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 Post subject: ETAP: Arc Flash FCT too long due to 51V not working
PostPosted: Thu Nov 30, 2017 11:46 am 

Joined: Thu Jul 06, 2017 9:19 am
Posts: 12
I'll try to summarize as best as I can.

In ETAP I've set up a multifunction relay (Multilin G30) with the correct CTs and PTs programmed as inputs and breaker interlocks put in place.

One of the functions in the multifunction relay I've enabled is 51V.

Attached is the coordination curve for this relay. The fault current is about 2.89kA, and the arcing current is about 2.78kA.

When I run the arc-flash study in ETAP, the relay trips offline at point 1, after about 12 seconds.

In reality, when the system voltage collapses, shouldn't the relay trip offline at about point 2, 2.22 seconds taking into account the voltage restraint 51V function?

I've attached the SLD and curve below...the SLD shows the arc flash results....


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 Post subject: Re: ETAP: Arc Flash FCT too long due to 51V not working
PostPosted: Mon Dec 11, 2017 1:35 pm 

Joined: Tue Apr 17, 2012 9:17 am
Posts: 12
A few questions:

1. Is the OCR labeled as "Multilin-750" on the 52-GM breaker the one with the 51V element?

2. You say the arcing fault current is 2.78kA but the total Ia on the bus is 15.067kA. Are you expecting one of breakers 52-G1, G2, or G3 clear the fault? Or should 52-GM clear it?

3. On your curve, one of them is labeled G30G1, but the other is not labeled. What is the other one?


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 Post subject: Re: ETAP: Arc Flash FCT too long due to 51V not working
PostPosted: Thu Dec 14, 2017 11:37 am 
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I don't see any details on sources in the diagrams. Why would 51V trigger? Is the voltage drop during fault conditions enough for transformer impedance to play a role? Compared to 15+ kA where a "collapse" as you refer to it happens, an arcing fault is only 20% of that number...not so much that I would assume that an undervoltage trip would occur. Try doing the transformer voltage calculation by hand and see how much of a drop you actually get.


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 Post subject: Re: ETAP: Arc Flash FCT too long due to 51V not working
PostPosted: Thu Dec 14, 2017 1:48 pm 

Joined: Thu Jul 06, 2017 9:19 am
Posts: 12
JeffBlichmann wrote:
A few questions:

1. Is the OCR labeled as "Multilin-750" on the 52-GM breaker the one with the 51V element?

2. You say the arcing fault current is 2.78kA but the total Ia on the bus is 15.067kA. Are you expecting one of breakers 52-G1, G2, or G3 clear the fault? Or should 52-GM clear it?

3. On your curve, one of them is labeled G30G1, but the other is not labeled. What is the other one?


1. No, the G30G1/G2/G3 relays are the ones with the 51V element. I should have clarified, this switchgear has 3 generators (not shown downstream), which are feeding the utility (not shown upstream).

2. 52GM clears the contribution from the utility side. 51G1/G2/G3 have to clear the fault from the generator side.

3. The other curve is the same relay but shifted to show an earlier tripping time due to the voltage restraint function.

PaulEngr wrote:
I don't see any details on sources in the diagrams. Why would 51V trigger? Is the voltage drop during fault conditions enough for transformer impedance to play a role? Compared to 15+ kA where a "collapse" as you refer to it happens, an arcing fault is only 20% of that number...not so much that I would assume that an undervoltage trip would occur. Try doing the transformer voltage calculation by hand and see how much of a drop you actually get.


When I run the protection / coordination study in ETAP, and I fault the bus CD4, it triggers the 51V and shows a quick trip time of ~2.2seconds.
However when i move into Arc-Flash mode, the software doesn't use the voltage restraint function and rather relies on the full range of the tripping curve to 'keep things more conservative'.

Since I posted this question, I confirmed this with ETAP customer support, and also confirmed this with the engineering company that performed the coordination study (arc-flash wasn't in their scope, hence I'm doing the arc-flash study).

ETAP customer support suggested that I note the tripping times from the star coordination study, and manually enter in those clearing times in the arc-flash study to obtain more realistic incident energy results and clearing times.

I would like to double check this (and for my own learning) use the hand calc that you mentioned. Can you point me to where I would get started on this calc? (I've relied on software primarily in the past, I know, i'm ashamed).


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 Post subject: Re: ETAP: Arc Flash FCT too long due to 51V not working
PostPosted: Thu Dec 14, 2017 2:48 pm 
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This uses the infinite bus assumption:
http://www.electricalaxis.com/2015/03/h ... e.html?m=1

For accuracy you need to calculate Thevinen equivalents of the transformed upstream impedance as seen through an ideal transformer. If the source bus is stiff this doesn’t matter. You can also convert everything to complex values and then just solve the Thevinen equivalent circuit using everything as an impedance. It’s faster if you understand it. For sequence values see a basic power system book because the math gets even easier. That’s the reason for using sequence values that etap uses.


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