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7SG15 MicroTAPP Description of Operation
©2013 Siemens Protection Devices Limited
Chapter 1 Page 18 of 38
Figure 12
Assume that the voltage is approaching the high voltage setting level. If a tap change is issued it will be an
instruction to lower the voltage, therefore for safety, the raise output logic can be blocked. The same argument
can be applied to the low voltage setting level.
The voltage blocks will take effect at (upper voltage level - total dead-band) and (lower voltage level + total dead-
band). Using typical settings as an example, if the monitor ‘high' alarm setting is 105% and the ‘low’ alarm setting
94% and the relay band setting is
±
1.5%, the ‘raise’ output will be blocked at 102% and the ‘lower’ output at 97%.
When the tap changer is operated manually from the tap change controls
at the tap changer
, the MicroTAPP can
still be used to prevent incorrect control signals by routing the tap changer raise and lower signals through the
MicroTAPP Block Raise and Block Lower output contacts respectively.
When the tap changer is operated manually from the relay, or from
a remote control centre
, all manual control
signals will be allowed. Such operations should therefore only be carried out if there is an indication of the voltage
level available at the point of control.
6.1.2 Voltage transformer faulty
Where a 3 phase VT is used the relay monitors all voltages in order to ensure the integrity of the VT secondary
output. Any abnormalities detected (HV fuse blown for instance) will inhibit the voltage raise outputs from the relay
and initiate an alarm. A VT failure is assumed when:-
5V
Vpps
when
10%
100%
x
Vpps
Vnps
≥
≥
A VT / VT fuse failure will result in misleading phase information being placed on the MPPC by the MicroTAPP
which has detected the failure. This will compromise the operation of all the MicroTAPP’s sharing load information
over the MPPC when they are set to operate in Circ. Current mode of control. For this reason, failure of a VT / VT
Fuse will result in all the MicroTAPP’s sharing the MPPC switching from Circ. Current to TAPP mode of control for
the duration of the failure. Care should be taken, therefore, to set a valid System Power Factor setting even when
using Circ. Current mode of control.
Note that this feature will only be available when the ‘MPPC Failure Detection’ is Enabled.