Electric Energy T&D - Index

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

Circle 54 on Reader Service Card
• The pressure peaks’ amplitude is
determined by the created arc. This
peak ranges from +1.5 to +13 bar
(+21.75 to +188.55 psi) for arc
energies from 0.01 MJ to more
than 2.4 MJ.
• The pressure peak’s values do not
strongly depend on the arc energy
since when comparing tests for which
pressure peaks respectively equal +8
bar (+116 psi) and +8.8 bar (127
psi), the maximum pressure only
varies in 0.8 bar (11.6 psi) while the
corresponding arc energies vary within
on order 10 of magnitude (0.1 MJ and
1 MJ respectively).
5) Tank withstand
To static pressure: To check the
mechanical properties of the transformers,
static tests were performed before
applying any low impedance fault. The
withstand limit was found to be +0.7 bar
(+10.15 psi) for the biggest transformer,
T3. This limit (+0.7bar, +10.15 psi)
was used in the analysis as a threshold
for tank depressurization during the
dynamic tests.
To dynamic pressure: Even if the local
pressure measured during the dynamic
tests is on average 6 or 10 times higher
than the static withstand limit, no
tank damage and no tank permanent
deformation occurs because the pressure
peaks are very short. In fact, the structure
can locally withstand high dynamic
pressure increases because of the walls’
elasticity and the prevention method
small inertia to operate.
6) operation of the Transformer protection
On average, the TP has activated after
about 0 milliseconds (minimum:
4.64ms, maximum: 45.7 ms) after the arc
was ignited. Because the pressure wave
propagation speed is finite, the maximum
distance between the arc location and the
TP is the parameter that matters the most
for the activation. In the worst situation,
8 I March-April 2008 Issue
the arc occurs in the transformer lower
part opposite the Depressurization Set
(location C).
The depressurization time is the time
between the TP opening and when the
pressure is definitely under the level of
+0.7 bar (+10.15 psi). On average, the
TP depressurizes the tank in 116ms,
with a minimum value of 19.7ms, and a
maximum of 347ms. This experimentally
proves the TP ability to depressurize the
transformer tanks within milliseconds
and prevent the explosion. The previous
experimental data are also used in the
numerical tool validation, which is the
subject of the next sections.
IV. NUMERICAL SIMULATIONS
A. Mathematical, Physical, and numerical
Modeling
The set of equations used to theoretically
and numerically describe the phenomena
is a model for 3D compressible two-phase
flows that is based on a set of Partial
Differential Equations (PDE), which governs
the hydrodynamic behavior of mixtures. It is
described in reference [1].
One of the major and most interesting model’s
characteristics is its ability to accurately
depict the pressure wave propagation inside
liquids and gases. Physical effects such as
gravity, viscosity, and heat transfers are added
in the modeling in order to be as close as
possible to reality. It is detailed in reference
[ ].
A Finite Volume Method is thus adopted
to numerically solve the PDE’s system (see
[1]). It allows describing precisely complex
geometries such as transformer tanks.
B. Numerical simulation results
As showed in [ ], simulations manage to give
results in accordance with the experimental
results, for a relatively low cost and without
any danger. They were thus used here to
compute the consequences of an electrical
arc appearing in a tank not equipped with
a TP and also to compute the protection
operation on a very large transformer.