2 Functions
62
7UM61 Manual
C53000-G1176-C127-3
2.9.2 Setting Notes
General Overload protection is only effective and accessible if address 116 Therm.Overload
is set to Enabled during configuration. If the function is not required Disabled is set.
Transformers and generators are prone to damage by extended overloads. These
overloads cannot and should not be detected by short-circuit protection. Time overcur-
rent protection should be set high so that it only detects faults, since short-circuit pro-
tection only permits short time delays. Short time delays, however, do not allow mea-
sures for unburdening the overloaded equipment nor do they permit advantage to be
taken of its (limited) overload capacity.
The 7UM61 protective relay features an overload protective function with thermal trip-
ping characteristic adaptable to the overload capability of the equipment being pro-
tected.
At address 1601 Ther. OVER LOAD the thermal overload protection ON or OFF can
be set, the trip command blocked (Block relay) or the protection function set
toAlarm Only. In the latter case no fault record is created should an overload occur.
If overload protection is switched ON, tripping is also possible.
K Factor The overload protection is set with quantities per unit. The nominal current I
N, machine
of the object to be protected (generator, motor, transformer) is typically used as base
current. The thermally admissible continuous current I
max prim
can be used to calculate
a factor k
prim
:
The thermally admissible continuous current for the equipment being protected is gen-
erally obtainable from manufacturers specifications. If no specifications are available,
a value of 1.1 times the nominal current rating is assumed.
The K-FACTOR (address1602) to be set on the device 7UM61 refers to the secondary
nominal current (= device current). The following applies for the conversion:
with
I
max prim
Thermally admissible continuous motor primary current
I
N Machine
Nominal Current of the Machine
I
N CT prim
Nominal primary CT current
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