Control/maintenance strategy fault tolerant mode and reliabilityanalysis for hydro power stations

Reliability analysis for hydro power stations for (N+M) and 2N fault tolerant modes (FTM) and their comparison from the standpoint of control and maintenance have been carried out in this paper. It shows that the (N+M) FTM is superior to 2N FTM in reliability and economics in most cases. So, (N+M) FTM is recommended for large power stations, especially for multi-unit plants. A simple method to choose the number of standby controllers is developed by optimizing the (N+M) FTM. The ideas and results proposed in this paper can be used not only for governing systems in hydro power stations but also for other industrial systems     

Frequency Dependence of Fading Statistics for Ultrawideband Systems

The variation of channel fading and dispersion statistics between the ultrawideband (UWB) channel subbands is characterized experimentally. Within the FCC-allocated UWB frequency range (3.1-10.6 GHz), the multipath characteristics in various subbands are found to be highly dissimilar. It is demonstrated that the higher subbands suffer greater attenuation, and the rms delay spread varies significantly with the center frequency. The impact of this variation on multiband-OFDM UWB system performance is evaluated, and a large performance variation across the band is reported     

Response of ductile metal sheet parameters to the cone angle of indenting conical tool and fracture toughness evaluation

The indentation and perforation of ductile metal sheet with a conical tool is accompanied by elastoplastic bending, stretching, plastic flow, and crack initiation and propagation. This ultimately results in material fracture in the form of petals. It has been observed that the perforation process is dependent upon the angle of the conical tool. Fracture toughness, crack initiation, work input before and after crack initiation, number of petals, and sheet and petal bending angles all depend on the tool angle. Crack initiation has resulted at minimum tool displacement for a tool angle α=45°, while minimum work input before and after the crack initiation is observed when the tool has an angle α=35°. The optimum range of tool angles for the indentation process is α=22.5 to 50°. In this range, the aluminum sheets showed minimum fracture toughness as well as minimum work input to overcome the offered resistance.     

Experimental results of a supplementary technique for auto-reclosing EHV/UHV transmission lines

A technique to discriminate between the line charging inrush current and the current resulting on closing or auto-reclosing a faulted EHV/UHV transmission line has been implemented on a 32-b digital signal processor (DSP) board. The technique is based on the voltage signals before switching and the current signals. This technique is proposed to be a supplement to the main protection schemes for both single-circuit and double-circuit lines. The scheme is tested on a physical model of a transmission system with a source at each end. Laboratory tests show that the proposed technique is effective for three-pole and single-pole reclosing on a faulted or healthy line.     

Malignant peripheral nerve sheath tumors. A clinicopathologic study of 28 cases

BACKGROUND: In order to improve management, the files and tissue sections of 28 cases of malignant peripheral nerve sheath tumors (MPNST) diagnosed at the University of Virginia Health Sciences Center between 1960 and 1990 were reviewed. METHODS: Clinical data tabulated included age, sex, race, the presence or absence of von Recklinghausen neurofibromatosis type 1 (NF-1), tumor size, tumor location, type of treatment, and status of surgical margins. Pathologic study included assessment of mitotic rate, divergent differentiation, cellular atypia, necrosis, and vascular reaction. RESULTS: The median disease-free survival time was 11 months, and the median overall survival time was 44 months. Overall survival and disease-free survival were significantly influenced by patient age, tumor location, tumor size, extent of surgery, and quality of margins. Patients with a family history of neurofibromatosis also had better disease-free survival. None of the other clinical variables correlated with survival. CONCLUSIONS: The authors recommended that patients with NF-1 be followed closely for MPNST development. For most cases, treatment should include aggressive surgery with wide surgical margins combined with adjuvant radiation therapy. Chemotherapy may have a role for treatment failures.     

Statistical timing analysis of combinational logic circuits

Efficient methods for computing an exact probability distribution of the delay of a combinational circuit, given probability distributions for the gate and wire delays, are developed. The derived distribution can give the probability that a combinational circuit will achieve a certain performance, across the possible range. This information can then be used to predict the expected performance of the entire circuit. The techniques presented target fast analysis as well as reduced memory requirements. The notion of a correct approximation, based on convex inequality, which never overestimates the percentage of circuits that will achieve any given performance is defined. It is shown that given the assumption that all the topologically longest paths are responsible for the delay, the computation technique provides a correct probabilistic measure in the sense given above. Methods are given to identify and to ignore false paths in the probabilistic analysis, so as to obtain correct and less pessimistic answers to the performance prediction question. Some practical results are given for a number of benchmark combinational circuits     

Variable structure and time-varying parameter control forhydroelectric generating unit

A variable-structure control strategy relative to the variable plant structure and output variation is developed, and a nonlinear compensation approach is proposed and analyzed. Moreover, for a certain plant structure with variable parameters a time-varying parameter control approach is developed, and its stability is analyzed and proved. Simulation results show the effectiveness of the proposed control strategy applied to an hydroelectric generating unit     

An H2 optimal adaptive power system stabilizer

Application of an H₂ optimal adaptive control algorithm as a power system stabilizer is described in this paper. The algorithm deals with disturbance attenuation in the sense of H₂ norm for nonlinear systems. Results of simulation and experimental studies suggest that the H₂ optimal control algorithm could be successfully used for the control of nonlinear systems such as synchronous generators