Reduction of Stray Losses in Flange–Bolt Regions of Large Power Transformer Tanks (original) (raw)
Related papers
International Conference on Electrical Machines (ICEM), 2012
In power transformers, the presence of stray currents in the structural elements near to the high current bushings can be considerable and this usually leads to hot spots. This work presents the analysis of the overheating of the screws that join the tank and the cover; these screws are near to the low voltage bushings of the transformer. Overheating results are analyzed and discussed for the case of a 420 MVA, 20/230 kV, OA/FOA transformer. The hot spots in the screws are discovered by thermal maps (thermography) that are obtained during the power transformer operation as part of the preventive maintenance program. This paper proposes the use of copper sill to ensure the connection of both the cover and the tank body because this solution significantly reduces the overheating of the screws. The proposed solution, which has been validated by measurements, significantly reduces the hot spots on the screws of power transformer tank.
Reduction of Stray Losses in Tertiary Voltage Bushings in Power Transformer Tanks
XVI IEEE Autumn Meeting of Power, Electronics and Computer Science ROPEC, 2014
This paper presents an analysis and computation of stray losses in the tank cover of a 75 MVA three-phase core-type transformer. Stray losses in the region surrounding high current bushings are estimated using 3D Finite Element (FE) simulations. In the considered region the stray losses are high and its reduction is important to avoid the presence of hot spots in the tank cover of power transformer. In this paper, an L-shape non-magnetic Stainless Steel Insert (SSI) is utilized to reduce the stray losses in the region of the Tertiary Voltage Bushings (TVBs) of the transformer. Stray losses in the tank cover are estimated for a level of overload of 30% considering two cases: 1) When there is no SSI and 2) When the SSI is considered. The reduction of stray losses in the tank cover of power transformers helps to avoid the presence of dangerous high temperature spots. Hot spots can degrade the transformer oil and they can produce a potential failure of the equipment during operation.
CBIP Publication
In large power transformers, extreme temperature rise can occur as a consequence of stray fields from heavy current carrying conductors (HCCC) and from windings so it should be taken in to account and calculated vigilantly. The field pattern and eddy current losses due to current carrying strip bus bars are evaluated in [1] for an aluminum sheet. The field of HCCC with constant excitation shows marked differences with Stray fields from windings with a constant flux as computed by many researchers [2], [3].The field of HCCC is greatly influenced by magnetic resistivity of metallic parts. In addition, the characteristic feature of the losses due to HCCC is that they are distributed along the conductors, mainly in the tank cover and walls. The distributions of magnetic flux and eddy current densities in metallic parts of different materials are calculated and according to the criterion of overheating, permissible lead current and limiting distance between HCCC and metallic parts and arrangements of HCCC are suggested in [4]. In [5] New types of measures, separated magnetic steel plates welded on a nonmagnetic steel plate and closed loop shield, are presented to reduce the eddy current losses and to prevent the tank cover and walls from local overheating due to HCCC. In the presented work, analysis methodology is validated with a scaled down model of bar-plate. The eddy current loss is calculated using finite element analysis in mild steel plate with varying current in copper bar and varying distance between copper bar and mild steel plate and it has been validated experimentally. The same methodology has been used to calculate eddy current losses in transformer tank wall caused by high current carrying copper bars with different connections. In Part of the tank which faces maximum incident field, materials has been changed from mild steel to stainless steel and after effects have been analyzed. One quarter model of the tank has been considered which is in the vicinity of HCCC. As the magnetic fields and the loss distribution in tank and structures in the transformers are three-dimensional; 3D analysis has been done. Due to presence of non linear magnetic materials, the sinusoidal source with 50 Hz frequency induces non-sinusoidally varying magnetic fields. A transient solution (which calculates time varying magnetic field) is required for calculating fields in non-linear materials. However, this requires more computational resources. Therefore Time harmonic solution (which calculates field at 50 Hz frequency) with linear magnetic materials is used for this analyses.
New method of Calculation of Temperature Distributions on Transformer Tanks
A formula that calculates the distribution temperature in the transformer tank zones close to bushings is deduced. The new formula can use analytically or numerically obtained loss distributions. The comparison of the analytically calculated temperature distribution, using proven loss distributions, with finite element simulations, that use a thermal-electromagnetic coupled problem, shows that there is a very good match between numerical and analytical results. Apart, the new formula requires much lower computational resources as compared to finite element simulations that require commercial or highly specialized software. Our formula can provide to designers a powerful tool to improve efficiency and to increase useful life of transformers.
Hot Spots Mitigation on Tank Wall of a Power Transformer using Electromagnetic Shields
International Conference on Electrical Machines (ICEM), 2014
The presence of stray losses in the tank walls, near the low voltage cable leads, can be substantial. This usually leads to high temperatures in power transformers. This paper presents the experimental analysis of overheating in the low voltage tank wall of a power transformer, which is near to the low voltage bushings. The power transformer considered in this paper is rated as 44/58/73 MVA, 230/34.5 kV, ONAN/ONAF/ONAF. Hot spots in the tank were discovered by thermography during a laboratory temperature rise test under overload condition (82 MVA). Gases were also detected in the transformer oil during this test. The hot spots problem was solved using aluminum electromagnetic shields in the tank wall. Temperatures were measured before and after installing the aluminum shields in the tank wall of the power transformer. It was found that aluminum shields considerably reduce stray losses and overheating in the tank wall. As a result, the proposed solution in this paper, which has been validated by measurements, significantly reduces the risk of potential failures of power transformers during service.
Mathematical Calculation of Stray Losses in Transformer Tanks with a Stainless Steel Insert
Mathematics
At present it is claimed that all electrical energy systems operate with high values of efficiency and reliability. In electric power systems (EPS), electrical power and distribution transformers are responsible for transferring the electrical energy from power stations up to the load centers. Consequently, it is mandatory to design transformers that possess the highest efficiency and reliability possible. Considerable power losses and hotspots may exist in the bushing region of a transformer, where conductors pass through the tank. Most transformer tanks are made of low-carbon steel, for economical reasons, causing the induction of high eddy currents in the bushing regions. Using a non-magnetic insert in the transformer tank can reduce the eddy currents in the region and as a consequence avoid overheating. In this work, analytical formulations were developed to calculate the magnetic field distribution and the stray losses in the transformer region where bushings are mounted, consi...
New Analytical Formula for Temperature Assessment on Transformer Tanks
2015
A rigorous analytical development is presented to find a formula that provides the temperature distribution in the tank zones close to bushings of distribution transformers. The new formula can be fed with a loss distribution obtained either analytically or numerically. This fact is shown using two proven loss distributions, combined with our new formula, and comparing their results with finite element simulations that use a pre-established loss distribution in one case and solve a thermal-electromagnetic coupled problem in the second case. An excellent match between numerical and analytical results is found, which are independently determined using completely different computation philosophies. As a result, it is clearly shown that our proposed formula is effective and accurate. Moreover, it requires much lower computational resources as compared to finite element simulations that require commercial or highly specialized software. Our formula will contribute to the better design of transformers, increasing their useful lives and reducing operating costs in power networks.