Functions
2.19 Undervoltage and Overvoltage Protection (optional)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
374
Overvoltage zero-sequence system
The zero-sequence voltage stages can be switched ON or OFF in address 3721 3U0>(>) (or Ux). They can
also be set to Alarm Only, i.e. these stages operate and send alarms but do not generate any trip commands.
If a trip command of the 2nd stage is still desired, the setting must be U>Alarm U>>Trip. This protection func-
tion can be used for any other single-phase voltage which is connected to the fourth voltage measurement input
U
4
. Also refer to Section 2.1.2.1 and see margin heading „Voltage Transformer Connection“.
This protection function also has two stages. The settings of the voltage threshold and the timer values depend
on the type of application. Therefore, no general guidelines can be established. The 3U0> stage (address
3722) is usually set with a high sensitivity and a longer delay time T 3U0> (address 3723). The 3U0>> stage
(address 3724) and its delay time T 3U0>> (address 3725) enables a second stage to be implemented with
less sensitivity and a shorter delay time.
Similar considerations apply if this voltage stage is used for a different voltage at the measuring input U
4
.
The zero-voltage stages feature a special time stabilisation due to repeated measurements allowing them to
be set rather sensitive. This stabilisation can be disabled in address 3728 3U0>(>) Stabil. if a shorter
pickup time is required. This parameter can only be altered in DIGSI at Display Additional Settings. Please
consider that sensitive settings combined with short pickup times are not recommended.
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
en
voltage of the set of voltage transformers is connected to U
4
and if this was already set in the Power
System Data 1 (refer also to Section 2.1.2.1 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
en
= 3·U
0
. When the
voltage triangle is fully displaced, the voltage will be √3
times the phase-to-phase voltage.
• If any other voltage is connected to U
4
, which is not used for voltage protection, and if this was already set
in the Power System Data 1 (refer also to Section 2.1.2.1 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
0
=|U
L1
+U
L2
+U
L3
|. When the
voltage triangle is fully displaced, the voltage will be √3
times the phase-to-phase voltage.
• If any other voltage is connected to U
4
, which is used for voltage protection, and if this was already set in
the Power System Data 1 (refer also to Section 2.1.2.1, 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
4
. Note that with a sensitive setting, i.e. close to operational values that are to be expect-
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-to-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 dropout to pickup ratio Uph-e<(<) RESET can be set in address 3759. This parameter can only be
altered in DIGSI at Display Additional Settings.
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