Siemens Siprotec 7VK61 Manual page 107

Overvoltage positive sequence systemU
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
1
U = / ·|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 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. If you want only an alarm to be generated, set address 3741 U2>(>) to
Alarm Only. If you want only one stage 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 this parameter to 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 equation
1
U = / ·|U + a
2 3 L1
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.
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 you want a trip command of the 2nd stage to be created anyway, 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 protective function also has two stages. The settings of the voltage threshold and the timer values depend
on the type of application. Here 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) allow you to implement a second stage 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 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.
SIPROTEC, 7VK61, Manual
C53000-G1176-C159-3, Release date 05.2009
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
. Also refer to Section 2.1.3.1 and see margin heading „Voltage Transformer Connec-
4
2.5 Under and over-voltage protection (optional)
Functions
.
4
107