Hitachi Relion 670 Series Applications Manual page 417

1MRK506375-UEN Rev. N
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.
22.1.3 Fault current
The current transformer requirements are based on the maximum fault current for faults in different
positions. Maximum fault current will occur for two-phase faults or single phase-to-earth faults. The
current for a single phase-to-earth fault will exceed the current for a two-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.
22.1.4 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 earth faults the loop includes the phase and neutral wire, normally twice the
resistance of the single secondary wire. For two-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. It is normally sufficient to consider just a single secondary wire for the two-
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-earth faults and the phase resistance, the resistance of a single
secondary wire, may normally be used in the calculation for two-phase faults.
As the burden can be considerable different for two-phase faults and phase-to-earth faults it is
important to consider both cases. Even in a case where the phase-to-earth fault current is smaller
than the two-phase fault current the phase-to-earth fault can be dimensioning for the CT depending
on the higher burden.
In isolated or high impedance earthed systems the phase-to-earth 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.
22.1.5 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 minimum operating value for all faults that are to be detected with the selected CT ratio. It
should also be verified that the maximum possible fault current is within the limits of the IED.
The current error of the current transformer can limit the possibility to use a very sensitive setting of a
sensitive residual overcurrent protection. If a very sensitive setting of this function will be used it is
Railway application RER670
Application manual
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Section 22
Requirements
M11613-3 v1
M11613-4 v4
M11614-3 v1
M11614-4 v5
M11615-3 v3
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