2.17 Undervoltage and Overvoltage Protection (optional)
335
7SD5 Manual
C53000-G1176-C169-1
For symmetrical voltages an increase of the positive sequence system corresponds to
an AND gate of the voltages. These stages are particularly suited to the detection of
steady-state overvoltages on long, weak-loaded transmission lines (Ferranti effect).
Here too, the U1> stage (address 3732) with a longer delay time T U1> (address
3733) is used for the detection of steady-state overvoltages (some seconds), the
U1>> stage (address 3734) with the short delay time T U1>> (address 3735) is used
for the detection of high overvoltages that may jeopardize insulation.
Note that the positive sequence system is established according to its defining equa-
tion U
1
=
1
/
3
·|U
L1
+a·U
L2
+a
2
·U
L3
|. For symmetrical voltages this is equivalent to a
phase-to-earth voltage.
If you want the voltage at the remote line end to be decisive for overvoltage detection,
you use the compounding feature. To do so, you must have set during the configura-
tion of the protective functions (Section 2.1.1.3) address 137 U/O VOLTAGE to
Enabl. w. comp. (enabled with compounding).
In addition, the compounding feature needs the line data, which have been set in the
General Protection Data (Power System Data 2) (Section 2.1.4.1):
Address 1111 x', address 1112 c' and address 1113 Line Length, as well as
address 1105 Line Angle. These data are vital for a correct compounding calcula-
tion. If the values provided here do not correspond to real conditions, the compound-
ing may calculate a too high voltage at the remote end, which will cause immediate
pickup as soon as the measured values are applied. In such a case, the pickup state
can only be reset by switching off the measuring voltage.
Compounding can be switched ON or OFF separately for each of the U1 stages: for the
U1> stage at address 3736 U1> Compound and for the U1>> stage at address 3737
U1>> Compound.
The dropout to pickup ratio (address 3739 U1>(>) RESET) is set as high as possible
with regard to the detection of even small steady-state overvoltages. This parameter
can only be altered with DIGSI
®
under Additional Settings.
Overvoltage Nega-
tive Sequence
System U
2
The negative sequence system voltage stages detect asymmetrical voltages. If such
voltages shall cause tripping, set address 3741 U2>(>) to ON. If you want only an
alarm to be generated, set address 3741 U2>(>) to Alarm Only. If you want to
create for these stages a trip command in addition to the alarms, choose the setting ,
in all other cases OFF.
This protective function also has two stages, one being U2> (address 3742) with a
greater time delay T U2> (address 3743) for steady-state asymmetrical voltages and
the other being U2>> (address 3744) with a short delay time T U2>> (address 3745)
for high asymmetrical voltages.
Note that the negative sequence system is established according to its defining equa-
tion U
2
=
1
/
3
·|U
L1
+a
2
·U
L2
+a·U
L3
|. For symmetrical voltages and two swapped
phases this is equivalent to the phase–to–earth voltage value.
The dropout to pickup ratio U2>(>) RESET can be set in address 3749. This param-
eter can only be altered with DIGSI
®
under Additional Settings.
Zero Sequence
System Overvolt-
age
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 you want a trip command
to be created anyway, the setting must be . This protection function 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“.
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