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

 

[waross]

Which leads to a question (if anyone can answer it): why isn’t this analog method not incorporated into the standard protection scheme to give a faster instantaneous clearing times during normal operation? I’m guessing there is a compromise ie, lack of multiple settings that is possible with the current digital instantaneous system?

Under the link that you gave,\:

The Arcflash Reduction Maintenance System is designed to
be used only during the time that a worker is exposed to the
flash hazard. The Arcflash Reduction Maintenance System is
not activated or armed continuously. This feature improves
overcurrent coordination when compared to a permanently
installed instantaneous trip element on the same circuit
breaker.

Does this answer the question?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[alehman]

I think so. The maintenance mode apparently also reduces the instantaneous pickup. Presumably coordination may be lost when it is activated.

 

[majesus]

Hey Bill,

No it doesn't. Basically what I'm asking:

Since this analog system improves the instantaneous clearing times of the circuit breakers, what are the draw backs preventing the ARMS technology to be designed for continuous applications?

I think so. The maintenance mode apparently also reduces the instantaneous pickup. Presumably coordination may be lost when it is activated.

Yeah that's my "hunch" too, anyone can confirm?

 

[Zogzog]

For what it is worth the AC PRO quick trip does the same thing, the Qt curve is different than the INST curve.

Not sure why when I posted this way back in the begining of this thread it was dismissed and ruled out, now the C-H version is the saving grace. I smell a C-H salesman in the group.

 

[EddyWirbelstrom]

I’ve used SKM PTW Arc Flash software to simulate the system described by Majesus.
For the Incomer I have used a Masterpact NW32 with Micrologic trip unit 5.0A.
The results show that reducing the Short-Time-Time-Delay (STTD) on the Incomer from 0.2 sec to 0.0 sec result in a reduction of Incident Energy at a working distance of 610mm ( 24 inches ) from 63.8 J / cm² or 15.2 Cal/cm² ( PPE Cat 3 ) to 33.2 J / cm² or 7.94 Cal/cm² ( PPE 2 ).
However the big issue is the arc flash energy of 587.9 J / cm² or 140.5 Cal/cm² at the line side of the Incomer ACB. No PPE can protect against the fatal pressure blast produced by this level of incident energy.
An intertrip to the HV side breaker is required.
Arc Flash Detection equipment tripping the incomer and HV breaker would reduce trip times and consequently incident energy even further.
Please see attached files for the Arc Flash study results.

 

[EddyWirbelstrom]

Please see attached tcc's which my previous posting on ArcFlash incident energy refers to.

 

[alehman]

The ARMS system reduces the instantaneous pickup to one of several settings between 2.5 and 10 In. It also reduces the maximum total clearing time from 80 to 40ms.

From the ARMS manual:

The Maintenance Mode Settings (labeled R5, R4, R3, R2, R1) provide the Arc Flash Reduction setting. R5 is the Maximum reduction setting which correlates to the lowest pickup value.
Nominal Trip current of Maintenance Mode Settings
Setting R5 2.5x Rating Plug Amperes
Setting R4 4.0x Rating Plug Amperes
Setting R3 6.0x Rating Plug Amperes
Setting R2 8.0x Rating Plug Amperes
Setting R1 10x Rating Plug Amperes

The setting changes as shown by mnewman for the Masterpact reduce the arc flash permanently, but may sacrifice coordination. Either way, I agree with his statement about arc flash upstream of the main. The main breaker basically cannot be racked out without first de-energizing the system.

 

[davidbeach]

I love the concept of dead work (de-energizing the system first). How is that done? First thing is that all known sources are turned off; then it is confirmed to be de-energized through direct measurement, and then grounds are applied. Now it is de-energized. But if it can't be safely approached while energized, how are the direct measurements to be made to confirm that it has been de-energized.

I've never seen an adequate explanation of how something that can't safely be worked while energized can be made to be un-energized for safe work. Until then the concept of dead work to avoid high arc flash hazard is merely that, a concept that can not be applied in the field.

 

[majesus]

For what it is worth the AC PRO quick trip does the same thing, the Qt curve is different than the INST curve.

Not sure why when I posted this way back in the begining of this thread it was dismissed and ruled out, now the C-H version is the saving grace. I smell a C-H salesman in the group.

Well Zogzog I did bring up the C-H ARMS and I'm not a salesmen. I was just posting material to inform the community on the findings from my work.

As well, just to let you know I do enjoy your posts and feedback. I did reply back in posting #6 about your suggestion on the AC Pro. I had a hypothesis on how they work and I did look at AC Pro's Quick Trip Instantaneous Time Current Curve. According to page 29 of the "Instruction Manual: I-AC-PRO-QT"

http://www.utilityrelay.com/PDFs/I-AC-PRO-QT-1.1.pdf

The trip time is shown as 50mS. However, I'm not clear if that is the total clearing time, or that the delay time for the relay to sense the fault? (Can you elaborate on that?)

While as in comparison with the C-H ARMS, it has a total clearing time of 40mS.

Now I'm saying go with manufacturer over another, it is up to you with what brand you prefer and whether those times mean much to your application. So please don't take it personally. I'm just in it for the discussion to learn.


Maj

 

[Zogzog]

"The quick-trip website shows a total clearing time of 50mS in their brochure. That is misleading, as those Curves aren't dependant on the QUICK-TRIPT product, but on the CB that is in use."

Isnt the same true about the C-H system?

I have installed and tested both systems, and the AC-PRO trip unit and arc flash reduction switch designs are superior. The trip unit has more programable settings, sturdier design, split core CT's, and security settings.

The quick trip has a padlock secured switch, leaving the switch in the "Reduction" mode sacrifices coordination, and 99% of the maintenance electricians do not understand that so having security for that switch is important.

 

[WDeanN]

"The quick trip has a padlock secured switch,..."

I like this feature. I am generally not a fan of the maintenance switches, because I think it puts the responsibility on the electrician, who may forget to switch it back when he's done. With the locked cover, it can become part of LOTO, and HOPEFULLY be less likely to be missed upon completion of the work.

 

[WDeanN]

davidbeach
"But if it can't be safely approached while energized, how are the direct measurements to be made to confirm that it has been de-energized."

You suit up to the calculated values. Here at my facility, they have embraced the "40 cal limit" myth, but still allow suiting up to the displayed levels to perform checks to confirm de-energization.

I'd hate to see that 330 cal suit, though!

 

[Zogzog]

Why do you call it the 40 CAl limit muyth?

 

[WDeanN]

I call it a myth because there are so many people that think that it is mandated that energized work be shut down if calculations show greater then 40 cal/cm. As I think we have discussed on another board, there is no such restriction in any of the codes. Not in IEEE, not in NFPA70E, not in the NEC, not if OSHA. There is language that it is advised against, but there is no total restriction. I agree with that it is ill advised to work above 40 cal/cm, by the way, and agree with my facility’s implementation, as far as it goes. I just hate seeing this limit presented like it is something that is mandated.

 

[Zogzog]

You are correct that it goes not violate any current codes or standards (It will be in the 2009 70E) however, sufficent reasearch has been conducted in labs showing the pressures from an arc >40cal are unsurvivable by most humans, so while I agree that the requirement is a myth, the hazard is not a myth.

 

[unclebob]

In our service group, it is forbidden to work if calculations show greater then 40 cal/cm.

 

[majesus]

Zogzog, according to C-H brochure, (see one of my posts for the brochure's link):

"A circuit breaker equipped with an Arcflash Reduction
Maintenance SystemTM can improve safety by providing a simple and reliable method to reduce fault clearing time. The Arcflash Reduction Maintenance System unit utilizes a separate analog trip circuit that provides faster interruption times than the standard dgital) "instantaneous" protection."

Mind you, the features with the quick trip product are great:
-more programable settings
-sturdier design
-split core CT's
-security settings
-padlock secured switch

 

[dpc]

It's interesting that they've returned to an analog circuit to speed up the tripping. It eliminates the sampling delay required with the digital units.

I've seen this in digital relays as well. If you look at the specifications, there is a definite delay in the response time of the "instantaneous" element.

 

[Zogzog]

Analog huh? That is suprising, using old technology to solve a new problem, makes sense, I see that digital lag all the time.

One major problem to this system I see is it only appears to be for C-H breakers. AC-PRO can be put on just about anything made after WWII.

 

[Zogzog]

Maybe we are looking at this the wrong way, what type of operations are you trying to protect people from?

Have you considered remote operators and remote racking solutions?

Arc resistant switchgear?