Power Transformer System Research Articles

Identification of the nonlinear vibration system of power transformers

This paper focuses on the identification of the nonlinear vibration system of power transformers. A Hammerstein model is used to identify the system with electrical inputs and the vibration of the transformer tank as the output. The nonlinear property of the system is modelled using a Fourier neural network consisting of a nonlinear element and a linear dynamic block. The order and weights of the network are determined based on the Lipschitz criterion and the back-propagation algorithm. This system identification method is tested on several power transformers. Promising results for predicting the transformer vibration and extracting system parameters are presented and discussed.

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.

Effects of temperature and aging on furfural partitioning in the oil-paper system of power transformers

Furfural content in oil is a valuable indicator of cellulose paper degradation and is widely utilized in aging assessment for power transformers. However, with the same aging degree of insulation paper, the actual furfural content in oil is influenced by temperature because furfural partitioning behavior in the oil-paper insulation system of power transformers changes with temperature. Thus, correction of the temperature effect on furfural partitioning is crucial for accurate assessment of the aging of power transformers. In this research, oil-paper samples with different furfural contents were prepared through an accelerated thermal aging test. A furfural equilibrium experiment was conducted at different temperatures to construct a furfural partitioning diagram. The mass fraction of furfural in oil and paper was calculated. Results showed that the mass fraction of furfural in oil increases with temperature rising and aging deepening, which means that furfural tends to migrate from paper to oil at high temperatures and deep aging conditions. The change in furfural partitioning between oil and paper at different temperatures causes a deviation in paper aging assessment. To correct the temperature effect on furfural partitioning, a furfural correction factor was developed to normalize furfural contents in oil at different temperatures to a reference temperature. The furfural correction factor was verified by experimental data and then introduced into the relation between degree of polymerization of paper and furfural content of oil. Verification results showed that the corrected equation can enhance the accuracy of paper aging estimation at different temperatures. The findings of this work indicated temperature effect on furfural partitioning should be considered when determining the furfural thresholds and evaluating the aging condition of power transformers.

Diagnostic Measurements for Power Transformers

With the increasing age of the primary equipment of the electrical grids there exists also an increasing need to know its internal condition. For this purpose, off- and online diagnostic methods and systems for power transformers have been developed in recent years. Online monitoring is used continuously during operation and offers possibilities to record the relevant stresses which can affect the lifetime. The evaluation of these data offers the possibility of detecting oncoming faults early. In comparison to this, offline methods require disconnecting the transformer from the electrical grid and are used during planned inspections or when the transformer is already failure suspicious. This contribution presents the status and current trends of different diagnostic techniques of power transformers. It provides significant tutorial elements, backed up by case studies, results and some analysis. The broadness and improvements of the presented diagnostic techniques show that the power transformer is not anymore a black box that does not allow a view into its internal condition. Reliable and accurate condition assessment is possible leading to more efficient maintenance strategies.

Correlation between stabilizer consumption and degree of polymerization of thermally upgraded paper aged in insulating natural ester and insulating mineral oil

Insulating paper holds significant importance in the insulation system of power transformers, and thus, its degradation is the subject of many studies. A successful evaluation of the degradation rate of such paper contributes to reducing downtime and avoiding equipment failure. In this work, samples of thermally upgraded paper were thermally aged in insulating natural ester (INE) and insulating mineral oil (IMO) and were evaluated by degree of polymerization (DP) and FTIR-ATR. It was possible to identify characteristic bands of dicyandiamide, an inhibitory compound of the thermal degradation of the paper, and to establish a correlation between the decrease in DP and the consumption of dicyandiamide during aging, which was observed to develop in three distinct steps for both IMO and INE.

KTX산천 귀선전류고조파가 일반선 TI-21형 AF궤도회로에 미치는 영향분석

KTX산천 고속차량에서 사용하는 전력변환시스템은 전력전자기술, 고속 대용량 반도체소자 및 마이크로프로세서의 기술발달에 힘입어 높은 성능과 효율, 안전성, 에너지 소비측면에서 뛰어난 제어능력을 갖추게 되었다. 하지만 고속스위칭소자인 IGBT를 사용함에 따라 발생하는 고조파로 인해 전기차량은 물론 변전소, 신호시스템, 데이터전송 및 감시시스템 등 선로변 신호설비에 문제를 야기시켰다. 특히 전력변환장치의 정수배 부근 주파수에서 발생하는 귀선전류고조파의 영향으로 TI-21 무절연AF궤도회로가 부정 동작하는 사례가 발생되었다. 본 논문은 고속전기차량과 궤도회로간 귀선전류고조파 이동경로 분석을 위하여 실제 현장에서 측정 및 분석방법을 제시하였다. 이러한 분석결과는 향후 신규로 도입되는 고속전기차량, AF궤도회로, 공동접지망 설계 시 폭넓게 반영될 것으로 기대된다. The power transformation system of High Speed rolling stocks like KTX-Sancheon has shown excellent control capacities in the areas of riding comfortability, switching efficiency, safety and energy consumption due to technical developments in power-electronics, high speed & large scale integrated semiconductors and microprocessors. However, harmonics from IGBT, a high speed switching device used in the Convertor & Invertor equipment of rolling stocks have given rise to various problems in transformer substations, signaling systems, data transmission systems and facility monitoring systems. Especially, TI21 non-insulated track circuits have malfunctioned due to the influence of returning current harmonics which were generated at around of integer times of the number of power transformation equipment in the frequency domain. This paper, measures and analyzes various schemes to analyze the traveling path of the returning current harmonics generated due to the relationship between the rolling stocks and track circuits on site. Ultimately, theseschemes will be used to design high speed rolling stocks, AF track circuits and a common grounding network.

Moisture estimation in power transformer oil using acoustic signals and spectral kurtosis

The aim of this paper is to present a new technique for estimating the contamination by moisture in power transformer insulating oil based on the spectral kurtosis analysis of the acoustic signals of partial discharges (PDs). Basically, in this approach, the spectral kurtosis of the PD acoustic signal is calculated and the correlation between its maximum value and the moisture percentage is explored to find a function that calculates the moisture percentage. The function can be easily implemented in DSP, FPGA, or any other type of embedded system for online moisture monitoring. To evaluate the proposed approach, an experiment is assembled with a piezoelectric sensor attached to a tank, which is filled with insulating oil samples contaminated by different levels of moisture. A device generating electrical discharges is submerged into the oil to simulate the occurrence of PDs. Detected acoustic signals are processed using fast kurtogram algorithm to extract spectral kurtosis values. The obtained data are used to find the fitting function that relates the water contamination to the maximum value of the spectral kurtosis. Experimental results show that the proposed method is suitable for online monitoring system of power transformers.

Eco-efficient control of the cooling systems for power transformers

Operation of large transformers requires heat transfer by the cooling systems. An appropriate control of the cooling system by Programmable Logic Controllers allows one to decrease the “loss of life” of the insulation system of the transformer. Additional diagnostic functions aimed at improved asset management may also be easily implemented. Practical examples of eco-friendly solutions implemented at PPH Energo-Silesia Ltd. are presented and discussed in detail. Asset management of transformer system may include techniques aimed at heat recovery and noise reduction.

Power System Transformation toward Renewables: An Evaluation of Regulatory Approaches for Network Expansion

We analyze various regulatory regimes for electricity transmission investment in the context of a power system transformation toward renewable energy. Distinctive developments of the generation mix are studied, assuming that a shift toward renewables may have temporary or permanent impacts on network congestion. We specifically analyze the relative performance of a combined merchant-regulatory price-cap mechanism, a cost-based rule, and a non-regulated approach in dynamic generation settings. We find that incentive regulation may perform better than cost-based regulation but only when appropriate weights are used. While quasi-ideal weights generally restore the beneficial properties that incentive regulatory mechanisms are well-known for, pure Laspeyres weights may either lead to over-investment or delayed investments as compared to the welfare-optimum benchmark. Laspeyres-Paasche weights, in turn, seem appropriate under permanently or temporarily increased network congestion. Thus, our analysis provides motivation for further research in order to characterize optimal regulation for transmission expansion in the context of renewable integration.

Optimization of Distributive Ratios of Apportioned Winding Configuration in HTS Power Transformers for Hysteresis Loss and Leakage Flux Reduction

In recent years, using high-temperature superconductor (HTS) transformers in transportation and electrical power systems has attracted much interest. The major advantages of HTS transformers compared with conventional ones are much smaller size and volume and lighter weight in confined spaces of urban areas or transportation devices. One of the most critical parameters to design, fabricate, and commercialize HTS transformers is hysteresis loss in HTS tapes. Using numerical methods, the accurate value of hysteresis loss and efficiency can be calculated or estimated in HTS transformers. In this paper, two innovative techniques to reduce the leakage flux and thereby hysteresis loss in HTS transformers have been proposed. In the first technique, the concentric windings of HTS transformer are apportioned into a certain number of sub-windings. In the second one, with optimizing the distributive ratios of sub-windings, the windings become unsymmetrical. In order to demonstrate the effectiveness of proposed techniques, an accurate and advanced numerical method based on finite element analysis has been used to design four symmetrical and unsymmetrical primary (P)-secondary (S)-P-S-P and P-S-P-S-P-S-P winding schemes. The simulation results prove the effectiveness of both techniques in reducing the radial and axial leakage flux magnitudes and thereby hysteresis loss in HTS windings.

Power Flow Control of a Power Conversion Circuit for Contactless Power Transformer Systems with Coil Misalignment

SUMMARYThe coupling coefficient between the coils of a noncontact power supply system is reduced by coil misalignment. This reduces the output voltage and the active power in the coils. To cope with these problems, an instantaneous current control method is applied to a single‐phase contactless power transformer system in order to apply the equivalent impedance of the PWM rectifier at the load side of the transformer. We propose a control method to supply power constantly to the load even when there is a misalignment in the receiving coils. The limit of the transferred power associated with the proposed control method is investigated. The proposed system, including the control method, is verified by experimental tests over a power range of several tens of watts.

CFD Study on Thermal Performance of Radiators in a Power Transformer: Effect of Blowing Direction and Offset of Fans

A focus on the present numerical study is on the fan air flow-used in the oil natural air-forced cooling system of a 25-MVA power transformer. An experiment is done to benchmark the present simulation methodology for the fan. Thermal performance of a radiator assembly is studied in two parts: 1) effect of horizontal and vertical blowing direction of two different fans at two speeds and 2) the effect of offset of one of the fans blowing in the horizontal direction. Vertical as compared to horizontal blowing is less efficient in dissipating heat from the radiator assembly considered here, as more air gets wasted from the sides. This is in contrast to the commonly encountered results of a heat exchanger-a cross instead of counter-flow heat exchanger is more efficient. The effect of offset of the top and bottom fan increases the heat dissipation. The factor of merit is proposed as a performance parameter to compare the different cases, considering the benefit in terms of heat transfer and cost incurred in terms of power consumption of the fan. Computational fluid dynamics are used more as an analysis than a design tool and lead to an improvement in the design of the radiator fan assembly.

Nonlinear Analysis of 220 kV Power Transmission Tower and Gradient Optimization

To meet the needs of the economic development, the transmission power, which is the key part of Power transmission and transformation system engineering , needs to be improved on quality and quantity. Yet the design method of the transmission power has not been improved today. The outdated way of designing has many disadvantages. They don’t benefit to carry out the work of making the transmission power, and they have an enormous influence on the quality and production cost as well. Therefore, optimizing the structure, reducing the consumption of steel and improving the design level is of great significance. This article mainly uses SAP2000Nonlinear analysis of transmission tower after dao heng gradient optimization in engineering software thus reducing the consumption of steel and it can be used as a reference in a real project.

Study on Fault Diagnosis of Power Transformer with Reduction Method of Attribute Significance

This paper studies the power transformer fault quality diagnosis using rough sets theory and neural network. It is rough sets reduction as the pre-unit of neural network based on reduction algorithm with the attribute significance. The paper describes the reduction algorithm and implementation method detailed. Through the training and testing results with practical data, it is proved that the reduction algorithm with the attribute significance can make the number of input samples shorter, the training speed faster and the diagnostic accuracy higher. The algorithm is feasible and effective for applying to the fault diagnosis system of power transformer.

ANALYSIS ON DEGRADATION AND DEFORMATION OF TRANSFORMER INSULATION SYSTEM

Performance of insulation system of power transformer is essential to ensure better performance of power network. Most of the power and distribution transformers in service are aged and, have experienced several thermal and electrical stresses due to varying loading conditions, in addition to ageing. The rate of degradation and hence, the useful life of the insulation system mainly depends on the variation of operational stresses. Due to the progress in insulation degradation, the dielectric strength of the insulating oil and paper over transformer winding may reach to a point such that it could not withstand any small abnormal currents and voltages. In order to maintain the quality of system operation it is necessary to keep the transformer in good condition. This needs appropriate maintenance based on reliable diagnostics. In this paper experimental results on the application of frequency response analysis (FRA) for the diagnosis of power transformers. Frequency Response of Transformer, also serves for assessing the deformation process.

Study on Online Monitoring System of Power Transformer

This paper puts forward an on-line monitoring system of power transformer which could online collect multiple physical parameters of power transformer at the same time without using electricity sensor. In this case, the sensor will not affected by the strong electric field, magnetic field and electromagnetic wave. The system analysis the running state of power transformer according to physical parameters accurately, as current technology can only measure single physical parameters, it can solve this problem that the measurement results lack of association. Moreover, the system improves the reliability of power transformer operation and security of online measurement, ensuring the safe operation of power system.

Power System Transformation Toward Renewables: Investment Scenarios for Germany

We analyze distinctive investment scenarios for the integration of fluctuating renewables in the German power system. Using a combined model for dispatch, transmission, and investment, three different investment options are considered, including gas-fired power plants, pumped hydro storage, and transmission lines. We find that geographically optimized power plant investments dominate in the reference scenarios for 2024 and 2034. In scenarios with decreased renewable curtailment, storage and transmission requirements significantly increase. In an alternative scenario with larger investments into storage, system costs are only slightly higher compared to the reference; thus, considering potential system values of flexible pumped hydro storage facilities that are not included in the optimization, a moderate expansion of storage capacities appears to be a no-regret strategy from a system perspective. Additional transmission and storage investments may not only foster renewable integration, but also increase the utilization of emission-intensive plants. A comparison of results for 2024 and 2034 indicates that this is only a temporary effect. In the long run, infrastructure investments gain importance in the context of an ongoing energy transition from coal to renewables. Because of long lead times, planning and administrative procedures for large-scale projects should start early.

A probabilistic detectability-based sensor network design method for system health monitoring and prognostics

Significant technological advances in sensing promote the use of large sensor networks to monitor engineered systems, identify damages, and quantify damage levels. Prognostics and health management technique has been developed and applied for a variety of safety-critical engineered systems, given the critical needs of system health state awareness. The prognostics and health management performance highly relies on real-time sensory signals that convey system health–relevant information. Designing an optimal sensor network with high detectability of system health state is thus of great importance to the prognostics and health management performance. This article proposes a generic sensor network design framework using a detectability measure while accounting for uncertainties in material properties and geometric tolerances. Our contributions in this article are threefold: (1) the definition of a detectability measure to quantify the diagnostic/prognostic performance of a given sensor network, (2) the development of detectability analysis based on physics-based simulation and health state classification, and (3) the formulation of a generic sensor network design optimization problem as a mixed integer nonlinear programming. We employ the genetic algorithms to solve the sensor network design optimization problem. The merit of the proposed methodology is demonstrated with a power transformer system, which suffers from core and winding joint loosening due to consistent vibration.

Feasibility Study on the Use of Vegetable Oil (Natural Ester) in Malaysia Power System Transformers

Power transformer insulation oil is critical to the useful life of the transformer. Although mineral base insulation oil is the most common type of transformer insulation oil in use, environmental and safety concerns has encouraged the development of biodegradable alternatives. Today, <TEX>$Envirotemp^{(R)}\;FR3^{TM}$</TEX> is in commercial use and this study is aimed at ascertaining the possibility of applying the oil in Malaysia power transformers. A sample of <TEX>$Envirotemp^{(R)}\;FR3^{TM}$</TEX> was tested to measure and compare the technical (including electrical, chemical and physical) properties of the oil according to Malaysian standards. The study found that the oil sample had better qualities, such as higher dielectric strength, lower dissipation factor, higher flash and fire points, higher moisture absorption capability, and less dissolved gases composition amongst others. However, it was also ascertained that further development in this area could be hindered due to Malaysia's lack of standards for biodegradable oil.

Lifetime estimation and monitoring of power transformer considering annual load factors

This paper focuses on the realistic remaining service life prediction and monitoring system of power transformers, using an algorithm developed by the authors. The power transformer is one of the vital components of power transmission and distribution substations. It is important to know the loading capacity and remaining service life of a power transformer to ensure its economical and secure operation. This information is directly related to the condition of cellulosic insulation. According to classical method of prediction, the power transformer is considered to be operated under constant load for the remainder of its lifespan. However, in reality the load factor varies continuously, and this is a key factor affecting the service life of power transformers. In this study, a realistic method for monitoring and predicting the service lifetime of power transformers is presented by considering annual load factors. The proposed method is applicable to any power transformer and can be updated by considering relevant standards for different liquid and solid insulation materials. As a case study, life-loss calculation and the remaining service life predictions are carried out using a thermal model that has been created considering a 250 MVA mineral oil-mersed power transformer.