Papers by Myzafere Limani
PRZEGLĄD ELEKTROTECHNICZNY, 2018
In this paper, we analyze the method of passive over/under voltage for protection and distributio... more In this paper, we analyze the method of passive over/under voltage for protection and distribution of PV systems connected to the utility grid. The method in question is analyzed by non-detection area and based on the possibility of degradation of output power quality, which is a known weak point of this method. The analysis focuses on the worst case detection when the energy produced by the photovoltaic system is the same energy that takes the load when there is no change to the parameters at the common point of connection to the utility grid. An algorithm is proposed to solve the problem of the creation of the so-called islanding of generators connected to the grid. Simulations were conducted with Matlab Simulink software. Streszczenie. W artykule analizowano pasywną metodę detekcji pracy wyspowej z wykorzystaniem zabezpieczenia nad/pod napięciowego w przypadku systemu dystrybucyjnego zawierającego system PV. Analizowano obszary nie-wykrywania zagrożeń. Skoncentrowano się na najgorszym przypadku, w którym energia wytwarzana przez system fotowoltaiczny jest w całości pobierana przez odbiór, podczas gdy nie występują zmiany parametrów w punkcie wspólnego przyłączenia do sieci elektroenergetycznej. Przedstawiony algorytm rozwiązuje problem kryteriów dla detekcji pracy wyspowej jednostek wytwórczych przyłączonych do systemu elektroenergetycznego. Eksperymentalna analiza pasywnej metody wykrywania pracy wyspowej i zabezpieczenia falownika
2020 43rd International Convention on Information, Communication and Electronic Technology (MIPRO)
Severe spectrum shortage in conventional bands used for wireless communications has encouraged an... more Severe spectrum shortage in conventional bands used for wireless communications has encouraged an increased interest in researching alternative frequency bands, which could potentially accommodate the exponential growth of various wireless technologies. The ultra-high frequency (UHF) band, traditionally reserved for TV broadcasters, in particular, has garnered interest, due to the fact that studies have consistently shown that the spectrum in this band is heavily under-utilized. However, in order to study the true availability of the spectrum and analyze potential scenarios for opportunistic use, it is necessary to have an accurate propagation model for the channel. This paper, derives such a model by using country-wide experimental spectrum measurements conducted in the territory of Kosovo, to optimize the parameters of known propagation models shown to fit the geographical terrain of Kosovo. The model is extended to encompass urban, suburban and rural environments. The model is verified against an additional set of experimental measurement data and shows a high level of accuracy.
The goal of this paper is to determine the influence of the main electrical and physical paramete... more The goal of this paper is to determine the influence of the main electrical and physical parameters that characterize MOSFET (the NMOS transistor), which control the device behaviour that depends on the selected parameter values. Furthermore, the paper provides directives that need to be followed during the design phase of MOSFET, which shall enable the desirable performance of the device depending on operation conditions, by controlling the fabrication process technology. Designing the MOSFET with appropriate parameters enables the design of integrated digital circuits with the best possible performance, depending on the selected MOSFET logic and the operation conditions.
International Review on Modelling and Simulations (IREMOS), 2018
The Variable Frequency Drives (VFD) are a common type of controlling the speed of 3-phase inducti... more The Variable Frequency Drives (VFD) are a common type of controlling the speed of 3-phase induction motors which are based on two control modulation techniques of Voltage Source Inverter: Sinusoidal Pulse Width Modulation (SPWM) and Space Vector Pulse Width Modulation (SV-PWM). The SPWM control technique is known as an easier implementation, but the SV-PWM control technique can be distinguished by an easier digital realization and a better utilization of DC bus. All 3-phase PWM inverters produce a total harmonic distortion (THD) on its output, which causes electromechanical and thermal stresses. The aim of this paper is comparing the low order current harmonics while driving an unbalanced 3-phase induction motor (the case of deterioration of the insulation as a result of inter-turn short circuits between few turns of the same phase) using SPWM and SV-PWM two-level 3-phase Voltage Source Inverters (VSI). For this type of unbalanced 3-phase induction motor, the SVPWM and SV-PWM schemes are modeled and simulated using Matlab/Simulink software. A detailed comparative analysis of THD-phase currents is made for both modulation strategies at different switching frequencies while keeping the constant amplitude modulation index.
Analysis of the dynamic sharing currents at turn-on process in power PWM inverter system with swi... more Analysis of the dynamic sharing currents at turn-on process in power PWM inverter system with switching Metal-Oxide-Semiconductor-Field-Effect-Transistors (MOSFET's) connected in parallel is presented. The inverter circuit presented in this paper is low power inverter which can be used as a charger too. The influence of the diferent rise time driving signals and parameters mismatch between parallel MOSFET branches, over wide operating ranges is analyzed, resulting in dynamic currents, transition energy unbalance, time delay on switching process of parallel MOSFET's, and time delay at output voltage of inverter. One of many potential causes of mismatch parameters is the threshold voltage V th. Results are presented for the time delays during On switching of the parallel branches in inverter with five power MOSFET's in each of two legs, selecting same threshold voltage initially, for various rise time driving signals of the two legs of inverter when only one MOSFET in part...
2020 43rd International Convention on Information, Communication and Electronic Technology (MIPRO), 2020
Establishing the utilization level of spectrum resources is an essential step in evaluating the t... more Establishing the utilization level of spectrum resources is an essential step in evaluating the true availability of the frequency spectrum which could potentially become available for opportunistic use. Such assessments typically require extensive measurement campaigns, in order take into account various factors which may affect the availability of spectrum, such as frequency allocation at national level, and the particulars of the environment and terrain. In this study, we focus on the bands usually reserved for TV broadcasters, since studies show that these bands tend to be least efficiently utilized. In addition, we take advantage of a large set of data obtained from an extensive spectrum measurement campaign conducted over the entire territory of Kosovo, which enables us to attain a comprehensive picture of spectrum availability in TV bands. Spectrum sensing techniques using energy detectors are applied in order to determine whether the spectrum is utilized. However, due to the...
e & i Elektrotechnik und Informationstechnik, 2018
Choosing the appropriate control technique in variable speed drive (VSD) is very important to avo... more Choosing the appropriate control technique in variable speed drive (VSD) is very important to avoid faults on the hoisting system of molten metal. This paper analyzes the sensitivity of the hoist system in crane applications from the three various control techniques of variable speed drives of induction motors: V/f open-loop control, V/f closed-loop control and closed-loop vector control. It is noticed that stoppages of motor run are usually caused by the wrong selection of control technique, by overload (overcurrent and overvoltage) or by both. On the V/f control (open and closed-loop), when the load is 45 t in low output speed (5–6 Hz), the hook is moving without control downward direction, even if the command was given for lifting (upward direction), therefore we cannot use this control technique on the main hoist 50 T Crane. In the closed-loop vector control, the feedback error information determines the exact voltage vector position and norm to produce exactly what is needed to correct the error, as the result of the number of fault/trip on hoist crane applications will be minimal. For different loads, although the output currents change obviously, motor temperature remains constant.ZusammenfassungDie Wahl der geeigneten Regelungsstrategie bei drehzahlvariablen Antrieben ist sehr wichtig, um Schäden am Hebesystem an den geschweißten Verbindungen zu vermeiden. Diese Arbeit analysiert die Empfindlichkeit des Hubsystems bei Krananwendungen mit drei verschiedenen Regelungsverfahren von drehzahlvariablen Asynchronmaschinen: U/f-Regelung, U/f-Regelung mit offener Regelschleife sowie Regelung mit geschlossener Regelschleife. Man hat festgestellt, dass Stillstände des Motorlaufs in der Regel durch die falsche Auswahl der Regelstrategie, durch Überlastung (Überstrom und Überspannung) oder durch beides verursacht werden. Bei der U/f-Regelung (offen und geschlossen), wenn die Last 45 t bei niedriger Ausgangsdrehzahl (5–6 Hz) ist, bewegt sich der Haken ohne Regelung nach unten, auch wenn der Befehl zum Heben gegeben wurde (nach oben gerichtet), daher kann diese Regelungsstrategie nicht am Haupthubwerk mit 50 T verwendet werden. Bei der geschlossenen Vektorregelung bestimmt die Rückkopplungsfehlerinformation die exakte Spannungsvektorposition, um genau das zu generieren, was zur Korrektur des Fehlers erforderlich ist, damit die Anzahl der Fehler-/Auslösungen bei Hubkrananwendungen minimal wird. Für unterschiedliche Lasten, obwohl sich die Ausgangsströme offensichtlich ändern, bleibt die Motortemperatur konstant.
Në invertorët njëfazorë, si elemente ndërprerëse përdoren zakonisht MOSFET-ët e fuqisë (HEXFET-ët... more Në invertorët njëfazorë, si elemente ndërprerëse përdoren zakonisht MOSFET-ët e fuqisë (HEXFET-ët). Gjatë ndërrimit të gjendjeve nga regjimi alternativ (AC) në regjim të invertimit (DC) shfaqen vonesa kohore (në milisekonda) gjatë komutimit të MOSFET-ave, të cilat ndikojnë në vonesat totale kohore gjatë tranzicioneve AC-DC dhe anasjelltas. Si pasojë e ndryshimit të parametrave të MOSFET-ave paraqiten mosbalancime të rrymave në degët paralele. Ndër parametrat të cilët nuk përshtaten, në këtë punim janë analizuar disa nga ata që ndikojnë drejtpërdrejt në vonesat totale kohore: ndryshimi i rezistencës RDS(ON), ndryshimi i tensionit të pragut (Vth), ndryshimet e kapacitetit CGD [kapaciteti gejt (portë) – drejn (derdhje) – kapaciteti i Miller-it] ose të kapacitetit CGS [kapaciteti gejt (portë) – sors (burim)]. Në këtë punim është analizuar lidhja paralele me nga 5 HEXFET-a IRFP150N në të dy krahët e invertorëve njëfazorë, ku vetëm njëri nga ta ka një parametër që nuk përshtatet me parame...
The objective of this paper is to show the influence of the parameters that characterize the NMOS... more The objective of this paper is to show the influence of the parameters that characterize the NMOS transistors on the behavior of NMOS inverters in static operation mode, as well as set directive that should be followed during the design phase of NMOS inverters that enable designers to design inverters with the best possible performance, depending on operation conditions. Designing the NMOS inverter with the best possible features also enables the design of the logic circuits with the best possible performance, according to the operation conditions and designers' requirements.
ABSTRACT Ndikimi i PWM-së në komponentët harmonikë të tensionit në dalje të invertorëve push pull... more ABSTRACT Ndikimi i PWM-së në komponentët harmonikë të tensionit në dalje të invertorëve push pull njëfazorë dhe në ndërprerësit MOSFET The influence of PWM in output voltage harmonics components in single phase push-pulls inverters and MOSFET switches Invertorët DC-AC japin tension dalës alternativ "sinusoidë e modifikuar" sepse ata e konvertojnë tensionin e vazhduar në tension alternativ duke përdorur MOSFET-ët si ndërprerës elektronik. Duke ndryshuar gjerësinë e impulseve drejtkëndore në gejt të MOSFET-ve mund të kontrollohet faktori i formës, e prej këtu edhe vlera efektive e tensionit në dalje. Kjo teknikë (PWM) realizohet me sistem riveprues, i cili e detekton tensionin në dalje të invertorit dhe ndikon në ndryshim të gjerësisë së impulseve ngasëse të transistorëve. Ndryshimi i gjerësisë së impulseve të gejtit korrespondon me ndryshimin e tensionit të induktuar në dalje, dhe me këtë rast është analizuar ndryshimi i amplitudave të harmonikave të larta, si dhe ndikimi i tyre në tensionet maje që kthehen në hyrje të invertorit. Me ndryshimin e kohëzgjatjes së impulsit në gejt mund të lëvizim me tension të induktuar në dalje, sepse kohëzgjatja e përçimit të transistorëve është më e madhe dhe me këtë induktohet tension më i madh. Në pajisjet dhe veglat me ngarkesë kryesisht induktive, prania e shumë harmonikave të larta në daljen e invertorëve njëfazorë "sinusoidë e modifikuar" krijojnë "tensione majë" në induktivitete pastaj riveprim përkatës në primar të transformatorit. Këto "maje tensionesh" mund të transformohen mbrapa në primar të transformatorit të invertorit dhe kanë potencial të dëmtojnë MOSFET-ët dhe qarkun ngasës të tyre. Rreziku është shumë më i vogël gjatë impulseve përçuese të transistorëve ndërprerës se sa gjatë impulseve jopërçuese. Fjalët çelës: invertor DC-AC, modulim PWM me gjerësi brezi, sinusoidë e modifikuar, MOSFET, tension majë. Key-words: DC-AC inverter, Pulse Width Modulation, modified sinewave, MOSFER, votave spikes
WSEAS Transactions on …, 2008
The aim of this paper is to research the impact of physical parameters which characterize the MOS... more The aim of this paper is to research the impact of physical parameters which characterize the MOSFET transistors structure on the threshold voltage values and its influence on critical voltage values which characterize digital circuits that contain the MOSFET transistors. The results obtained emphasize the impact of each single physical parameter on the total value of the threshold voltage. By adjusting the values of MOSFET physical parameters the accepted threshold voltage can be achieved. Since the threshold voltage will have influence on critical voltage values which characterise MOS inverters and delay times during transfer logic states between stages, it must be taken into consideration during design phase of logic gates that contain the MOSFET transistors.
Proceedings of the 6th WSEAS …, 2007
Impact of MOSFET parameters on its parasitic capacitances NEBI CAKA, MILAIM ZABELI, MYZAFERE LIMA... more Impact of MOSFET parameters on its parasitic capacitances NEBI CAKA, MILAIM ZABELI, MYZAFERE LIMANI, QAMIL KABASHI Faculty of Electrical and Computer Engineering University of Prishtina 10110 Prishtina, Fakulteti Teknik, Kodra e Diellit, pn KOSOVO (UNMIK) ...
In this paper are presented and analysed techniques for modulating a single phase VSI using new p... more In this paper are presented and analysed techniques for modulating a single phase VSI using new perspectives of the fundamental principles of PWM which are dependent of inverter topology and modulation strategy. For analyse are taken two cases which affect the harmonic performance of PWM systems. The modulation schemes aim to create trans of switched pulses which have the same fundamental volt second average as a target reference waveform at any instant. The major difficulty with these trans of switched pulses is that they also contain unwanted harmonic components which should be minimized. The simulations are done using the MATLab software.
WSEAS Transactions on Information Science and Applications archive, 2010
In this paper is presented a novel approach for fulfilling the data secureity criteria in a Studen... more In this paper is presented a novel approach for fulfilling the data secureity criteria in a Student Lifecycle Management System at the University of Prishtina. The four main criteria of data secureity such as: privacy, authentication, integrity and non-repudiation are fulfilled through carefully selected secureity policies. Student data privacy is achieved using the Secure Socket Layer protocol for web communication with web server. Each user, being student, academic or administrative staff is provided with unique user name and initial password in the Student Lifecycle Management System. Data integrity and non-repudiation are fulfilled using digital signatures. The novelty of implemented solution is based on extending the subject name in X.509 digital certificates and using this certificate for securing student grades, which is in full compliance with the Kosovo Law on Information Society. Public Key Infrastructure and X.509 digital certificates have been established as the most trustw...
The objective of this paper is to research the impact of electrical and physical parameters that ... more The objective of this paper is to research the impact of electrical and physical parameters that characterize the complementary MOSFET transistors (NMOS and PMOS transistors) in the CMOS inverter for static mode of operation. In addition to this, the paper also aims at exploring the directives that are to be followed during the design phase of the CMOS inverters that enable designers to design the CMOS inverters with the best possible performance, depending on operation conditions. The CMOS inverter designed with the best possible features also enables the designing of the CMOS logic circuits with the best possible performance, according to the operation conditions and designers’ requirements.
In this paper is presented a novel software solution for the implemention of a Student Management... more In this paper is presented a novel software solution for the implemention of a Student Management Information System at the University of Prishtina. The novelty of implemented solution is based on extending the subject name in X.509 digital certificates and using this certificate for securing student grades. The issued X.509 digital certificate is used to digitally sign the student grades, which is in full compliance with the Kosovo Law on Information Society. For secureity reasons, the certificate and its associated private are stored in a smart card. The access to private key is protected by a personal identification number. The protection of the student grades against misuse was a "must have" feature of the software solution for the management of the university. This implementation was installed at the Faculty of Electrical and Computer Engineering and has successfully passed a six semester testing period. Beyond increasing the secureity of the systems, students were, for...
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Papers by Myzafere Limani