Expodev Inc.

Montreal, Canada

Expodev Inc.

Montreal, Canada

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Rizk F.A.M.,Expodev Inc.
2010 30th International Conference on Lightning Protection, ICLP 2010 | Year: 2010

For the first time, a model gives due consideration to the effect of space charge produced by trigger-wire glow-corona on continuous upward leader onset, in rocket triggered lightning under negative cloud. © 2010 University of Cagliari.


Rizk F.A.M.,Expodev Inc.
2010 30th International Conference on Lightning Protection, ICLP 2010 | Year: 2010

It has been shown that a positive upward connecting leader from an overhead conductor spans a considerable part of the lateral attractive distance of the conductor. This justifies the effort to search for means to suppress streamers/upward leaders from such conductors. © 2010 University of Cagliari.


Rizk F.A.M.,Expodev Inc.
IEEE Transactions on Power Delivery | Year: 2011

In this paper, it has been shown that a positive upward connecting leader from an overhead conductor spans a considerable part of the lateral attractive distance of the conductor. This justifies the effort to search for means to suppress streamers/upward leaders from such conductors. A new simplified model is introduced to account for the initiation and time growth of glow corona from a cylindrical conductor above ground under linearly rising fields due to cloud charges or negative downward leaders. The model was applied to assess lightning exposure of a streamer-inhibited ground wire as well as its application to lightning protection of adjacent power conductors. The paper confirms the prospect of successful application of streamer inhibition due to space charge shielding (ultra-corona) as a means for lightning protection. Streamer inhibition by using larger conductor diameters or conductors provided with an insulating layer was also considered and the limitations of both techniques were pointed out. © 2010 IEEE.


Rizk F.A.M.,Expodev Inc.
IEEE Transactions on Power Delivery | Year: 2011

This paper presents a model for the critical conditions required for a successful launch of rocket-triggered lightning of the classical type under negative cloud. For the first time, the model gives due consideration to the effect of space charge produced by trigger-wire glow-corona on continuous upward leader onset. The model includes the effects of space charge from bush corona resulting in non-uniformity of the ambient field into which the rocket is launched, as well as the effects of rocket speed and ambient relative air density. The paper shows that the effect of trigger-wire glow-corona on continuous upward leader onset is quite substantial, emphasizing the basic difference in this respect between tall static structures and rocket-triggered lightning. The work confirms the importance of two necessary and sufficient criteria for continuous upward leader onset, a global criterion relating to the space potential at the wire-tip height and a local criterion regarding the ambient field aloft. The model has been applied to several previous field experiments characterized by a wide variety of rocket speeds, bush coronas and ambient relative air densities. Comparison between model and experimental results, including the more recent and better instrumented, was found very satisfactory. © 2010 IEEE.


Rizk F.A.M.,Expodev Inc.
IEEE Transactions on Electromagnetic Compatibility | Year: 2010

The paper comprises the first application of the concept of upward leader inception and propagation to meet the downward negative leader in a final jump, to assess shielding of substations against direct lightning strikes. Expressions for the attractive radius of a slender mast and the lateral attractive distance of a shield wire are used to determine the respective protection zones. The analysis is extended to account for lightning protection provided by double masts as well as by four masts placed at the corners of a rectangular area. Lightning protection by double ground wires is also analyzed. The model takes into account not only the different lightning protection characteristics of masts and ground wires, but also distinguishes between rod-type and conductor-type protected objects. The model considers not only flashes descending on the substation from above, but also accounts for shielding against side flashes. A systematic approach is presented in sufficient detail to allow practical application to shielding design of substations. Extensive comparisons of the findings of the model to results obtained by the electrogeometric model and also to a practical empirical formula are included. © 2010 IEEE.


Rizk F.A.M.,Expodev Inc.
43rd International Conference on Large High Voltage Electric Systems 2010, CIGRE 2010 | Year: 2010

The paper describes a new physical approach to assess lightning exposure of buildings and massive structures. This comprises a generalization of previous modelling of lightning exposure of slender masts and power conductors. The analysis outlines an alternative to the widely used rolling sphere method which has only limited scientific foundation. The paper addresses the limitations of the rolling sphere method which does not adequately account for the role of the air terminal in the lightning attachment process. The paper specifically addresses the effect of a massive structure, immersed in an ambient ground field due to both cloud charges and downward negative stepped leader charges, on the distribution of space potential and on inception of an upward connecting leader. Special attention is paid to vulnerable points such as building corners and edges as well as the application of appropriate lightning rods in different locations. It has been shown that, for the same height above ground, the critical ground field for upward connecting leader inception from a building edge is substantially higher than the corresponding value for a slender rod. An attractive distance ratio ADR is defined as the ratio between the attractive distance of a building corner or edge to that of a slender rod of the same height. It is shown that the attractive distance ratio of a building edge or corner is always below unity but increases steadily with the prospective return stroke current. It is also shown that the attractive radius of a corner is less sensitive to variation of the prospective return stroke current than a rod of equal height.


Rizk F.A.M.,Expodev Inc.
IEEE Transactions on Power Delivery | Year: 2010

This paper starts with a review of anode corona modes with particularly streamer-free glow known as ultra-corona, with the view of exploring its application in lightning protection. Due to the inherent intensification of the ambient electric field atop a tall structure, strict conditions have to be imposed on the streamer-free space charge generation and the stability of that mode of corona when exposed to rapid field variations due to remote lightning. A novel ultra-corona electrode satisfying the above conditions is introduced. High voltage test results on such electrodes are presented including corona current, charge and laboratory air gap breakdown voltage. By applying dimensional analysis, a generalized formula for corona currents from such a device atop a tall structure is presented which includes the effects of the ambient ground field, structure height, ambient electric field at the top of the structure, electrode dimensions, and wind speed. © 2010 IEEE.


Rizk F.A.M.,Expodev Inc.
IEEE Transactions on Power Delivery | Year: 2010

Corona characteristics of lightning rods exposed to ambient fields due to cloud charges prior to negative leader descent are analyzed, taking into account rod slenderness and relative air density. A new formula is introduced to account for the effect such corona on upward connecting leader initiation. This quantifies the tendency of a sharp lightning rod towards self-shielding, which can be detrimental to the protection of a nearby object, particularly if this object is less susceptible to corona formation. The effect of corona due to cloud charge fields on the attractive radius of a solitary rod is also investigated. Based on discharge physics, the characteristics of a non-self shielding air terminal are then defined. Since large electrodes are susceptible to deteriorating corona performance due to protrusions and water drops, preference is given to a corona-free rod with the smallest diameter, under cloud charge fields. For tall rods or rods exposed to high ambient fields, however, the required diameter of a hemispherically capped rod may be impractical. The physical approach described in this paper satisfactorily accounts for the lightning performance of competing sharp and blunt rods in a previous field investigation. © 2010 IEEE.

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