ABB RELION REX640 - Page 391

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
Max current
selector
Thermal
counter
AMB_TEMP
START
BLOCK
Temperature
estimator
I3P
I_A
I_B
I_C
Figure 229: 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:
Θ
final
ref
ref
I
I
T=








⋅
2
(Equation 35)
I highest measured phase current
I
ref
the set value of the Current reference setting
T
ref
the set value of the Temperature rise setting (temperature rise (°C) with the
steady-state current I
ref
The ambient temperature value is added to the calculated final temperature
rise estimation. If the total value of temperature is higher than the set operate
temperature level, the START output is activated.
The Current reference setting is a steady-state current that gives the steady-state
end temperature value Temperature rise . It gives a setting value corresponding to
the rated power of the transformer.
The Temperature rise setting is used when the value of the reference temperature
rise corresponds to the Current reference value. The temperature values with
the corresponding transformer load currents are usually given by transformer
manufacturers.
1MRS759142 F
Protection functions
REX640
Technical Manual
391