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Grounding riddle No.9 - Grounding transformer instead of grounding resistor
According to electrical basic design of one industrial plant, contractor shall supply suitable grounding transformer with an additional NGR in its neutral connection for limitation of ground fault current system to 400 A. He decided to observe a standard bid. After receiving of different proposal, he note one of vendors have submitted oemission of grounding resistor. He clam his earthing transformers can limit ground fault current to 400 A without applying any NGR in its neutral point (without any overcharge).

What is your opinion? Whether electrical system contractor can assent manufacturer proposal?
Author : Hamid - From: Iran
 
#1
Thu, November 6, 2008 - 13:38
this transformer will absorb nearly such current even in healthy and sound conditions and it's not good.   
Author : BEHNAM - From: IRAN
 
#2
Sat, August 28th, 2010 - 17:57
Where this energy during fault condition will be dissipated the transformer has not enough resistance   
Author : Iqbal Khan - From: Paksitan
 
#3
Sun, August 29th, 2010 - 09:29
The equivalent impedance of NGR is quit holmic but we face inductive impedance in grounding transformers approximately. One of important difference between resistive and inductive impedance is their behavior against through fault current harmonics. Therefore when the grounding system is chosen with zero sequence harmonic current considerations (telecommunications interference and so on), the type of impedance, resistive or inductive, will be important. For example the equivalent impedance of an inductor with 5 ohm (in 50 HZ) will be 15 ohm for third harmonics (X= L.ω).
Also the type of grounding impedance is important in transient stability study of network when an earth fault current occurs, indeed the system shall be stable against through earth fault current losses which can occurred in neutral point of power system.
Meanwhile in impedance grounding system particularly, the peak of overvoltage due to an earth fault depends to X0/R0 of earth fault path, therefore it shall be considered in best chosen.  
Author : Hamid - From: Iran
 
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