4.1.6.4 Operation principle
The function can be enabled and disabled with the Operation setting. The
corresponding parameter values are "On" and "Off".
The operation of the three-phase thermal overload, two time constant protection for
power transformers can be described using a module diagram. All the modules in
the diagram are explained in the next sections.
ALARM
OPERATE
BLK_CLOSE
I_A
Max
current
selector
I_B
I_C
BLOCK
Thermal
counter
Temperature
estimator
START
AMB_TEMP
GUID-FF965F1C-6039-4A01-9A4F-B378F8356279 V2 EN
Figure 118: 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=

ï£






â‹…
2
GUID-06DE6459-E94A-4FC7-8357-CA58988CEE97 V2 EN (Equation 7)
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.
1YHT530004D05 D Section 4
Protection functions
615 series 233
Technical Manual
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