Siemens Siprotec 7VK61 Manual page 108

Functions
2.5 Under and over-voltage protection (optional)
The dropout to pickup ratio 3U0>(>) RESET can be set in address 3729. This parameter can only be altered
in DIGSI at Display Additional Settings.
When setting the voltage values please observe the following:
• If the U voltage of the set of voltage transformers is connected to U
en
System Data 1 (refer also to Section 2.1.3.1 under margin heading „Voltage Connection", address 210 U4
transformer = Udelta transf.), the device multiplies this voltage by the matching ratio Uph /
Udelta (address 211), usually with 1.73. Therefore the voltage measured is √3·U
voltage triangle is fully displaced, the voltage will be √3 times the phase-to-phase voltage.
• If any other voltage is connected to U
in the Power System Data 1 (refer also to Section 2.1.3.1 under margin heading „Voltage Connection", e.g.
U4 transformer = Usy2 transf. or U4 transformer = Not connected), the device calculates the
zero sequence voltage from the phase voltages according to its definition 3·U
voltage triangle is fully displaced, the voltage will be √3 times the phase-to-phase voltage.
• If any other voltage is connected to U
the Power System Data 1 (refer also to Section 2.1.3.1, under margin heading „Voltage Connection", U4
transformer = Ux transformer), this voltage will be used for the voltage stages without any further
factors. This „zero sequence voltage protection" then is, in reality, a single-phase voltage protection for any
kind of voltage at U
ed, not only the time delay T 3U0> (address 3723) must be greater, but also the reset ratio 3U0>(>)
RESET (address 3729) must be set as high as possible.
Phase-earth undervoltage
The phase voltage stages can be switched ON or OFF in address 3751 Uph-e<(<). In addition to this, you can
set Alarm Only, i.e. these stages operate and send alarms but do not generate any trip command. You can
generate a trip command for the 2nd stage only in addition to the alarm by setting U<Alarm U<<Trip.
This undervoltage protection function has two stages. The Uph-e< stage (address 3752) with a longer setting
of the time T Uph-e< (address 3753) operates in the case of minor undervoltages. However, the value set
here must not be higher than the undervoltage permissible in operation. In the presence of higher voltage dips,
the Uph-e<< stage (address 3754) with the delay T Uph-e<< (address 3755) becomes active.
The settings of the voltages and times depend on the intended use; therefore no general recommendations for
the settings can be given. For load shedding, for example, the values are often determined by a priority grading
coordination chart. In case of stability problems, the permissible levels and durations of overvoltages must be
observed. With induction machines undervoltages have an effect on the permissible torque thresholds.
If the voltage transformers are located on the line side, the measuring voltages will be missing when the line is
disconnected. To avoid that the undervoltage levels in these cases are or remain picked up, the current criterion
CURR.SUP. Uphe< (address 3758) is switched ON. With busbar side voltage transformers it can be switched
OFF. However, if the busbar is dead, the undervoltage protection will pick up and expire and then remain in a
picked-up state. It must therefore be ensured in such cases that the protection is blocked by a binary input.
If the 7VK61 relay is operated without current transformer connection, the current criterion is of no use and as-
sociated parameters are hidden.
Phase-phase undervoltage
Basically, the same considerations apply as for the phase voltage stages. These stages may replace the phase
voltage stages or be used additionally. Depending on your choice, set address 3761 Uph-ph<(<) to ON, OFF,
Alarm Only or U<Alarm U<<Trip.
As phase–to–phase voltages are monitored, the phase–to–phase values are used for the settings Uph-ph<
(address 3762) and Uph-ph<< (address 3764).
The corresponding times delay are T Uph-ph< (address 3763) and T Uphph<< (address 3765).
108
, which is not used for voltage protection, and if this was already set
4
, which is used for voltage protection, and if this was already set in
4
. Note that with a sensitive setting, i.e. close to operational values that are to be expect-
4
and if this was already set in the Power
4
= 3·U
en 0
= |U + U + U
0 L1 L2
SIPROTEC, 7VK61, Manual
C53000-G1176-C159-3, Release date 05.2009
. When the
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L3