Arc resistant design:
Introduction
Personal safety stands on the top of agenda of many companies. When talking about medium voltage electric compartments then arc fault is one of the main concerns. Uncontrolled electric arc may have devastating impact on the personnel around and of course on the hardware itself.
In this introduction we will briefly explain what is an electric arc, what is the origin of arc flash as well as its effects. Afterwards we gradually transfer to aspects of arc resistant VFD design which will then be closely elaborated in our next post.
What is an electric arc flash?
Electric arc is very intensive electrical discharge inside normally non-conductive gas such as air. The gas is ionized and a plasma is produced (plasma is fourth state of aggregation, apart from solid, liquid and gaseous forms). In plasma, atoms are divided in electric particles, such as electrons and ions. The temperature of plasma in case of arc flash can reach incredible 20 000°C (35 000°F). Some sources go even higher stating 30 000 K. For comparison:
- An ordinary flame does not exceed 4 000°C in the hottest spot.
- When burning ozone the temperature can go up to 5 700°C.
- Temperature at the surface of the Sun may go as high as 6 300°C.
As we can see the temperature reached during arc fault may be 3 to 4-times as high as the temperature on the surface of the Sun.
The higher the temperature the higher the degree of ionization.
Conductivity of plasma heavily depends on the current. For high currents (kA range) the plasma has similar conductivity like ordinary conductors; for low currents the conductivity has the same order of magnitude as insulators. In switching apparatus there is a generation of plasma during electric arc between the contacts.
The discharge in the video above is a well known flash-over similar to atmospheric lightning. The air is ionized and plasma is generated. However, this particular discharge has very low energy and looks quite innocent compared to an arc flash that we will talk about further.
Effects of arc flash
We have just learnt that arc flash is associated with extremely high temperatures. The gas (in most cases air) is heated up and starts to expand. When the electric arc burns inside a metallic enclosure like MV switchgear or MV VFD, the internal pressure rapidly increases. If the arc is not quickly extinguished or the pressure is not relieved, the enclosure will likely explode. This is something that must be avoided by any means for all equipment located in areas where people have access while the equipment is energized.
Take away: Electric arc is associated with high temperature and rapid increase of pressure creating a pressure wave. The discharge is followed by acoustic boom (sound wave). All these effects can seriously hurt or even kill a human being. Maximum shall be done to avoid any injury due to arc flash.
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Root cause of an arc fault
In general, the occurrence of an arc fault is very rare. Root cause of an arc flash may be a breakdown of the insulation, maloperation, faulty assembly or installation, aging under electric stresses, overvoltages and surges, insufficient maintenance, pollution and other environmental factors or error by personnel (e.g. forgotten tools).
Sources of fault current
An arc resistant equipment is linked with specific fault current rating. What are the sources of current and power that may supply an internal arc fault inside the VFD? These include but are not limited to:
- Supply grid
- Electric motor
- Intermediate DC link
- Additional filters, power compensation equipment etc.
To analyze each of the potential fault current contributors, additional factors need to be considered, such as: grid connection, type of motor, type of load, realization of DC link etc.
Protection against arc flash
One point is the protection of personal by use of proper personal protective equipment (PPE). The PPE shall be arc rated according to appliable class. The standard governing protective clothing against electric arc is IEC 61482-2 and EN ISO 11612 in Europe and ASTM F1506 ARC / NFPA 70E in the USA.
Another aspect to to design the equipment to be arc resistant. The industry has developed multiple approaches that provide protection against electric arc and minimize the consequences of an arc fault. A deeper look into the solutions will be provided in the next posts of this short series.
Arc related standards
In this section we will briefly discuss applicable safety and arc related standards. Until recently there was no dedicated safety standard for medium voltage VFDs. It was a common practice to apply the switchgear standard IEC 62271-200 instead. Since mid of 2018 the IEC 62477-2 is available for medium voltage power electronic converters.
Switchgear
IEC 62271-200
IEEE C37.20.7
Drives and Power Converters in general
IEC 61800-5-1
IEC 62477-2
Do you want to know more about arc resistant design of medium voltage VFDs? Purchase our paper presenting and comparing 5 most common concepts to achieve arc resistant VFD design. The analysis focuses on
- level of human and hardware protection
- response time and possible damages
- complexity of the solution
- suitability for “special” VFD topologies
- maturity of the solution
- engineering effort
- implications on the system
- summary of pros and cons
In product brochures and marketing material each manufacturer praises “his solution” as the best one. With our paper you get a bit wider view and look at the problem from different angles and pespective. That helps you to draw your own opinion.
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References
[1] When safety and reliability are not optional, https://www.abb-conversations.com/2014/04/when-safety-and-reliability-are-not-optional/
[2] How to choose a medium voltage VFD: Protection concept, https://mb-drive-services.com/choosing-mv-vfd-protection/
1 Comment
Arc resistant design: Solutions - MB Drive Services · October 2, 2021 at 9:22 am
[…] our introduction last time we have explained the danger of an electric arc flash for the personnel and the […]
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