7SG15 MicroTAPP Application Guide
In a situation where a load fluctuates wildly, such a when running large motors or furnaces intermittently,
automatic voltage control can result in an excessive number of tap change operations caused by the impedance
drop through the transformer. In this instance the menu selection ‘transformer voltage drop compensation’ can be
disabled. The disabling of this compensation allows a normal dead-band sensitivity to be applied while at the
same time reducing the number of corrective tap-change operations to a practical number.
Figure 6 shows a situation where a heavy load causes a real voltage drop from A to B. After an initial time delay
four tap-change operations will bring the voltage to the normal level at C. As the load is removed the busbar-
voltage increases to a point D, where it may be excessive, and after a short delay if fast tap down is enabled, 5
tap-change operations are required to return the voltage to normal.
With the ‘transformer voltage drop compensation’ set to Disabled, the ‘measured’ voltage reduces to B when the
load is applied and no corrective tap-change operations are required. As the load is removed the ‘measured’
voltage increases to E where, in this example, a single corrective tap change operation takes place after a normal
time delay. This will reduce the number of tap changes and hence wear, but at the expense of some variation in
the busbar voltage. To allow the transformer voltage drop to be ignored the VT must be located at the opposite
side of the transformer from the load.
Normal Band
A
B
C
D
E
F
D
4 Taps
5 Taps
Figure 6
6 Network Control
6.1 Switching
Out
When a transformer, as a member of a group, is switched out of service, an increase in load on the remaining
units will cause a drop in the supplied power system voltage. If the transformers are high impedance this voltage
drop can be very significant. Figure 7 shows the effect on the busbar voltage when a transformer is switched out.
At switch-out the voltage falls from A to B and an initial time delay will take place before the first corrective tap-
change operation at C which is then followed by further tap-change operations (determined by the inter-tap delay)
until the voltage is returned to normal at D.
Figure 7
The MicroTAPP relay has functionality that enables automatic corrective tap changing to take place prior to
switch-out. On receipt of a ‘switch-out’ signal the MicroTAPP relays operate the tap-changers such that, when the
transformer is switched out, minimal change in voltage will be seen. The initiation instruction can be sent as serial
data over a communication system or as a latched signal applied to a status input. On completion of the operation
a signal (ready for switch-out) is returned either over the communication system or by operation of an output
©2011 Siemens Protection Devices Limited
Chapter 5 Page 10 of 21
