Siemens SIPROTEC 4 7VK61 Manual page 108

Functions
2.5 Under and over-voltage protection (optional)
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 overvol-
tages that may jeopardise insulation.
Note that the positive sequence system is established according to its defining equation
U = 1 / ·|U + a·U
1 3 L1
The dropout to pickup ratio (address 3739 U1>(>) RESET) is set as high as possible with regard to the detec-
tion of even small steady-state overvoltages. This parameter can only be altered in DIGSI at Display Additional
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. If such conditions will only be reported, set address 3741 U2>(>) to Alarm
Only. If only one stage is desired to generate a trip command, 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 OFF.
This protection function also has two stages, one being U2> stage (address 3742) with a longer time delay T
U2> (address 3743) for steady-state asymmetrical voltages and the other being U2>> stage (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 2 ·U + a·U
L1 L2
earth voltage value.
The resetting ratio can be changed U2>(>) RESET using the address 3749. This parameter can only be
altered in DIGSI at Display Additional Settings.
Overvoltage zero-sequence system
The zero-sequence voltage stages can be switched in address 3721 3U0>(>) (or Ux) ON or OFF. In addi-
tion to this, you can set Alarm Only, i.e. these stages operate and also send alarms but do not generate any
trip command. If a trip command of the 2nd stage is still desired, the setting must be U>Alarm U>>Trip.
This protection function can be used for any other single-phase voltage which is connected to the fourth
voltage measurement input U
tion".
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 stage 3U0>
(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 imple-
mented 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
The zero-voltage stages feature a special time stabilization due to repeated measurements allowing them to
be set rather sensitive. This stabilization 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 drop out to pick up ratio can be changed 3U0>(>) RESET using the address 3729. This parameter can
only be altered in DIGSI at Display Additional Settings.
When setting the voltage values please observe the following:
108
+ a 2 ·U |. For symmetrical voltages this is equivalent to a phase-to-earth voltage.
L2 L3
2
|. For symmetrical voltages and two swapped phases this is equivalent to the phase-to-
L3
. Also see section
4
2.1.3.1 Setting Notes at margin heading "Voltage Connec-
C53000-G1176-C159-5, Edition 05.2018
= 1 / ·|
2 3
.
4
SIPROTEC 4, 7VK61, Manual