Fault Current - ABB RELION RET670 Applications Manual

Section 25
Requirements
25.1.3
25.1.4
986
acceptable at all maximum remanence has been considered for fault cases critical for
the 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
1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
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