Fault Current; Secondary Wire Resistance And Additional Load; General Current Transformer Requirements - Hitachi Relion 670 Series Applications Manual

1MRK511407-UUS Rev. N
The remanence in the current transformer core can cause unwanted operations or minor additional time
delays for some protection functions. As unwanted operations are not 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.
23.1.3

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.
23.1.4

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.
Therefore, the resistance of the single secondary wire can always be used in the calculation for this kind
of power systems.
23.1.5

General current transformer requirements

The current transformer ratio is mainly selected based on power system data for example, maximum load
and/or maximum fault current. It should be verified that the current to the protection is higher than the
Phasor measurement unit RES670
Application manual
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Section 23
Requirements
M11613-3 v1
M11613-4 v4
M11614-3 v1
M11614-4 v5
M11615-3 v3
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