Electric Energy T&D - Index

Electric Energy T&D - EEMag March / April 2008 - Index

1) what would happen without protection during the cEpEl tests?
Experimental testing would be dangerous if the transformer is
not protected against explosion so numerical simulations were
performed instead. Performing computations for a geometry and
for arcing conditions similar to those of a CEPEL test shows that,
after the arc feeding, the average pressure remains close to an
equilibrium state equal to 7 bar (100 psi), much higher than the
static withstand limit pressure.
Thus, during this test, if the transformer had not been equipped
with the TP, the inner average pressure would have risen up to the
static overpressure withstand limit. The transformer would have
exploded as soon as the tank wall elasticity limits were over, i.e.
as soon as the tank walls could not store any more mechanical
energy due to the pressure increase.
2) Numerical simulation results: Explosion prevention on a large
Transformer (400mva)
A 400 MVA transformer (7.8 m (25.6 ft) long and 4 m (13 ft)
high) is considered in that section. An electrical arc (11 MJ-arc
generating about 3.3m 3 of gas) ignites near a bushing, generating
a 11 bar (160 psi) gas bubble.
Figure 4: Chronology of the Prevention Technology Operation
up to 50 ms
When the transformer is equipped with a TP, Figure 4
and Figure 5.a clearly show the pressure propagation
inside the tank and the drain operation as soon as the
first pressure peak has activated the depressurization set
(4 ms after the arc occurrence, Figure 4).
The drained oil velocity is represented by vectors which color
accounts for the velocity magnitude, V, ranging from 0 to 10 m/s
(0 to 33 ft/s).
The drain gives place to the pressurized fluids so that after
120 ms, the pressure is back to safe levels (see Figure 5.a).
March-April 2008 Issue I
Figure 5: Inner Tank Pressure Evolutions a) with and b) without
protection – Protection Efficiency Illustration
Otherwise, when the tank is not equipped with any protection system,
and if it is subjected to a similar low impedance fault, the tank is still
exposed to very dangerous pressure levels (up to 15 bars, 17 psi) after
1 0 ms (Figure 5.b): without the tank protection, the static pressure
stabilizes around 7.5 bars (109 psi) and the transformer explodes. A
technology based on a fast tank drain has thus a very positive effect on
the tank protection.
V. CONCLUSION
TPC’s vocation is to study the prevention of explosion for all
transformers and all types of rupture of insulation and its research
program philosophy is to maintain a strong connection between
experiments and the theoretical developments.
The experiments made by EDF as well as CEPEL showed the efficiency
of the explosion prevention method. This one is based on the fast
tank depressurization induced by the quick oil drainage out of the
transformer. The oil drainage is triggered by the direct and passive
mechanical response of a depressurization set to the pressure wave.
Thus during a transformer short circuit, the TP is activated within
milliseconds by the first dynamic pressure peak of the shock wave,
avoiding transformer explosions before static pressure increases.
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