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Transformer Study
Continued from Page 48
The rupture disks manufactured for the TRANSFORMER
PROTECTOR are made of stainless steel and dimensioned for
different tarred pressures. For this study, a 0.8 bar set point
depressurization pressure has been settled.
Figure 8: Rupture Disk opening principle
Rupture disks therefore split open at tarred pressure and
release the excess of fluid, Figure 8.
The SERGI TRANSFORMER PROTECTOR depressurization
chamber is generally located on the upper part of the
transformer vessel.
Research regarding the opening time and behaviour of the
rupture disk was conducted by SERGI. Its results were applied
to the power plant transformer study.
The rupture disks complete opening time, versus diameters
and fault levels are shown in Figure 9.
Figure 9: Rupture disk complete opening for different faults
The SERGI rupture disk complete opening time depends
on the fault type, and varies from 0.6 millisecond for a very
sharp pressure rise to 1.8 millisecond for a small short-circuit,
as shown Figure 10:
Figure 10: Rupture disk response versus fault type
50
Figure 11: 34,5 kA short-circuit TRANSFORMER PROTECTOR
response, pressure in bar and psi
Also, if the Rupture Disk is submitted to a 0.8 bar pressure
step, the opening time depicted in Figure 9 is of 2.4 ms.
7. DEPRESSURIZATION CALCULATION
The SERGI TRANSFORMER PROTECTOR is based on
the reactive opening of a rupture disk integrated in a depressurization
chamber. In order to simulate the depressurization, 3
steps were followed:
• Different depressurization chamber diameters were created
in the transformer tank in order to simulate the pressure evolution
during short-circuit, versus size;
• The depressurization chamber conditions to the limits
were modified (speed let free, suppression of heat exchange
parameters on the opened area…);
• Exhaust speeds were calculated recursively, according to
transformer pressure and depressurization chamber diameter.
The simulations were conducted for each short-circuit type
and with different exhaust diameters Figure 11 to Figure 13.
Figure 12: 118 kA short-circuit TRANSFORMER PROTECTOR
response, pressure in bar and psi
The diameter of the rupture disk
plays a significant role in the depressurization
process. The tarred pressure is
also of utmost importance, but in order to
protect the Rupture Disk during maintenance
period, it was decided to settle the
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