ABB Relion 650 Series Applications Manual page 92

Section 7
Current protection
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
heat content using a set cooling time constant. Energizing of the transformer can be blocked until
the heat content has reached a set level.
7.3.3 Setting guideline
The parameters for the thermal overload protection, two time constants (TRPTTR, 49) are set via
the local HMI or through the Protection and Control Manager (PCM600).
The following settings can be done for the thermal overload protection:
GlobalBaseSel : Selects the global base value group used by the function to define ( IBase ), ( VBase )
SBase ).
and (
Operation : Sets the mode of operation. Disabled switches off the complete function.
IRef : Reference level of the current given in % of IBase . When the current is equal to IRef the final
(steady state) heat content is equal to 1. It is suggested to give a setting corresponding to the
rated current of the transformer winding. Transformer rated current /
IBase1 : Base current for setting given as percentage of IBase . This setting shall be related to the
status with no COOLING input. It is suggested to give a setting corresponding to the rated current
of the transformer with natural cooling (OA).
IBase2 : Base current for setting given as percentage of IBase . This setting shall be related to the
status with activated COOLING input. It is suggested to give a setting corresponding to the rated
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Busbar protection REB650
Application manual