid membrane extraction and liquid chromatography-ultraviolet detection system.
- 2008
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Ingår i: Journal of Chromatography. B. - : Elsevier BV. - 1873-376X .- 1570-0232. ; 870:2, s. 203-208
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Tidskriftsartikel (refereegranskat)abstract
- A hollow-fibre supported liquid membrane (HF-SLM) extraction method has been developed for determination of 11 heterocyclic aromatic amines (HCAs) in human urine samples by using high performance liquid chromatography (HPLC) equipped with an ultraviolet (UV) absorbance detector. These compounds were extracted from an alkaline urine sample (donor phase) into the organic solvent residing in the pores of a polypropylene hollow fibre and then back extracted into an acidic solution (acceptor phase) inside the lumen of the hollow fibre. After extraction, HCAs were analyzed by injecting the analyte enriched acceptor phase into the HPLC. The analyte enrichment factors ranged between 241 and 339 obtained in a 90min extraction time, and method detection limits (MDL) ranged between 0.1 and 0.5mugL(-1) with relative standard deviation (RSD) values between 3.4% and 11%. The extraction technique employed in this work is easy to use and rapid as it involves only a few minutes manipulation of each sample. It is the most economical sample preparation/preconcentration technique to our knowledge as compared to other microextraction techniques.
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| 17. |
- Ullah, Farman, et al.
(författare)
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Enantioselective Organocatalytic Conjugate Addition of Fluorocarbon Nucleophiles to alpha,beta-Unsaturated Aldehydes
- 2009
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 15:39, s. 10013-10017
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Tidskriftsartikel (refereegranskat)abstract
- Highly chemo- and enantioselective organocatalytic conjugate additions of fluorocarbon nucleophiles to alpha,beta-unsaturated aldehydes and the construction of a quaternary carbon stereocenter bearing a fluorine atom with high enantioselectivity was reported. After extensive screening of catalysts and conditions, it was found that chiral amines 5-9 catalyzed the conjugate addition of FBSM 2a to enal 1a to form the corresponding bfluorobis(sulfonyl)methyl aldehyde 3a, which was reduced in situ to the alcohol derivative 4a, with enantiomeric excess (ee) values ranging from nearly racemic to 95%. Chiral protected diarylprolinols catalyzed the transformation with the highest stereoselectivity under reaction conditions and mediated the formation of 3a with high chemoselectivity. The highest enantioselectivity was achieved in toluene. The enantioselectivity was improved, possibly due to stabilization of the iminium intermediate, by the addition of an acid additive and performing the transformation in ethanol.
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| 18. |
- Ullah, Farman, et al.
(författare)
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Enantioselective organocatalytic conjugate addition of fluorocarbon nucleophiles to α,β-unsaturated aldehydes
- 2009
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Ingår i: Chemistry - A European Journal. - Weinheim : Wiley-VCH Verlag GmbH & Co. KGaA. - 0947-6539 .- 1521-3765. ; 15:39, s. 10013-10017
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Tidskriftsartikel (refereegranskat)abstract
- A highly chemo- and enantioselective organocatalytic addition of fluorocarbon nucleophiles, such as 1-fluoro-bis(phenylsulfonyl)methane, toα,β-unsaturated aldehydes is presented (see scheme). The reactions are catalyzed by simple chiral amines and give access to optically active fluorine derivatives in good yields and up to 95 % ee. Notably, the methodology can be applied to the formation of a chiral quaternary carbon center bearing a fluorine atom with high enantioselectivity.
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| 19. |
- Ullah, Muhammad Muneeb, et al.
(författare)
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Towards Robust Place Recognition for Robot Localization
- 2008
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Ingår i: 2008 IEEE INTERNATIONAL CONFERENCE ON ROBOTICS AND AUTOMATION, VOLS 1-9. ; , s. 530-537
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Konferensbidrag (refereegranskat)abstract
- Localization and context interpretation are two key competences for mobile robot systems. Visual place recognition, as opposed to purely geometrical models, holds promise of higher flexibility and association of semantics to the model. Ideally, a place recognition algorithm should be robust to dynamic changes and it should perform consistently when recognizing a room (for instance a corridor) in different geographical locations. Also, it should be able to categorize places, a crucial capability for transfer of knowledge and continuous learning. In order to test the suitability of visual recognition algorithms for these tasks, this paper presents a new database, acquired in three different labs across Europe. It contains image sequences of several rooms under dynamic changes, acquired at the same time with a perspective and omnidirectional camera, mounted on a socket. We assess this new database with an appearance based algorithm that combines local features with support vector machines through an ad-hoc kernel. Results show the effectiveness of the approach and the value of the database.
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| 20. |
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| 21. |
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| 22. |
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| 23. |
- Ullah, Nayeem, 1978
(författare)
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Grid reinforcing wind generation
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, effects of some grid assisting possibilities that wind turbines could be equipped with, are investigated. The background is that concerns regarding the stability of the already existing power system in the presence of large wind farms have been risen. Variable speed wind turbines have power electronic based frequency converters already included in their design and the ideaused in this thesis is to utilize the fast controllability of these converters to assist the grid.It is found that the transient stability of conventional synchronous generators located nearby a wind farm, can be increased if a suggested contingency operational mode is incorporated into the control of the wind farm. Another finding is that large wind farms integrated into the transmission level with their control properly modified, can increase the steady state power transfer limit of thetransmission line as well as assist the grid to delay or even prevent a voltage collapse event. It is also shown that wind farms integrated into the grid in the distribution level together with different loads, have the possibility to increase the short term voltage stability of the network provided that their controls have been modified accordingly.Finally, the suggested grid assisting methods are incorporated into the control of two large planned wind farms in the southern part of the Swedish grid and tested in the CIGR\'{E} Nordic32 test grid, which is taken as a representation of the Swedish transmission network. It is concluded that care has to be taken if voltage control mode of operation is utilized in a wind farm located closeto a conventional synchronous generator. The interaction between the operational mode of a wind farm and the overall synchronous generator control may interact with each other and could lead to a reduction of damping of power oscillations in nearby transmission lines instead of increasing the damping.
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| 24. |
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| 25. |
- Ullah, Nayeem, 1978, et al.
(författare)
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Improving voltage stability by utilizing reactive power injection capability of variable speed wind turbines
- 2005
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Ingår i: The Eighth IASTED International Conference on POWER AND ENERGY SYSTEMS October 24-26, 2005 Marina del Rey, CA, USA. - 0889865507
-
Konferensbidrag (refereegranskat)abstract
- The impact that wind turbines have on the voltage stability is investigated in this paper. In particular, the effect of utilizing the reactive power injection capability of modern wind energy converters is investigated. It is found that reactive power injection from the wind turbine can increase the voltage stability of the power grid substantially, as well as moderately increase the steady-state power transfer limit. For a high wind speed situation, where the wind turbine converter is fully utilized, it is found that it isworth reducing the active power production from the wind turbine in order to make room for reactive power injection, from a voltage stability point of view. An interesting observation is that a modern variable speed wind turbine constantly operating at maximum power factor does not provide much voltage stability improvement compared to a traditional fixed-speed system under its usual operating condition, i.e. at lower wind speeds. The finding isthat the worst case to handle, from a voltage stability point of view, is the case where there is a high load demand, irrespective of the wind speed situation.
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| 26. |
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| 27. |
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| 28. |
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| 29. |
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| 30. |
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| 31. |
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| 32. |
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| 33. |
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| 34. |
- Ullah, Nayeem, 1978
(författare)
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Wind Power - Added Value for Network Operation
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This dissertation deals with the investigation on different value added properties of variable speed wind turbines (VSWT) that stem from the flexible controllability of converter interfaced wind turbines (WT). Improvements in voltage and transient stability of a nearby grid, small-signal stability improvement on the power system, frequency control support for the network operation, as well as thetechnical and the economic issues related to the reactive power ancillary service provision, are among the issues covered. To demonstrate that additional control functions can be incorporated in real installations, a real situation is presented where the short-term voltage stability is improved as an additional feature of an existing voltage source converter (VSC) high voltage direct current (HVDC) installation.A finding is that the voltage and the transient stabilityperformance of the used Cigre Nordic 32-bus test system during disturbances is improved when the wind farm (WF) is complying with the E.ON code compared to the traditional unity power factor operation. Further improvements are noticed when the example grid code is modified (increased slope of the reactive current support line and extended reactive current support).The damping of the inter-area mode and the local mode (where the WF is connected) of oscillation of the studied two-area power system are increased in the presence of the WF. It is also noticed that, the damping associated with the inter-area mode is slightly better when the constant reactive power mode is applied compared to the voltage control mode of operation. Another finding is that the example WT exhibits a slow well damped system mode which depends on the WT controllers (torque and pitch controllers, and pitch compensator).Due to the non-minimum phase characteristic of hydro dominated systems, a temporary active power support from WFs, utilizing the stored rotational energy in the moving turbine blades, could be helpful in reducing the network frequency fall. In this regard, it is found that an example WT system can provide 0.1pu extra active power support for 10s without any larger effect on the WT operation. When this arrangement is used, both the temporary droop and thereset time of the existing speed governing system need to be reduced to maintain certain benchmark stability properties.From the case investigated, it is found that grid-side converters (GSC), designed to handle only rated active power, cost around 1.5% of the total investment of the WF. However, a 50% over-rated GSC would cost around 2.25% of the total investment of the WF, and would be capable of providing 0.65~pu reactive power at the grid connection point under nominal conditions. Another finding is that higher wind speed prediction errors, i.e.~a WF site with a high degree of wind variations, may result in higher payments to the WF for the reactive power service, mainly due to the increased lost opportunity cost (LOC) component.
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| 35. |
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| 37. |
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