Fault Current - ABB RELION 670 SERIES Applications Manual

Section 25
Requirements
25.1.3
25.1.4
918
security, for example, faults in reverse direction and external faults. Because of the almost
negligible risk of additional time delays and the non-existent risk of failure to operate the
remanence have not been considered for the dependability cases. The requirements below
are therefore fully valid for all normal applications.
It is difficult to give general recommendations for additional margins for remanence to
avoid the minor risk of an additional time delay. They depend on the performance and
economy requirements. When current transformers of low remanence type (for example,
TPY, PR) are used, normally no additional margin is needed. For current transformers of
high remanence type (for example, P, PX, TPX) the small probability of fully
asymmetrical faults, together with high remanence in the same direction as the flux
generated by the fault, has to be kept in mind at the decision of an additional margin. Fully
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.
1MRK 506 369-UUS -
Line distance protection REL670 2.2 ANSI
Application manual