Fault Current - ABB REL650 Applications Manual

1MRK 506 334-UUS A
18.1.3
18.1.4
Application manual
asymmetrical fault current will be achieved when the fault occurs at approximately zero
voltage (0°). Investigations have shown that 95% of the faults in the network will occur
when the voltage is between 40° and 90°. In addition fully asymmetrical fault current will
not exist in all phases at the same time.

Fault current

The current transformer requirements are based on the maximum fault current for faults
in different positions. Maximum fault current will occur for three-phase faults or single
phase-to-ground faults. The current for a single phase-to-ground fault will exceed the
current for a three-phase fault when the zero sequence impedance in the total fault loop is
less than the positive sequence impedance.
When calculating the current transformer requirements, maximum fault current for the
relevant fault position should be used and therefore both fault types have to be considered.
Secondary wire resistance and additional load
The voltage at the current transformer secondary terminals directly affects the current
transformer saturation. This voltage is developed in a loop containing the secondary wires
and the burden of all relays in the circuit. For ground faults the loop includes the phase and
neutral wire, normally twice the resistance of the single secondary wire. For three-phase
faults the neutral current is zero and it is just necessary to consider the resistance up to the
point where the phase wires are connected to the common neutral wire. The most common
practice is to use four wires secondary cables so it normally is sufficient to consider just
a single secondary wire for the three-phase case.
The conclusion is that the loop resistance, twice the resistance of the single secondary
wire, must be used in the calculation for phase-to-ground faults and the phase resistance,
the resistance of a single secondary wire, may normally be used in the calculation for
three-phase faults.
As the burden can be considerable different for three-phase faults and phase-to-ground
faults it is important to consider both cases. Even in a case where the phase-to-ground fault
current is smaller than the three-phase fault current the phase-to-ground fault can be
dimensioning for the CT depending on the higher burden.
In isolated or high impedance grounded systems the phase-to-ground fault is not the
dimensioning case and therefore the resistance of the single secondary wire always can be
used in the calculation, for this case.
Section 18
Requirements
491