ABB RELION REX640 - Page 589

(Equation 104)
Maximum fault current value is obtained when fault resistance ( R
F
) is zero, in case of
Equation 104 can be written as:
(Equation 105)
Interpretation of Equation 105 is that fault current has resistive part due to network
damping and imaginary part due to network detuning. Fault current magnitude
increases when damping or detuning increases.
The effect of fault resistance to fault current magnitude can be written as:
(Equation 106)
where
(Equation 107)
Interpretation of Equation 106 is that effect of fault resistance to fault current
magnitude is proportional to scaling the fault current magnitude at zero fault
resistance with the per unit value of residual voltage ( ) due to fault resistance.
Minimum fault current magnitude is always obtained at resonance, when detuning
( I
v
) is zero, in case Equation 104 can be written as:
(Equation 108)
At resonance, fault current is only due to system resistive shunt losses i.e. due to
network damping ( I
d
) and possible harmonic components.
In case of unearthed network, detuning value ( I
v
) equals the uncompensated earth-
fault current due to total network phase-to-earth capacitance value. In case of high
resistance earthed network detuning value ( I
v
) equals the uncompensated earth-
1MRS759142 F
Protection functions
REX640
Technical Manual
589