Mohamed A. A. Wahab

Novel modeling for the prediction of aged transformer oil characteristics

The effect of aging on transformer oil physical, chemical and electrical properties has been studied using the international testing methods for the evaluation of transformer oil quality. The study has been carried out on twelve transformers in the field and for monitoring periods up to 8 years. The properties which are strongly time dependent have been specified and those which have a great impact on the transformer oil breakdown voltage have been defined. Mathematical models for the breakdown voltage, total acidity and water content as a function of service periods have been given. The validity and applicability of these models for future prediction of these properties have been verified by the good agreement between the measured end predicted values. A multiple linear regression model for each transformer oil breakdown voltage as a function of its water content, total acidity and service period has been introduced and its adequacy has been illustrated by statistical analysis. Another multiple linear regression model has been developed by combining the results of a group of transformers into that of a single equivalent transformer. This model has been validated by predicting the properties of some other transformers and comparing them with the measured values. The comparison showed a good agreement for the results of transformers which have either been used or not in the derivation of the model.

The Effects of the Span Configurations and Conductor Sag on the Electric-Field Distribution Under Overhead Transmission Lines

The precise evaluation and mitigation of the electric field generated by overhead transmission lines has gained great interest due to its impact on health and environmental issues. This paper presents a more generalized technique to calculate the electric field generated by power transmission lines in three dimension coordinates. This technique has been evolved, formulated, and applied to an Egyptian 500-kV single-circuit transmission line to evaluate the effects of line topology and terrain topography on the computed electric field. The results are compared with those produced by a 2-D technique.

Bayesian Networks for Fault Diagnosis of a Large Power Station and its Transmission Lines

Abstract This article proposes a simplified fault-diagnosis system based on Bayesian networks with noisy-OR/AND nodes to estimate the faulty item/section(s) of a large power station and its transmission lines. The proposed method utilizes the final information of protective relays and corresponding circuit breakers to construct the Bayesian fault diagnosis model for each section. The learning algorithm for Bayesian network parameters takes the sum of the mean-squared error between the expected values and the computed values of certain target variables as the minimizing optimization function to adjust the network parameters continuously. By comparing the result beliefs of possible faulty sections, the faulty item/section(s) becomes a candidate. In order to test the validity and feasibility of that method, a computer simulation of the High Dam power station and its 500-kV transmission lines is used. It is shown that the proposed diagnosis method has many merits, such as rapid reasoning, less storage memory and processing time, easy correctness of diagnosing results, flexibility, and application into a large power station and its transmission lines for real-time fault diagnosis. Finally, it assists and supports the operator of the control room to make the right decision, especially in case of communication loss.

A newly modified forced oil cooling system and its impact on in-service transformer oil characteristics

This paper introduces a newly modified forced oil-cooling system in which a bypass filter (BPF) has been used. The BPF was introduced in such a way that its intake oil is the hot oil from the transformer top and the outlet is connected to the input of the oil-circulating pump. This system has been used for two transformers. Periodical measurements of the physical, chemical, and electrical transformer oil characteristics by standard testing methods before BPF operation, in operation, and after stopping its operation in the transformer have been carried out. Before BPF operation, results revealed that these characteristics are continuously deteriorating with the increase in transformer oil service period. However, when BPF is in operation, this deterioration not only has been reduced but also some of the characteristics have been improved. After the operation of BPF has been stopped, some of the characteristics resumed their deterioration. However the rates by which these characteristics are deteriorating are noticeably smaller than their initial values. The deteriorated transformer oil characteristics (without installation of BPF) were predicted by polynomial regression, multiple linear regression, and general linear multiple regression models. The efficiency and feasibility of the new cooling system in preserving in-service transformer oil characteristics have been proved and justified by quantitative evaluation of the measured and theoretically predicted deteriorated (without installation of BPF) characteristics.

A proposed strategy for capacitor allocation in radial distribution feeders

A new strategy for capacitor allocation in radial distribution systems is presented in this paper. This strategy adds a new constraint to the well-known constraint (allowed voltage violation). The new constraint is the sectional ohmic loss in each branch of the feeder. Sizes and locations of the capacitors are selected in order to achieve the maximum reduction in the total losses in the system. Sometimes this reduction is not achieved in all the branches but the loss in any individual branch of the system is not allowed to increase than it was before the capacitor placement. The proposed strategy is applied to two different distribution systems with 9 and 34-buses respectively. The results of the proposed strategy are compared with previous works. The comparison showed the validity and the superiority of this strategy.

Ramp rate control and voltage regulation for grid directly connected wind turbines

This paper introduces a methodology to enhance the quality of the voltage and power produced from fixed-speed direct-connected wind turbines. The method uses an electrolyzer/fuel cell combination to be connected to the point of common coupling via limited rating converters. The control scheme is designed to keep the power ramp rate and voltage fluctuation within their limits and to minimize the rating of the converters used to interface the electrolyzer and the fuel cell. Intensive simulation results are carried out to demonstrate and validate the proposed technique.

Simple maximum power extraction control for permanent magnet synchronous generator based wind energy conversion system

A new and simple maximum power extraction control strategy is proposed for the permanent magnet synchronous generator (PMSG) based variable speed wind energy conversion system (VSWECS). The PMSG is connected to the grid through a switch mode rectifier and a three phase voltage source (VSI) inverter. The generator side switch mode rectifier is controlled to achieve maximum power from wind. Simple estimating of PMSG generator speed, using estimated generator speed to calculate mechanical power generated from wind, and optimum power coefficient can be achieved from the relation governed the generator speed and mechanical power. The grid side voltage source inverter uses a hysteresis current controller to supply power at unity power factor into the grid. Extensive simulations have been performed using MATALB/SIMULINK. Simulation results show that the controller can extract maximum power from the wind and achieve unity power factor at the grid with different wind speed.

Exact analytical model of single electron transistor for practical IC design

Abstract This paper presents a new exact analytical model for single electron transistor (SET) applicable for circuit simulation. It has been developed based on orthodox theory of single electronics using master equation where a scheme has been suggested to determine the most probable occupied electron states. The proposed model is more flexible and is valid for single or multi-gate, symmetric or asymmetric devices and can also consider the background charge effect. It can be used for large drain–source voltage range whatever the device is biased under symmetric or asymmetric bias conditions. SET characteristics produced by the proposed model have been verified against widely accepted single electron circuits Monte Carlo simulator SIMON and show a good agreement. Moreover, the model has been implemented in a widely used commercial circuit simulator SPICE to enable simulation with conventional electronic elements and a single electron inverter has been simulated and verified with SIMON results.

Control of a grid connected variable speed wind energy conversion system

This paper presents a simple control strategy for a variable speed wind energy conversion system with a permanent magnet synchronous generator (PMSG). PMSG is connected to the grid through a switch mode rectifier and a voltage source inverter. The grid should be supplied by certain levels of powers for certain times from wind energy system according to a certain schedule. Batteries bank is connected to the DC-link voltage through a DC-DC bidirectional converter to maintain the DC voltage at constant value. Also it is used to make batteries bank stores a surplus of wind energy and supplies this energy to the grid during the wind power shortage. Voltage source inverter, at the grid side, uses a relatively complex vector control scheme to supply power at unity power factor into the grid. Simulation results have been performed using MATLAB/SIMULINK. It demonstrates that the controller performs very well in spite of variation in wind speed and demanded grid power. Also, maximum power extraction from wind turbine can be achieved through the switch mode rectifier control at the generator side.

Semi-fragile Image Authentication Using Robust Image Hashing with Localization

The great advance in image processing demands a guarantee for assuring integrity of images. In this paper, a semi-fragile image authentication technique is proposed, which can detect and localize the malicious alteration distinguishing the content-preserving manipulations, such as JPEG compression, from the content alterations. The original image is divided into large blocks (sub-images) that are also divided into 8x8 blocks. Secure image hashing is utilized to generate the sub-image hash (signature) that may slightly change when the content-preserving manipulations are applied. Then, two sub-image hash copies are embedded into relatively-distant sub-images using a doubly linked chain, which prevents the vector quantization (VQ) attack. The hash bits are robustly embedded in chosen discrete cosine transform (DCT) coefficients exploiting a property of DCT coefficients that is invariant before and after JPEG compression. The experimental results show that the proposed technique successfully detects and localizes the content alterations only. Furthermore, It can effectively resist many attacks such as vector quantization attacks.