Identification; Application - ABB REB670 Applications Manual

Section 7
Current protection
7.5.1
7.5.2
198

Identification

Function description
Thermal overload protection, two time
constants

Application

Transformers in the power system are designed for a certain maximum load current
(power) level. If the current exceeds this level the losses will be higher than expected. As
a consequence the temperature of the transformer will increase. If the temperature of the
transformer reaches too high a value, the equipment might be damaged;
• The insulation within the transformer experiences forced ageing. As a consequence
of this, the risk of internal phase-to-phase or phase-to-ground faults increases.
• There might be hot spots within the transformer, which degrades the paper insulation.
It might also cause bubbling in the transformer oil.
In stressed situations in the power system it can be required to overload transformers for
a limited time. This should be done without the above mentioned risks. The thermal
overload protection provides information and makes temporary overloading of
transformers possible.
The permissible load level of a power transformer is highly dependent on the cooling
system of the transformer. There are two main principles:
• OA: The air is naturally circulated to the coolers without fans and the oil is naturally
circulated without pumps.
• FOA: The coolers have fans to force air for cooling and pumps to force the circulation
of the transformer oil.
The protection can have two sets of parameters, one for non-forced cooling and one for
forced cooling. Both the permissive steady state loading level as well as the thermal time
constant is influenced by the cooling system of the transformer. The two parameters sets
can be activated by the binary input signal COOLING. This can be used for transformers
where forced cooling can be taken out of operation, for example at fan or pump faults.
The thermal overload protection estimates the internal heat content of the transformer
(temperature) continuously. This estimation is made by using a thermal model of the
transformer which is based on current measurement.
IEC 61850 IEC 60617
identification identification
TRPTTR
SYMBOL-A V1 EN
1MRK 505 337-UUS -
ANSI/IEEE C37.2
device number
49
Application manual