We have already discussed the importance of an arc flash study for low voltage equipment. You can read more about that blog by clicking here.
Though one might wonder, how big of an arc flash hazard is present in the case of medium voltage switchgear?
The answer: A LOT!
Before delving into the details of how to prevent arc flash for MV switchgear, we will first take a look at the characteristics of an arc flash in MV switchgear.
Therefore, what this entails is that engineers should be well aware of design alternatives and mitigation techniques that can reduce arc flash hazards in medium-voltage systems. In an arc-flashover, the current always travels where it is not intended to be in the first place, it may protrude from one phase to another phase or even from one phase to the ground/earth.
This can happen easily in the case of low voltage open air arcs, but what if we consider a medium voltage switchgear? For an arc flash to happen in a medium voltage switchgear, it would need to breakdown the dielectric strength of the air which is typically 3 MV/m. This phenomenon is called a dielectric breakdown and it happens when the charge buildup due to the arc flash exceeds the electrical limit of the air.
This means that an arc across a minute length such as a centimeter might take several kilovolts to achieve dielectric breakdown. A round shaped arc flashover will have a higher breakdown than a sharp vertical one. The addition of a third object between the air gap can further decrease the value of the breakdown voltage.
Once the arc takes over the breakdown voltage, it then starts to behave like a flexible conductor, with a temperature of approximately 35000 F. This can easily burn through material such as copper and aluminum and vaporize them resulting in the occurrence of secondary faults.
Hello there! On a related topic, we previously wrote a blog about Importance of Arc Flash Hazard Analysis. If this peaks your interest, check it out and let us know what you think
There are several reasons for an arc flashover taking place in a medium voltage switchgear. What causes arc flash can be summarized as follows:
Electrical equipment can be damaged as well with flashovers over 50kV resulting in permanent damages to the insulation, electrical fires etc. When you combine all this, then what you get in the end is not only the cost of the repairs to the machine but the added cost of an injury to the working personnel.
OSHA has a strict stance on the requirements with respect to the arc flash hazard. It states in the subsection 1910.335(b):
"to warn and protect employees from hazards which could cause injury due to electric shock, burns or failure of electric equipment parts".
Next up, we will examine some of the techniques on how to prevent arc flash. Since a medium voltage switchgear supplies power to LV feeds therefore a blackout on the switchgear could mean a blackout on a major portion of your facility. Concurrently, all medium voltage protective devices are operated with a relay which must be set to operate with a greater time delay in order to ensure optimal coordination with the downstream feeders. Moreover, medium voltage circuit breakers have a higher contact opening time which, when added to the relay signaling time, makes up a fault current opening time which is significantly higher when compared to low voltage circuit breakers. It is therefore imperative for facility owners to look for the best methods available for arc flash mitigation.
Some of the more effective methods include the following:
The selection of the method will depend on the type of system under consideration. This will require a comprehensive arc flash calculations complete with mitigation solutions for the equipment in scope. NFPA 70E requirements dictate that an arc flash study every 5 years is optimal for maintaining site safety.
They use superconductors which essentially means that they operate with variable impedance with respect to the current magnitude. which operate on low impedance under normal conditions of current flow through the system. In the case of a fault, the limiter will insert additional impedance into the system which will limit the overall fault current magnitude. We shall be discussing fault current limiters with a practical example in a future article.
These are manually operated switches which have to activated manually by the working personnel before any maintenance activity can be undertaken. Once activated, they allow for adjustments to the 50P element which results in swift clearance of the arcing fault (typically 10 to 20ms). On the other hand, they are inactive under normal operating conditions. This allows for the clearing of arcing faults during scheduled maintenance, all the while preserving coordination with the downstream feeders under normal operating conditions.
During the course of an arc flash within enclosed equipment, severe pressure can build up over time which can result in a massive explosion which can prove to be fatal to personnel nearby. Therefore, the arc resistance switchgear has been designed in order to channel the flow of the pressurized arcing energy away from adjacent personnel and equipment. It has a strong structure capable of withstanding the pressure and discharging the gases through a built-in exhaust supported by vents.
This is a mechanism which allows the clearance of internal faults without any intentional time delay, all the while maintaining protection coordination with the other protective devices. It is normally employed in equipment such as generators and transformers. Only for internal faults will the differential protection be deployed. For external faults, no tripping or clearance will occur due to the fault being outside the zone of protection.
They are special type of protective relays which act upon the intensity of the light being emitted by the occurring arc flash. The greater the intensity, the faster the tripping time and fault clearance will be. The sensing is done through photo-electric receptors which take in as much light as possible, after which a signal is sent to the attached protective relay for the tripping function to occur. This method is still relatively new which is why they are yet to become a popular mitigation technique.
Arc flash studies are provided by our team of certified professional engineers here at AllumiaX, who will assist in the evaluation of your system and deliver state-of-the-art recommendations and arc flash solutions for your power system's protection. We work closely with our clients in collecting the data, modeling the system, identifying the hazards, simulating the incident energy levels & providing solutions in compliance with the latest industrial standards including OSHA, NEC, IEEE & NFPA 70E.
About The Author
Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.