After tripping by the thermal overload protection, the transformer will cool down over time.
There will be a time gap before the heat content (temperature) reaches such a level so that the
transformer can be taken into service again. Therefore, the function will continue to estimate
the heat content using a set cooling time constant. Energizing of the transformer can be
blocked until the heat content has reached a set level.
7.8.3 Setting guideline
M13250-3 v8
The parameters for the thermal overload protection, two time constants (TRPTTR) are set via
the local HMI or Protection and Control IED Manager (PCM600).
The following settings can be done for the thermal overload protection:
Operation
:
Off
/
On
Operation
: Sets the mode of operation.
Off
switches off the complete function.
GlobalBaseSel
: Selects the global base value group used by the function to define (IBase),
(UBase) and (SBase).
IRef
: Reference level of the current given in % of
IBase
. When the current is equal to
IRef
the
final (steady state) heat content is equal to 1. It is suggested to give a setting corresponding
to the rated current of the transformer winding.
IRefMult
: If a binary input ENMULT is activated the reference current value can be multiplied by
the factor
IRefMult
. The activation could be used in case of deviating ambient temperature
from the reference value. In the standard for loading of a transformer an ambient temperature
of 20°C is used. For lower ambient temperatures the load ability is increased and vice versa.
IRefMult
can be set within a range: 0.01 - 10.00.
IBase1
: Base current for setting given as percentage of
IBase
. This setting shall be related to
the status with no COOLING input. It is suggested to give a setting corresponding to the rated
current of the transformer with natural cooling (OA).
IBase2
: Base current for setting given as percentage of
IBase
. This setting shall be related to
the status with activated COOLING input. It is suggested to give a setting corresponding to
the rated current of the transformer with forced cooling (FOA). If the transformer has no
forced cooling
IBase2
can be set equal to
IBase1
.
Tau1
: The thermal time constant of the protected transformer, related to
IBase1
(no cooling)
given in minutes.
Tau2
: The thermal time constant of the protected transformer, related to
IBase2
(with cooling)
given in minutes.
The thermal time constant should be obtained from the transformer manufacturers manuals.
The thermal time constant is dependent on the cooling and the amount of oil. Normal time
constants for medium and large transformers (according to IEC 60076-7) are about 2.5 hours
for naturally cooled transformers and 1.5 hours for forced cooled transformers.
The time constant can be estimated from measurements of the oil temperature during a
cooling sequence (described in IEC 60076-7). It is assumed that the transformer is operated at
a certain load level with a constant oil temperature (steady state operation). The oil
temperature above the ambient temperature is DQ
o0
. Then the transformer is disconnected
from the grid (no load). After a time t of at least 30 minutes the temperature of the oil is
measured again. Now the oil temperature above the ambient temperature is DQ
ot
. The thermal
time constant can now be estimated as:
1MRK 511 358-UEN A Section 7
Current protection
145
Application manual
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