ABB RELION REX640 Technical Manual page 320

Section 4
Protection functions
314
account the selectivity requirements, switching-in currents, and the thermal and
mechanical withstand of the transformer and outgoing feeders.
Traditionally, overcurrent protection of the transformer has been arranged as shown in
Figure 177. The low-set stage PHLPTOC operates time-selectively both in
transformer and LV-side busbar faults. The high-set stage PHHPTOC operates
instantaneously making use of current selectivity only in transformer HV-side faults.
If there is a possibility, that the fault current can also be fed from the LV-side up to the
HV-side, the transformer must also be equipped with LV-side overcurrent protection.
Inrush current detectors are used in start-up situations to multiply the current start
value setting in each particular protection relay where the inrush current can occur.
The overcurrent and contact based circuit breaker failure protection CCBRBRF is
used to confirm the protection scheme in case of circuit breaker malfunction.
GUID-64E2E56E-A14B-4530-96AB-4E2F52C04455 V1 EN
Figure 177: Example of traditional time selective transformer overcurrent
protection
The operating times of the main and backup overcurrent protection of the above
scheme become quite long, this applies especially in the busbar faults and also in the
transformer LV-terminal faults. In order to improve the performance of the above
scheme, a multiple-stage overcurrent protection with reverse blocking is proposed.
Figure 179 shows this arrangement.
Overcurrent protection with line differential protection
The line differential protection relay has also four separate overcurrent functions
which can be used as the backup protection of line differential function for lines and
cables. There are three stages available with definite or inverse time characteristics
and an instantaneous stage.
1MRS759142 C
REX640
Technical Manual