• ONAN: The air is naturally circulated to the coolers without fans, and the oil is
naturally circulated without pumps.
• OFAF: The coolers have fans to force air for cooling, and pumps to force the
circulation of the transformer oil.
The protection has several parameter sets located in the setting groups, for example
one for a non-forced cooling and one for a forced cooling situation. Both the
permissive steady-state loading level as well as the thermal time constant are
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 237: 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
Table continues on next page
1YHT530004D05 D Section 4
Protection functions
615 series 237
Technical Manual
To Next Page
Loading...
Need help?
General
