Operation Principle - ABB RELION 620 Series Technical Manual

1MAC504801-IB E
4.5.2.4
620 series ANSI
Technical Manual
function using a thermal model with two settable time constants. If the temperature rise
continues, 49T 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.

Operation principle

The function can be enabled and disabled with the Operation setting. The corresponding
parameter values are "Enable" and "Disable".
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.
GUID-FF965F1C-6039-4A01-9A4F-B378F8356279-ANSI V2 EN
Figure 175: 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 estimator.
Temperature estimator
The final temperature rise is calculated from the highest of the three-phase currents
according to the expression:
2
 
I
Θ =   ⋅ T
final ref
 
I
ref
 
GUID-06DE6459-E94A-4FC7-8357-CA58988CEE97 V2 EN
I highest measured phase current
Current reference setting
I the set value of the
ref
Temperature rise setting (temperature rise (°C) with the steady-state current I
T the set value of the
ref
Section 4
Protection functions
(Equation 24)
ref
381