11KV overhead line

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Anonymous Poster	11KV overhead line 10/25/2009 1:33 AM
	Recently we have installed an 11KV overhead line for 1.25MVA transformer. the line is being tapped to an existing overhead line which is powered from the power plant switchgear. the new line is protected by using two on-load disconnector switches with fuse rated at 160A installed at the T-off(tapping point) and at the terminal pole near the transformer. During the initial testing of the line, one surge arrestor failed causing the switchgear at the power plant to trip on earth fault without blowing the line fuses. our client has asked to replace the fuse with another 80A to solve the problem and hence the new line fuse will blow faster and disconnect the line separately in case of earth fault or s/c without tripping the power plant switchgear. please help and advise whether this can solve the problem??
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Anonymous Poster	#1 Re: 11KV overhead line 10/25/2009 1:53 AM
	11KV, 1.25MVA I =1250/11√3 = 65.6A Fuse rating must be 82A ie 80A is OK as client has suggested. What was the breaker setting ?
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Shyam Guru Join Date: Aug 2006 Location: AM-51, Deen Dayal Nagar, Gwalior, Madhya Pradesh, MP 474001, India Posts: 3418 Good Answers: 32	#2 In reply to #1 Re: 11KV overhead line 10/25/2009 11:44 PM
	I think I =1250/(11√2) = 83.33A should be taken giving more margin for load based fluctuations and inductive serge build up. Fuse in the range of 85A looks more reasonable for fast fuse. __________________ Prof. (Dr.) Shyam, Managing Director for Sensors Technology Private Limited. Gwalior, MP474001, India.
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vinukumar Active Contributor Join Date: Nov 2008 Location: Bangalore Posts: 22	#3 Re: 11KV overhead line 10/26/2009 12:22 AM
	Surge arrester failure was due to defects in surge arrester, microscopic crack may leads to earth fault. If your transformer loading is up to full load, then recommended fuse rating is fine, otherwise select the fuse rating as per the load pattern. __________________ Let Noble Thoughts Comes to Us From All The Sides
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ruk_eee35 Participant Join Date: Oct 2009 Location: Bangalore Posts: 2	#4 Re: 11KV overhead line 10/26/2009 6:46 AM
	As one of the guest has rightly calculated, the fault current is 65.607A. Therefore the fuse rating of 80A is sufficient
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stahir Active Contributor Join Date: Mar 2008 Posts: 15 Good Answers: 3	#5 Re: 11KV overhead line 10/26/2009 11:14 AM
	Hi, If you install a fuse of 80A as per your client advice, the problem will not solve. Why? The fuse blowing time is higher than the response time of protection relay which is operating the circuit breaker. From where this line is coming? Is there any 11KV circuit breaker on the new overhead line befor the starting point? Can you post the single line diagram? After checking the single line diagram, a better solution can suggested.
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Anonymous Poster #1	#6 Re: 11KV overhead line 08/04/2014 9:44 AM
	If an arrester breaks down in a phase and it is replaced, the other two arresters in the other phases should be also replaced, or they should at least be examined to determine if they have also been damaged. It is thus recommended that all three arresters be sent to the manufacturer for examination. It bears mentioning that a MO arrester fulfils its protection function even in a case of overloading. The voltage decreases towards zero due to the fact that an earth or short circuit is produced and in this way the devices connected in parallel to the arrester are protected against excessively high voltages. The protection that takes place in an overload case, is deliberately used in some special cases as the last possibility to protect very important and expensive electrical equipment. If the aim is to overstress a rrester at a predetermined point - such as the exterior of a building - this arrester is dimensioned deliberately weaker, from the voltage point of view, than the other arresters in the installation.
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Anonymous Poster (2); ruk_eee35 (1); Shyam (1); stahir (1); vinukumar (1)

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