Update Documentation P44x/EN AD/E44
MiCOM P441, P442 & P444
Page 47/82
The thermal protection also provides an indication of the thermal state in the measurement
column of the relay. The thermal state can be reset by either an opto input (if assigned to
this function using the programmable scheme logic) or the relay menu, for example to reset
after injection testing. The reset function in the menu is found in the measurement column
with the thermal state.
9.6.3 Setting guidelines
9.6.3.1
Single time constant characteristic
The current setting is calculated as:
Thermal Trip = Permissible continuous loading of the plant item/CT ratio.
Typical time constant values are given in the following table.
The relay setting, ‘Time Constant 1’, is in minutes.
Time constant τ (minutes)
Limits
Air-core reactors 40
Capacitor banks 10
Overhead lines 10
Cross section ≥ 100 mm
2
Cu or 150mm
2
Al
Cables 60 - 90 Typical, at 66kV and above
Busbars 60
TYPICAL PROTECTED PLANT THERMAL TIME CONSTANTS
An alarm can be raised on reaching a thermal state corresponding to a percentage of the trip
threshold. A typical setting might be ‘Thermal Trip’ = 70% of thermal capacity.
9.6.3.2
Dual time constant characteristic
The current setting is calculated as:
Thermal Trip = Permissible continuous loading of the transformer / CT ratio.
Typical time constants:
τ1 (minutes) τ2 (minutes)
Limits
Oil-filled transformer 5 120 Rating 400 - 1600 kVA
An alarm can be raised on reaching a thermal state corresponding to a percentage of the trip
threshold. A typical setting might be ‘Thermal Alarm’ = 70% of thermal capacity.
Note that the thermal time constants given in the above tables are typical only. Reference
should always be made to the plant manufacturer for accurate information.
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