1MAC050144-MB C
Section 4
Protection functions
615 series ANSI
299
Technical Manual
4.5.2.2
Function block
Figure 142:
Function block
4.5.2.3
Functionality
The three-phase thermal overload, two time constant protection function 49T 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, 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.
4.5.2.4
Operation principle
The function can be enabled and disabled with the
Operation
setting. The corresponding
parameter values are Enable and Disable.
The operation of three-phase thermal overload, two time constant protection can be
described using a module diagram. All the modules in the diagram are explained in the
next sections.
Figure 143:
Functional module diagram
Max current selector
The sampled analog phase currents are pre-processed and the TRMS value of the phase
current is derived for each phase current. These phase current values are fed to the
function. The max current selector of 49T checks continuously the highest phase current
value and 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:
49T
I_A
I_B
I_C
BLOCK
TRIP
PICKUP
ALARM
BLK_CLOSE
AMB_TEMP
ALARM
TRIP
BLK_CLOSE
I_A
Max
current
selector
I_B
I_C
BLOCK
Thermal
counter
Temperature
estimator
PICKUP
AMB_TEMP
Содержание 615 Series ANSI
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