4.1.7.3 Functionality
The three-phase thermal overload, two time constants, protection function T2PTTR
protects the transformer mainly from short-time overloads. The transformer is
protected from long-time overloads with the oil temperature detector included in
its equipment.
The alarm signal gives an early warning to allow the operators to take action
before the transformer trips. The early warning is based on the three-phase current
measuring function using a thermal model with two settable time constants. If the
temperature rise continues, T2PTTR operates based on the thermal model of the
transformer.
After a thermal overload operation, the re-energizing of the transformer is inhibited
during the transformer cooling time. The transformer cooling is estimated with a
thermal model.
4.1.7.4 Operation principle
The function can be enabled and disabled with the
Operation
setting. The
corresponding parameter values are "On" and "Off".
The operation of T2PTTR can be described using a module diagram. All the modules
in the diagram are explained in the next sections.
The function uses ambient temperature which can be measured locally or remotely.
Local measurement is done by the protection relay. Remote measurement uses
analog GOOSE to connect AMB_TEMP input.
If the quality of remotely measured temperature is invalid or
communication channel fails the function uses ambient temperature set
in
Env temperature Set
.
ALARM
OPERATE
BLK_CLOSE
I_A
Max
current
selector
I_B
I_C
BLOCK
Thermal
counter
Temperature
estimator
START
AMB_TEMP
Figure 171: Functional module diagram
Max current selector
The max current selector of the function continuously checks the highest measured
TRMS phase current value. The selector reports the highest value to the thermal
counter.
Temperature estimator
The final temperature rise is calculated from the highest of the three-phase
currents according to the expression:
Protection functions
1MRS757644 H
344 620 series
Technical Manual
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