Operation Of Power Transformers Research Articles

Parallel operation of three-phase power transformers with different short-circuit voltages

Parallel operation of three-phase power transformers with different short-circuit voltages has been studied using a lab bench. The circuit diagram of the bench and experimentally measured vector diagrams of the currents in primary and secondary windings of the transformers have been presented. It has been found that, when the difference of short-circuit voltages of parallel operating transformers is more than 10%, both an arithmetic difference of currents and a great phase shift of secondary-phase voltages occur. As a result, one of the transformers becomes overloaded, while the other is underloaded. It has experimentally been found that the load current is determined by the geometrical sum of currents of secondary windings in transformers. It has been shown that one of the studied transformers works in the conversion mode and its currents are shifted by 180° with respect to the phase, which is the main reason for failure of the normal operational conditions of parallel transformers.

Measuring the Pressboard Water Content of Transformers Using Cellulose Isotherms and the Frequency Components of Water Migration

The measurement of water within power transformer insulation is necessary for the unit's reliable operation. Probes can be inserted into the oil of a transformer to measure water activity, and then this water activity is used to determine the water content of transformer cellulosic insulation. The migration of water between the insulation media is driven by differences in water vapor pressure around the insulation system. Over time, this water reaches an equilibrium partition when cellulosic adsorption isotherms are used to calculate the concentration of water absorbed by the cellulose. However, the insulation of a normally operating transformer is highly unlikely to reach this state, as the migration of water between the insulation media is much slower than the rate of change of temperature caused by the varying load of the transformer. This leads to inaccuracies in analyzing water vapor pressure, which is a function of both oil water content and temperature. The frequency spectrum of the oil's water activity data was used to determine whether very slow changes in dissolved water, attributed to long diffusion time constants, could be separated by a filter from fast changes caused by temperature, leading to a better measurement technique. This filter was then used on data from three operating power transformers, which had their insulation water content measured using dielectric response. The results are discussed in this paper.

Condition Assessment of Paper Insulation in Oil-Immersed Power Transformers Based on the Iterative Inversion of Resistivity

The resistivity of oil impregnated paper will decrease during its aging process. This paper takes paper resistivity as an assessment index to evaluate the insulation condition of oil impregnated paper in power transformer. The feasibility of this method are discussed in two aspects: reliability and sensitivity. Iterative inversion of paper resistivity was combined with finite element simulation. Both the bisection method and Newton’s method were used as iterative methods. After the analysis and comparison, Newton’s method was selected as the first option of paper resistivity iteration for its faster convergence. In order to consider the spatial distribution characteristic of paper aging and enhance the calculation accuracy, the resistivity calculation is expanded to a multivariate iteration based on Newton’s method, in order to consider the spatial distribution characteristic of paper aging and improve the calculation accuracy. This paper presents an exploratory research on condition assessment of oil impregnated paper insulation, and provides some reference to the security and economy operation of power transformers.

Influence of thermal aging on the breakdown characteristics of transformer oil impregnated paper

For a long time, thermal effect, as a major factor for insulation aging, is a great challenge to the safe operation of power transformers. During the decades-long operation, cellulose fibers contained in the oil-paper insulation, break and deform, accompanied by the generation of new chemical products and variation of mechanical/electrical properties. Therefore, in this work, the influence of thermal aging on the electrical performance of the transformer oil paper insulation was investigated. 50% breakdown voltage and average AC breakdown voltage of oil impregnated paper were experimentally determined, and the water content measurement, surface morphology observation and frequency domain spectroscopy measurement were carried out on the oil impregnated paper samples with different aging degrees. The test results indicate that the electrical performances of aged oil impregnated paper improved during the aging process, which could be attributed to the forming of micro globules caused by the effect of thermal aging.

Health indexes for power transformers: a case study

Proper operation of power transformers is critical to ensuring transmission and distribution of electrical power. Most transformers have an electrical insulation system based on oil and paper. The state of the insulation system is the major factor influencing the state of the transformer. During service the dielectric materials within the transformer deteriorate, and small concentrations of impurities such as water, carbon monoxide, carbon dioxide, and furan compounds accumulate in the oil. Since it is easy to obtain oil samples from power transformers, the information most commonly collected by transformer fleet managers relates to the physical and dielectric properties of the oil. These properties include dielectric strength, dissipation factor, color, interfacial tension, and concentrations of dissolved gases, furans, acids, and moisture. Using these properties it is possible to determine whether a transformer has developed certain specific faults, e.g., partial discharges, arcing, sparking, overheating, etc. On the other hand, various health indexes have been proposed to characterize the general condition of a transformer [1]-[3]. The factors taken into account in these indexes vary, and are given different statistical weightings depending on their influence on the general condition of the transformer. In this article we evaluate the condition of a fleet of operating power transformers, using two recently proposed health indexes, and compare the results.

From the editors' desk

The first article in this issue is "Health indexes for power transformers: A case study" by Felix Ortiz and collaborators from the University of Cantabria, Santander, Spain. Using the results of measurements on oil samples taken from a transformer, e.g., dielectric strength, dissipation factor, color, interfacial tension, and concentrations of dissolved gases, furans, acids, and moisture, it is possible to determine whether a transformer has developed certain specific faults, e.g., partial discharges, arcing, sparking, overheating, etc. On the other hand, various health indexes have been proposed to characterize the general condition of a transformer. The factors taken into account vary between indexes, and are given different statistical weightings depending on their influence on the general condition of the transformer. In this article the authors present the results of a study of the condition of a fleet of 52 operating power transformers, using two recently proposed health indexes.

Power transformer protection using chirplet transform

This study presents a novel differential protection algorithm (DPA) for power transformer using chirplet transform (ChT). The proposed method combines the features of biased restraint characteristic (BRC) of the conventional differential relay and out-turn of ChT in a two-stage algorithm. In the first stage, the BRC plane is divided into three zones: namely, high-set (HS), non-trip and vulnerable zones. The tripping decisions are carried out in the first two zones based on differential and biased current. However, if the operating condition of the power transformer falls in the vulnerable zone, then there is an ambiguity in discriminating internal fault, inrush current and current transformer saturation cases. Therefore, in the second stage, ChT is applied to differential current signal to obtain an energy distribution on the time-frequency plane with respect to time, frequency and chirp rate. Then, using the mean and standard deviation of the normalised energy, power transformer operating conditions are classified. Also, most of the DPAs available in the literature are system dependent. However, the proposed novel DPA can be effectively used for any system. The proposed scheme is validated for two power transformer systems using PSCAD to simulate various operating conditions and MATLAB to implement the algorithm.

DYNAMICS MODEL OF MOISTURE IN PAPER INSULATION-TRANSFORMER OIL SYSTEM IN NON-STATIONARY THERMAL MODES OF THE POWER TRANSFORMER

Introduction. An important problem in power transformers resource prognosis is the formation of moisture dynamics trends of transformer insulation. Purpose. Increasing the accuracy of power transformer insulation resource assessment based on accounting of moisture dynamics in interrelation with temperature dynamics. Working out of moisture dynamics model in paper insulation-transformer oil system in conjunction with thermodynamic model, load model and technical maintenance model. Methodology. The mathematical models used for describe the moisture dynamics are grounded on nonlinear differential equations. Interrelation moisture dynamics model with thermodynamic, load and technical maintenance models described by UML model. For confirming the adequacy of model used computer simulation. Results. We have implemented the model of moisture dynamics in power transformers insulation in interrelation with other models, which describe the state of power transformer in operation. The proposed model allows us to form detailed trends of moisture dynamics in power transformers insulation basing on monitoring data or power transformers operational factors simulation results. We have performed computer simulation of moisture exchange processes and calculation of transformer insulation resource for different moisture trends. Originality. The offered model takes into account moisture dynamics in power transformers insulation under the influence of changes of the power transformers thermal mode and operational factors. Practical value. The offered model can be used in power transformers monitoring systems for automation of resource assessment of oil-immersed power transformers paper insulation at different phase of lifecycle. Model also can be used for assessment of projected economic efficiency of power transformers exploitation in projected operating conditions.

Power transformer diagnosis based on mechanical oscillations due to AC and DC currents

The reliable operation of power transformers is an essential factor for the high availability of electric energy. Reliability demands can be enforced using condition based asset management which relies on different diagnosis measurements and continuous monitoring. This contribution determines the abilities of asset monitoring by regarding transformers' mechanical oscillations during operation. The physical origins of oscillation sources are introduced as well as the common measurement technique using accelerometers. The contribution evaluates the method's principle diagnostic abilities by correlating the actual mechanical status of an active part and resulting mechanical vibrations in a laboratory setup. The practical usability is determined by long-term comparison of measurements on different power transformers in service. Several influencing factors are evaluated: the sensor position, the changing transformer's load and operating temperature during service as well as the position of the on-load tap changer. Additionally, mechanical oscillations are used to determine the influence of superimposed, undesirable DC components on 3-limb power transformers originated by coupling between nearby AC and HVDC lines.

An oscillograms processing algorithm of a high power transformer on the basis of experimental data

The paper presents the studies on digital processing of oscillograms of the power transformer operation allowing determining the state of its windings of different types and degrees of damage. The study was carried out according to the authors' own methods using the Fourier analysis and the developed program based on the following application software packages: MathCAD and Lab View. The efficiency of the algorithm was demonstrated by the example of the waveform non-defective and defective transformers on the basis of the method of nanosecond pulses.

Power transformers’ condition monitoring using neural modeling and the local statistical approach to fault diagnosis

On-line monitoring of electric power transformers can provide a clear indication of their status and ageing behavior. This paper proposes neural modeling and the local statistical approach to fault diagnosis for the detection of incipient faults in power transformers. The method can detect transformer failures at their early stages and consequently can deter critical conditions for the power grid. A neural-fuzzy network is used to model the thermal condition of the power transformer in fault-free operation (the thermal condition is associated to a temperature variable known as hot-spot temperature). The output of the neural-fuzzy network is compared to measurements from the power transformer and the obtained residuals undergo statistical processing according to a fault detection and isolation algorithm. If a fault threshold (that is optimally defined according to detection theory) is exceeded, then deviation from normal operation can be detected at its early stages and an alarm can be launched. In several cases fault isolation can be also performed, i.e. the sources of fault in the power transformer model can be also identified. The performance of the proposed methodology is tested through simulation experiments.

LMI-Based Method for Estimating Passive Blackbox Models in Power Systems Transient Analysis

Power system transient analysis relies on the deployment of models with ever-increasing accuracy that represents the system's frequency-dependent components with respect to the phenomena being scrutinized. A key issue in this context is the construction of a model yielding passive properties for each system component. Such properties are commonly compromised by corrupting noise in measurements that serve as input to the computation of blackbox models. In addition, model passivity could be further compromised during parameter estimation. This paper undertakes the task of developing a proposal to build an optimization methodology based on linear matrix inequalities (LMI-based) which is intended for obtaining an optimal, passive, blackbox model for a $n$ -terminal power transformer. The resulting model can be further realized as a circuit representation in order to simulate the transformer dynamics in an Eletromagnetic Transient program (EMT type). The results derived from two operating power transformers are later provided to confirm how effective the procedure herein proposed is.

An Effective Detection of Inrush and Internal Faults in Power Transformers Using Bacterial Foraging Optimization Technique

Power transformers in transmission network are utilized for increasing or decreasing the voltage level. Power Transformers fail to connect directly to the consumers that result in the less load fluctuations. Power transformer operation under any abnormal condition decreases the lifetime of the transformer. Power Transformer protection from inrush and internal fault is critical issue in power system because the obstacle lies in the precise and swift distinction between them. Due to the limitation of heterogeneous resources, occurrence of fault poses severe problem. Providing an efficient mechanism to differentiate between faults (i.e. inrush and internal) is the key for efficient information flow. In this paper, the task of detecting inrush and internal fault in power transformers is formulated as an optimization problem which is solved by using Hyperbolic S-Transform Bacterial Foraging Optimization (HS-TBFO) technique. The Gaussian Frequency- based Hyperbolic S-Transform detects the faults at much earlier stage and therefore minimizes the computation cost by applying Cosine Hyperbolic S-Transform. Next, the Bacterial Foraging Optimization (BFO) technique has been proposed and has demonstrated the capability of identifying the maximum number of faults covered with minimum test cases and therefore improving the fault detection efficiency in a wise manner. The HS-TBFO technique is evaluated and validated in various simulation test cases to detect inrush and internal fault in a significant manner. This HS-TBFO technique is investigated based on three phase power transformer embedded in a power system fed from both ends. Results have confirmed that the HS-TBFO technique is capable of categorizing the inrush and internal faults by identifying maximum number of faults with minimum computation cost as compared to the state-of-the-art works.

Maximum acceptable concentrations of dbds, sulphur mercaptan and optimal concentration of passivators for safe and prolonged operation of power transformers

The study presents the results of experimental investigation of finding out the maximum acceptable concentrations of DBDS, sulfur mercaptan, and optimal concentration of passivator for safe and prolonged operation of power transformers in the local environmental conditions of Saudi Arabia. For experimental investigation of optimal concentrations of DBDS and sulfur mercaptan, new oil free of sulfur compounds was acquired. Four specimens of this were spiked with different concentrations of DBDS and another four with free sulfur mercaptan RSH concentrations. The covered conductor deposition (CCD) tests were performed in accordance with IEC 62535 method on all the specimens. Next, to study the effect of passivator concentrations and temperatures on corrosive sulfur formation, 10 specimens from new oil were spiked with different concentrations of BTA and Irgamet 39. Copper strips were immersed in these specimens and the vials containing the oil and the copper strips were subjected to different temperature from 50 to 150°C for 24 hours in the oven. The experimental results indicated that the concentration of DBDS should always be <4 ppm and sulfur mercaptan < 1 ppm in the new oils before putting in to the service. Based on the effectiveness of the passivators, BTA was found to be effective with 50 ppm concentration at 150°C compared to Irgamet 39 which was effective only with 150 ppm concentration.

Consolidating Learning in Power Systems: Virtual Reality Applied to the Study of the Operation of Electric Power Transformers

Within the field of electric power systems, the study of electrical equipment can be frustrating and demotivating because of the lack of a clear vision of how this equipment functions and operates in a real environment. The use of virtual reality can provide a more concrete representation for students, who rarely have the opportunity to visit a power substation or generating plant to acquire an understanding of how the equipment operates when integrated into an actual system. This paper describes work undertaken to improve the classroom study of such equipment , and to motivate students, by providing a clear overview of the equipment, approximating theory and practical functions, and simulating real situations that would be encountered on the equipment in an actual system. A virtual reality authoring tool was developed to provide students with a visualization of an autotransformer installed in an electric power substation, and to allow them to perform virtual maintenance and operating procedures on it. The influence of the software on students' levels of motivation and their performance was evaluated using questionnaires and reports. Finally, a framework is proposed that can help others create virtual reality trainings.

Comparative Study of Two Thermal Aging Estimating Methods for Power Transformers

In this paper a comparison between two methods for the calculation of the hot spot temperature of a power transformer is presented. The first method is proposed by C57.91 IEEE Standard which is based on daily averages and exponential equations; on the other hand, the second method is based in differential equations and hourly measurements. To compare these methods, the ageing of a currently operating power transformer has been estimated. A comparison of the results of degradation using actual values and a hypothetical overload were performed. A brief description of each method, aging results, time calculation and analysis of the results is presented.

Method to evaluate the degradation condition of transformer insulating oil - establishment of the evaluation method and application to field transformer oil

To operate power transformers long-term, as well as ensuring their insulating reliability, it is important to study age-related decline in various insulating oil characteristics and the method used to evaluate the same adequately. Previous studies showed that age-related decline in insulating oil characteristics was caused by trace components in oil produced during oxidation degradation. To maintain and manage aged insulating oil rationally, a specific diagnostic method must be studied, based on measurement of the trace components in oil that cause degradation in these insulating oil characteristics. The present study evaluated the sensitivity to detect various components based on the amount of trace components produced in oil during oxidation degradation for field-aged insulating oil for 34 transformers in various degradation conditions. A study was also conducted to evaluate the degradation condition based on the ratio of trace components produced in oil. Consequently, the detection sensitivity levels were in the order of carbonyl value, saponification value, peroxide value, and total acid value. As the ratio of the saponification value – the so-called final product – increased, the degradation of insulating oil developed further. With the above study results, methods of evaluating the dissociation property in the preceding study and the breakdown voltage with the degree of water saturation taken into consideration were combined to establish a comprehensive method of evaluating aged oil. This was then applied to field-aged insulating oil as an example, whereupon the need to replace insulating oil could be evaluated.

Comparison of DOA Algorithms Applied to Ultrasonic Arrays for PD Location in Oil

To reduce the number of elements and the cost of follow-up circuit of a $16 \times 16$ ultrasonic array sensor, two ultrasonic array sensors are designed. One is L shaped with 13 elements and the other is cross shaped with 25 elements. Based on these two ultrasonic array sensors, this paper compares the directional performance of a two-sided correlation transformation algorithm, which is a broadband focusing algorithm, an array expanded algorithm based on fourth-order cumulants, and a combination of these two algorithms, namely, synthesis algorithm. The broadband focusing algorithm can adjust to the broadband ultrasonic signal emitted by partial discharge (PD) source. After virtual expansion, the L-shaped and cross-shaped ultrasonic arrays can achieve the effects of an array with 97 and 193 elements, respectively. The adaptive capacity of broadband ultrasonic signal emitted by the PD source, and the precision of direction to PD are compared for the three algorithms. Directing simulation and experiments with single or multiple PD sources are carried out. The results show that the directing precision is highest with the synthesis algorithm, the error is <2°, the peaks of spatial spectrum are sharp, and the best broadband adaptive capacity and noise suppression ability are achieved. This paper should provide a foundation for the application of array technology in enhancing the reliability of power transformer operation.

Development of a moisture-in-solid-insulation sensor for power transformers.

Moisture is an important variable that must be kept under control to guarantee a safe operation of power transformers. Because of the hydrophilic character of cellulose, water mainly remains in the solid insulation, while just a few parts per million are dissolved in oil. The distribution of moisture between paper and oil is not static, but varies depending on the insulation temperature, and thus, water migration processes take place continuously during transformers operation. In this work, a sensor is presented that allows the determination of the moisture content of the transformer solid insulation in the steady state and during the moisture migration processes. The main objective of the design is that the electrodes of the sensor should not obstruct the movement of water from the solid insulation to the oil, so the proposed prototype uses a metallic-mesh electrode to do the measurements. The measurement setup is based on the characterization of the insulation dielectric response by means of the frequency dielectric spectroscopy (FDS) method. The sensitivity of the proposed sensor has been tested on samples with a moisture content within 1% to 5%, demonstrating the good sensitivity and repeatability of the measurements.

A Load Test Research on Large Power Transformers

Due to the large capacity of large power transformers, normally the power capacity of a load test is difficult to meet the test requirement, which makes the ancillary devices such as induction regulator, intermediate transformer etc. difficult to meet the test demands too. The transformer factories with limited power capacity can only perform load test under less than fifty-percent of the rated load current, which not obeys the standards and technical protocols. Thus, a field load test method for large power transformers is proposed in this paper. It carries out the reactive power compensation with compensation capacitors, which reduces the capacity of test power supply and makes the field load test possible. This method also gives a guarantee for the safe and stable operation of power transformers.