7SG15 MicroTAPP Description of Operation
©2013 Siemens Protection Devices Limited Chapter 1 Page 17 of 38
6 Protection
A comprehensive monitoring of the voltage control system is incorporated that will detect and prevent abnormal
power system voltages either from incorrect operation of a tap changing mechanism or from incorrect control
signals.
6.1 Voltage and Current
6.1.1 Measured voltage outside normal range
The voltage monitor is connected to separate inputs on the relay and not those used for voltage measurement,
i.e. where a 3 phase VT is used A-B phase may be for measurement and B-C phase for measurement monitoring,
see Figure 11.
If the measured system voltage is less than a pre-set under-voltage limit or greater than a pre-set over-voltage
limit, the relay inhibits the appropriate tap control outputs to the relevant transformer but allows tap change
operations that will correct the abnormal voltage. An under or over-voltage alarm is generated if the abnormal
voltage reaches excessive levels for a time equal to the Alarm Time setting. The under-voltage and over-voltage
alarm level is settable to accommodate the range of voltage allowable by the voltage control settings.
Voltage Measure
Voltage Monitor
C25
C26
C27
C28
VT phase A
VT phase B
VT phase C
Figure 11
As a guideline, the maximum voltage that should normally be seen is = Target Level + Band + LDC. Note that the
Target Level might be the "Target Voltage" setting or any relevant "Auxiliary Target" setting. So, for example, if
the Target Voltage is 98%, the Band is +/- 1.5% and the LDC is 5%, the upper alarm setting should be at least
98% + 1.5% + 5% = 104.5%. In practice a further 1% should be added for tolerance. The actual Over-voltage
Alarm level would therefore be 105.5%.
The minimum voltage that should normally be seen is = Target Level - Band. Note that the Target Level might be
the "Target Voltage" setting or any relevant "Auxiliary Target" setting. So, for example, if the Target Voltage is
98%, the Band is +/- 1.5% and the minimum Auxiliary Target is 97%, the lower alarm setting should be no more
than 97% - 1.5% = 95.5%. In practice a further 1% should be deducted for tolerance. The actual Under-voltage
Alarm level would therefore be 94.5%.
The voltage monitor automatically blocks control signals that would drive the system voltage in the wrong
direction. The overall action of the monitor is shown in Figure 12.
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