Mohamed Ahmed Abdel-Rahman

Application of Series Voltage Boosting Schemes for Enhanced Fault Ridethrough Performance of Fixed Speed Wind Turbines

This paper presents new series grid interface topologies for enhancing the low voltage ridethrough capability of self-excited induction generator-based wind turbines. Two proposed schemes: static synchronous series compensator (SSSC) and controllable series braking resistor (CSBR) have been designed and simulated. The test system simulated represents a wind turbine connected to an electric grid with alternatively employing SSSC and CSBR. The potential of the two schemes is evaluated and analyzed using positive and negative-sequence reference frames in response to balanced and unbalanced fault scenarios. The simulation results using PSCAD/EMTDC demonstrate the performance of the proposed schemes for improving the fault ridethrough capability and transient stability margin in response to severe symmetrical and asymmetrical grid faults. The schemes have shown successful and fast mitigation of voltage disturbance consequences on the mechanical system. The schemes helped the wind turbine to remain connected with considerable isolation from grid faults.

Can a Novel Echocardiographic Score Better Predict Outcome after Percutaneous Balloon Mitral Valvuloplasty

Objective: The assessment of patients with mitral stenosis before percutaneous balloon mitral valvuloplasty (PBMV) is crucial to predict outcome after the procedure. We tried to explore the prognostic power of a novel echocardiographic score to predict immediate postprocedural outcome in comparison to the standard score. Methods: We enrolled 50 consecutive patients with moderate to severe mitral stenosis admitted to undergo PBMV. For all patients, we assessed both the standard Massachusetts General Hospital (MGH) score and a novel score based on calcification (especially commissural) and subvalvular involvement. Patients underwent PBMV with the double balloon technique. Thereafter, patients were classified into two groups: group 1 (poor outcome) defined as no procedural success and/or increase of MR by more than 1 grade and group 2 (optimal outcome) defined as procedural success with increase of MR by 1 grade or less. Results: The total MGH score did not differ significantly between the two groups. However, among the individual parameters of the score, both calcification and subvalvular affection were significantly higher in group 1 versus group 2 (2.8 ± 0.4 versus 1.7 ± 0.8, and 2.4 ± 0.5 versus 1.6 ± 0.4, respectively, P < 0.01 for both). The total novel score and its two individual parameters (calcification and subvalvular involvement) were all significantly higher in group 1 versus group 2 (6 ± 1.5 versus 2.9 ± 1.9, 4.9 ± 1.2 versus 2.4 ± 1.5, 1.5 ± 1.6 versus 0.5 ± 0.9, respectively, P < 0.01 for all). Multivariate analysis demonstrated the novel score to be the only independent predictor of poor outcome. Conclusion: The novel score is more reliable and correlates with outcome better than the standard score.

Resection arthrodesis for the management of aggressive giant cell tumor of the distal femur

Background: Giant cell tumors (GCTs) of bone are aggressive benign tumors. Wide resection is reserved for a small subset of patients with biologically more aggressive, recurrent, and extensive tumors. Wide resection and mobile joint reconstruction are preferable for treating tumors around the knee. In certain situations, resection arthrodesis or an amputation is suggested. In this prospective study we report the outcome of 8 patients of aggressive GCT of lower end of femur treated with resection arthrodesis. Materials and Methods: Eight patients with mean age of 37.25 years (range 30–45 years) with Campanacci Grade III (Enneking stage III) giant cell tumors at the distal femur were treated with wide resection and arthrodesis using dual free fibular graft and locked intramedullary nail from January 2003 to January 2008. There were four males and four females patients. The mean follow-up was 48.75 months (range 30–60 months). The functional evaluation was done using the standard system of musculoskeletal tumor society with its modification developed by Enneking et al. Results: At the final follow up the functional score ranged from 20 to 27 out of total score of 30. Graft union was achieved in all cases in a duration mean of 14.5 months (range 12-20 months).One case required secondary bone graft due to delayed union, and one case had superficial wound infection which healed on systemic antibiotics. At final followup, all the patients were disease free. Conclusion: Wide resection and arthrodesis in aggressive GCTs of the distal femur with involvement of all muscle compartments is a good treatment option. Resection arthrodesis offers a biological reconstruction alternative to amputation in a special group of patients when extensive resection precludes mobile joint reconstruction.

Application of Balanced Realizations for Model-Order Reduction of Dynamic Power System Equivalents

This paper reviews and applies the balanced realization (BR) theory to obtain reduced-order models from dynamic system equivalents of electric power networks. BR allows obtaining reduced-order models via a process in which the original asymptotic stable system is internally balanced. The balanced system is truncated according to its dominant dynamics. It can be proven that the truncated, that is, reduced-order, system is also stable. This paper applies the BR method to dynamic system equivalents represented as frequency-dependent network equivalents (FDNEs). Furthermore, it shows that an assumed reduced-order FDNE can be further reduced via the BR process. Four case studies, involving one transmission network and three wind power plants, are presented.

Power line carrier for real time load management

Demand Side Management (DSM) is presently gaining momentum in nearly all the world. Lack of primary energy resoureces, lack of generation capacity, and the need to reduce emissions all combined together are pushing world utilities into exploring the extent DSM techniques can play in their favor. To that end, a cheap and reliable communication means is necessary to make DSM projects of a sound economic value. Power line communication (PLC) technology has a huge potential to become the favored communication technology for DSM, since there is no extra investment is put in dedicated communication cables. This paper presents a prototype for a DSM regime. The prototype is composed of a central station, which monitors different loads. Load data is collected at the load stations and sent back to the central station via PLC. The central station contains the DSM algorithm, whether peak limiting, load shifting, etc. Control signals are carried back to load stations. The prototype has been successfully experimented with dummy loads.

Low-voltage Ride-through Capability Characterization of Wind Farms

Abstract This article characterizes low-voltage ride-through capability of wind farms. Critical clearing time has been used in the literature as a measure of the transient stability of induction generators and, hence, an indication for low-voltage ride-through capability of single wind turbine generators. Therefore, this article aims to aggregate entire wind farms into a single induction generator so that low-voltage ride-through capability can be characterized for this wind farm equivalent. The proposed model has the potential to be used for power systems analysis with large amounts of wind energy plants as the wind energy becomes more dominant in the generation mix. Case studies are further presented to show the validity of the introduced equivalent technique compared to exact wind farm modeling.

Extended curettage and adjuvant therapy for benign tumors of the talus

PURPOSE Benign tumors of the talus are rare, and their management is controversial. Recent efforts have extended the safety margin of intralesional excision and curettage by chemical and physical means. Cryotherapy as adjuvant therapy is associated with risks of local wound and bony complications (delayed healing, potential for pathological fractures). METHODS We retrospectively reviewed six cases of benign talar tumors (three giant cell tumors, two aneurysmal bone cysts, one osteoblastoma) treated by extended curettage and adjuvant cryotherapy. Talar bone stock was restored by bone grafting the residual cavity with fibular strut grafts and/or cancellous bone grafts. Patients were followed for a mean of 40 months. RESULTS At latest follow-up, each patient had a mobile painless ankle and good functional outcome. No patient experienced recurrence or complications related to cryotherapy. CONCLUSIONS Intralesional extended curettage with bone grafting and cryotherapy is a successful, safe, and efficient treatment of benign tumors of the talus.

Floquet domain for open-loop transient simulation of switched devices

This paper presents a Floquet-based time-domain (TD) simulation approach of switched devices in open-loop operation. According to the coordinate transformation provided by Floquet theory, a linear time-periodic (LTP) system becomes a system with a constant state matrix via a transformation matrix. The paper primarily shows that Floquet transformation is readily applicable to pulse-width modulation (PWM) schemes employed in modern electrical networks. Furthermore, the paper demonstrates that switched networks modeled in Floquet coordinates exhibits significant computational savings compared to solving the original LTP state-space system by traditional TD techniques. Two switched networks, i.e., an HVDC system and a system equipped with three PV generators, in open-loop operation, are presented as case studies to support the statements above.

Radius of proximal isovelocity surface area in the assessment of rheumatic mitral stenosis: Connecting flow to anatomy and hemodynamics

Background Echocardiographic assessment of left atrial pressure (LAP) in mitral stenosis (MS) is controversial. We sought to examine the role of the radius of the proximal isovelocity surface area (PISA-r) in the assessment of the hemodynamic status of MS after fixing the aliasing velocity (Val). Methods and results We studied 42 candidates of balloon mitral valvuloplasty (BMV), for whom pre-BMV echocardiography was done and LAP invasively measured before dilatation. PISA-r was calculated after fixing aliasing velocity to 33 cm/s. In addition, the ratio IVRT/Te’–E was also measured, where IVRT was isovolumic relaxation time, and Te’–E was the time difference between the onset of mitral flow E-wave and mitral annular early diastolic velocity. IVRT/Te’–E and PISA-r showed a strong correlation with LAP (r = −0.715 and −0.637, all p < 0.001) and with right-sided pressures. In addition, PISA-r correlated with mitral valve area by planimetry method (MVA) and with left ventricular outflow tract stroke volume (r = 0.66 and 0.71, all p < 0.001). Receiver operator characteristic curve (ROC-curve) showed that PISA-r was not inferior to IVRT/Te’–E in differentiating LAP ⩾25 from <25 mmHg. Conclusion Provided that Val is set to a constant of 33 cm/s, PISA-r can assess the hemodynamic status of MS, and seems a simple alternative to the tedious IVRT/Te’–E for estimation of LAP.

Cardiac rehabilitation improves the ischemic burden in patients with ischemic heart disease who are not suitable for revascularization.

BACKGROUND Ischemic heart diseases including stable angina & acute events, represent a huge burden on both the individual & the society and represent an important source of disability. AIM We aimed to identify the effect of cardiac rehabilitation program (CRP) on the ischemic burden in patients with ischemic heart disease (IHD) unsuitable for coronary revascularization. METHODS The study included 40 patients with IHD who were not suitable for coronary revascularization either by PCI or CABG (due to unsuitable coronary anatomy, co morbidities, high surgical/procedural risk or patient preference). All patients were subjected to sophisticated CRP protocols, including patient education, nutritional, medical, psychological and sexual counseling and group smoking cessation. All patients participated in low intensity exercise program twice weekly. The patients symptoms, vitals and medications were evaluated at each visit and clinical and laboratory data, echocardiography and stress myocardial perfusion imaging (SPECT) were evaluated before and 3months after the end of the study. RESULTS The mean age was 56.8±3.1years and only 2 patients (5%) were females. 22 (55%) patients were diabetic, 21 (53%) were hypertensive and 30 (75%) were smokers. It was found that 3months after completion of CRP, there was a significant decrease in BMI (30.3±2.9 vs. 31.2±1.9, p<0.001), and mean blood pressure (93.4±11 vs. 105±10.6mmHg, p<0.001). There was also a favorable effect on lipid profile and a significant improvement of the functional capacity in terms of NYHA functional class (2.1±0.62 vs. 1.4±0.6, p<0.001). Despite that wall motion score index did not significantly change after CRP, there was a strong trend toward a better ejection fraction (53.7±7.8 vs. 54.5±6.3 %, p=0.06) and significant improvement of Canadian cardiovascular class (1.42±0.6 vs. 1.95±0.5, p<0.001) post CRP. Importantly, the difference between the SPECT-derived summed segmental scores at peak stress and at rest (SDS) was significantly lower after CRP (4.4±3 vs. 7.2±3, p<0.001). CONCLUSION Participation in cardiac rehabilitation program improves ischemic burden in patients with IHD who are unfit or not suitable for conventional cardiac revascularization. In addition the decreased ischemic burden, functional capacity, hemodynamic and metabolic profiles also improve for this group of patients and thus, cardiac rehabilitation should be implemented for routine management of those patients.