ABB RELION REF615R, Техническое руководство

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operation must be both very fast and selective, which is usually achieved by using coarse
current settings.
The purpose is also to protect the transformer from short circuits occurring outside the
protection zone, that is through-faults. Transformer overcurrent protection also provides
protection for the LV-side busbars. In this case the magnitude of the fault current is
typically lower than 12xI
n
depending on the fault location and transformer impedance.
Consequently, the protection must operate as fast as possible taking into 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 56 . The low-set stage 51P operates time-selectively both in transformer and LV-
side busbar faults. The high-set stage 50P-1/2 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 pickup value setting in each particular protection
relay where the inrush current can occur. The overcurrent and contact based circuit
breaker failure protection 50BF is used to confirm the protection scheme in case of circuit
breaker malfunction.
A070978-ANSI V2 EN
Figure 56: Example of traditional time selective transformer overcurrent protection
Section 4
1MRS240050-IB C
Protection functions
120 REF615R
Technical Manual