The following figure presents the principle operation of the harmonic blocking in the transformer
differential. When the transformer is energized, both the fundamental frequency and the 2
nd
harmonic
increase significantly. In this example the harmonic blocking limit was set to 15 % (the ratio between
the 2
nd
harmonic and the fundamental frequency, all phases), which seems more than sufficient for this
transformer. The pick-up in the example is set to 30 %. Now, when the flux in the transformer core
starts to catch up, the saturation in the core is reduced and the current for magnetizing is reduced as
well. The blocking remains active until the setting is reached after which the blocking is released
for each phase separately. With our example transformer the harmonic blocking limit could be set to 30
% and the energizing would still be successful because the 2
nd
harmonic is still heavily present by the
time the fundamental currents are reduced below the differential stage's pick-up limit.
Figure. 5.4.29 - 165. Inrush blocking by using the 2
nd
harmonic (relative to fundamental frequency).
Figure. 5.4.29 - 166. Example of transformer magnetizing inrush currents.
A conservative setting recommendation for standard type transformers:
• enabling the 2
nd
harmonic blocking
A
AQ
Q-M257
-M257
Instruction manual
Version: 2.07
© Arcteq Relays Ltd
IM00021
311
Summary of Contents for AQ-M257
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