Mohamed M. Hamada

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.

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.

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.

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.

Coordinated reactive power management in distribution networks with renewable energy resources

Due to increasing penetration level of renewable energy based Distributed Generation (DG), voltage/reactive power control in distribution networks is currently an important topic. This paper proposes two coordinated voltage/reactive power control approaches in active distribution grids. The proposed coordination control is achieved based on predefined procedures to increase speed response and to reduce the complexity of computations. Simulations were carried out on a realistic Medium Voltage (MV) network using measured data of DG output power. The results were taken for different three weeks in the year representing different seasons. The results of the proposed approach were compared with previously published centralized control approach. The results indicate that the proposed approaches reduce the number of OLTC operations, reduce system losses, minimize the voltage fluctuation in the distribution system, and increase the hosting capacity of DG power.

Computation of electric field and human body induced current under overhead transmission lines

In this paper the Charge Simulation Method (CSM) is implemented to compute the electric field distribution around Egyptian three phase 500kV transmission line, with the presence and absence of the human model. The human body is simulated as a conducting cylinder object with a hemisphere for his head. Three positions of human body model are considered to compute the induced current in it. These positions are defined as under maximum, minimum and average conductor heights. A method to estimate the human body model induced current from the electric field computed without the presence of the human body model is proposed and its results are in good agreement with those computed with the presence of human body model under the transmission line.

Fast Detection of Voltage Collapse Proximity in Radial Distribution Systems

This paper presents new method for detecting voltage collapse proximity in radial distribution systems. Two indicators for evaluating voltage stability of radial distribution systems were derived. These indicators were based on the reduction of the actual distribution system into two-bus equivalent system. The parameters for the equivalent two-bus system are computed only once at the critical loading at certain bus for a specified power factor. Then the critical loading at this bus at any different power factor can be evaluated using the new indicator. This method is implemented for detecting voltage collapse proximity in some practical radial distribution systems. The results then compared with the results obtained by applying other methods. The comparison showed that the new method gives accurate results despite its simplicity and fastness. This may recommend this method to be applied for investigating voltage stability of radial distribution systems for on-line applications

Artificial Neural Network Modeling Technique for Voltage Stability Assessment of Radial Distribution Systems

This paper presents an artificial neural network (ANN) based modeling technique for predicting the voltage stability of radial distribution systems. The modeling technique is based on a new voltage stability index for assessment of radial distribution systems Lv . The index is implemented to investigate a 33-bus distribution system. An ANN model which has an input layer with two input vectors (P, Q), one hidden layer, and an output layer, which gives the predicted value for the voltage stability index Lv is suggested to predict the value of this index. The performance of the ANN model is tested by using the results of the 33-bus distribution system. Then the ANN model is checked by two model evaluation indices namely mean absolute percentage error and actual percentage error. Plotting of the simulated results with the ANN output is used to evaluate visually the accuracy of simulation. Extensive testing of the proposed ANN based technique have indicated its viability for voltage stability assessment