electrodynamic forces during short · pdf fileconcepts on how to calculate electrodynamic...
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Consultancy, R&D and Training Ltd
Severiano da Fonseca, 68 / 203 *** Rio de Janeiro – RJ – Brazil - CEP 21931-220 Phone: 55-21-2465 3689 *** 55-21-33934600 *** Cell phone. 55-21-9 88874600
E-mail: [email protected] Site: http://www.cognitor.com.br
ELECTRODYNAMIC FORCES DURING SHORT CIRCUITS IN SWITCHGEAR & SUBSTATIONS
Author name: Affiliation: Sergio Feitoza Costa COGNITOR – Consultancy, Research and Training Ltd. Email: [email protected] Site : www.cognitor.com.br Keywords: IEC, CIGRÈ Simulations, High Power Tests, Switchgear, Switchboards, Bus-bar systems, Testing Laboratory, Internal Arcs, Overpressure, Temperature rise, Electrodynamic forces, stresses, Short time currents, EMC, Magnetic fields, Electric Fields, Substations, certification.
A. CONCEPTS
Concepts on how to calculate electrodynamic forces and stresses which occur during short circuits are a must in the design of switchgear, busbar systems in local equipment and in complete substations arrangements. The results of the studies to determine forces and their consequences are used to design structures, size of the busbar conductors, number of insulators and supports, etc.…
As the currents in equipment and substations are each time higher and the size of equipment tend to be each time smaller it is more difficult to adjust the design to support the consequences of the high mechanical forces and stresses during a short circuit. If you do calculations on a medium voltage or low voltage switchgear at short circuit levels higher than, let’s say 31,5 kA you may get forces in the range of tons. This is because the forces are approximately proportional to the square of the values of currents and inversely proportional to the distance between phases.
The equations to calculate forces and stresses in two neighbor conductors circulated by short circuit currents are like the ones showed in Figure 1.
Figure 1 – Forces created in two neighbor conductors circulated by short circuit conductors
Consultancy, R&D and Training Ltd
Severiano da Fonseca, 68 / 203 *** Rio de Janeiro – RJ – Brazil - CEP 21931-220 Phone: 55-21-2465 3689 *** 55-21-33934600 *** Cell phone. 55-21-9 88874600
E-mail: [email protected] Site: http://www.cognitor.com.br
These forces depend on the currents, geometry and distances, materials used, etc.…. Inside a switchgear there are dozens of conductors, each one influencing the other. So, in order to calculate to calculate the forces in conductors and in the insulators it is necessary to use a computer program like SwitchgearDesign or others. In Switchgear Design you just enter with the values of currents, distances and other relevant data (Figure 2)
Figure 2 – Input data for calculation of forces and stresses in MV and a LV switchgear
(extracted from Report 071/2014: Validation Of Software Switchgeardesign_307 For Simulation Of High Power Tests (Temperature Rise, Short Time And Crest Current Tests – Electro Dynamical Forces / Stresses And Overpressures From Internal Arc - download in http://www.cognitor.com.br/TR_071_ENG_ValidationSwitchgear.pdf )
Consultancy, R&D and Training Ltd
Severiano da Fonseca, 68 / 203 *** Rio de Janeiro – RJ – Brazil - CEP 21931-220 Phone: 55-21-2465 3689 *** 55-21-33934600 *** Cell phone. 55-21-9 88874600
E-mail: [email protected] Site: http://www.cognitor.com.br
Certain approximations sometimes used in substations as considering that all the conductors are parallel, frequently do not work well for compact equipment like switchgear or a busway.
As a consequence of these uncertain calculations, to avoid failing in the expensive type tests in testing laboratories, the designer tends to use high “safety” factors . So, at the end, the project become over dimensioned with a higher weight of copper and aluminum and a much higher number of supports and insulators than necessary.
It is impressive to see the number of possibilities of optimizations and reduction of materials in the existing designs a lot of money and resources are thrown away due to exaggerated projects that we see in the market. In many cases gains of 20 to 30% are easily achievable by a more precise calculation.
This occur not only in the aspect of mechanical forces but also with temperature rise aspects.
To assess the withstand ability of insulators and conductors include the following steps:
Calculate the static forces distributions
To convert static forces in "dynamic"
Calculate forces on insulators, shear forces and bending moment diagram.
Calculate mechanical stresses on the conductors = bending moment / resistant moment
Compare stresses in conductors and forces in insulators with permissible limits.
The determining parameters for the design are:
Geometry and distances between phases
Materials
Short circuit currents and its asymmetry
Supportability (tensile, compression and bending) and distance between insulators
Consultancy, R&D and Training Ltd
Severiano da Fonseca, 68 / 203 *** Rio de Janeiro – RJ – Brazil - CEP 21931-220 Phone: 55-21-2465 3689 *** 55-21-33934600 *** Cell phone. 55-21-9 88874600
E-mail: [email protected] Site: http://www.cognitor.com.br
Aspects of resonance and types of short circuit shall be considered properly to arrive to the final values of forces and in the assessment of the supportability
B. SIMULATIONS OF SHORT TIME CURRENT AND CREST TESTS
When all the parameters are properly considered and a calculation is done using SwitchgearDesign results as the ones showed below are obtained. The upper curve shows the forces acting in insulators and supports of the busbars. The lower curve shows the mechanical stresses which will be applied in the busbars tending to bend them. With a basic training it is easy to the designer to check if these values are acceptable for a successful type test after.
The complete method of calculation and how to the use the calculation tool SwitchgearDesign may be seen in the parts 8a and 8b of the training detailed in this link http://www.cognitor.com.br/en/site/index.php?sec=3
Consultancy, R&D and Training Ltd
Severiano da Fonseca, 68 / 203 *** Rio de Janeiro – RJ – Brazil - CEP 21931-220 Phone: 55-21-2465 3689 *** 55-21-33934600 *** Cell phone. 55-21-9 88874600
E-mail: [email protected] Site: http://www.cognitor.com.br
The author of this article is Mr. Sergio Feitoza Costa. Sergio is an electrical engineer, M.Sc in Power Systems and director of COGNITOR. Sergio has 25 years of experience in design, operation and management of high power, high voltage and other testing laboratories. After leaving the activities in testing labs Sergio acquired 16 years of experience in the use of testing simulations in support to manufacturers and certification companies. Sergio is the author of the simulations software SwitchgearDesign In these links there are some videos and reference material, inclusive for validation of testing simulations:
Cognitor site and CV http://cognitor.com.br/en/site/index.php?sec=1 *** YouTube movies (free) about concepts and simulations in (some in English, in Spanish & Portuguese)
https://www.youtube.com/channel/UCyQtdE7dQTvsZPHBw3ostMg/videos?view=0&sort=dd&shelf_id=0
Articles: http://www.cognitor.com.br/en/site/index.php?sec=5
Information on trainings http://www.cognitor.com.br/en/site/index.php?sec=3
More information on trainings, forum, etc..: http://www.cognitor.com.br/en/site/index.php?sec=6
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