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Sudden voltage change function (SVC)
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The sudden voltage change function, SVC, is intended for single-phase systems and is
applicable in systems supplied from static frequency converters. It can however also be
applied in two-phase systems but will only measure voltage change in phase UL1.
The function is intended to prohibit the distance protection tripping due to heavy load.
The function measures the voltage derivative (du/dt) and releases the distance protec-
tion tripping. It is a complement to the sudden current change function (SCC). If the
converter is working close to the current limit before the fault, it is not sure that suffi-
cient current change is obtained for the release of the impedance protection from SCC.
A quickly dropping of the voltage is obtained instead, which activates sudden voltage
change function.
The output signal from the step voltage function is configured to the logical element for
release of tripping from the impedance function, see figure in section )XQFWLRQDOLW\
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The voltage measuring element in one of the built-in digital signal processors continu-
ously measures the voltages between the phase conductor and earth. The voltage signal
is conditioned by a filter before the fundamental frequency, RMS-value is calculated.
The difference between the RMS-value and the value one period earlier is calculated.
This difference is compared to the set operate value du/dt.
Figure 64 shows the simplified logic diagram for the step voltage function.
The output signal SVC-START is activated instantaneously if the change in voltage is
greater than the set operate value du/dt. A timer with the settable time t delays the return
of the output signal SVC-START.
The step voltage function is blocked if the input signal SVC-BLOCK is activated.
The signal SVC-VTSU, which is normally connected to the fuse-failure supervision
function, prevents unwanted operation of the step voltage change function.
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