ABB Relion 670 Series Applications Manual page 182

Section 8
Current protection
8.8.1 Identification
Function description
Thermal overload protection, two
time constants
8.8.2 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.
If the heat content of the protected transformer reaches a set alarm level a signal can be given to
the operator. Two alarm levels are available. This enables preventive actions in the power system
to be taken before dangerous temperatures are reached. If the temperature continues to increase
to the trip value, the protection initiates a trip of the protected transformer.
After tripping by the thermal overload protection, the transformer will cool down over time. There
will be a time gap before the heat content (temperature) reaches such a level so that the
transformer can be taken into service again. Therefore, the function will continue to estimate the
176
IEC 61850
identification
TRPTTR
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IEC 60617 ANSI/IEEE C37.2
identification device number
49
SYMBOL-A V1 EN-US
1MRK 511 401-UUS F
M14877-1 v2
M15341-3 v5
Bay control REC670
Application manual