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.
7.4.3 Setting guideline
M13250-3 v9
The parameters for the thermal overload protection, two time constants (TRPTTR)
are set via the local HMI or Protection and Control IED Manager (PCM600).
The following settings can be done for the thermal overload protection:
Operation: Off/On
Operation: Sets the mode of operation. Off switches off the complete function.
GlobalBaseSel: Selects the global base value group used by the function to define
IBase, UBase and SBase as applicable.
IRef: Reference level of the current given in %. 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.
IRefMult: If a binary input ENMULT is activated the reference current value can be
multiplied by the factor IRefMult. The activation could be used in case of deviating
ambient temperature from the reference value. In the standard for loading of a
transformer an ambient temperature of 20°C is used. For lower ambient
temperatures the load ability is increased and vice versa. IRefMult can be set within
a range: 0.01 - 10.00.
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 current of the transformer with forced cooling
(FOA). If the transformer has no forced cooling IBase2 can be set equal to IBase1.
Tau1: The thermal time constant of the protected transformer, related to IBase1 (no
cooling) given in minutes.
1MRK 505 388-UEN B Section 7
Current protection
Busbar protection REB650 2.2 IEC 119
Application manual
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