ABB RER670 Applications Manual page 298

Section 12
Control
292
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 124, 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).
*) *)
Raise Cmd
0 Ublock Umin
*) Action in accordance with setting
IEC06000489 V2 EN-US
Figure 124: Control actions on a voltage scale
During normal operating conditions the busbar voltage U
deadband (interval between U1 and U2 in figure 124). In that case no actions will
be taken by TR1ATCC. However, if U
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
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 falls outside this range will be regarded as an abnormal
B
situation.
When U falls below setting Ublock, or alternatively, falls below setting Umin but
B
still above Ublock, or rises above Umax, actions will be taken in accordance with
settings for blocking conditions (refer to table 32).
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
Security Range
D U D
U
Lower Cmd
DU DU
in in
U Uset U Umax
1 2
becomes smaller than U1, or greater than
B
. The inner deadband ΔU
in
1MRK 506 375-UEN A
*)
Voltage Magnitude
IEC06000489_2_en.vsd
, stays within the outer
B
, setting
in
Railway application RER670 2.2 IEC
Application manual