Arc resistant design solutions
In our introduction last time we have explained the danger of an electric arc flash for the personnel and the equipment. The arc reaches extremely high temperatures and releases pressure and sound waves into the surrouding. Besides personal protective equipment (PPE) the risk of an arc fault can be minimized by design → We talk about arc resistant design solutions. That is our subject today.
5 common arc resistant design solutions
Multiple approaches have been developed by the industry to provide protection against electric arc and to minimize the consequences of an arc fault. In the next paragraphs we will briefly describe 5 commonly used solutions to achieve an arc resistant design.
A. Reinforced cabinet plus pressure relief system
This solution is based on increased mechanical strength of the cabinet (enclosure) and is usually combined with a pressure relief system. The cabinet shall withstand certain increased level of pressure. A pressure relief system may be required in addition. It is often realized by means of overpressure flaps. The pressure relief system must respond before the maximum withstand pressure of the enclosure is reached. Pressure relief shall direct the gases in such a way that the personnel is not endangered, e.g. through openings on the top of the enclosure. More advanced systems may incorporate a duct to guide the gas and plasma safely out of the electric room.
B. Interlocked room without access during operation
Such approach restricts the access of personnel to the installation of power hardware when the equipment is energized. Practically it usually means that the control cabinet is separated from the power section and is installed in a completely separate room or separate pre-fabricated building. The power hardware can only be accessed when the equipment is de-energized and earthed (for service and maintenance purpose). The interlocking system shall be fail-safe.
C. Use of current limiting fuses
Use of a fuse is a classical method to interrupt high fault currents. Medium voltage current-limiting fuses can principally be used to disconnect the power source in case of short circuit or arc fault. In low/medium power range the fuses can be integrated in so called fused contactors. Attention shall be paid to select a fuse with suitable characteristic (“fast fuses”). In addition, there is a limited availability of high-power fuses on medium voltage level. Fuses are also subject of aging and once triggered they need to be replaced on site.
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D. Use of external short-circuiting device
Another option is to create an external short circuit that takes over the fault current. The short-circuiting device shall be able to handle high peak current. Moreover, it shall have very short response time.
Such devices can be classified as active arc fault protection equipment. Some of them can be used repeatedly while other need to be replaced once they were triggered (similar as a fuse) as the device supports just one closing.
E. Fast detection and elimination
The principle is based on fast arc detection and elimination. An arc is detected with conventional overcurrent protection and may be optionally extended with optical arc detection sensors. After a detection, the reaction is similar to the external short-circuiting device described above. In case of a VFD all active semiconductor switches are turned on. The fault current is distributed into several paths. Consequently, the energy supplied to the fault drops significantly. The arc cannot be self-sustained anymore and is extinguished. As the elimination of arc is extremely fast (range of few milliseconds) the energy is limited before the cabinet can reach critical overpressure and potential rupture or explosion.
What arc resistant design solution is the best?
After presenting the 5 common solution the logical question comes: Which of those solutions is the best?
While all of them shall provide personal safety (top priority) the protection against hardware damages is on very different level. And there are several other factors to take into account such as reliability, complexity or maturity of the solution. Finally, the choice of arc resistant design solution also heavily depends on the VFD topology.
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.
Purchase price: CHF 19.90
References
[1] Arc resistant design: Introduction, https://mb-drive-services.com/arc-resistant-design-introduction/
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Arc resistant design: Comparison - MB Drive Services · October 23, 2021 at 1:55 am
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