Arc Flash Mitigation 2

[nightfox1925]

We have a stepdown transformer connected to a downstream switchgear. This transformer is also fed from an upstream switchgear feeder. We are getting high arc flash energy values on both bus side and line side of the downstream switchgear. We have set a 'maintenance' setting on the main circuit breaker of the downstream switchgear to instantaneous and brought down the bus arc flash result to Level 2.

However, the line side of the breaker remains high. We tried to set the instantaneous of the uptream switchgear breaker to slightly override the transformer inrush but could not bring down the arc flash level on the line side of the secondary circuit breaker.

Does anyone have suggestion or other option to bring the line side A/F level?

 

[tem1234]

Differential protection

or something with an arc flash detection sensor like this

http://www.selinc.com/WorkArea/DownloadAsset.aspx?id=3897

 

[jghrist]

Use the instantaneous on the upstream breaker as a maintenance setting. The maintenance setting would not be in place while the transformer is energized so there would not be any inrush.

 

[dpc]

Also, if there are CTs on the secondary of the transformer you can use a relay on the low side CTs to trip the primary breaker. This relay does not see the transformer inrush.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[nightfox1925]

Thanks for all the responses folks.

The transformer is a 4.16-0.6kV step down connected to a downstream 600V switchgear.

Is it possible to set the overcurrent pickup of the primary protection to be just above the transformer FLA for purpose of reducing the line side arc flash temporarily during maintenance?

I am proposing to use IOC 2 = 1.75*FLA primary for 'maintenance' purposes only. The present settings are based on 1.75*highest Ifault secondary.

 

[nightfox1925]

Thanks for all the responses folks.

The transformer is a 4.16-0.6kV step down connected to a downstream 600V switchgear.

Is it possible to set the overcurrent pickup of the primary protection to be just above the transformer FLA for purpose of reducing the line side arc flash temporarily during maintenance?

I am proposing to use IOC 2 = 1.75*FLA primary for 'maintenance' purposes only. The present settings are based on 1.75*highest Ifault secondary.

P.S.: CAN I SET IT TO VALUE BELOW FLA TOO?

 

[jghrist]

You need to set it higher than the maximum load current (which may be higher or lower than the nameplate FLA). You need to set it lower than the minimum fault current.

 

[nightfox1925]

Thanks Jighrist for reminding.

Right now, these settings are going to be used ONLY for commissioning the 600V switchgear. That means, the transformer will not be loaded at all.

If we could not reduce the A/F level at FLA setting, then we have no choice but to set it lower than FLA.

My logic that since the transformer will operating at NO-LOAD, only NO LOAD current will be flowing on the transformwer primary...hence we have the flexibility to set it lower as long as it is higher than the NO-LOAD current. Again this is for commissioning purposes ONLY.

Is my option acceptable?

 

[dpc]

You can set it as low as you want if it is only for commissioning and you can live with possible nuisance trips.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[nightfox1925]

Thanks dpc and to all..

 

[PHovnanian]

nightfox1925,

What will you do for arc flash mitigation once the system has been placed in service? The settings for commissioning may be fine, but you'll need a (higher) IOC maintenance setting that will accommodate (some) loads eventually.

Depending on what sorts of procedures might be acceptable to the AHJ and/or owner, it might be permissible to have a low (below full load) maintenance trip setting. One sufficient to run critical loads, but make disconnecting non-critical loads a part of the maintenance procedure.

 

[nightfox1925]

I'll take a note on that. Thanks PHovnanian.

Basically, for now, these settings will be strictly used for commissioning the 600V switchgears at this point of time only. We will ensure that everybody in construction will know that.

A separate study will be performed for the settings intended for maintenance wherein in this case the transformer will loaded somewhere below FLA.

 

[TZilla]

Setting the pickup level (51) a little higher or lower probably won't have any impact on arc-flash mitigation. If you're clearing on the time-over current portion of the curve, you're probably into the 0.5-1.0+ second timeframe anyway - so incident energy levels will be high regardless of where you set the TOC pickup.

The real issue is whether or not the instantaneous element will be able to "see" through the transformer when an arcing fault occurs on the secondary of the transformer. Source-impedance, arc-impedance, and the turns ratio could result in very low primary currents (not to mention a L-G fault on the secondary of a D-Yg would result in a further reduction of primary line currents - which aren't addressed in 1584). So, relying on the 50 element to reduce incident energy levels isn't a good idea to begin with, and setting it artifically low will probably result in miscoordination with downstream devices.

Arc-detecting relays are nice, but if you're tripping the primary side breaker, that 3-5 cycle interrupting time can still result in significant energy levels.

 

[davidbeach]

setting it artifically low will probably result in miscoordination with downstream devices

But that's the whole point of a maintenance mode setting, you give up all hope of coordination in exchange for the fastest possible clearing. The maintenance mode setting is only for use during "man in the circuit" conditions and is disabled at all other times.

 

[dpc]

and is disabled at all other times...[\quote]

Except the times they forget to switch back to normal.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[davidbeach]

I'd put an amber strobe over the electrical room door (outside the room) that is turned on by switching to maintenance mode. I'd also include a SCADA/DCS/EMS/ETC. alarm point that would issue an alarm if in maintenance mode for more than 8 hours. True, not a total and complete solution, but far, far better than just a switch on the wall.

 

[jghrist]

If we could not reduce the A/F level at FLA setting, then we have no choice but to set it lower than FLA.

I don't understand this. As long as the instantaneous pickup is above the minimum fault current level (transferred to the high side), the arc flash energy will be limited by the instantaneous relay time and breaker clearing time. The amount of current doesn't change. Surely the current will be above FLA. Otherwise, you wouldn't be able to set the time-overcurrent pickup to allow full load and still clear a fault.

 

[Maak]

Time to find out how optical sensor based arc-flash protection could help? It is de facto standard in some countries.

If only a very short arcing time is allowed, the answer is arc eliminator.

Manufacturers with a lot of experience: ABB and Vamp.

 

[dede61]

I have tested and commisioned 2 complete low-voltage (400V) panels with dedicated ARC flash protection (flash sensors + overcurrent) last year. This also was connected to the scada for alarming when a flash (only flash), overcurrent (only overcurrent) or a trip occured. The selectivity included tripping only the current feeding breaker and in case the breaker response time was to slow we tripped the MV CB feeding the panel also when the Flash was detected on the cable compartement we would directly trip the MV CB.
The customer suffored an incidend some months before that caused a complete plant shut down of several weeks. Before the incidend the customer was not convinced about Arc Flash protection, now he is a big believer.

regards

 

[JMD83]

Does your design include high resistance grounding for the 4160/600 transformer?