26
6
F
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0
7
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2.5 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 transformers 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 2.5.1 and Figure 2.5.2 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.
Summary of Contents for GRT100 Series
Page 142: ... 141 6 F 2 S 0 7 8 9 Appendix A Block Diagram ...
Page 144: ... 143 6 F 2 S 0 7 8 9 Appendix B Signal List ...
Page 159: ... 158 6 F 2 S 0 7 8 9 ...
Page 160: ... 159 6 F 2 S 0 7 8 9 Appendix C Variable Timer List ...
Page 162: ... 161 6 F 2 S 0 7 8 9 Appendix D Binary Output Default Setting List ...
Page 165: ... 164 6 F 2 S 0 7 8 9 ...
Page 166: ... 165 6 F 2 S 0 7 8 9 Appendix E Details of Relay Menu and LCD and Button Operation ...
Page 174: ... 173 6 F 2 S 0 7 8 9 Appendix F Case Outline Flush Mount Type Rack Mount Type ...
Page 179: ... 178 6 F 2 S 0 7 8 9 ...
Page 180: ... 179 6 F 2 S 0 7 8 9 Appendix G External Connections ...
Page 185: ... 184 6 F 2 S 0 7 8 9 ...
Page 200: ... 199 6 F 2 S 0 7 8 9 ...
Page 201: ... 200 6 F 2 S 0 7 8 9 Appendix J Return Repair Form ...
Page 205: ... 204 6 F 2 S 0 7 8 9 Customer Name Company Name Address Telephone No Facsimile No Signature ...
Page 206: ... 205 6 F 2 S 0 7 8 9 ...
Page 207: ... 206 6 F 2 S 0 7 8 9 Appendix K Technical Data ...
Page 220: ... 219 6 F 2 S 0 7 8 9 ...
Page 221: ... 220 6 F 2 S 0 7 8 9 Appendix M Symbols Used in Scheme Logic ...
Page 224: ... 223 6 F 2 S 0 7 8 9 ...
Page 225: ... 224 6 F 2 S 0 7 8 9 Appendix N Implementation of Thermal Model to IEC60255 8 ...
Page 228: ... 227 6 F 2 S 0 7 8 9 ...
Page 229: ... 228 6 F 2 S 0 7 8 9 Appendix O IEC60870 5 103 Interoperability and Troubleshooting ...
Page 241: ... 240 6 F 2 S 0 7 8 9 Appendix P Modbus Interoperability ...
Page 255: ... 254 6 F 2 S 0 7 8 9 ...
Page 256: ... 255 6 F 2 S 0 7 8 9 Appendix Q Inverse Time Characteristics ...
Page 259: ... 258 6 F 2 S 0 7 8 9 ...
Page 260: ... 259 6 F 2 S 0 7 8 9 Appendix R Failed Module Tracing and Replacement ...
Page 266: ... 265 6 F 2 S 0 7 8 9 Appendix S Ordering ...
Page 269: ... 268 6 F 2 S 0 7 8 9 3 1 Oct 2 2017 Republished under spin off company ...
Page 270: ......