Reducing arc flash hazard on switchgear

Install a protective relay on the secondary side of the transformer and trip a primary side fault interrupting device

07/08/2014


An increasing number of industrial facilities now have their arc flash analysis completed and are in compliance with NFPA 70E requirements for labeling of equipment with regard to the having a current study performed and the equipment labeled.

This has raised the awareness of high arc flash energy areas on facility’s main switchgear where incident energy values may exceed category 4 (40 cal/cm2). The question being asked now is “How do I reduce the arc flash hazard category of my main switchgear?” 

Basic principles

NFPA 70E article 100 defines the arc flash hazard as “A dangerous condition associated with the possible release of energy caused by an electric arc”. Incident energy is the amount of energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. Incident energy is dependent on the duration of the arcing source - how long the source is present before being interrupted – and is measured in cal/cm2. 

The most controllable factor in reducing incident energy is the duration of the arcing fault.  In most cases, adjusting a circuit breaker setting so that the arcing fault current falls into the instantaneous region of the upstream circuit breaker will reduce the incident energy.  However, for the facility’s main switchgear, the upstream protective device is typically a fuse on the line side of the substation transformer, which is not adjustable. Even if a relay and circuit breaker combination is used on the line side of the substation transformer, it is difficult to obtain low arc flash values on the low voltage switchgear. 

The problem with transformers

The typical arc flash mitigation technique of adjusting the circuit breaker settings to reduce the incident energy becomes a problem when transformers are involved.  The switchgear on the secondary side of the transformer is protected by the overcurrent protective device on the primary side of the transformer.

Primary overcurrent protective devices have to be set high enough to accommodate transformer inrush.  Upon startup of a transformer, a large magnitude transient current flows for several cycles that is many times the rated amperes of the transformer. The overcurrent protective device on the line side of the transformer must be able to ride through this inrush current. This hampers the ability of the protection system to reduce the arc flash energy.

The solution

One solution to this problem is to install a protective relay on the secondary side of the transformer and trip a primary side fault interrupting device.  This solution in essence creates a circuit breaker for the secondary side.  The arc flash mitigation system can trip the primary fault interrupting device independent of the transformer inrush current and allows a much faster tripping in the event of a fault. 

This is possible because the secondary side of the transformer is not affected by the transformer inrush and does not have to coordinate with it.  The instantaneous setting on the secondary side can be set as low as the equipment downstream permits.

Food and beverage industry example

The facility under evaluation has 18 substations each consisting of a 12.47kV high voltage load break primary switch, a 1500kVA cast coil transformer and a 480/277V switchboard.  After performing the analysis, it was determined that all of the 480/277V switchboards had an incident energy rating over 85 cal/cm2.  Since there was no safe method of working on equipment with an incident energy level over 40 cal/cm2, a solution was needed to reduce the arc flash energy. 

The high voltage switch was replaced with a vacuum circuit breaker which was protected by a protection relay. This particular relay monitors both the primary and the secondary side of the substation transformer and was set up to provide over current protection in addition to transformer differential protection.  The relay set point for the secondary protection was selected to act fast in the case of an arcing fault.  This setup effectively reduced the arc flash hazard category to 1 (4 cal/cm2) for the entire 480/277V switchgear.

Upon successful implementation of the design, the solution was applied to three other facilities totaling 55 separate arc flash mitigation systems.

Things to watch for

All electronic relays require control power to operate. In this particular facility, a reliable source of control power was not available and there was not adequate room to put a control power transformer on the 12.47 kV side of each substation.  The 480V side of the unit substation would not provide a reliable power source because once the breaker tripped, there wouldn’t be control power to power up the relay.  A small UPS unit was selected and installed to provide the necessary control power to the relay. 

Another issue with the application of the arc flash mitigation systems is the footprint of the medium voltage load interrupting device. The new medium voltage load interrupting device, including the isolating switch, the circuit breaker and the control equipment had to fit in the same space as the previous primary switch.

This kind of arc flash mitigation design is an effective way to reduce incident energy on the secondary sides of transformers.  Replacing the existing high voltage load break switch with a new circuit breaker with improved relay protection reduces the arc flash hazard and better protects downstream equipment.

Justin Schroeder is a registered professional engineer in the state of Georgia and is a senior field application engineer for Schneider Electric Engineering Services. 



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