Figure. 5.4.4 - 181. Low voltage blocking.
The recommended setting for low voltage blocking is the maximum tap increase positions effect. For
example, if the tap changer has a ± 9 × 1.67 % control range, the undervoltage blocking should be set
to 15 % (9 × 1.67 %).
The last part of the AVR configuration is to make sure that an overcurrent or a short-circuit fault on the
load side does not cause a tap change operation due to the load-side voltage drop. If the regulator's
operation is not blocked during the short-circuit fault when the transformer is under heavy overcurrent,
the tap changer controls the voltage up to compensate for the voltage drop; this most probably ends
up causing damage to the tap changer equipment. However, the blocking can also be achieved by
internal overcurrent blocking (if the phase currents are measured with the AVR) or by a pick-up signal
from the external overcurrent relay or transformer protection relay (GOOSE or a wired signal to the
AVR's digital input).
Measured input
The AVR measures phase-to-phase voltages for voltage controlling. Optionally, the phase currents can
be measured for overcurrent blocking.
Table. 5.4.4 - 172. Measurement inputs of the automatic voltage regulator function.
Signal
Description
Time base
UL12 System
Phase-to-phase system voltage UL12
5ms
UL23 System
Phase-to-phase system voltage UL23
5ms
UL31 System
Phase-to-phase system voltage UL31
5ms
IL1
Phase current IL1
5ms
A
AQ
Q-T215
-T215
Instruction manual
Version: 2.04
278
