ABB RELION REX640 Technical Manual page 374

Section 4
Protection functions
368
If the quality of remotely measured temperature is invalid or
communication channel fails the function uses ambient temperature
set in Env temperature Set.
I_A
I_B
I3P
I_C
AMB_TEMP
BLOCK
GUID-7E17E25E-8791-4F1A-AC90-05059AB4FCED V1 EN
Figure 204: 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:
2
I
 
T
Θ =  ⋅
final  ref
I
 ref 
GUID-972A5942-30C8-4674-A0AF-5206D041FA53 V1 EN
I highest measured phase current
Current reference setting
I the set value of the
ref
T 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.
Temperature
Max current
selector estimator
1MRS759142 C
START
OPERATE
Thermal
ALARM
counter
BLK_CLOSE
Technical Manual
(Equation 34)
ref
REX640