ABB ret650 - Page 200

On the HV side, the three-phase current is normally required in order to feed the
three-phase over current protection that blocks the load tap changer in case of over-
current above harmful levels.
The voltage measurement on the LV-side can be made single phase-earth.
However, it shall be remembered that this can only be used in solidly earthed
systems, as the measured phase-earth voltage can increase with as much as a factor
√3 in case of earth faults in a non-solidly earthed system.
The analog input signals are normally common with other functions in the IED for
example, protection functions.
The LV-busbar voltage is designated U
B
, the load current I
L
and
load point voltage U
L
.
Automatic voltage control for a single transformer
SEMOD159053-73 v6
Automatic voltage control for tap changer, single control TR1ATCC measures the
magnitude of the busbar voltage U
B
. If no other additional features are enabled
(line voltage drop compensation), this voltage is further used for voltage
regulation.
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
71, 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-US
Figure 71: 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
71). 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
Section 11 1MRK 504 169-UEN A
Control
194 Transformer protection RET650 2.2 IEC
Application manual