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2.9.2
Setting Hints
General
K–Factor
7UM62 Manual
C53000-G1176-C149-3
Thermal overload protection is only effective and accessible if address 0116
Therm.Overload was set to Enabled. Set Disabled if the function is not required.
Transformers and generators are prone to damage by overloads which last for an
extended period of time. For this reason, short circuit protection elements such as the
overcurrent protection should not be used to protect against overload. The short time
delays associated with short circuit protection do not allow sufficient time for the
orderly curtailment of load by operating personnel. In addition, short circuit protection
set to trip for overload will not allow short-duration, non-damaging overloads – a
practice which is often required in real operating situations.
The 7UM62 protective relay features an thermal overload protective function with a
thermal tripping characteristic curve which may be adapted to the overload tolerance
of the equipment being protected.
At the address 1601 Ther. OVER LOAD you can set the thermal overload protection
OFF or ON, you can block the trip command (Block relay for trip commands);
or set the protection function to Alarm Only. In that last case no fault record is
created n case of an overload. If the overload protection is switched ON, tripping is also
possible.
The overload protection is set in per unit quantities. The nominal current I
the object to be protected (generator, motor, transformer) is typically used as base
current. The thermally permissible continuous current I
calculate a factor k
:
prim
I
max prim
------------------------- -
k
=
prim
I
N, Machine
The thermally permissible continuous current for the equipment being protected is
indicated in the manufacturer's specifications. If no specifications are available, a
value of 1.1 times the nominal current rating is assumed.
The K-FACTOR to be set at the 7UM62 (address 1602) refers to the secondary
nominal current (= device current). The following applies for the conversion:
Setting value K-FACTOR =
where I
Maximum permissible continuous motor current in primary
max prim
amperes
Nominal machine current
I
N Machine
Current transformer nominal primary current
I
N CT prim
Example: Generator and current transformer with the following data:
Permissible continuous current:
Generator nominal current
Current transformer
Settingxvalue K–FACTOR
Thermal Overload Protection (ANSI 49)
max prim
I
max prim
------------------------
I
N Machine
I
= 1.15 · I
max prim
I
= 483 A
N Machine
500 A/1 A
483 A
,
⋅
≈
1 15
-------------- -
1.11
=
500 A
N, machine
can be used to
I
N Machine
⋅
------------------------
I
NCT prim
N, Machine
of
59
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