ABB REG670 - Page 558

TR1ATCC then compares this voltage with the set voltage, USet and decides which
action should be taken. To avoid unnecessary switching around the setpoint, a
deadband (degree of insensitivity) is introduced. The deadband is symmetrical around
USet, see figure
282, and it is arranged in such a way that there is an outer and an inner
deadband. Measured voltages outside the outer deadband start the timer to initiate tap
commands, whilst the sequence resets when the measured voltage is once again back
inside the inner deadband. One half of the outer deadband is denoted ΔU. The setting
of ΔU, setting Udeadband should be set to a value near to the power transformer’s tap
changer voltage step (typically 75–125% of the tap changer step).
Voltage Magnitude
Umax
U
2
UsetU
1
UminUblock0
Raise Cmd
*) Action in accordance with setting
Security Range
Lower Cmd
IEC06000489_2_en.vsd
DD U U
DU
in
DU
in
*) *) *)
IEC06000489 V2 EN
Figure 282: Control actions on a voltage scale
During normal operating conditions the busbar voltage U
B
, stays within the outer
deadband (interval between U1 and U2 in figure 282). In that case no actions will be
taken by TR1ATCC. However, if U
B
becomes smaller than U1, or greater than U2, an
appropriate raise or lower timer will start. The timer will run as long as the measured
voltage stays outside the inner deadband. If this condition persists longer than the
preset time delay, TR1ATCC will initiate that the appropriate ULOWER or URAISE
command will be sent from TCMYLTC or TCLYLTC function block to the
transformer tap changer. If necessary, the procedure will be repeated until the
magnitude of the busbar voltage again falls within the inner deadband. One half of the
inner deadband is denoted ΔU
in
. The inner deadband ΔU
in
, setting UDeadbandInner
should be set to a value smaller than ΔU. It is recommended to set the inner deadband
to 25-70% of the ΔU value.
This way of working is used by TR1ATCC while the busbar voltage is within the
security range defined by settings Umin and Umax.
A situation where U
B
falls outside this range will be regarded as an abnormal situation.
When U
B
falls below setting Ublock, or alternatively, falls below setting Umin but still
above Ublock, or rises above Umax, actions will be taken in accordance with settings
for blocking conditions (refer to table
59).
If the busbar voltage rises above Umax, TR1ATCC can initiate one or more fast step
down commands (ULOWER commands) in order to bring the voltage back into the
security range (settings Umin, and Umax). The fast step down function operation can
be set in one of the following three ways: off /auto/auto and manual, according to the
Section 15 1MRK 502 071-UEN -
Control
552 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual