Siemens 7sg15 microtapp Operation Manual page 9

7SG15 MicroTAPP Description of Operation
Figure 3
The initial delay timer increments (A) until the timer display is full, when a tap-change signal is issued (B). At this
time the display resets and the inter-tap delay timer increments, shown by C, while the initial timer is held at the
operative value. If the voltage is still abnormal after the first tap-change operation the inter-tap timer will continue
to increment and initiate a further corrective tap-change operation.
If after completion of the tap-change the voltage is normal the initial delay timer will 'run back' while the displayed
timer continues to increment (D). When the 'hidden' initial timer is equal to the displayed time the display will now
'run back' (E) to the reset position when the default display is restored.
Normally, voltage deviations take place slowly and are caused by changes in network loading. When a substantial
change in voltage is seen it is most likely the result of a network abnormality. As an abnormally high voltage can
cause damage to equipment if not corrected immediately the definite time delay of the MicroTAPP can be by-
passed by a fast tapping feature in the event of substantial voltage excursions above the set band.
A fast tap occurs when the voltage rises to a level at least = Top of Dead Band + 2% of normal voltage for 2
seconds.
If the relay is allowed to make fast response to a substantial low voltage deviation, which is of a transient nature
(such as for an auto-reclose sequence) and the tap change is operated to correct the deviation, an unwanted
over-voltage will occur when the transient problem is corrected. For safety reasons, therefore, the IDMTL
characteristic and the fast tap response for the DTL are only enabled for voltages above the relay dead-band.
Following an initial tap change operation (IDMTL or DTL) any subsequent corrective signals for the same voltage
deviation will be delayed by a separate inter-tap time delay (definite time lag characteristic).
Monitoring of the voltage level is via separate connections and inputs to those used for voltage measurement.
Under and over-voltage blocking functions inhibit operation of the regulating relay when the supplied voltage falls
below, or rises above limits which are within the set alarm levels. Tap change operations that will correct the
abnormal voltage are allowed.
Where a 3 phase VT is used, each phase is monitored as a check against fuse failure.
Raise and Lower commands operate normally open relay contacts. Output contacts can be mapped to internally
generated alarms or lockout signals.
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