Siemens 7SR242 Duobias, Technical Manual

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©2015 Siemens Protection Devices Limited Chapter 7 Page 33 of 64
2.8 Thermal Overload (49)
Thermal protection is provided to supplement the Winding Temperature device. This function provides a general
overload thermal protection i.e. not a winding hot spot protection.
Outputs can be assigned to both alarm and trip levels. The default settings are recommended if transformer data
is not available, these settings correspond to the lowest level of thermal withstand for an oil filled transformer.
Transformer overloading can result in:-
Reduced transformer life expectancy.
Lower insulation voltage withstand due to degradation of the insulation.
Increased mechanical stress due to expansion.
Gas bubble production in the mineral oil at extreme levels of overload.
2.8.1 Settings Guidelines
49 Overload Setting
The 49 Overload Setting is expressed as a multiple of the relay nominal current and is equivalent to the factor
k.I
B
as defined in the IEC255-8 thermal operating characteristics. It is the value of current above which 100% of
thermal capacity will be reached after a period of time. This setting should be set to 110% of the secondary
current flowing when the transformer is at its full rating and on its minimum voltage tap position.
49 Time Constant Setting
A transformer may be required to temporarily run overloaded e.g. 150% of rating for two hours or 200% of rating
for one hour.
The thermal time constants required to match these specifications are:
150% for two hours Time constant = 178 minutes
200% for one hour Time constant = 186 minutes
These times are applicable to an overload occurring from no load with the transformer at ambient temperature.
The actual tripping time will depend on the loading level prior to the overload occurring.
The operate time can be calculated from:
Time to trip
2 2
2
) (I I
I
ln t(mins)
The steady state % thermal capacity used can be calculated from:
100
) (I
I
used capacity thermal %
2
2
Where:
I = applied current in terms of x I
n
I = thermal pick-up setting x I
n