Undervoltage Protection - Siemens SIPROTEC 7SJ62 Instruction Manual

6.11.2.4 Undervoltage Protection

Pickup Values
Dropout Setting
Current
Supervision
7SJ62 Manual
C53000-G1140-C121-1
Undervoltage protection can be turned 21, 2)), or set to $ODUP RQO\ at address
 )&7 . When undervoltage protection is turned 21, tripping by the undervolt-
age elements is allowed.
There are not clear cut procedures for setting the pickup values of the undervoltage
relay elements. However, because the undervoltage protection function is primarily in-
tended to protect induction machines from voltage dips and to prevent stability prob-
lems, the pickup values will usually be between 60% and 85% of the nominal voltage.
The time delay settings should be selected to prevent voltage dips from causing un-
stable conditions, however, the time delay should be long enough to avoid tripping due
to momentary voltage dips.
Undervoltage protection includes two definite time elements. The pickup value of the
27-2 element is set at either address  or   3,&.83 (depending on the
voltage transformer connection) while the time delay is set at address  
'(/$<. The pickup of the 27-2 element is typically set low while the time delay is set
short, thus this element is used for fast protection against severe undervoltage condi-
tions. The pickup value of the 27-1 element is set at either address  or  
 3,&.83 (depending on the voltage transformer connection) while the time delay is
set at address   '(/$<. The pickup of the 27-1 element is typically set high
while the time delay is set long, thus this element is used for slower protection against
less severe undervoltage conditions. Setting the 27-2 and 27-1 relay elements in this
matter allows the undervoltage protection function to closely follow the stability behav-
ior of the system.
While the drop out setting on the 27-2 element is set permanently to 105% of the pick-
up setting, the drop out setting on the 27-1 element can be set at address  
 '287 5$7,2 as a multiple of the pickup setting. However, the following limitations
are to be observed:
(Address ) x (Address ) cannot exceed 130 V when address 0213 = Vab,
Vbc, VGnd.
(Address ) x (Address ) cannot exceed 225 V when address 0213 = Van,
Vbn, Vcn.
Note:
If a setting has a value of greater than 130 V or 225 V results for the drop out setting,
the drop out setting will be automatically limited. No error message occurs.
The 27-2 and 27-1 elements can be supervised by the current flow monitoring setting
(BkrClosed I MIN) entered at address . If address  &855(17 683(59 is
switched on, then the 27-2 and 27-1 elements will not pickup until the current flowing
through the circuit breaker exceeds the setting entered at address  (typically set
very sensitive). In other words the sustained pickup of the 27-2 and 27-1 elements is
dependent on the circuit breaker being closed, as determined by "%NU&ORVHG ,
0,1." A benefit of current supervision is that the feature prevents an immediate gen-
eral-pickup of the device that would otherwise be caused by the 27 elements when the
device is powered-up without voltage being present.
Functions
6-117