ELECTRICAL SAFETY ASPECTS IN TRANSFORMERS
J.M.MALIK,
BHARAT HEAVY ELECTRICALS LIMITED, BHOPAL
1.
SYNOPSIS:
Electrical safety in transformer is the most important aspect, because failure of
transformer could prove to be highly disastrous. One of the infrequent but costly
event of power or distribution transformer is “Transformer on fire”. Transformer
fires can lead to significant direct & indirect costs, potential to casualties &
resulting liabilities, prolonged outages & unfavorable publicity.
This paper describes about the various fire protection systems used in
power transformers, use of dry transformer in critical areas like cinema hall, high
rise building & power stations and use of non inflammable oil i.e. silicone oil
filled transformer in Roof Top thermal power stations.
2.
FIRE PROTECTION MEASURES IN TRANSFORMERS:
Power transformer is a very vital link of present complex power systems, which
demand change in voltage level at different locations of the generation,
transmission & distribution system network. With the steep rise in power
demand, the reliability and protection of power transformer/associated
equipments is of prime concern. Most of the power transformers being filled
with oil are quite vulnerable to fire hazard & therefore protection of transformers
from fire is of paramount importance. The present fire protection of transformers
not only aims at protection from fire but are also proactive & aim at a faster
recovery time to restore the system.
2.1
Risk of fire:
Power transformer may be considered as a metal tank filled with oil i.e.
flammable liquid. The inside active part of transformer can be considered as
“insulation materials under high dielectric stress” & “conducting parts i.e. copper
etc. carrying very high current”.
Owing to this any internal or external fault on a transformer may
trigger a fire & the volume of oil in the transformer is sufficient to cause a major
fire accident/explosion in spite of the safety devices like pressure relief valve,
Buccholz relay etc.
2.2
Reasons of fire:
i. Internal arcing fault due to insulation failure.
ii. Internal Fault generated during external Short circuit fault.
iii. Internal Fault generated during lightning/switching impulse stroke.
iv. Improper preventive maintenance of transformer for e.g. oil quality etc.
v. Improper operation of transformer i.e. overloading, over voltage etc.
vi. Bypassing the safety & protection devices.
vii. Inherent design/manufacturing defect.
viii. Ageing of transformer
ix. Manufacturer’s recommendations not followed.
2.3
Various Fire Protection Systems For Transformer:
i.
Automatic High Velocity Water (HVW) spray system.
ii.
Nitrogen injection method (Drain & stir method)
iii.
Carbon dioxide flooding (For indoor Transformers)
iv.
External mobile/portable fire extinguishers (For small Transformers)
The first two methods are generally used & are elucidated as follows:
2.3.1
Automatic High Velocity Water (HVW) Spray System:
This is a commonly used method for fire protection of power
transformer. In this method water jet at a high velocity is used for fire extinction.
In case of fire, the bulb detectors sense the fire & send signal to open the deluge
valve, which in turn sends water at high pressure to the nozzles mounted near
transformer wall & water spray starts. The typical electric & hydraulic drawing
of the system is given in FIG-I
Working of HVW spray system:
High Velocity Spray Nozzles are used to spray water at a high velocity
on the transformer in case of fire. A number of high velocity spray nozzles are
fitted all along the periphery of transformer at all vulnerable locations. The
number, location & size of nozzles depend on the size of transformer & oil
volume.
All the nozzles are fitted on a common
ringmain, which surrounds the periphery of transformer. This ringmain receives
water supply at a high pressure from the water header through deluge valve in
case of fire, which is converted to high velocity water spray through nozzles.
Deluge valve is electrically controlled (through a solenoid coil) & is
normally closed. In case of fire, the valve opens automatically & releases the
water from header to ring mains & then sprayed on all the faces of the
transformer.
Quartzite bulb detectors are the sensing devices, which sense the fire &
send signal to solenoid of deluge valve in case of fire. Normally these are made
of glass & filled with heat sensing liquid & in case of fire these bulb breaks
causing drop in the pressure of wet line system. This sends signal to deluge valve
to open.
Pressure switches are used for automatic control of compressors &
pressurised water control.
This method is generally used where water supply is abundant.
2.3.2 Nitrogen Injection Drain & Stir Method:
This is a modern method & it uses partial oil draining, which is followed by
nitrogen injection for stirring & for fire extinction.
In this method initially a calculated amount of oil is drained (Drain
operation) & then dry nitrogen is injected from the bottom of transformer tank to
stir the transformer oil thermally (Stir operation) & thereby ensuring fire
extinction and preventing any chance of re-combustion.
FIG-I (HVW SPRAY SYSTEM)
Working of Nitrogen injection drain & stir method: As soon as the fire is
detected, immediately signal is sent to the quick drain valve & within fractions of
a second (0.2secs) drain valve is opened to release the pressure & partially drain
the oil. The conservator oil is isolated from main transformer by closing of
automatic shutter valve so that the conservator oil does not replenish the tank.
Within 20 seconds the partial oil draining is complete.
The nitrogen injection valve opens thereby injecting dry Nitrogen into the
transformer from bottom. Explosive gases released from decomposition of
oil/insulation are evacuated. Because of thermal stirring, the surface temperature
is dropped below the flash point & fire is extinguished in approximately 5
minutes. The Nitrogen injection is continued for approximately 45 minutes to
sufficiently cool the transformer & to prevent any recombustion. The typical
working of the system is shown in FIG-II & described as below:
ITEMS DESCRIPTION:
1. BUCCHOLZ RELAY
2. TEMPERATURE DETECTORS
3. SHUTTER VALVE
4. FIRE PROTECTION SYSTEM
5. TRANSFORMER QUICK DRAIN VALVE
6. NITROGEN CYLINDER
7. NITROGEN PYROTECHNIC VALVE
8. PRESSURE REDUCER
FIG-II (NITROGEN INJECTION DRAIN &STIR SYSTEM)
Transformer drain valve is normally located at 15-20 cms below the top
cover of transformer. This automatically opens in case of fire & calculated
amount of hot transformer oil is drained to the oil soak pit.
Nitrogen injection valves open automatically after drain operation &
inject dry nitrogen from tank bottom. 1 -4 valves are sufficient for power
transformer depending upon size. Fire detectors are mounted on all vulnerable
starting points of fire in a transformer. Minimum 6 detectors are recommended
for a power transformer.
Shutter valve is provided between main tank & conservator. It closes
automatically during the drain operation so that that the drained oil of main tank
is not replenished from the conservator.
This method is quite effective & requires lesser maintenance.
3
USE OF NON INFLAMMABLE LIQUID IN TRANSFORMER.
Looking into the flammable nature of transformer oil, non-inflammable
liquid i.e. silicone oil is also being used for transformers.
Silicone oil is non-hydrocarbon, non fire-propagating oil & is used in
electrostatic precipitator transformer installed over rooftop of Thermal power
stations (Refer P1).
This oil is manufactured by Dow Corming USA company
(DC-971) and in event of fire the insulation of transformer burns but quenches
due to non-hydrocarbon nature of this oil. The flash point & fire point of this oil
is very high. Special considerations are required in the design aspects of these
transformers.
A
large
number
of
power
station
customers
like
NTPC/NLC/BSES/TATA Power etc are adopting these transformers inorder to
resort to the safety from fire at critical locations where fire protection is of
paramount importance.
Although this concept is adopted in small rating transformers i.e 7 to 100
kV & 100-200 KVA transformers, but can also be adopted in large power
transformer where fire safety aspect is important.
4
USE OF NON-INFLAMMABLE DRY TYPE TRANSFORMER
These transformers adopt non-inflammable insulation materials of
Temperature class F/H, which are fire proof. These transformer are used at
location i.e Power houses, indoor installations, static excitation equipment
Generator rooms etc.
These transformers are manufactured in the range of 100 kVA to 600
kVA & voltage rating of 11or 33 kV primary & 415 V secondary voltage. These
transformers have no oil & they use air as the cooling media.
Photographs of cast resin dry type transformers are enclosed (P2-P5) for
reference.
5
CONCLUSION:
The risk of fire is almost inevitable for a product like transformer, which
is under high dielectric stress & under heavy current. Most of the power
transformers being oil filled are highly vulnerable to fire & for these
transformers various methods like HVW (High Velocity Water Spray System)
and Nitrogen injection Drain & Stir method are available, the later being a
modern & effective method. For critical applications, non-inflammable silicone
oil filled transformer and cast resin dry type transformer are also being used
where fire protection is of utmost importance.
The installation of transformer also plays important role i.e proper
terminations, adequate earthing, provision of oil sump & surge arrestors.
Periodic maintenance instructions are to be followed as per manufacturer’s
recommendations.
6.
ACKNOWLEDGEMENT:
The author would like to thank Management of BHEL Bhopal for
permitting the publication of this paper.
(PI) ESP TRANSFORMERS ON ROOF TOP THERMAL POWER STATION
(P2) CAST RESIN DRY TYPE TRANSFORMER
(P3) CAST RESIN DRY TYPE TRANSFORMER
(P4) CAST RESIN DRY TYPE TRANSFORMER
(P5) CAST RESIN DRY TYPE TRANSFORMER
10