kV Power Transformer Research Articles

Mitigation of magnetic inrush current during controlled energization of coupled un-loaded power transformers in presence of residual flux without load side voltage measurements

This paper presents a novel approach for minimizing magnetic inrush current to a level as low as no load current during controlled energization of coupled un-loaded power transformers in presence of residual fluxes without employing load side voltage measurements. The suggested method has been evaluated for power transformers having various winding connections, design configurations (magnetically dependent and independent type), its side of energization (Δ or Y) and different level & polarities of residual fluxes. The effect of breaker performance in terms of variation in dielectric properties and operating time of circuit breakers has also been explored. The method is applied successfully on various 765/400/33 kV power (auto) transformers with delta connected tertiary winding on an existing Indian power transmission network and results, for a couple of cases, are presented. Based on simulation results, it has been observed that the inrush current for a number of transformers has been managed below 5% of its full load current rating. This validates applicability of the proposed technique in real system applications.

Measurement and analysis of transient overvoltage distribution in the low-voltage winding of 1000 kK power transformer without oil immersion

High frequency voltage oscillations will be generated when transient overvoltages intrude into power transformers. These oscillations may cause damage to the insulation of transformers. In this paper, a capacitive coupling sensor array was presented for measuring voltage distributions and voltage gradient distributions in low-voltage winding of a 1000 kV power transformer. A variable impedance transmission line between impulse generator and winding was also designed to avoid distortion of the applied impulses. The results indicate that under the lightning impulse voltages, voltage distributions have been restrained below applied voltage at sandwich-interleaved section but exceed the applied voltage in continuous coils in the first half of the winding and lead to a very nonuniform voltage distribution. Longer duration time will result in a more nonuniform distribution. The amplitude of the transient voltage under steep front square wave reduces faster downward the winding than that under lightning impulse voltages. A shorter rise time will lead to a much worse distribution. In addition, a dominated oscillation of 63 kHz is found under both impulse voltages and steep front square waves.

Electrical Equipment Life Cycle Monitoring

This article describes the problems associated with the electrical equipment condition control and life cycle monitoring at electrical substations. This article contains a basic approach of electrical equipment life cycle assessment derived from the actual equipment condition analysis. A graphical interpretation of the 110 kV power transformer life cycle based on the statistical analysis is also figured out.

Development of 110-220 kV Power Transformer Model for Equipment Functional State Assessment System

This paper addresses the problems, connected with implementation of 110-220 kV power transformer structural model for automated equipment functional state assessment system based on test and technical diagnostics data. This article describes the basic construction principles of hybrid neural network using Takagi-Sugeno fuzzy method. The paper also provides the statistical data analysis results for power transformers (of real energy grid part) to define fuzzy neural network criteria (layers).

Partial discharge detection by RF coil in 161 kV power transformer

The partial discharge (PD) sensor based on the copper coil will be used to on-line detect the radio frequency (RF) signal emitted from PD in 161 kV power transformer. The coil sensor is attached onto the surface of the power transformers to increase the sensitivity of PD signal detection. In the oil-filled power transformer, according to the electromagnetic (EM) wave propagation theory, the PD-induced EM wave could propagate to the metal tank, and most of the electric field attenuates within the short range (i.e. skin depth) of the inner surface of metal tank. However, the magnetic field of the EM wave could pass through the metal tank. Thus, the PD signal can be detected with RF coil sensor placed the outside surface of metal tank. Three RF sensor arrays with line configuration are designed to achieve PD detection and localization. From the amplitude variation among the sensors, we can predict where PD occurs. In the open-tank examination, the results show that the PD source location in power transformer is consistent with the experimental results of proposed RF detection method. Therefore, in this paper, the proposed RF detection method provides a simple and reliable early-stage on-line examination for the power apparatuses.

765 kV power transformer losses upon energizations: A comparison between field test measurements and EMTP-RV simulations

Transformer energization tests were performed in Hydro-Québec Chateauguay substation on a 300 MVA single-phase 765/125/12.5kV Yg-Yg-Δ power transformer in order to improve power transformer modeling for harmonic over-voltage evaluations upon energizations and fault removals close to HVDC converters with associated filters and capacitor banks. From these results, an enhanced transformer model has been developed in EMTP-RV Software. Subsequent simulations show that accurate modeling of the power transformer losses does only have minor influences on harmonic over-voltages damping. It is shown however that the usage of the proposed model is essential to predict adequately the risks of ferroresonance and generated over-voltages in power systems.

Ageing assessment of transformer paper insulation through post mortem analysis

A post mortem analysis of scrapped transformers provides an opportunity to directly measure the properties of in-service aged paper insulation. Investigations on 10 scrapped 275 kV and 400 kV power transformers were carried out, where Tensile Index (TI) and Low Molecular Weight Acid (LMA) of the paper insulation sampled from various locations were measured. Multiple-layer TI profile demonstrated the effect of oxidized oil on paper ageing which was confirmed by laboratory ageing experiments. It is LMA rather than High Molecular Weight Acid (HMA) that accelerates the paper degradation. LMA acts as the paper ageing accelerator and also as the paper degradation by-product, supporting the view that self-accelerated hydrolysis process is the main paper ageing mechanism. Finally, a generic relationship between LMA in paper and TI of paper was revealed based on both in-service and laboratory ageing data, i.e. the lower the TI of paper, the higher value of LMA in paper is measured. It implies that LMA in oil can be a potential paper ageing indicator pro viding a link between LMA in oil and paper is found.

A new method for evaluation of transformer drying process using transfer function analysis and artificial neural network

Abstract Since a few years ago, there is an increasing interest for utilization of transfer functions (TF) as a reliable method for diagnosing of mechanical faults in transformer structure. However, this paper aims to develop the application of TF method in order to evaluate the drying quality of active part during the manufacturing process of transformer. To reach this goal, the required measurements are carried out on 50 MVA 132 KV/33 KV power transformer when active part is placed in the drying chamber. Two different features extracted from the measured TFs are then used as the inputs to artificial neural network (ANN) to give an estimate for required time in drying process. Results show that this new represented method could well forecast the required time. The results obtained from this method are valid for all the transformers which have the same design.

Testing and Diagnostics of Neutral Earthing Current on 500kV/150kV Inter Bus Transformer in Tasikmalaya EHV Main Sub Station

Abstract The burning of phase-R, 166 MVA, 500 kV/150 kV inter bus Power Transformer of Tasikmalaya EHV main substation on 28th January 2009, has been disturbing the JAMALI (JAVA-MADURA-BALI) interconnection system. The investigation on root cause of the trouble is still under studied. After replacement of the inter bus power transformer, the existing IBT transformer still show some abnormalities on the neutral earthing current. Neutral earthing current found relatively higher than other IBT transformer even though the load level only minimal. Within this paper, measurement and simulation and analysis will be done to verify the cause of abnormality neutral earthing current on this Ynyd configured IBT 500/150 kV power transformer. The measurement on neutral earthing current were conducted by true rms current logging for one week on three area of similar configuration of EHV main substation including the Tasikmalaya EHV main substation to compare with other normal conditions. Measurement with true rms current meter, will investigate the effectiveness of delta tertiary winding for reducing zero sequence current on IBT transformer and also if there is dc component on neutral current due to geomagnetic field interference. Analysis and simulation was also conducted to verify the measurement result. The analysis and simulation show that if it is in the normal condition whether distorted, balanced or unbalanced the total resultant neutral earthing current should be zero when there is no insulation leakage to ground.

Economic Benefit Analysis of 220 kV Energy-saving Power Transformer

Power transformer serves as one of the most widely used electrical equipments in power grid. During the operation, terrible losses are produced. With the development of loss reduction technology of power transformers, in order to save energy saving and reduce emissions, the old power transformer should be replaced. The paper summarizes the main method to reduce the losses of power transformers and brings up the improved Total Owning Cost (TOC) algorithm, which applies to 220 kV power transformers’ comprehensive benefit analysis. Using the improved Total Owning Cost (TOC) algorithm, based on today 220 kV energy-saving power transformer manufacturing level, the economic benefits of new energy-saving power transformer and the return period of investment are analyzed. Finally, combined with energy-saving effect, the appropriate replacement proposal of 220 kV power transformers has been given.

Reliability analysis of 220 kV power transformers in Egypt

In this paper, outage data analysis of 220 kV power transformers is considered. Outage-data are obtained from the Egyptian Electricity Transmission Company. Outage-data analysis over 8 years from 2002 to 2009 of 252 transformers with MVA rating ranging from 40 MVA to 150 MVA is presented. Several indices are determined for evaluating the reliability of power transformers. The knowledge of these indices is essential for proper system planning and operation. Forced outages due to correct and false action of transformer’s protection systems are carefully considered. The trend of the determined hazard function indicated that a significant number of the transformers is probably being operated in the wear-out phase; however, detailed aging analysis is required to determine the reliability state of each transformer. The determined values for the maintainability and availability show significant variations with time. The results are compared with the IEEE 1979 survey.

Electrostatic charging tendency and correlation analysis of mineral insulation oils under thermal aging

In order to investigate the electrostatic charging tendency (ECT) of transformer oil under thermal aging and understand its correlations to the other insulation parameters, a series of accelerated aging tests were carried out on two types of mineral insulation oils, A and B, both of which were mainly used in 500 kV power transformers in China. Oil samples were pre-processed by degassing and dehydrating, and then aged at 120°C for various periods up till to 675 h, meanwhile, different contents of oxygen and copper/pressboard/silicon steel sheet were also added into the oil samples for sensitive study. The results indicate that the ECT of oil A increases notably with aging time, the ECT of oil B however does not change so much during the whole aging period. Injection of oxygen in the samples enhances the ECT of oil A, such a promotion effect is minor for oil B. The correlations between the ECT and other parameters, such as tanδ, volume resistivity (ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</sub> ) and acid number were analyzed as well. There are linear correlations between ECT and tanδ, acid number and log (ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</sub> ) of oil A, of which ECT and tanδ has the highest correlation coefficient of 0.881. Regarding oil B, there is only a positive linear correlation between ECT and acid number.

Multipoint Fiber-Optic Hot-Spot Sensing Network Integrated Into High Power Transformer for Continuous Monitoring

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A multipoint fiber-optic temperature sensor network integrated inside a power transformer for continuous monitoring of hot-spots on windings, cellulose insulations, and oil, is demonstrated and tested. The temperature sensors are based on proprietary encapsulated fiber Bragg grating (FBG) sensors and the optical interrogation unit uses a special designed narrowband high power broadband fiber source. The fiber-optic sensing network is integrated into a 20 MVA, 345/<formula formulatype="inline"><tex>$\surd 3$</tex></formula> kV <formula formulatype="inline"><tex>$-20$</tex></formula> kV power transformer (CORE type) having 12 temperature sensing points, distributed over several physical locations inside the transformer (windings, cellulose insulators, magnetic circuit, and cooling oil entrance and exit). </para>

Sensitivity studies on power transformer ferroresonance of a 400 kV double circuit

The ability to predict ferroresonance significantly relies on the accuracy of the transformer model and the power system's parameters. The accomplishment of a suitable simulation model allows the sensitivity studies to be performed to determine the degree of influence of various components and parameters of the ferroresonance phenomenon such as line length, point-on-wave switching and transformer core loss. The modelling work carried out in ATP (commercially available software) on a 1000 MVA 400/275/13 kV power transformer model is described and the simulation with field ferroresonance test recordings is verified. The maps that define the boundaries between safe and ferroresonant (fundamental, subharmonic, chaotic) regions as a function of system parameters are created through the sensitivity studies performed.

History of Development and Review of Future for Power Transformer

The history of development for the power transformer is equal to the history of development for the alternating current transmission systems. The rated voltage and capacity of power transformer were increased depend on the great demand of electrical power. On the early stage of development for the power transformer, the main technology had been imported, but Japan has leaded the reduction of the partial discharge and the wise solution for the problem of the oil flow electrification with the development of 500kV power transformer. In the future, it will be important to make a solution for development of UHV transformer and environmental problem.

Seismic qualification of transformer high voltage bushings

The paper is born as a contribution to the IEC 36A Working Group 2 "Seismic qualification of bushings" in order to evaluate the coefficients K (amplification factor between the ground and the transformer bushing flange) and R (response factor of the bushing itself) as defined in the IEC 61463. These coefficients allow the definition of the seismic severity at the flange level to be used for static calculations and vibrational tests on high voltage bushings. For the evaluation of these coefficients, both experimental on-site dynamic tests and numerical finite element analyses have been performed for a 160 MVA, 230/135 kV power transformer. The natural frequencies and the seismic response of the whole transformer (tank, oil conservator and HV bushings), subjected to different earthquakes, have been calculated. A comparison between the calculated coefficients (K and R) and those established by the IEC 61463 is illustrated.

Development and practical operation of perfluorocarbon immersed 275 kV transformers with compressed SF/sub 6/ gas insulation

A perfluorocarbon (PFC) immersed 275 kV power transformer with compressed SF/sub 6/ gas insulation has been under development. This paper clarifies the AC partial discharge inception voltage and time characteristics of PFC immersed insulation and also clarifies that a prototype 275 kV 100 MVA three-phase transformer could be operated without any trouble during long-term overvoltage tests. This prototype proved that it had AC partial discharge inception strength of higher than 1.5 times the AC test voltage and lightning impulse breakdown strength of 1.5 times the test voltage. A 275 kV, 250 MVA three-phase power transformer was developed and operated at the outdoor substation of the Chubu Electric Power Co., Japan. This transformer has been successfully operated to date and a detailed internal inspection of the transformer was carried out after one year and 9 months of successful practical operation. No significant abnormal condition was recognized. >

Eddy current fields and overheating problems due to heavy current carrying conductors

Large complex problems, eddy current fields and local overheating due to heavy current carrying conductors (HCCC) including leads and connecting bus bars in a 360 MVA/500 kV power transformer, are analyzed by the boundary element method. To improve numerical accuracy, this paper chooses triangle polar coordinate mapping and addition-substraction techniques to evaluate singular integrals. The distributions of magnetic flux and eddy current densities in metallic parts of different materials are discussed. According to the criterion of overheating, permissible lead current and limiting distance between HCCC and metallic parts and arrangements of HCCC are suggested. >

Allowance for insulation ageing in the new concept of accelerated life tests of high-voltage power transformers

It is pointed out that the existing system of type and acceptance tests of high-voltage transformer insulation does not take into account insulation aging, which is particularly objectionable with respect to equipment with reduced insulation levels. The authors propose a novel concept of accelerated life tests based on integrated simulation of basic operating loads both periodic (surge) and long-term ones, by performing a long-term accelerated test simulating the working conditions, with exposure of the test object and/or its insulation to periodic operating surges (overvoltages and overcurrents). This test replaces a group of conventional individual acceptance tests and provides more ample and more precise information on the performance and dependability of the equipment. The test procedure was checked in a test of a small lot of 1600 kVA 35 kV power transformers. >

Computation of eddy current losses by heavy current leads and windings in large transformers using IEM coupled with improved R- Psi method

A method combining the improved R- Psi method (in which the electrical vector potential R only exists in the conducting region) and the IEM (integral equation method) is described which synthesizes the advantages of the two methods and reduces the CPU time and the storage space required to solve 3-D eddy-current problems in open regions. The IEM is used to compute the magnetic field and losses due to heavy-current leads and the improved R- Psi method is used to calculate the magnetic field and losses due to windings. The coupled method is used to calculate the three-dimensional sinusoidal eddy-current field related to the open region in a 360 MVA/500 kV power transformer. The eddy current and loss computation and the estimation of the local hot spots makes it possible to prevent a large transmitter from local overheating.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>