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# Protection Riddle No.60 -Role of resistor in high impedance REF

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• #430
alireza

What is the reason of using resistor in high impedance relay?
What is the reason of using resistor in high impedance relay?

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• #1802
Hamid

The main role of external resistor in high impedance REF circuit is expansion of available protection setting range for suitable setting selection and increasing of protection stability against out of zone earth fault which can be caused CT saturation regarding probability variation wide of used current transformers characteristics. Indeed the prime task in REF high impedance calculating settings is to calculate the value of the stabilizing resistor Rstab and stability factor K. A stabilizing resistor is required to ensure through fault stability when one of the secondary CT

#3169
s.balasubramanian

I am not able to follow the following para extracted from your riddle no 60The required earth fault setting current Iop is 250A (assume the maximum earth fault current is limited by the earthing resistor to 1000A). The chosen E/F CT has an exciting current Ie of 1%, and hence using the equation: Iop = CT ratio x (IS + nIe) where: n = no of CT

#3172
Hamid

CT ratio = 600/1=600 Ie = 0.01 A n = 4 ( 3 ph+1 N ) 250 = 600 x ( Is + 0.04) Is= 0.37666

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