Arc Flash Reduction Methods On Secondary of Sub-unit 4

[majesus]

I've seen this application time and time again, but I would appreciate any advice and comments on my findings for the following project I’m working on.

My project is a new 3MVA dry type sub-unit installation (step down 12.47kV to 600V) for a Grain Processing Facility.

Approximating the Impedance Data for the XFMR based on "Cutler-Hammer Reference Data" (p.1.5-11 of the 2006 Consulting Application Guide). The dry-type XFMR (80C rise) is modeled as having:
Z=5.75%
X/R=10.22

Note: The following is based on the secondary side.
The short circuit current is about 58 kVA because the Grain Processing load (on the secondary) is about 95% induction motors. (Largest motor is 400hp.) The load current is: 2700 A.



Modeling in Etap, the secondary of the XFMR is assumed to be a switchgear type equipment with an air gap between conductors of 32mm.) My simulations calculate an Arc Flash energy of 330cal/cm2 which (exceeds the Cat 4 level.) This is what I expect.

Now my design goal is to install a secondary protection device to bring the arc flash hazard down to a Cat 2. GOOD LUCK, because this is where I am having my difficulties:

Option 1) Secondary Protection Using Power CB
Power CBs (with ratings greater than 3000A) have typically an instantaneous 4 cycle clearing time. Provided that I coordinate the CB so that the arching current will trip in the instantaneous region, this brings the arc flash to a Category 3 (about 9.66 cal/cm2). (This is the best that a CB can do.) I've been looking at Schneider's Masterpact NW and NT Circuit Breakers with Arc Flash Protection that have faster clearing times, but they are available only up to 2000A. My CB options seem limited.

Option 2) Current Limiting Fuses
In the past, I’ve done research looking at the fastest fuses available for applications that require load currents between 1-5kA. The Bussman KTU fast acting fuse is the fastest available in a Class L format. Using a fuse rated above the XFMR’s secondary current, even with this protection device, the arc flash current flowing through the fuse lies in the clearing time’s tolerance range that using the worst case scenario, the arc flash is not significantly improved. I can always select a smaller current fuse, however, it will conflict with the XFMR’s load capacity.

Option 3) Increasing the Impedance of the XFMR.
I’ve simulated the XFMR as having an impedance of Z=7.5%. The arc flash reduced slightly, but was still in the Cat 3 region.

I’ve looked at the very interesting document “Distribution Equipment Modernization to Reduce Arc Flash Hazards” obtained from here:

http://www.eng-tips.com/viewthread.cfm?qid=203769&page=14

Their scenario is similar to mine. I have simulated their SLD (Figure 2 on page 29) and I’ve obtain similar result as them. However, their SLD has smaller secondary load then mine.


So now I'm stuck as I excercised my options. I would appreciate any comments.
Thanks,
Maj

 

[stevenal]

davidbeach,

In this case, perhaps you could isolate all sources and then test for potential and apply grounds on the high side rather than the low. It may be counterintuitive that 12.5 kV is safer to work hot than 600 V, but the arc flash hazard is usually much lower due to lower currents and faster clearing times. Would your work rules allow for the assumption that the state of the secondary follows that of the primary?

 

[davidbeach]

stevenal,

That's an interesting solution. It is certainly the only thing I've seen that seems to address the issue without taking a head in the sand approach to "dead work".