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Figure. 5.5.1. - 116. Low voltage blocking prevents the tap changer operations to avoid the control to maximum position when
the feeding voltage returns to nominal level.
Setting suggestion for the low voltage blocking is the maximum tap increase positions effect. For
example if the tap changer has +/- 9 x 1.67% control range, undervoltage blocking should be set to 9 x
1.67% = 15 %
Last part of the AVR con guration is to make sure that load side overcurrent/short circuit fault does not
cause tap change operation due to load side voltage drop. If the operation of AVR is not blocked during
the short circuit fault when the transformer is under heavy overcurrent, tap changer controls the
voltage up to compensate the voltage drop causing most probably damage to the tap changer
equipment. Internal overcurrent blocking can be used to achieve this (if the phase currents are
measured with the AVR) or pick-up signal from the external overcurrent relay or transformer protection
relay may be used either with goose or wired signal to the AVR digital input.
Measured input values
AVR measures phase to phase voltages for voltage controlling. Optionally phase currents can be
measured for overcurrent blocking.
Table. 5.5.1. - 171. Measurement magnitudes used by the AVR function.
Signal
Description
Time base
U12 System
Phase to phase system voltage U12
5 ms
U23 System
Phase to phase system voltage U23
5 ms
AQ-T257
Instruction manual
Version: 2.00
© Arcteq Relays Ltd
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