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2. Application Notes
GRT100 is applied to both main protection and backup protection for the following transformers
and reactors:
• Two-winding or three-winding power transformers
• Auto-transformers
• Generator-transformer units
• Shunt reactors
2.1 Protection Scheme
GRT100 provides the following protection schemes with measuring elements in parentheses.
Appendix A shows the block diagrams of the GRT100 series.
• Current differential protection (DIFT)
• Restricted earth fault protection (1REF-3REF)
• Time-overcurrent protection (1OC-3OC, 1OCI-3OCI, 1EF-3EF and 1EFI-3EFI)
• Thermal overload protection (THR)
• Frequency protection (FRQ)
• Overexcitation protection (V/F)
• Trip and/or indication of external devices (Buchholtz relay, pressure or temperature sensing
devices etc.)
The DIFT, provided with DIF and HOC elements and the REF are applied for main protection. For
details, see Sections 2.2, 2.3 and 2.10.
They provide transformer protection coverage as follows:
REF: protection for winding to earth faults of star-winding side
DIF: protection for all internal transformer faults (The DIF can be blocked by 2F or 5F
element.)
HOC: protection for all internal transformer faults, specifically for heavy internal faults,
high-speed operation (The HOC is not blocked by 2F or 5F element. The sensitivity is
set above the estimated maximum inrush current.)
Differential current
REF
HOC
DIF
Small
Large
For earth fault only
The number of measuring elements for the restricted earth fault protection and time-overcurrent
protection is dependent on the relay models.
Figure 2.1.1, 2.1.2 and 2.1.3 show typical application and the relationship between AC inputs and
the measuring elements applied in each model.