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Requirements
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All testing was made without any remanence flux in the current transformer core. The
requirements below are therefore fully valid for a core with no remanence flux. It is dif-
ficult to give general recommendations for additional margins for remanence flux. They
depend on the reliability and economy requirements.
When current transformers of low remanence type (TPY) are used, practically no addi-
tional margin is needed.
For current transformers of high remanence type (e.g. TPX), the small probability of a
fully asymmetrical fault, together with maximum remanence flux 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 zero
voltage (0°). Investigations have proved that 95% of the faults in the network will occur
when the voltage is between 40° and 90°.
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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-earth faults. The current for a single phase-to-earth fault will exceed the cur-
rent 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 should
be used and therefore both fault types have to be considered.
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The current transformer saturation is directly affected by the voltage at the current
transformer secondary terminals. This voltage, for an earth fault, is developed in a loop
containing the phase and neutral conductor, and relay load. For three-phase faults, the
neutral current is zero, and only the phase conductor and relay phase load have to be
considered.
In the calculation, the loop resistance should be used for phase-to-earth faults and the
phase resistance for three-phase faults.
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The current transformer ratio should be selected so that the current to the protection is
higher than the minimum operating value for all faults that are to be detected. The min-
imum operating current is 10-30% of the nominal current.
All current transformers of high remanence and low remanence type that fulfil the re-
quirements on the rated equivalent secondary e.m.f. E
below can be used. The current
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transformers should have an accuracy class comparable to 5P or better. The character-
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