DTDX991A ABB distributed control system industrial DCS module card

Description

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Transformer protection
Insertion current
When the LV/LV transformers are powered up, very strong
currents occur, which must be considered when selecting
the protective device. The peak value of the first current
wave often reaches a value between 10 and 15 times the
transformer’s effective rated current.
For power ratings below 50 kVA, it may reach between
20 and 25 times the rated current. This transient current
decreases very rapidly with a time constant T varying
from several ms to 10, 20 ms.
Main protection on the primary side
The tables below are the result of a set of tests on
co-ordination between circuit-breakers and BT/BT
transformers. The transformers used in the tests are
normalized. The table, referring to a primary supply
voltage of 230 or 400 V and to single-phase and
three-phase transformers, indicate which circuit-breaker
should be used according to the transformer power rating.
The transformers considered have the primary winding
outside the secondary winding.
The circuit-breakers suggested allow:
• transformer protection in the event of maximum
short-circuit;
• prevention of unwanted tripping when the primary
winding is powered up using
1. modular circuit-breakers with a high magnetic threshold,
curve D or K
• 2. circuit-breakers with magnetic only releaser;
• guaranteed circuit-breaker electrical life.