1MAC050144-MB C
Section 4
Protection functions
615 series ANSI
303
Technical Manual
After the trip, the transformer needs to cool down to a temperature level where the
transformer can be taken into service again. 49T 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 299: Conversion table between one and two time constants
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.
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
Содержание 615 Series ANSI
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Страница 389: ...Section 4 1MAC050144 MB C Protection functions 382 615 series ANSI Technical Manual ...
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Страница 707: ...Section 15 1MAC050144 MB C Applicable standards and regulations 700 615 series ANSI Technical Manual ...
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