Functionality; Operation Principle - ABB RELION 620 Series Technical Manual

Section 4
Protection functions
4.1.7.3
4.1.7.4
326

Functionality

The three-phase thermal overload, two time constants, protection function T2PTTR
protects the transformer mainly from short-time overloads. The transformer is
protected from long-time overloads with the oil temperature detector included in its
equipment.
The alarm signal gives an early warning to allow the operators to take action before the
transformer trips. The early warning is based on the three-phase current measuring
function using a thermal model with two settable time constants. If the temperature
rise continues, T2PTTR 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 "On" and "Off".
The operation of T2PTTR can be described using a module diagram. All the modules
in the diagram are explained in the next sections.
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.
I_A
Max
I_B
current
I_C selector
BLOCK
AMB_TEMP
GUID-FF965F1C-6039-4A01-9A4F-B378F8356279 V2 EN
Figure 161: 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
Thermal
counter
1MRS757644 F
START
OPERATE
ALARM
BLK_CLOSE
620 series
Technical Manual