Undervoltage Protection 27 - Siemens siprotec SJ62 User Manual

2 Functions
Figure 2-43 Logic diagram of the overvoltage protection
2.6.3

Undervoltage Protection 27

Application
Function
137
The undervoltage protection function detects voltage collapses on transmission lines
and electrical machines and prevents the persistance of inadmissible operating states
and a possible loss of stability.
With three-phase connection, undervoltage protection uses the positive sequence fun-
damental component or, optionally, also the actual phase-to-phase voltages. The
latter case applies the smallest of the phase-to-phase voltages.
If only one voltage transformer is connected, the function is provided with the phase-
to-ground or phase-phase fundamental component voltage in accordance with the
type of connection.
Undervoltage protection consists of two definite time elements (27-1 PICKUP and
27-2 PICKUP). Therefore, tripping can be time-graded depending on how severe
voltage collapses are. Voltage thresholds and time delays can be set individually for
both elements. The voltage limit values are configured as phase-to-phase quantities.
Thus, either the positive sequence system value V1 · √3 or, optionally, the smallest of
the phase-to-phase voltages is evaluated.
The dropout ratio for the two undervoltage elements (= V
set.
/V
dropout value pickup value
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
) can be