Differential Protection for Unearthed 4 Winding Transformers

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namah1984 Member Join Date: Dec 2008 Posts: 8	Differential Protection for Unearthed 4 Winding Transformers 04/22/2013 6:28 AM
	Hello, Here is my question. I have 4 winding transformer primary Y/ ( Y/Y/D)secondaries. All the neutrals of star windings including primary also unearthed. But source is earthed on primary of transformer. Here two questions: 1) Delta winding is feeding a filter to suppress the harmonics on load side. If I want to implement differential protection for this 4 winding transformer, does it cover faults in delta winding also ? My doubt is that Is there any influence of filter currents on differential scheme ?? Rest of the two secondary windings are feeding a converter based drive. 2) Does this scheme detect in-zone earth faults ( transformers are unearthed) if earth fault current is more than the bias setting of relay ? Or can I go for REF scheme for each winding seperately ? Thank you Namah.
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	Pathfinder Tags: converter transformer protectio Differential Protection Transformer Protection

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LAA_Lucke Guru Join Date: Feb 2010 Posts: 1013 Good Answers: 36	#1 Re: Differential Protection for Unearthed 4 Winding Transformers 04/23/2013 6:30 AM
	If the secondary windings have no earthing connections, then faults to earth in the secondary circuit will not be detected. No current can normally flow to the primary earth circuit. Each winding is isolated: Therefore, the earthing must be done for each secondary winding to enable fault to earth detection.
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SHOCKHISCAN Guru Join Date: Nov 2007 Location: Eastern Arizona mountains on Route 666 about a mile from God's country Posts: 1676 Good Answers: 122	#2 Re: Differential Protection for Unearthed 4 Winding Transformers 04/23/2013 1:39 PM
	Differential protection is only acheivable on the primary winding due to the Y neutrals being "earthed" and on the "D" winding(s). The "un-earthed" Y neutral secondary windings cannot be monitored by nor protected by differential due to the "B" phase voltage being unstable and of much higher magnitude than the other two phases. More than likely the primary "Y" protection will be activated by a ground/earth fault in the secondary as the fault current will induce an unbalance (differential) in the primary winding. However, you would be much safer and better off to incorporate ground fault and REF protection on each the individual secondary windings. __________________ They said; "Brain size?" I heard; "Train size?" so I said: "I'll take a small one, thank you."
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