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
Temperature of the hot spot
Θ
o
top oil temperature
H
gr
hot-spot factor
k
load factor Ι/Ι
N
(measured)
Y winding exponent
In this aspect, the load factor Ι/Ι
N
is determined from the currents of that side to which the overload protection
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.
Functions
2.9 Thermal Overload Protection
204 SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
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