NAME : Pukel Georg
COUNTRY : Austria
REGISTRATION NUMBER : 1179
GROUP REF. : A2
PREF. SUBJECT : 2
QUESTION N° : 2-3
Question 2-3: What other eco aspects should be considered e.g. low noise, integration, etc.?
What other design and/or construction techniques should be considered at the same time?
Eco Aspects - Tank Rupture Prevention & Low Noise (Ester Liquids)
G. J. Pukel, H. Pregartner, R. Labinsky, E. Taschler
Siemens AG Österreich Transformers Weiz
Elingasse 3, A-8160 Weiz, Austria
Tank Rupture Prevention
An internal arc causes numerous effects, even a tank rupture, which are researched, identified and dealt with. So
the probability of a tank rupture is reduced significantly, to minimize damages to environment, to reduce losses
for our customers and, finally, to enhance grid reliability.
From internal arc to tank rupture:
If an internal arc appears, which sometimes cannot be avoided, the released energy decomposes the ambient
insulation fluid, mineral oil for example. The duration of the arc can be neither known nor calculated. Now, the
internal pressure increases, due to the formation of a gas bubble. As a consequence, the raising pressure deforms
the transformer tank. If pressure still increases, the tank breaks - what is known as a “tank rupture”. The leaking
hot insulation fluid in combination with atmospheric oxygen ignites, fire starts.
Tanks are not pressure vessels. They are designed to withstand full vacuum and a
pressure 25% above the normal operating pressure. Most rectangular tanks will
sustain 1.4 to 2.10 bar before distortion or rupture. A transformer tank has to
resist following kinds of loads: acceleration forces of the active part at
transportation and lifting, hydrostatic pressure, vacuum pressure and in case of a
fault - the arc pressure.
How to ensure safety from tank rupture:
Transformers have been saved from rupture by common protection devices like real time gas monitors, gas
detector relays, rapid pressure rise relays and pressure relief devices. But tank rupture prevention is the most
effective way to reduce damage costs. Following steps describe the calculation procedure:
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In the calculation the internal pressure is increased in steps and the volumetric
flexibility of the tank is obtained by static FEM.
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The 3D-CAD-model of the transformer tank was imported to ANSYSSpaceclaim, to create a mid surface-model (from shell-model, less knots to solid
model, which means faster computation), as shown on the right: a half model with
106000 knots.
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Further the arc pressure is calculated for each step with this formula.
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This procedure proceeds until arc pressure is equal to applied pressure. Finally the
stresses in the tank are calculated. The model is changed until the calculated
stresses are lower than the mechanical strength of the steel used.
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Additionally the tank is designed in a way that the weakest part is near the top. So
in the event of leakage, the amount of insulation fluid loss is minor.
Design Adjustments:
Aligned tank and cover stiffeners, which are extremely beneficial.
Symmetry of the tank, which homogenizes stress distribution effectively.
Controlled deformation of the tank. The choice of material is significant.
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Tank rupture improved tank design together with high flash point liquids like esters will increase the safety
aspect and so the eco aspect as well.
Low Noise
Transformers are now, more than ever, located where acoustic environmental protection is of crucial
significance. Limited space in cities and strict noise performance requirements create extra efforts in
building power transformers.
The load noise emitted by energized transformers consists of noload noise, load current induced noise and cooling equipment
noise. The reduction of core vibration, a classic noise source, is a
major task in low noise transformer design. DC - magnetization
must be considered to avoid amplification of core vibration.
The biggest challenge was the development of a calculation
method for load current noise as the understanding of load current
effects is the basis for noise cancellation activities. Calculation
models relating to magnetic forces, magnetostriction and dynamic
mechanical models, in particular, brought the break through innovation and desired noise reduction results.
Standard transformer design vs. low noise transformer design/420 MVA/sound pressure
condition
100% Urated
no-load
100% Irated
load current
fans, pumps
cooler
TOTAL
Note:
standard design
dB(A)
65.5
70.3
74.5
76.3
low noise design
dB(A)
46.5
51.1
39.2
52.4
noise reduction
dB(A)
- 19.0
- 19.2
- 35.3
- 23.9
420 MVA autotransformer,
emits only 52.4 dB(A)
For the human ear 56 dB(A) sound pressure level is comparable with a domestic refrigerator.
Sound intensity measurement method is 2 - 4 dB(A) below sound pressure method.
Low noise and ester liquids
Significant differences in the dielectric and thermal behavior between mineral oil and environmentally
friendly ester liquids lead to specific designs for ester filled transformers.
Even for these designs our noise calculation programs deliver low noise optimized designs.