ABB RELION REX640 Technical Manual page 1288

Section 7
Condition monitoring functions
1282
model may also differ from IEC and IEEE markets, though the underlying principle
of temperature calculation remains the same. The transformer constants are decided
based on a user defined setting to select the preferred type of thermal model (IEC,
IEEE or Manual).
For most transformers in service, the top oil temperature inside a winding is difficult
to measure. On the other hand, the top oil temperature at the top of the tank is well
known, either by measurement or by calculation. The winding hot-spot temperature
rise above the top oil temperature is calculated. By adding this value to the measured
or calculated top oil temperature, the winding hot-spot temperature is obtained. A
pictorial representation of the location of top oil temperature sensor is shown in the
Figure 698.
GUID-AAFCFF00-4741-4F52-9954-65C2F678BA37 V1 EN
Figure 698: Location of top oil sensor
WARNING and ALARM signals are provided for the operator to take corrective actions
to reduce the temperature inside the transformer. Corrective actions include switching
on coolers, reducing the load on the transformers and so on.
Normally, warning level is set to indicate if the temperature is nearing
the limiting value and alarm is generated when the limiting condition
has reached. Hence, alarm level is set at a higher value than the
warning level.
The function only calculates hot-spot temperature based on the load factor of the
winding. This function does not calculate the rise in actual hot-spot temperature,
which occurs due to oil flow blockage or malfunction of cooler groups. That said, the
set oil and winding properties can be updated manually to indicate this change.
Based on the calculated hot-spot temperature, the function calculates the relative
ageing, that is, the accelerated or decelerated ageing of the insulation at current load
compared to its ageing at rated load. The variation of ageing acceleration factor with
1MRS759142 C
REX640
Technical Manual