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 443: 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
The default
Max temperature
setting is 105°C. This value is chosen since even
though the IEC 60076-7 standard recommends 98°C as the maximum allowable
temperature in long-time loading, the standard also states that a transformer can
withstand the emergency loading for weeks or even months, which may produce
the winding temperature of 140°C. Therefore, 105°C is a safe maximum temperature
value for a transformer if the
Max temperature
setting value is not given by the
transformer manufacturer.
4.1.6.7 Signals
1MRS759142 F
Protection functions
REX640
Technical Manual
395
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