Siemens SIPROTEC Manual page 373

Overvoltage positive sequence system U
You can use the positive sequence voltage stages instead of or in addition to previously mentioned overvoltage
stages. Depending on your choice, set address 3731 U1>(>) to ON, OFF, Alarm Only or U>Alarm
U>>Trip.
For symmetrical voltages an increase of the positive sequence system corresponds to an AND gate of the volt-
ages. 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 (ad-
dress 3734) with the short delay time T U1>> (address 3735) is used for the detection of high overvoltages
that may jeopardise insulation.
Note that the positive sequence system is established according to its defining equation
U = ·|U + a·U
1
/
1 L1
3
If the voltage at the remote line end is to be decisive for overvoltage detection, you can use the compounding
feature. This requires that address 137 U/O VOLTAGE is already set to Enabl. w. comp. (enabled with
compounding) when configuring the protection functions (Section 2.1.1.3).
In addition, the compounding feature needs the line data, which have been entered 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 and address 1105 Line Angle. These data are vital to correctly calculate the
compounding. If the values provided here do not correspond to real conditions, the compounding may calculate
an excessively 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 de-
tection of even small steady-state overvoltages. This parameter can only be altered in DIGSI at Display Addi-
tional Settings.
Overvoltage negative sequence system U
The negative sequence voltage stages detect asymmetrical voltages. If such voltages should cause tripping,
set address 3741 U2>(>) to ON. Or you can set address 3741 U2>(>) to Alarm Only. In this case the con-
dition will be reported but no trip signal will be generated. If only one stage is desired to generate a trip com-
mand, choose the setting U>Alarm U>>Trip. With this setting a trip command is output by the 2nd stage only.
If negative sequence voltage protection is not required, set this parameter to OFF.
This protection 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 calculated according to its defining equation
U = ·|U + a
1
/
2 L1
3
phase-to-earth voltage value.
The dropout to pickup ratio U2>(>) RESET can be set in address 3749. This parameter can only be altered
in DIGSI at Display Additional Settings.
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
1
2
+ a ·U |. For symmetrical voltages this is equivalent to a phase-to-earth voltage.
L2 L3
2
2
·U + a·U |. For symmetrical voltages and two swapped phases this is equivalent to the
L2 L3
2.19 Undervoltage and Overvoltage Protection (optional)
Functions
373