OVERPRESSURE MEASUREMENTS during INTERNAL ARC TESTS. A MUST TO SAVE MONEY
image credit: Image prepared by Sergio Feitoza
- May 26, 2020 12:15 pm GMT
- 267 views
The occurrence of internal arc in enclosed power equipment is an impacting event. The overpressures are more dangerous if the equipment is more compact and compactness is frequently a design goal. Technical Standards specify requirements and tests to qualify equipment for this purpose. In this YouTube video you may find more information https://www.youtube.com/watch?v=dYpV9D2Mg9c
Several articles and documents have been published about internal arc in panels and switchgear. The more complete document in this theme is the brochure CIGRÈ 602 / 2014 “Tools for The Simulation of the Effects of the Internal Arc in Transmission & Distribution Switchgear”. That document covers diverse aspects as the theoretical fundaments, calculations, simulations, and practical issues of the daily life, for example:
· Calculation of the over pressure using proper computation models
· Explanations about the internal arc testing
· Standardization and test experience
· Effect of internal arc on structures (mechanical stress on switchgear and building walls)
· Burn-through effect
· Internal arc simulation review and tools for the simulation of the effects in T&D switchgear
· Comparison of actual and reference switchgear using design parameters
· Equations for pressure rise calculation
· Pressure sensors: types and accuracies
· Cotton indicators: energy absorption and flammability
· Comparisons between effects in air and SF6 equipment
I am one of the coauthors of this publication and can assure that it is very useful for designers of switchgear. In that brochure there are suggestions for improvements in the technical standard IEC 62271-200 (medium voltage switchgear) and other related high voltage and low voltage standards. One of these suggestions is about the importance of measuring the overpressure during internal arc tests. It is almost unbelievable that after so many years of use of these IEC standards and knowing that the overpressure is the most relevant parameter which influence test results, the measurement is not mandatory in IEC 62271-200 nor in IEC 61641. The brochure present objective information about the importance and how to do the measurements of overpressure.
As the overpressure measurement is not mandatory in the technical standards, the testing laboratories, for saving time, do not perform it unless it is formally requested by the client of the laboratory which will pay for the test. A lot of useful information is simply lost because this measurement is not performed. The new IEC 62271-307 (medium voltage switchgear) gives guidance for the extension of the validity of test reports made in accordance with IEC 62271-200. I explained about this in a previous post. However, for practical purposes, this is possible only when the overpressure has been measured in the test of the original equipment.
If a manufacturer is doing a test in a “first of a series” equipment and do not request the overpressure measurement to be included in the test report it is throwing away the opportunity to save hundreds of thousands of dollars in the execution of future tests. Simple like this.
The information given by this easy overpressure measurement can be used for improving designs and solutions and even to clarify doubts that arise during testing in laboratories. An example is when the test conditions become more severe than is specified in the technical standard because extra arcs were produced due to some failure in the (laboratory) cables which feed the test. This is difficult to identify even for experienced designers and testing teams. If the overpressure curve is available, it is easy to identify the causes and to avoid undue manufacturer expenses with test repetitions.
Internal arc tests on AIS equipment are type tests for medium voltage panels (IEC 62271-200) but are not yet (why?) type tests for low voltage panels (IEC 61439). The buyer market, however, increasingly requires, on a sound basis, that manufacturers of low voltage panels prove that the equipment has been tested and approved in the IEC TR 61641 test.
In the brochure there is a review of more than 100 papers and technical standards and a collection of test data from dozens of internal arc tests involving enclosure sizes ranging from less than 5 liters to large GIS tanks (1200 liters). Fault currents ranged from 12 kA to 63 kA, with fault durations ranging from 10 ms to 1.2 s. It included cases with SF6 and air. The analysis covered the three main effects of an internal arc (a) the overpressure (b) mechanical stresses in the enclosure, installations and walls caused by internal overpressure, and (c) the "burn-through" effect trying to make holes in the enclosure. One of the references is the text "Guidelines for the use of simulations and calculations to replace some tests specified in international standards". Cognitor Guide 2010. Per the Guide, the minimum input data to be recorded in laboratory test reports are:
· The circulating electric current,
· Materials used in conductors, insulation parts and fluid inside a compartment.
· The position and spatial geometry of conductors
· The volume of the fluid inside the compartment and the area of the overpressure relief devices.
· The ventilation entrance and exit areas (if exist devices that close them during an internal arc)
· The relative position of the equipment in relation to the walls and ceiling.