Electric Energy T&D - Index

Electric Energy T&D - EE Magazine March / April - Index

2) Experiments
To study in detail the pressure wave propagation influence, the
electrical arcs were ignited at three different locations, as shown in
Figure : on the top cover close to the Decompression Set location
(position A), on the top cover opposite the Depressurization Set
location (position B), and in the lower part of the tank opposite
the Depressurization Set location (position C). The position D
shown in Figure is the location where the depressurization set
was installed.
Most of the tests were carried out with electrical arcs with currents
ranging from 5 to 15 kA, and fed during 83 milliseconds. This
duration corresponds to the average response time of an old circuit
breaker and was chosen to maximize the generated gas volume.
B. Transformer Explosion Prevention: Test Results Analysis
1) generated gas
During the CEPEL test campaign, the electrical arc produced from
1 to 2.3 m 3 (35 to 88 ft 3 ) of gas. For the tested energy range,
the gas volume generated during an electrical arc is a logarithmic
type function of the arc energy, which seems in accordance with
the vaporization process and especially with the saturation of the
vaporization for high energy arcs: the arc remains in the generated
gas volume using its energy to crack the oil vapour rather than
continuing directly vaporizing the oil, which results in a smoother
vaporization process.
2) pressure profile Evolution at a Single location
The pressure at a specific location in the transformer after an
electrical arc has occurred is transient as shown in 3, where an
experimental curve of the pressure evolution close to the arc
location after the arc ignition is displayed.
March-April 2008 Issue I
Figure 3: Experimental Pressure Measurements
After the arc ignition the pressure locally rises and reaches a
maximum level; the waves, generated by the arc, propagate at a
finite speed through the transformer and burst the rupture disc
with a pressure gradient of 3900 bar/s (56000 psi/s). Three
milliseconds after the rupture disk burst, the pressure is back to
the activation level. Some secondary peaks, much lower than the
first pressure maximum, can be observed; they are due to wave
reflections off the tank walls and reflected waves interactions.
As soon as the TP has activated, it can be noted that the arc can
be fed for a period much longer than the standard opening time of
a circuit breaker. Even in this severe condition, the pressure would
remain at harmless levels for the transformer tanks.
3) pressure wave propagation
In Figure 3, three experimental pressure profiles are displayed.
Each curve shows what happens near each sensor located in
positions A, B and C (see Figure ).
The arc is generated in C and the shock wave propagation can be
followed step by step because of the pressure peak’s displacement
from C to A. The other pressure peaks (smaller than the main
peak) are due to wave reflections off the walls.
The pressure does not rise spatially uniformly in the tank. The
experiments show the pressure waves propagate in the oil at a
finite speed.
4) pressure peaks
• Only one main pressure peak has been noticed for each test.
The pressure profiles show variations after that main peak but
their magnitude remains low compared to the first pressure
peak level. They are due to waves reflections.
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