Method and device for prevention against explosion and fire of electrical transformers

An electrical transformer which is filled with a combustible coolant may experience a break in the electrical insulation within the transformer. This break in the electrical insulation may lead to an explosion or fire. A pressure sensor and a vapor sensor are preferably coupled to the enclosure to monitor the pressure and vapor content of the enclosure. An increase in pressure of the enclosure may indicate that an insulation breakdown has occurred. When an increase in pressure is detected, the coolant is partially drained from the enclosure. After draining some of the coolant, an inert gas may be injected into the bottom of the enclosure to stir the remaining coolant.

The present invention relates to the field of the prevention against 
explosion and fire of electrical transformers cooled by a combustible 
fluid. 
Electrical transformers exhibit losses both in the windings and in the 
core, for which reason the heat produced needs to be dissipated. 
High-power transformers are thus generally cooled using oil. The oils used 
are dielectric and can ignite above a temperature of the order of 
140.degree.. Since transformers are very expensive components, particular 
attention must be paid to protecting them. 
Fires in power transformers insulated by dielectric oil generally occur 
because of an internal electrical insulation break, causing an often very 
violent deflagration. This results in extensive rupture of the enclosure 
of the transformer and combustion of the oil which spreads the fire to 
other on-site equipment which may also contain large quantities of 
combustible products. 
Explosions can be caused by overloads, voltage surges, progressive 
deterioration of the insulation, an insufficient oil level, the appearance 
of moisture or mould or the failure of an insulating component. 
Safety valves, which trigger in the event of an overpressure within the 
enclosure of the transformer, are known in the prior art. However, these 
valves are not suited to the consequences of an internal insulation fault 
in the transformer. 
Fire protection systems for electrical transformers are also known which 
are actuated by temperature detectors. However, these systems are 
implemented with a significant time lag, when the oil of the transformer 
is already burning. A compromise is then made to limit the combustion to 
the equipment in question, and to prevent the fire from spreading to the 
neighbouring plant. 
The object of the present invention is therefore to provide a method which 
protects both against overpressure inside the transformer, due to 
deflagration when the internal electrical insulation breaks, and against 
the fire which results from such insulation breaks. 
A further object of the invention is a device for prevention against 
explosion and fire, which allows immediate detection of the electrical 
insulation break. 
According to the invention, the method of prevention against explosion and 
fire in an electrical transformer equipped with an enclosure filled with a 
combustible coolant, comprises the following steps: 
detecting a break in the electrical insulation of the transformer, using a 
pressure-sensor means, 
partial draining of the coolant contained in the enclosure, using a valve, 
and 
cooling the hot parts of the coolant by injecting a pressurized inert gas 
into the bottom of the enclosure in order to stir the coolant and flush 
the oxygen located in proximity. 
According to the invention, the device for prevention against explosion and 
fire in an electrical transformer equipped with an enclosure filled with a 
combustible coolant comprises a means for sensing the pressure in the said 
enclosure and a means for partially draining the coolant contained in the 
enclosure. 
An insulation fault firstly generates a strong electric arc which causes 
actuation of the electrical protection systems which trigger the supply 
cell of the transformer (circuit breaker) . The electric arc also causes 
resultant dissipation of energy, which generates an increase in the 
internal pressure of the transformer, sufficient to cause its enclosure to 
rupture. 
The device for prevention against explosion and fire is preferably equipped 
with a means for detecting the triggering of the supply cell of the 
transformer, and with a control unit which receives the signals emitted by 
the sensor means of the transformer and can emit control signals. 
The device for prevention against explosion and fire preferably comprises a 
means for cooling the hot parts of the fluid, by injecting inert gas into 
the bottom of the enclosure, which is controlled by a control signal from 
the control unit. The reason for this is that some parts of the coolant 
undergo heating which can cause them to ignite. Injecting an inert gas at 
the lower part of the enclosure causes stirring of the coolant, which 
equilibrates the temperature and makes it possible to flush the oxygen 
present in proximity to the fluid.

As illustrated in the figures, the transformer 1 comprises an enclosure 2, 
resting on the floor 3 by means of feet 4, and is electrically supplied 
via wires 5 surrounded by insulators 6. 
The enclosure 2 is filled with a coolant 7, for example dielectric oil. In 
order to guarantee a constant level of coolant 7 in the enclosure 2, the 
transformer 1 is equipped with a make-up tank 8 communicating with the 
enclosure 2 via a conduit 9. 
The conduit 9 is provided with an automatic check valve 10 which closes off 
the conduit 9 as soon as it detects a rapid movement of the fluid 7. Thus, 
in the event of an explosion in the enclosure 2, the pressure in the 
conduit 9 drops abruptly, which causes fluid 7 to start to flow, and this 
is stopped rapidly by the closing of the automatic check valve 10. This 
prevents the fluid 7 contained in the make-up tank 8 from feeding the fire 
in the transformer 1. 
The enclosure 2 is equipped with a pressure sensor 11 which can immediately 
detect the variation in pressure due to the deflagration caused by the 
break in the electrical insulation of the transformer 1. The pressure 
sensor 11 may, in particular, consist of a safety valve which is equipped 
with an electrical contact and is thus capable of transmitting information 
relating to the pressure variation detected. The enclosure 2 is also 
equipped with temperature sensors 12, located at several points in the 
enclosure 2, in order to ascertain the temperature of the fluid 7. 
However, these temperature sensors 12 have a delay estimated at 20 or 30 
seconds relative to the pressure detector 11, because of the fact that 
heat propagates more slowly than pressure. 
The enclosure 2 comprises a sensor 13 detecting the presence of coolant 
vapour, also referred to by the term buchholz, mounted at an upper point 
in the enclosure 2, in general on the conduit 9. The deflagration due to 
an electrical insulation break rapidly causes the release of vapour from 
the fluid 7 in the enclosure 2. A vapour sensor 13 is therefore highly 
expedient for detecting a break in the electrical insulation. 
The transformer 1 is supplied via a supply cell (not shown) which comprises 
means for cutting off the supply, such as circuit breakers, and which is 
equipped with triggering sensors 21. 
The enclosure 2 is equipped with drainage means, comprising a conduit 14 to 
which it is connected at the desired height of the drainage level. The 
conduit 14 is closed by a valve 15 with a large diameter, for example 100 
to 150 mm. The enclosure 2 comprises a means for cooling the fluid 7 by 
injecting an inert gas 16, such as nitrogen, into the bottom of the 
enclosure 2. The inert gas 16 is stored in a pressurized tank 17 equipped 
with a valve 18, a pressure reliever 19 and a pipe 20 which conveys the 
gas 16 to the enclosure 2. 
The pressure sensor 11, the temperature sensors 12, the vapour sensor 13, 
the triggering sensors 21, the valve 15 of the conduit 14 and the valve 18 
of the pipe 20 are connected to a control unit 22 intended to control the 
operation of the device. The control unit 22 is equipped with 
data-processing means which receive the signals from the various sensors 
and can emit control signals intended for the valves 15 and 18. 
The device is actuated by a high-pressure signal coming from the pressure 
sensor 11 coinciding with a triggering signal coming from the triggering 
sensors 21 of the supply cell of the transformer 1, in order to prevent 
explosion and fire. The device may also be actuated by a high-temperature 
signal coming from one of the temperature sensors 12 coinciding with a 
vapour-presence signal coming from the vapour sensor 13, in order to 
initiate the extinguishing of a fire. There is thus a requirement that two 
sensors supply concordant information, in order to avoid premature 
triggering. 
Under normal conditions, the device is triggered by the high-pressure 
information in accordance with the information relating to the triggering 
of the supply cell, which immediately initiates the step 23 of opening the 
drainage valve 15, which allows immediate decompression of the enclosure 2 
of the transformer 1, most of whose components will therefore remain 
intact, with the exception of those located in a region very close to the 
electric arc generated by the insulation fault. Opening the valve 15 makes 
it possible to avoid overflows of ignited fluid 7 when the inert gas 16 
will be injected into the possibly damaged enclosure 2. Finally, opening 
the valve 15 causes decompression in the conduit 9, which leads to the 
automatic check valve 10 closing. The make-up tank 8 is thus isolated and 
the fluid 7 which it contains does not feed the fire. Opening the valve 15 
quickly also reduces the risks of explosion and raises the probability 
that the enclosure 2 of the transformer 1 will remain intact. 
The risks of fire are therefore reduced, but after partial draining of the 
enclosure 2, the step 24 of injecting the inert gas 16 into the bottom of 
the enclosure 2 is systematically initiated after a given time delay of, 
for example, 20 seconds, in order to stir the fluid 7 so as to equilibrate 
its temperature and also to suffocate any possible flames on the surface 
of the fluid 7 by flushing the oxygen. The reason for this is that the 
fluid 7, in general oil, can only ignite at a temperature above its 
flash-point, that is to say about 140.degree.. Moreover, in the case of a 
fire in the transformer 1, due to an electric arc, only the surface of the 
fluid 7 reaches this value, whereas the average temperature is at most 
80.degree. C. Stirring the fluid 7 therefore makes it possible to reduce 
the temperature of the hottest parts. For safety reasons, the tank 17 
containing the inert gas 16 is intended to be able to inject the inert gas 
16 for a time lasting of the order of 45 minutes, which is much greater 
than the predicted time for extinguishing the fire. 
The transformer 1 may be equipped with one or more on-load tap changers 25, 
used as interfaces between the said transformer 1 and the electrical 
network to which it is connected, in order to ensure a constant voltage in 
spite of the variations in the current delivered to the network. The 
on-load tap changer 25 is connected via a conduit 26 to the conduit 14 
intended for drainage. The on-load tap changer 25 is actually also cooled 
by an inflammable coolant. Because of its small volume, the explosion of 
an on-load tap changer is extremely violent and may be accompanied by the 
spraying of jets of ignited coolant. The conduit 26 is provided with a 
calibrated diaphragm 27, capable of rupturing in the event of a 
short-circuit, and therefore of overpressure inside the on-load tap 
changer 25. This prevents the enclosure of the said on-load tap changer 25 
from exploding. This changer also comprises a pressure sensor 28 which is 
connected, on the one hand, to the supply cell of the transformer 1 in 
order to trigger it and, on the other hand, to the control unit 22 in 
order to initiate operation of the prevention device in the event of a 
short-circuit in the on-load tap changer 25. 
The invention thus provides a method and a device for prevention against 
explosion and fire in a transformer, which require few modifications to 
existing components, which detect insulation breaks extremely rapidly, and 
which act almost simultaneously so as to limit the resulting consequences. 
This allows both the transformer and the on-load tap changer to be 
safeguarded from loss, and also makes it possible to minimize the damage 
due to short-circuiting.