Functions
2.8 Undervoltage and Overvoltage Protection 27/59 (Optional)
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
127
Undervoltage Phase-to-Phase
Basically, the same considerations apply as for the phase undervoltage elements. These elements may
replace the phase voltage elements or be used additionally. Accordingly set address
3761
27-Vph-ph Mode
to
ON
,
OFF
,
Alarm Only
or
V<Alarm V<<Trip
.
As phase–to–phase voltages are monitored, the phase–to–phase values are used for the settings
27-1-Vpp
PICKUP
(address
3762
) and
27-2-Vpp PICKUP
(address
3764
).
The corresponding times delay are
27-1-Vpp DELAY
(address
3763
) and
27-2-Vpp DELAY
(address
3765
).
The dropout to pickup ratio
27-Vph-ph RESET
can be set in address
3769
. This parameter can only be set
in DIGSI at
Display Additional Settings
.
If the voltage transformers are located on the line side, the measuring voltages will be absent when the line is
disconnected. To avoid that the undervoltage levels in these cases are or remain picked up, the current criterion
CURR.SUP 27-Vpp
(address
3768
) is switched
ON
. With bus-sided voltage transformers it can be switched
OFF
. However, if the busbar is dead, the undervoltage protection will pick up and expire and then remain picked
up. It must therefore be ensured that the protection is blocked by a binary input.
Undervoltage Positive Sequence System V
1
The positive sequence undervoltage elements can be used instead of or in addition to previously mentioned
undervoltage elements. Accordingly set address
3771
27-V1 Mode
to
ON
,
OFF
,
Alarm Only
or
V<Alarm
V<<Trip
.
Basically, the same considerations apply as for the other undervoltage elements. Especially in case of stability
problems, the positive sequence system is advantageous, since the positive sequence system is relevant for
the limit of the stable energy transmission.
To achieve the two-element condition, the
27-1-V1 PICKUP
element (address
3772
) is combined with a
greater time delay
27-1-V1 DELAY
(address
3773
), and the
27-2-V1 PICKUP
element (address
3774
) with
a shorter time delay
27-2-V1 DELAY
(address
3775
).
Note that the positive sequence system is established according to its defining equation
V
1
=
1
/
3
·|V
A
+ a·V
B
+ a
2
·V
C
|. For symmetrical voltages this is equivalent to a phase-ground voltage.
The dropout to pickup ratio
27-V1 RESET
can be set in address
3779
. This parameter can only be set in DIGSI
at
Display Additional Settings
.
If the voltage transformers are located on the line side, the measuring voltages will be missing when the line is
disconnected. To avoid that the undervoltage levels in these cases are or remain picked up, the current criterion
CURR.SUP. 27-V1
(address
3778
) is switched
ON
. With bus-sided voltage transformers it can be switched
OFF
. However, if the busbar is dead, the undervoltage protection will pick up and expire and then remain in a
picked-up state. It must therefore be ensured in such cases that the protection is blocked by a binary input.
