Ahmad Gholami - Academia.edu (original) (raw)

Papers by Ahmad Gholami

International Journal of Engineering, 2015

In order to restrain ferroresonance in coupling capacitor voltage transformers (CCVT), three kind... more In order to restrain ferroresonance in coupling capacitor voltage transformers (CCVT), three kinds of ferroresonance suppression circuits (FSCs) have been presented. The resonant type, the fast-saturation reactor type and electronic type which are connected to secondary side of CCVTs, are proposed in this paper. Frequency domain analyses, for voltage ratio magnitude, are carried out for CCVTs to obtain preferable FSC. Furthermore, using surge suppressors such as metal oxide varistors (MOV) to improve the performance of the FSCs is investigated. Time domain simulations demonstrate that ferroresonance can be cleared in few cycles in presence of FSCs. In addition, to mitigate ferroresonanse in inductive transformers (VTs), we connect the damping resistor to the open-delta-arranged auxiliary winding.

2009 Transmission & Distribution Conference & Exposition: Asia and Pacific, 2009

ABSTRACT According to reports that had been published in many papers, switching operation in gas ... more ABSTRACT According to reports that had been published in many papers, switching operation in gas insulated substation (GIS) lead to malfunction in the substation equipments. These improper functioning contain of malfunction electronic equipments and deformation of transformer winding and bushing failures. The reason of malfunction of electronic equipments is due to coupling voltages on data and control cables. The reasons of these problems are travelling waves which is generated during switching operations in a gas-insulated substation (GIS). To evaluate these cases, it must be performed an accuracy analysis on the wave shape and level of the very fast overvoltage (VFTO) and very fast transient currents (VFTC). In this paper the effective factors on the wave shape and level of the VFTC and VFTO is investigated and the beneficial approaches for the industry to finding the optimum approaches for VFT mitigation, is presented. These factors are included trapped charge, entrance capacitor, compensation resistor, hybrid compensation L and T types and variation of arrangement of switchgear. As variation of switchgear arrangement is included variation of lengths, branches (bypasses) and series capacitor (disconnectors opened).

Journal of Physics D: Applied Physics, 2012

ABSTRACT A three-dimensional (3D) transient model is developed to investigate plasma current defo... more ABSTRACT A three-dimensional (3D) transient model is developed to investigate plasma current deformation driven by internal and external magnetic fields and their influences on arc stability in a circuit breaker. The 3D distribution of electric current density is obtained by solving the current continuity equation along with the generalized Ohm's law in the presence of an external magnetic field, while the magnetic field induced by the current flowing through the arc column is calculated by the magnetic vector potential equation. The applied external field imposes a rotational electromagnetic force on the arc and influences the plasma current deformation, which is discussed in this paper. In SF6 circuit breakers when gas interacts with the arc column, the fundamental equations such as Ampere's law, Ohm's law, turbulence model, transport equations of mass, momentum, and energy of plasma flow have to be coupled for analysing the phenomenon. The coupled interactions between the arc and the plasma flow are described within the framework of magnetohydrodynamic equations in conjunction with a K–ε turbulence model. Simulations are focused on sausage and kink instabilities in the plasma (these phenomena are related to the electromagnetic field distribution and define the plasma deformations). The 3D simulation reveals the relation between plasma current deformation and instability phenomena, which affects the arc stability during the operation. Plasma current deformation is a consequence of coupling between electromagnetic forces (resulting from internal and radial external magnetic fields) and the plasma flow that are described in the simulations.

Journal of Economic Development Research, Jun 15, 2011

Journal of Optometry, 2016

This study was performed to investigate whether occupationally related color vision deficiency ca... more This study was performed to investigate whether occupationally related color vision deficiency can occur from welding. A total of 50 male welders, who had been working as welders for at least 4 years, were randomly selected as case group, and 50 age matched non-welder men, who lived in the same area, were regarded as control group. Color vision was assessed using the Lanthony desatured panel D-15 test. The test was performed under the daylight fluorescent lamp with a spectral distribution of energy with a color temperature of 6500K and a color rendering index of 94 that provided 1000lx on the work plane. The test was carried out monocularly and no time limit was imposed. All data analysis were performed using SPSS, version 22. The prevalence of dyschromatopsia among welders was 15% which was statistically higher than that of nonwelder group (2%) (p=0.001). Among welders with dyschromatopsia, color vision deficiency in 72.7% of cases was monocular. There was positive relationship between the employment length and color vision loss (p=0.04). Similarly, a significant correlation was found between the prevalence of color vision deficiency and average working hours of welding a day (p=0.025). Chronic exposure to welding light may cause color vision deficiency. The damage depends on the exposure duration and the length of their employment as welders.

IEEE Transactions on Applied Superconductivity, 2000

ABSTRACT In this paper, the thermal performance of a 10-MW-class wind turbine-based high-temperat... more ABSTRACT In this paper, the thermal performance of a 10-MW-class wind turbine-based high-temperature superconducting (HTS) synchronous generator is studied. The proposed generator is designed and modeled through a finite-element method (FEM), and the generator losses, including iron losses and electrical losses, are obtained. As the output power substantially depends on thermal rising of the generator, heat transfer analysis of the proposed HTS generator is carried out using both lumped parameters network (LPN) and FEM analysis. Then, temperature distribution of the components of the machine is evaluated and discussed for different generator speeds. The proper temperature distribution guarantees the desirable operation of the generator. This paper shows that it is feasible to design a 10-MW-class HTS synchronous generator from thermal analysis point of view.

As wind farms grow rapidly throughout the world, the grounding system and ground impedance of win... more As wind farms grow rapidly throughout the world, the grounding system and ground impedance of wind turbine towers under transients have got more important. As a matter of fact, an exact model to testify dynamic properties of grounding system should represent nonlinear effects of soil ionization and be frequency dependent. Some new researches conducted in this issue, all are based circuit theory which does not allow for accurate analysis of aforementioned behaviors. This paper has come to express a new approach to model any grounding system only by few RLC branches in wide frequency range. A new model and some simple formula that is recommended in this paper, represent all effects of transient phenomena of lightning strikes and all analysis have got more easier than other approach. By using this method, there’s no difference between those grounding system buried in cylindrical soils or conical one, multi-layer oil or not, humid soil or not and so on, they all are available for transi...

20th Iranian Conference on Electrical Engineering (ICEE2012), 2012

Abstract This paper presents a model of lightning performance of a 400kV double circuit overhead ... more Abstract This paper presents a model of lightning performance of a 400kV double circuit overhead transmission line using ATP/EMTP. Including the models of transmission line, tower surge response, footing impedance, impulse electric strength of line insulator and ...

2006 IEEE International Power and Energy Conference, 2006

2012 11th International Conference on Environment and Electrical Engineering, 2012

Abstract This paper presents a model of lightning performance of a 230 kV double circuit overhead... more Abstract This paper presents a model of lightning performance of a 230 kV double circuit overhead transmission line and substation using ATP/EMTP. Including the models of transmission line, tower surge response, footing impedance, impulse electric strength of ...

Http Dx Doi Org 10 1080 03772063 2014 961977, Nov 7, 2014

Engineering Technology Applied Science Research, Jun 1, 2011

International Universities Power Engineering Conference, 2007

Principle role of a Power System Stabilizer (PSS) is to increase damping of oscillations of gener... more Principle role of a Power System Stabilizer (PSS) is to increase damping of oscillations of generator rotor by control of its excitation by using of auxiliary stabilizer signals. The design of a PSS can be performed by suitable state and optimum feedback that roots of case study is transferred to suitable points as designed results. As the large searching techniques,

2007 IEEE Power Engineering Society General Meeting, 2007

Abstract This paper presents a genetic algorithm based technique for on-line estimation of synchr... more Abstract This paper presents a genetic algorithm based technique for on-line estimation of synchronous generator parameters, using an excitation disturbance. To illustrate and verify the proposed algorithm, some excitation disturbance tests are conducted on an actual hydro generator. Machine response to the applied disturbance is processed by the proposed technique using genetic algorithm. The estimated values of the parameters are compared with manufacture's data and also validated by comparing measured and simulated d-axis ...

Energy and Power Engineering, 2011

2010 Conference Proceedings IPEC, 2010

ABSTRACT Reportedly, switching operation in gas insulated substation (GIS) leads to deformation o... more ABSTRACT Reportedly, switching operation in gas insulated substation (GIS) leads to deformation of transformer winding and bushing failures. This paper presents practicable method that is investigated for mitigation of overvoltage magnitude and resonance frequency amplitude of very fast transients (VFT) at the power transformer. Properties of this method are simple and inexpensive with at least changes in installed GIS. This method can be changing in the type and length of terminal components. Also alteration of load side terminal and source side terminal are examined. Computer simulation has been done using the alternative transients program (ATP), a widely used version of EMTP.