influenced by the transformer cooling system. The active setting group can be
changed by a parameter, or through a binary input if the binary input is enabled
for it. This feature can be used for transformers where forced cooling is taken out
of operation or extra cooling is switched on. The parameters can also be changed
when a fan or pump fails to operate.
The thermal overload protection continuously estimates the internal heat content,
that is, the temperature of the transformer. This estimation is made by using a
thermal model of the transformer which is based on the current measurement.
If the heat content of the protected transformer reaches the set alarm level, a signal
is given to the operator. This enables the action that needs to be taken in the
power systems before the temperature reaches a high value. If the temperature
continues to rise to the trip value, the protection initiates the trip of the protected
transformer.
After the trip, the transformer needs to cool down to a temperature level where
the transformer can be taken into service again. T2PTTR continues to estimate the
heat content of the transformer during this cooling period using a set cooling time
constant. The energizing of the transformer is blocked until the heat content is
reduced to the set level.
The thermal curve of two time constants is typical for a transformer. The thermal
time constants of the protected transformer are given in seconds with the
Short
time constant
and
Long time constant
settings. If the manufacturer does not state
any other value, the
Long time constant
can be set to 4920 s (82 minutes) for a
distribution transformer and 7260 s (121 minutes) for a supply transformer. The
corresponding
Short time constants
are 306 s (5.1 minutes) and 456 s (7.6 minutes).
If the manufacturer of the power transformer has stated only one, that is, a single
time constant, it can be converted to two time constants. The single time constant
is also used by itself if the p-factor
Weighting factor p
setting is set to zero and
the time constant value is set to the value of the
Long time constant
setting. The
thermal image corresponds to the one time constant model in that case.
Table 332: Conversion table between one and two time constants
Single time constant
(min)
Short time constant
(min)
Long time constant
(min)
Weighting factor p
10 1.1 17 0.4
15 1.6 25 0.4
20 2.1 33 0.4
25 2.6 41 0.4
30 3.1 49 0.4
35 3.6 58 0.4
40 4.1 60 0.4
45 4.8 75 0.4
50 5.1 82 0.4
55 5.6 90 0.4
60 6.1 98 0.4
65 6.7 107 0.4
70 7.2 115 0.4
75 7.8 124 0.4
Protection functions 1MRS757644 H
348 620 series
Technical Manual
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