Siemens 7sg15 microtapp Operation Manual page 75

relay. The preparation sequence is shown in Figure 8. On receipt of a switch-out signal (A), the nominated
MicroTAPP will immediately tap down until its measured power factor is unity and send a warning signal to other
relays in the same group. The transformers that are to remain in service react to maintain the busbar voltage at
the correct level as shown by the tap change operations A-B. On switch-out at C there may be relatively small
resistive voltage drop that may or may not require a corrective tap-change (D).
6.2 Switching In
When a transformer, as a member of a group, is switched in, a reduction in load on the transformers already in
service will result in a voltage increase. When energised but before being put on load, the relay will adjust its open
circuit terminal voltage such that no change in voltage will occur when the transformer is closed onto the busbar.
The energised but unloaded transformer should be allowed to complete tap changes until the dead band is
reached. It can then be loaded by closing the LV CB. At this point some circulating current will flow and the relays
will tap to minimise it, but will hold the system voltage at the normal level.
6.3 Falling Frequency
When available generation cannot meet demand the power system frequency will fall. When the frequency falls to
a pre-defined value, circuit breakers are opened to shed load and thus return the frequency to normal.
Where loads are resistive a controlled voltage reduction can facilitate a load reduction before the operation of
circuit breakers, a general standard for the reduction being two stages, each of 3%.
The MicroTAPP can be set to automatically effect a voltage reduction if the power system frequency falls below a
set value, which would be higher than the trip out value, thereby reducing the possibility of loss of supply.
6.4 Tap Stagger
When in parallel with other transformers a MicroTAPP can be made to control voltage and at the same time
import or export a fixed reactive current. The initiation instruction can be sent as serial data over a communication
system or a signal applied to a status input.
6.5 Reactive Compensators
When power factor correction reactors are used to reduce power system losses the position of the reactive
compensator can affect the operating point of the relay. Consider Figure 9 that shows a reactive compensator
teed from the secondary connections from transformer 2 with the CT used for load current measurement located
in the incoming side of the transformer circuit breaker.
©2011 Siemens Protection Devices Limited
7SG15 MicroTAPP Application Guide
Figure 8
Chapter 5 Page 11 of 21