Fault Current; Secondary Wire Resistance And Additional Load - ABB RET670 Applications Manual

1MRK504116-UUS C
2.1.3
2.1.4
Application manual
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, TPS, 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.
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.
Section 2
Requirements
21