Siemens SIPROTEC 4 7UT6 Series Manual page 204

Functions
2.9 Thermal Overload Protection
to acquire the temperature at up to 6 points of the transformer tank. Up to two RTD boxes of this type can be
connected to a 7UT6x.
The device calculates the hot-sport temperature from these data and the settings of the main properties.
When a settable threshold (temperature alarm) is exceeded, an annunciation and/or a trip is generated.
Hot-spot calculation is done with different equations depending on the cooling method.
For ON-cooling and OF-cooling:
[thermueberl-heisspunkt-on-021026-rei, 1, en_GB]
For OD-cooling:
[thermueberl-heisspunkt-of-021026-rei, 1, en_GB]
Θ
h
Θ
o
H
gr
k
Y
In this aspect, the load factor Ι/Ι
is assigned. The phase information is taken from the concerned phase in case of generators, motors, etc., or y-
or z-connected transformer windings; in case of delta-connected transformer windings the difference current
is taken. The rated current is that of the corresponding side.
Ageing Rate Calculation
The life-time of a cellulose insulation refers to a temperature of 98 °C or 208.4 °F in the direct environment of
the insulation. Experience shows that an increase of 6K means half the life-time. For a temperature which
defers from the basic value of 98 °C (208.4 °F), the relative ageing rate B is given by
[thermueberl-alterung-021026-rei, 1, en_GB]
The mean value of the relative ageing rate L is given by the calculation of the mean value of a certain period of
time, i.e. from T1 to T2
[thermueberl-alterungsrate-021026-rei, 1, en_GB]
With constant rated load, the relative ageing rate L is equal to 1. For values greater than 1, accelerated ageing
applies, e.g. if L=2 only half of the life-time is expected compared to the life-time under nominal load condi-
tions.
According to IEC, the ageing range is defined from 80 °C to 140 °C. This is the operating range of the ageing
calculation: Temperatures below 80 °C (176 °F) do not extend the calculated ageing rate; values greater than
140 °C (284 °F) do not reduce the calculated ageing rate.
The above-described relative ageing calculation only applies to the insulation of the winding and cannot be
used for other failure causes.
204
Temperature of the hot spot
top oil temperature
hot-spot factor
load factor Ι/Ι (measured)
N
winding exponent
is determined from the currents of that side to which the overload protection
N
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016