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Protection Riddle No.11 – Earthing point location
In an industrial power plant including four individual power generation units and related step up power transformers, the delta side of transformers is connected to generators and star side to main substation busbars via relevant incoming circuit breakers.
Earthing system type that is considered in main substation is impedance grounding. Also the related resistor is not connected to star point of that mentioned transformers. They installed on star point of two zigzag earthing transformers that are connected to main bus bars of substation.

Indeed the earthing point of electrical system; because of some location problems are moved from typical place to substation room.
In the other hand, all main substation incomings are equipped by many earth fault relays (51N) that shall be protected the main bus bar against probable earth faults on it.
Next investigations shown that earth fault relays are not able to do works that assigned for it.

How can you explain the reason of that protection system inability?
Author : Hamid - From: Iran
Tue, December 25, 2007 - 11:20
The CTs for the 51N should be located at the resistor point, ie at connection of zig zag transformer and not on incoming circuit breaker level 
Author : kissan from Mauritania
Wed, December 26, 2007 - 12:09
I think the 51N relay which located at star point of earthing transformer can not apply selective protection for all sources. 
Author : Hamid
Wed, December 26, 2007 - 20:12
The relaying point being the star point of earthing transformer will be able to detect the earth fault at bus bar level, whilst the CTS on individual sources will not be able to detect the earth fault as  they can see only  an overcurrent.
It is evident that, in case of an earth fault, the feeding sources will have to  be tripped, i.e with tie breaker open condition, the feeding circuit breakers on the relecant section on which the fault occurs will have to be tripped.
For tie breaker closed position the control circuit of the protection devise has to be wired , so that they can trip the related source.
The above configuration is not a very common one and thus it has its inherent difficulties. 
Author : kissan from Mauritania
Thu, December 27, 2007 - 15:53
Yes you are right.
In main substation, selection of earthing transformers location prevents ground fault protection operation against main busbar earth faults, indeed because of moving of earthing point from star connection of power transformer to  switchgear, the related ground fault relays can not have proper responds against earth faults in main  bus bars and that protection behave as a restricted earth fault inherently.

Author : Hamid
Tue, November 25, 2008 - 14:20
I feel  the earth fault relay 51-N shall be connected in series with resistor for detection of earth fault 
Author : V.Ambarani - From: INDIA
Thu, June 11th, 2009 - 11:00
The scheme will help to protect the Generator from E/F.  That is the part behind the relay will have the protection.  Not the Bus bar 
Author : Abdul Hameed Pathiyil - From: India working in Qatar
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