By: Guillaume Perigaud, Sébastien Muller, Gaël de Bressy, Ryan Brady and Philippe Magnier, Transformer Protector Corp.
Power transformers are one of the most dangerous pieces of electrical equipment because of the large quantity of oil they contain, which is in direct contact with high voltage elements. Under such circumstances, low impedance faults that result in arcing can appear in transformer tanks once the oil loses its dielectric properties. Oil is then vaporized and the generated gas is pressurized because the liquid inertia prevents its expansion. The pressure difference between the gas bubbles and the surrounding liquid oil generates a dynamic pressure peak, which propagates and interacts with the tank and builds up static pressure. Then, the static pressure rises, leading to tank explosion and possible fire.
To avoid such damages, the transformer explosion and fire prevention strategy described in this article, called the transformer protection (TP), is based on the direct mechanical response of a depressurization set (DS) to the tank inner dynamic pressure peak due to an electrical fault. Since transformers always rupture because of the static pressure at their weakest point, the DS is designed to be this weakest point in term of inertia to break with the dynamic pressure peak before the tank explodes. Thus, during a transformer short circuit, the TP is activated within milliseconds by the first dynamic pressure peak of the shock wave generated by the electrical fault and before static pressure increases. It depressurizes the tank by expelling the oil and gas mixture.
This fast direct-tank depressurization method has been experimentally studied with arcing tests in industrial-size oil-immersed transformers. Physical modeling and numerical tools, validated on collected experimental data, have been developed to test TP reliability in various operation conditions. Here, we present a brief description of the protection, the experimental campaign carried out on arcing in large transformers and the theoretical and numerical developments, which prove the reliability of the whole prevention strategy.
The Depressurization Strategy
Mitigating the effects of a transformer explosion consists of either limiting the explosion consequences by extinguishing the subsequent fire, usually with fire walls or sprinklers, or preventing tank explosion by using mechanical technologies that absorb the high overpressures generated by the electrical arc, thus preventing the tank rupture and the subsequent fire.
Indeed, the electrical fault generates a dynamic pressure peak, which travels at the speed of the sound inside the transformer oil, 1,200 meters per second. This dynamic pressure peak burst a rupture disc located in the DS (Figure 1, #1). Oil and gas are then expelled out of the transformer tank through the DS (#2) to an oil gas separation tank (#3). The explosive gases are then channeled away to a remote and safe area. Nitrogen is then injected (#4) to make the whole transformer safe, cool and ready for repairs. Note that additional DS can be placed in order to protect the OLTC or the OCB ( #5).
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The Experimental Campaigns
Two test campaigns were performed, both under worst-case conditions, by creating low impedance faults leading to electrical arcs inside the transformer tank dielectric oil. In 2002, Electricité de France performed 28 tests. In 2004, a second campaign was carried out by CEPEL, the Brazilian independent high voltage laboratory.
Thirty-four tests were conducted in transformer tank dielectric oil by CEPEL in 2004 to study the vaporization process induced by an electrical arc and the resulting pressure wave propagation and to evaluate a fast-direct-tank-depressurization-based method to prevent transformer tank explosions.
Three standard transformers were used for the CEPEL tests. The maximum distance between an electrical arc and the protection system was 8.5 meters. Various physical parameters such as pressure, gas temperature, applied current, arc voltage and tank acceleration were studied in detail.
Each transformer was equipped with:
- Arcing generation devices
- Temperature probes
- Accelerometers and
- Pressure sensors at different locations in the transformer, to study the pressure wave propagation.
The transformer protection was also installed on each transformer to study the ability to mitigate tank rupture by a tank depressurization method based on the fast and direct passive mechanical response of a depressurization set to the pressure wave.
Experiments and Results Analysis
To study the pressure wave propagation influence in detail, the electrical arcs were ignited at three different locations (Figure 2): on the top cover close to the decompression set location (position A), on the top cover opposite the depressurization set location (position B), and in the lower part of the tank opposite the depressurization set location (position C). Position D is the location where the depressurization set was installed.
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Most of the tests were carried out with electrical arcs with currents ranging from 5 to 15 kA and fed during 83 milliseconds. This duration corresponds to the average response time of an old circuit breaker and was chosen to maximize the generated gas volume.
Generated Gas: During the CEPEL test campaign, the electrical arc produced from 1 to 2.3 m3 of gas. For the tested energy range, the gas volume generated during an electrical arc is a logarithmic type function of the arc energy, which seems in accordance with the vaporization process and especially with the saturation of the vaporization for high energy arcs: the arc remains in the generated gas volume using its energy to crack the oil vapour rather than continuing directly vaporizing the oil, which results in a smoother vaporization process.
Pressure profile evolution at a single location: The pressure at a specific location in the transformer after an electrical arc has occurred is transient as shown in Figure 3 (page 174), where an experimental curve of the pressure evolution close to the arc location after the arc ignition is displayed.
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After the arc ignition the pressure locally rises and reaches a maximum level; the waves, generated by the arc, propagate at a finite speed through the transformer and burst the rupture disc with a pressure gradient of 3,900 bar/s (56,000 psi/s). Three milliseconds after the rupture disc burst, the pressure is back to the activation level. Some secondary peaks, much lower than the first pressure maximum, can be observed; they are due to wave reflections off the tank walls and reflected waves interactions.
As soon as the TP has activated, the arc can be fed for a much longer period than the standard opening time of a circuit breaker, and the pressure remains at harmless levels for the transformer tanks.
Pressure Wave Propagation: In Figure 3, three experimental pressure profiles are displayed. Each curve shows what happens near each sensor located in positions A, B and C as shown in Figure 2.
The arc is generated in C and the shock wave propagation can be followed step by step because of the pressure peak’s displacement from C to A. The other pressure peaks (smaller than the main peak) are due to wave reflections off the walls. The pressure does not rise spatially uniformly in the tank. The experiments show the pressure waves propagated in the oil at a finite speed.
Pressure Peaks: Only one main pressure peak has been noticed for each test. The pressure profiles show variations after that main peak but their magnitude remains low compared to the first pressure peak level. They are due to waves reflections.
The pressure peaks’ amplitude is determined by the created arc. This peak ranges from +1.5 to +13 bar (+21.75 to +188.55 psi) for arc energies from 0.01 MJ to more than 2.4 MJ.
The pressure peak’s values do not strongly depend on the arc energy since when comparing tests for which pressure peaks respectively equal +8 bar (+116 psi) and +8.8 bar (127 psi), the maximum pressure only varies in 0.8 bar (11.6 psi) while the corresponding arc energies vary within on order 10 of magnitude (0.1 MJ and 1 MJ respectively).
Tank Ability to Withstand Static Pressure: To check the mechanical properties of the transformers, static tests were performed before applying any low impedance fault. The withstand limit was found to be +0.7 bar (+10.15 psi) for the biggest transformer, T3. This limit (+0.7bar,+10.15 psi) was used in the analysis as a threshold for tank depressurization during the dynamic tests.
Tank Ability to Withstand Dynamic Pressure: Even if the local pressure measured during the dynamic tests is on average six or 10 times higher than the static withstand limit, no tank damage and no tank permanent deformation occurs because the pressure peaks are very short. In fact, the structure can locally withstand high dynamic pressure increases because of the walls’ elasticity and the prevention method small inertia to operate.
Operation of the Transformer Protection: On average, the TP has activated after about 20 milliseconds (minimum: 4.64ms, maximum: 45.7ms) after the arc was ignited. Because the pressure wave propagation speed is finite, the maximum distance between the arc location and the TP is the parameter that matters the most for the activation. In the worst situation, the arc occurs in the transformer lower part opposite the depressurization set (location C).
The depressurization time is the time between the TP opening and when the pressure is definitely under the level of +0.7 bar ( +10.15 psi). On average, the TP depressurizes the tank in 116 ms, with a minimum value of 19.7 ms, and a maximum of 347 ms. This experimentally proves the TP’s ability to depressurize the transformer tanks within milliseconds and prevent the explosion.
The previous experimental data are also used in the numerical tool validation, discussed in the following sections.
The set of equations used to theoretically and numerically describe the phenomena is a model for 3D compressible two-phase flows that is based on a set of partial differential equations (PDE), which govern the hydrodynamic behavior of mixtures. One of the major and most interesting of the model’s characteristics is its ability to accurately depict the pressure wave propagation inside liquids and gases. Physical effects such as gravity, viscosity and heat transfers are added in the modeling to be as close as possible to reality.
A finite volume method is thus adopted to numerically solve the PDE’s system. It allows for describing precisely complex geometries such as transformer tanks. Simulations manage to give results in accordance with the experimental results for a relatively low cost and without any danger. They were used here to compute the consequences of an electrical arc appearing in an unprotected tank and also to simulate the protection operation on a large transformer.
Experimental testing would be dangerous if the transformer was 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 show 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 to the static overpressure withstand limit. The transformer would have exploded as soon as the tank wall elasticity limits were passed, that is, as soon as the tank walls could not store any more mechanical energy due to the pressure increase.
Large Transformer Explosion Prevention
Consider a 200 MVA transformer (5.75 meters long, 3.25 meters high and 2.5 meters wide): An electrical arc (11.5 MJ-arc generating about 3.4 m3 of gas) ignites near a winding at the opposite side of the TP, generating an 11 bar abs (160 psi) gas bubble.
When the transformer is equipped with a horizontal TP, the pressure wave propagates throughout the transformer, reflecting and otherwise interacting with the tank structure (Figure 4b). Within 3 milliseconds, a large pressure peak reaches the entry of the first bushing, as shown in Figure 4b. This first dynamic pressure peak’s magnitude is similar to those measured during the CEPEL test campaign. Furthermore, the pressure wave triggers the depressurization set (DS) activation within about 10 milliseconds after the gas bubble creation. The operation of the DS induces the rapid evacuation of fluid from the transformer tank, which thus generates rarefaction waves spreading throughout the transformer. After 60 milliseconds, the pressures throughout the transformer stabilize well below dangerous levels, as shown in Figure 4b.
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When the tank is not equipped with any protection system, and if it is subjected to a similar low impedance fault, the tank is exposed to very dangerous pressure levels. For instance, 30 ms after the arc occurrence, the pressure in a bushing reaches more than 10 bars abs (145 psi) as shown in Figure 4a. Moreover, without the tank protection, the static pressure stabilizes around 6 bars abs (87 psi) and the transformer would violently explode, as transformer tanks are designed to withstand static pressure up to about 2.2 bars abs. A technology based on a fast fluid evacuation thus has a very positive effect on the tank protection. Indeed, the simulations showed that the TP fast opening prevents the occurrence of violent pressure waves in the bushings, and the TP depressurizes the whole tank within milliseconds, thus avoiding the high static pressure build up that the tank could not withstand.
Authors: Guillaume Perigaud is research department manager, Transformer Protector Corporation (TPC). He holds a Ph.D. in mechanics and heat transfers from the Université de Marseille I, France. Sébastien Muller is a TPC researcher. He holds a Ph.D. in fluid mechanics, Université d’Orléans, France. Gaël de Bressy is a TPC researcher. He holds a diploma of mechanical engineering, IFMA, Clermont Ferrand, France. Ryan Brady is a TPC researcher. He holds a Master of Science in physics, UNT, Denton, Texas. Philippe Magnier is TPC chairman. He holds a doctorate in Nuclear Physics from Université Paris Orsay.