Operational performance of a PV generator feeding DC shunt and induction motors with MPPT

This paper presented the transient and operational behavior of a parallel Combination of DC Shunt Motor and IM fed by a photovoltaic generator at different solar irradiance levels. The maximum power point of current/voltage (I/V) characteristic of the PV generator was achieved for different solar intensities, by utilizing an open circuit voltage method. The nonlinear operational behavior of (I/V) characteristics of the PV generator at various solar intensities and the magnetization curve of the ferromagnetic material of the DC shunt motor were both modeled by high order polynomial mathematical expressions. The study investigated the response of the system at different solar irradiance levels and changing the torque loads for both motors and then following step change in solar intensity levels with fixed loading torques for both motors. All numerical simulations were executed using MATLAB software

On the effect of AVR gain on bifurcations of subsynchronous resonance in power systems‟‟.Electr Power Energy System 2010;32(6):60–76

a b s t r a c t This paper presents the effect of the automatic voltage regulator (AVR) gain on the bifurcations of subsynchronous resonance (SSR) in power systems. The first system of the IEEE second benchmark model of SSR is chosen for numerical investigations. The dynamics of both axes damper windings of the generator and that of the power system stabilizer (PSS) are included. The bifurcation parameter is the compensation factor. Hopf bifurcation, where a pair of complex conjugate eigenvalues of the linearized model around the operating point transversally crosses from left-to right-half of the complex plane, is detected in all AVR gains. It is shown that the Hopf bifurcation is of subcritical type. The results also show that the location of the Hopf bifurcation point i.e. the stable operating point regions are affected by the value of the AVR gain. The variation of the location of the Hopf bifurcation point as function of the AVR gain for two operating conditions is obtained. Time domain simulation results based on the nonlinear dynamical mathematical model carried out at different compensation factors and AVR gains agree with that of the bifurcation analysis

Entwurf, Optimierung, Konstruktion und Prüfung von Seltenerde-Permanentmagnet Elektrischen Maschinen mit neuer Topologie für Wind Energienanlagen

Diese Dissertation behandelt Entwurf, Optimierung, Konstruktion und meßtechnische Untersuchungen von langsam laufenden Seltenerde-Hochenergie Permanentmagnet Maschinen mit neuer Topologie. Zwei von den entworfenen Maschinen können, als direkt gekoppelte Windenergiegeneratoren genutzt werden. Die gebaute Prototypmaschine wurde als drehzahlvariabler langsamlaufender Generator unter verschiedenen Belastungsbedingungen geprüft. Gute Übereinstimmung zwischen den theoretisch vorausgesagten und experimentell erhaltenen Ergebnissen sind erreicht worden. Die hergestellte Prototypmaschine wurde als Einphasengenerator dimensioniert. Zwei Dreiphasige Entwürte sind theoretisch behandelt worden. Die typischen Merkmale dieser Maschinen sind der Stator mit seiner Ringwicklung sowie der Rotor mit den Permanentmagneten in der Anordung zur Flußkonzentration auf einem unmagnetischen Rotorkörper. Das Ergebnis dieser Entwurfsmethode ist ein recht hoher Wirkungsgrad mit 84,6% bei einer Leistung von etwa 1200W. Die zwangsläufig auftretenden Rastmomente konnten durch den Einsatz von speziellen magnetischen Nutkeilen reduziert werden. Es erfolgte auch ein Berechnung der Rastmomente sowie der relevanten Parameter mittels der Finite Elemente Methode.This thesis presents design, optimization, construction and test of radial-flux low-speed rare-earth high-energy permanent-magnet (PM) electrical machines with new topology. Two of the designed machines can be used as directly driven wind energy generators. Due to absence of the field current and field winding, permanent magnet generators exhibit high efficiency in operation, simple and robust structure in construction and high power to weight ratio. The attractiveness of the permanent magnet generators is further enhanced by the availability of high-energy permanent magnet materials like NdFeB. Based on the equivalent magnetic circuit approach and permanent magnet load line characteristics, iterative preliminary design for the proposed generator was firstly carried out. The aim of this simple linear pre-design tool was to have an initial geometry for the detailed investigations of the finite element technique (FET) where the electromagnetic behavior of the machine was optimized. The stator of the machine was slotted. Slotted configuration was chosen as it permits lower effective air gap length and therefore thin magnets can be used. This, in fact, largely decreases the cost of active material as it is dominated by that of the magnets. However, cogging torque comes as a consequence of slots. The cogging torque of the machine was estimated using the flux-MMF technique enhanced by the FET. Cogging torque is an oscillatory torque caused by the variation of the co-energy of the permanent magnets with respect to the rotor position, resulting from the variation of the magnetic circuit seen by the magnets. It is an inherent characteristic of slotted permanent magnet machines. It should be studied and minimized in the applications where minimizing torque ripple, vibration and noise are essential requirements. The configuration of the rotor corresponding to the lowest cogging torque was adopted. The prototype machine was constructed using novel updated high-energy permanent magnets with a remanent of 1.41T. The stator laminations of the machine were cut using laser technology methods. The slots of the machine were flat and the windings were of toroidal type (torus) with short ends. This, in turn, reduced the cost and weight of active material and improved the efficiency. The permanent magnets were rectangular blocks of NdFeB with flux concentration arrangement and magnetized tangentially on the rotor support structure. Soft magnetic material is attached to both poles of the permanent magnets, which not only produced an easy path for the flux penetration but also reduced the leakage flux and therefore a typical ‘rule of thumb’ value for the leakage flux coefficient was achieved. The rotor support structure of the machine was manufactured from nonmagnetic light material (Aluminum). To further reduce the total weight of the machine, longitudinal holes were excavated inside the rotor support structure. The constructed prototype machine was tested as variable low-speed generator with different loading conditions. Good agreement between the theoretically predicted and experimentally obtained results has been achieved. The inertia of the system together with the cogging torque necessitated a peak starting torque of 64% of the rated torque. To reduce this starting torque, the slot openings were filled up with bonded soft magnetic material with a relative permeability equals 10. The peak value of the starting torque has been reduced to 43% of the rated torque. The cogging torque of the machine was estimated again and the performance of the slot-filled machine was studied and compared with the previous case. The manufactured prototype machine was single phase. Three-phase is also possible. The number of phases is usually determined based on the type of the load and the rated power demand. Finite element analysis and theoretical study were carried out on two three-phase machines of the same topology. The first one is a four-pole three-slot machine and the second one is ten-pole six-slot design

Bifurcation Analysis Of Subsynchronous Resonance Of The Ieee Second Benchmark Model With Tcsc

This paper presents the bifurcation analysis of the IEEE second benchmark model of Subsynchronous Resonance (SSR) when the transmission line is compensated by Thyristor-Controlled Series Capacitor (TCSC). Varying the firing angle of the thyristors changes the equivalent capacitance, which in turn controls the compensation factor of the transmission line. The dynamics of damper windings, Automatic Voltage Regulator (AVR), Power System Stabilizer (PSS) and Turbine Governor (TG) are included. As the series compensation increases, the power transfer capability of the transmission line increases. However, the operating point of the system loses stability via supercritical Hopf bifurcation in case of conventional compensation. In this study, it is found that the operating point of the thyristor-controlled compensated system never loses stability at any realistic firing angle and therefore all bifurcations of the system are eliminated

Characteristics and outcomes of COVID-19 patients admitted to hospital with and without respiratory symptoms

Background: COVID-19 is primarily known as a respiratory illness; however, many patients present to hospital without respiratory symptoms. The association between non-respiratory presentations of COVID-19 and outcomes remains unclear. We investigated risk factors and clinical outcomes in patients with no respiratory symptoms (NRS) and respiratory symptoms (RS) at hospital admission. Methods: This study describes clinical features, physiological parameters, and outcomes of hospitalised COVID-19 patients, stratified by the presence or absence of respiratory symptoms at hospital admission. RS patients had one or more of: cough, shortness of breath, sore throat, runny nose or wheezing; while NRS patients did not. Results: Of 178,640 patients in the study, 86.4 % presented with RS, while 13.6 % had NRS. NRS patients were older (median age: NRS: 74 vs RS: 65) and less likely to be admitted to the ICU (NRS: 36.7 % vs RS: 37.5 %). NRS patients had a higher crude in-hospital case-fatality ratio (NRS 41.1 % vs. RS 32.0 %), but a lower risk of death after adjusting for confounders (HR 0.88 [0.83-0.93]). Conclusion: Approximately one in seven COVID-19 patients presented at hospital admission without respiratory symptoms. These patients were older, had lower ICU admission rates, and had a lower risk of in-hospital mortality after adjusting for confounders