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surajsivadasan
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Join Date: Jul 2013
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Fault Current Calculation

12/12/2013 11:17 AM

hi in my site we have a ge make 6.6kv/0.415kv delta-star connected transformer(Dyn11) where its star side neutral is earthed. we have a ct on the neutral side. secondary of the transformer is connected to 0.415kv LT switchgear for auxillary feeders like auxillary oil pumps,boiler mcc,etc.....

questions are as follows

1. for ref protection ps class ct is not used in neutral of star point of transformer?
2. how the neutral ct sizing is made?
3. how phase fault and earth fault current in 0.415kv Switchgear is calculated.
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PWSlack
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#1

Re: Fault Current Calculation

12/12/2013 1:09 PM

A3) Using the contents of British Standard 7671.

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dj95401
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#2

Re: Fault Current Calculation

12/12/2013 11:28 PM

Being an Electrical Engineer, I would thought you would have known the calculations already? Or, haven't they covered that in class yet?

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7anoter4
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#3

Re: Fault Current Calculation

12/13/2013 12:19 AM

See[cr4 thread of 2010]:

Why to use class PS CT's for Differential Protection?

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Wal
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#4
In reply to #3

Re: Fault Current Calculation

12/13/2013 6:23 AM

I just tried searching CR4 for that and came up with zip. (could be operator error)

Throw the OP a bone....do you have a URL for that?

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7anoter4
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#7
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Re: Fault Current Calculation

12/13/2013 11:36 AM

http://cr4.globalspec.com/thread/62622/Why-to-use-class-PS-CT-s-for-Differential-Protection

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Wal
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#8
In reply to #7

Re: Fault Current Calculation

12/13/2013 10:49 PM

Thanks for that.

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vyral2311
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#5

Re: Fault Current Calculation

12/13/2013 6:30 AM

1. your que. is not clear.

2. Neutral CT sizing means ratio? if yes than it depends on whether neutral is solidly earthed or impedance earthed. For your application it would be solidly earthed I guess (because 0.415kV) and hence its CT ratio would be equal to the Full load current of LV side of transformer (you can select it equal to LV side line CT ratio)

3. phase fault current can be approximately calculated as

fault current = (Full load current on LV side / percent impedance) x 100

and earth fault current would be more or less same as that of phase fault value if it is solidly grounded.
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Rohan_EEPL
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#6

Re: Fault Current Calculation

12/13/2013 6:45 AM

1. Considering sensitvity of this protecton PS class of CT must be used in order to avoid inadvertant differential current in application of normal 5P or 10P class of CTs for REF protection. However being solidly earthed sysem we can set the same REF upto 50% of setting and hence the nuisance tripping due to CT error can be avoidded. One must note that in any case Neutral CT and Phase side CT should match with each other in all respects like Accuracy, ALF, VA and Magnetising charctersitics of CT. Not sure do you have details like Rct and Im in case of exisitng CT.

2. Neatral CTR should be identical to phase side CTs which must have been calulated considering FLA of TR.

3. Easy way to calculate the same = FLA / % Z of TR and it would be more or less same for Phase as well as Ground fault though it would be little higher for Ground fault being Delta-Star TR.
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Users who posted comments:

7anoter4 (2); dj95401 (1); PWSlack (1); Rohan_EEPL (1); vyral2311 (1); Wal (2)

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