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2.4 Overcurrent Protection
GRT100 provides definite time and inverse time overcurrent elements for both phase faults and
earth faults, separately for each transformer winding. Three phase currents from each set of line
CTs are used for the phase fault protection elements, while the earth fault protection is based on
the neutral CT input.These elements can be used selectively depending on the requirements of the
particular application, but the following points should be noted:
• In the case of large power transformers, overcurrent protection is usually employed only as
back-up protection for terminal faults, and for uncleared LV system faults. In such cases, the
overcurrent elements can be applied either on one or both sides of the transformer as
required.
• Coverage of internal transformer faults is generally limited.
• It is common practice to apply IDMTL phase and earth fault overcurrent protection as
back-up for the LV system. Current and time settings must be arranged to grade with
downstream relays and fuses. The phase fault current setting must also be set to exceed the
maximum overload current.
• High-set instantaneous overcurrent protection can be applied on the primary side to provide
back-up protection for terminal faults. The current setting must be higher than the maximum
through-fault current to ensure that the element does not operate for faults on the LV side.
One of the following IEC-standard-compliant inverse time characteristics or one long time inverse
characteristic is available for the inverse current protection.
• standard inverse IEC 60255-3
• very inverse IEC 60255-3
• extremely inverse IEC 60255-3
Up to three definite time elements (1OC to 3OC) and inverse time elements (1OCI to 3OCI) input
three phase currents from line CTs in the transformer windings.
Up to three definite time elements (1EF to 3EF) and inverse time elements (1EFI to 3EFI) input
neutral currents from CTs in the neutral circuit.
Figure 2.4.1 and Figure 2.4.2 show the scheme logic of overcurrent protection. Each element can
perform time-delayed tripping of up to five breakers. The breaker tripping signals are blocked by
the scheme switch settings.
The number of overcurrent elements applied depends on the relay models.