Load Current; Tap Changer; Tap Change Runaway - Siemens 7sg15 microtapp Operation Manual

6.1.3 Load Current

If the true load current is greater than a pre-set limit, the relay system will inhibit tap control outputs to the and
generate an over-current alarm, unless the situation is caused by circulating current flowing between
transformers, in which case tap changing will be enabled that will reduce the circulating current, see Figure 12.
6.2

Tap Changer

The tap changer operation is monitored for a mechanism, wiring or relay fault. The following is provided: -

6.2.1 Tap Change Runaway

Following a tap change instruction the first incorrect tap change operation is detected immediately by the
MicroTAPP and further operations are inhibited. An incorrect tap change operation is defined as 'a tap change
operation that is not initiated by a true control signal'. As an example, a slow to clear 'raise' contactor may allow a
motor drive to continue driving the mechanism at the end of a tap change cycle such that the tap change
maintaining switch recloses thus allowing the tap change to 'run away'.
The MicroTAPP intelligently monitors the relationship between the initiating control signals, the tap change 'in
progress' inputs and the tap position.
For a tap change to be correct, the following sequence must take place:-
1 A control signal must be issued to initiate the process
2 The tap position must change to a new position
3 The tap change mechanism must stop completely
A 'Tap In Progress' (TIP) auxiliary switch is closed while the tap is not at the rest position and used to indicate and
monitor the operation of the mechanism. Figure 13 shows the MicroTAPP input status signals during a normal tap
change operation.
Raise/Lower
In Progress
TPI
Lock Out
IP Indication
To confirm correct operation a definite break between successive tap change operations must be detected.
On detection of the completion of each signal the MicroTAPP checks that the mechanism has stopped by
monitoring for no further signals, shown in the Figure as the 'reclaim time', set at 2 seconds for the MicroTAPP. If
a further operation is detected in the reclaim time, a runaway condition is assumed and the relay will lockout
The TIP LED is maintained until all signals are removed, the tap position has changed and the reclaim time has
expired.
©2013 Siemens Protection Devices Limited
Operation
Time
7SG15 MicroTAPP Description of Operation
Reclaim
Time
Figure 13
Normal Operation
Chapter 1 Page 19 of 38