918 PCBs AND ASSOCIATED AROMATICS
4) Tank rupture with scattering fluid, gaseous
decomposition products, solid insulation, steel
components and molten conductor leading to
ignition of nearby combustible building materials
and furnishings by the transformer.
5) Scattered materials in static condition within
minutes after tank rupture resulting in ignition of
combustible building materials and furnishings.NEMA has recognized that the principal underlying cause
of transformer failures involves insulation breakdowns.
A typical failure was described as beginning as an insulation
failure in the high voltage winding which initiates a low cur-
rent, high impedance fault. This turn to turn or layer to layer
fault progresses rapidly, involving additional turns and layers.
The coil impedance decreases and the fault current increases.
An alternative failure model begins with a high impedance
path to ground along the surface of insulation (tracking).
The following scenario was formulated to serve as the
basis for a worst case failure:1) Due to malfunction, the electrical breakers and/
or fuses which are directly associated with the
particular transformer are inoperative. The sus-
tained presence of a severe overload current there-
fore goes undetected, permitting the insulating oil
to overheat significantly.
2) As a consequence of the overload, the average oil
temperature in the tank reaches 135°C. (Note, the
NEMA study accepted this temperature as “an
arbitrary but reasonable level to assume under the
circumstances.”)
3) Insulating deterioration occurs, causing charring
or tracking of the high voltage insulation. A fault
to ground develops through the train of bubbles or
along the insulation surface to ground. The track-
ing fault then rapidly flashes into a long, high
current arc.
4) Once the long arc has developed the sequence
involves the following:a. The long arc rapidly decomposes the surround-
ing oil into gases.
b. A high pressure (several hundred psi) gas bub-
ble develops around the arc under the oil.
c. The pressurized bubble rapidly accelerates
the oil upward towards the gas space and also
subjects the tank walls below liquid level to a
severe overpressure.
d. Depending on the tank geometry, failure occurs
either near the bottom, below the oil surface or
at the top by ejection of the cover.5) Ignition of the hot oil spray results from one or
more of the following sources:a. The hot solid particles of insulation and con-
ductor which are produced by the arcing fault.b. The arc drawn by the ejected bushing.
c. The hot gas bubble of arc decomposed oil which,
at a temperature of about 3000°C initially drove
the oil out of the tank, is itself ejected.The theory of pressure phenomena due to arcing in liquid
filled transformers has been studied and relationships derived
between arcing and tank pressure (EPRI project 325^88 ). Tests
were conducted on different types of arcs to corroborate the
results. For example, current flowing through an expulsion
fuse produces a higher arc- voltage gradient than does the
same current in a free arc in oil. Higher voltage resulted in
higher arc energy, and was accompanied by higher peak
pressure in the tank. Faults contained within the coil’s wind-
ings were found to produce less pressure on the transformer
than either an open arc or an expulsion fuse. In particular,
short length winding faults were found to be less severe than
one inch arcs drawn directly in oil. Peak values of below-oil
pressures were observed when the melting of a fuse wire
initiated the arc, since this resulted in a high, near instanta-
neous rise in arc current. The maximum pressures developed
under the oil for fuse initiated arcs were found to be very
high, in the range of 20–30 atmospheres.
Electrical failure resulting in rupture of cylindrical 10
kVA tanks has been reported to exceed 100 kW-sec. but for
rectangular tanks this value increases to 800–1400 kW-sec.
Values for cylindrical tanks are found to grow rapidly with
increasing tank size. Therefore, eventful failure is much
more likely in small cylindrical tanks.
The real world test parameters discussed above have been
incorporated into the fire hazard assessment model used by
UL (Webber^89 ) to determine the compliance of transformer
fluids with section 450.23 of the National Electrical Code. In
order to be complaint with the code, transformers insulated
with less-flammable liquids are permitted to be installed
without a vault in Type I and Type II building of approved
noncombustible materials in areas in which no combustible
materials are stored, provided there is a liquid confinement
area, the liquid has a fire point of not less than 300°C and the
installation complies with all restrictions provided for in the
listing of the liquid. UL has identified the need for pressure
relief devices and current limiting fusing to limit the effect
of possible high current arcing faults.
The purpose of the tests was to determine the flammabil-
ity of dielectric liquids after an explosion. Approximately ten
galloons of each of the fluids under test were placed in sepa-
rate transformer tanks and preheated to 150°C. Each container
had internal electrodes designed to force the arc upward into
the gas space. Although the temperature at the point of arcing
was several thousand degrees, the duration was only for a few
cycles and therefore the temperature of the fluid in a container
the size of even a small distribution transformer cannot be sig-
nificantly affected. The 150°C used in the test was probably
higher than would reasonably by expected to occur and there-
fore the same tests were also run at 120°C. It was found that a
fireball was not produced at the lower fluid temperature.
When the test conditions were made sufficiently
severe to expel liquid and gas from the transformer at highC016_003_r03.indd 918C016_003_r03.indd 918 11/18/2005 1:12:41 PM11/18/2005 1:12:41 PM