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Grounding Riddle No.32 - Voltage transformer grounding
Is it necessary to connect star point of Primary of VT (Y-Y) to earth in a resistance grounded system?
Author : Mahesh - From: India
Sun, September 11th, 2011 - 08:00
Primary shoudn't be grounded at all . In case of three separate single phase VT to form a three phase VT or Shell type VT , also  secondary shouldn't be grounded . Only in case of core type VT , secondary is allowed and to say it's better to be grounded.  
Author : GLAVIZH - From: Iran
Sun, September 11th, 2011 - 09:04
For similar discussion you can refer to Transformer Riddle No.1 . 
Author : Hamid - From: Iran
Sun, September 18th, 2011 - 17:33
Thanks Glavizh & Hamid for your recommendations.
The 3 VTs are single phase and connected in wye-earthed / wye earthed configuration.
If the primary side & secondary side VT star point earthing is remove as per your recommendation, what are the consequences on the protection & metering fuctions provided by Numerial Protection relay 50/51,  50N/51N, 46, 50BF,  27D,  27R,  27,  59,  59N , 47,  81H/81L ,  25 and V, A,  kW, kWH.
Author : Mahesh - From: India
Mon, September 19th, 2011 - 09:48
In three phase core type measuring transformer just removing of primary grounding is recommanded. I think it will not cause any relay malfunctions when you use three seperated VT. 
Author : Hamid - From: Iran
Mon, September 19th, 2011 - 11:08
In my opinion the VT primary impedance is very very high (I do not have actual VT primary impedance value) as compared to the grounding reststor value 1500 Ohm. Hence the VT star point will provide path for very negligible earth fault current. Hence what is the reason to recommed removal of VT primary star point earthing? Kindly elaborate. 
Author : Mahesh - From: India
Mon, September 19th, 2011 - 17:57
In three single voltage transformer with independent magnetizing flux path, regarding protection requirments it is not important. However sharing of ground fault current path with primary power supply in VT neutral point can to damage the VT when its primary side is grounded.

Author : Hamid - From: Iran
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