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In this article, novel methods and ideas are introduced, which altogether lead to an accurate model of power transformer windings. First, by the determination of non-dominant (hidden) resonances from frequency-response tests, a ladder model is proposed. Next, it is improved by assigning different values for similar elements of each section of the model. The parameters are obtained by minimizing the error function via a genetic algorithm. Sensitivity analysis is then applied to the obtained model to achieve further examinations and tests. Measurements have been driven from the windings of a 20/0.4-kV, 1600-kVA transformer. Modeling, methodologies, and sensitivity analysis in this article can be very useful for future research aiming to find internal faults of the transformer with the frequency response analysis.

A novel approach to diagnostics of transformer winding deformations, caused by mechanical forces from short-circuit currents, is presented. We employ a simple model of a transformer as a two-dimensional parallel plate waveguide. The upper plate represents the transformer tank wall and the lower plate represents the iron core which carries the magnetic flux. Between the two plates, we model the transformer winding by a set of parallel conductors. We utilize commercial simulation software to simulate the generation and measurement of microwave radiation at both ends of the winding structure. The radiation interacts with the metallic structures in the model waveguide. The measured responses from the model waveguide are expected to be sensitive to mechanical deformations of the transformer winding. We use conventional waveguide theory to solve the direct propagation problem, and an optimization method to solve the inverse problem. In particular, we determine the locations of winding segments, and obta...

Frequency response analysis is commonly used as an effective diagnostic tool to detect mechanical defects within power transformers. The frequency response analysis result is affected by the test condition. This article presents a new method based on transmission line diagnostics (characteristic impedance) to discriminate between axial displacement and radial deformation in transformer winding. This method is independent of external circuit elements. To study this method, an appropriate model of the transformer winding in the frequency range of kHz–MHz is used, and different mechanical defects are analyzed. The mechanical defects of the transformer winding are simulated using ATP/EMTP in order to obtain current and voltage signals. The simulation results show the sensitivity and capability of the proposed method using wavelet coherence technique in condition monitoring analysis.

In this work, we developed a finite element package to investigate the dependence of electromechanical coupling coefficient on ring-shaped piezoelectric transformer parameters, and proposed how to design the transformer in achieving optimum efficiency using the ceramic PZT-5H as an application. The ring transformer firstly used full driven electrode to observe piezoelectric vibration characteristic with varying the ring dimensional (structural) parameters. Then, with the optimized ring parameters, electrode layout was designed in accordance to the distribution of electrical potential within the ring transformer, where both alternating and non-alternating electrode layouts were considered. From the calculation, results showed that the piezoelectric vibration characteristic strongly depends on the ring-shaped transformer dimension. Specifically, with increasing piezoelectric ceramic thickness and the width between inner- and outer-ring-radius, the resonance frequencies of the transformer decrease. On the ot...

In this article, a new algorithm based on time-frequency analysis of differential current is presented for power transformer protection. Since the differential currents are non-stationary signals, the hyperbolic S-transform can be used as a powerful signal processing tool that yields the complete information in both time and frequency domains. A criterion function is proposed based on some extracted features from the obtained hyperbolic S-matrix and frequency contours. By selecting the proper threshold value, the internal fault can be detected correctly from other conditions. Various conditions of internal and external faults, transformer energization, over-excitation, and different levels of current transformer saturation are simulated using PSCAD/EMTDC software (Manitoba HVDC Research Center Inc., Manitoba, Canada), while the important parameters that have a direct effect on the differential current waveform are considered. Current transformers are simulated using accurate Jiels–Atherton model,...

The transfer function is a well-known method to detect mechanical defects of transformer windings. After measurement of transfer functions, they must be evaluated to detect any anomaly in transformer windings. One of the main problems in this stage is lack of transparency. This article tries to solve this problem through a vector space-based perspective to diagnose radial deformation and axial displacement of transformer windings. To determine the severity of the winding deformation or displacement, the angular proximity index is used. The measurement results show the effectiveness of this index. Based on the verified model of the transformer, a dimensionality reduction technique is applied to transformer's transfer function to map it from its original high-dimensional vector space to a lower-dimensional one. It is shown that transfer functions of radially deformed and axially displaced windings are well separated visually in the reduced space.

In this study, DC-DC converter using the thickness–vibration-mode piezoelectric transformer for driving high brightness LED was manufactured and its driving properties were investigated. LED output voltage of DC 4.11 V which can be feasibly turned on was observed. At the time, driving frequency of 1.12 MHz was measured. When the high brightness LED was turned on for 40 min., the surface temperature of the piezoelectric transformer under 2.5 W input power showed the temperature rise of 17°C. Its temperature was saturated after 25 min. after turning on and driving frequency slightly decreased as the temperature of the piezoelectric transformer increased. The high brightness LED was stably turned on at the frequency of 1.12 MHz.

Based on the principle of a quasi-stationary electromagnetic field, a hybrid lumped parameter model of transformer winding under very fast transient over-voltage (VFTO) is proposed in this article. The hybrid model consists of a single-turn model combined with a two-coil model. The single-turn model can be used to calculate the inter-turn over-voltage under a steep-front impulse. The simulation time can be significantly reduced with the help of the two-coil model in the proposed hybrid model. The calculation method for the model parameters is given, and the simulation study for the voltage distribution of a practical transformer winding under a chopped-wave impulse has been performed. The simulation result has been compared with the actual data measured at the Chongqing ABB Transformer Factory to verify the effectiveness of the model. The result shows that the model can predict the voltage distribution inside a transformer winding correctly.

A novel interference-free transformer (IFT), is proposed to suppress conduction interference. The fabrication of IFT is materialised through an air-gap shunt added to the magnetic circuit of this transformer, while an equivalent step-down inductor can be obtained between the primary and the secondary windings. The inductor is connected with a parallel resonance circuit in series to form a filter-network which makes the high-frequency difference-mode (DM) interferences be attenuated without affecting the fundamental signal. Two windings in opposite directions are connected in series with the primary winding. These added passive components can suppress the common-mode (CM) interferences effectively. A design procedure is proposed, and a prototype is built and analysed. The simulation and experimental results validate the analysis and method for optimising the design for the proposed transformer.

Experimental experiences prove that partial discharge (PD) is a major source of inner insulation system failure in power transformers. If the deterioration of the insulation system caused by PD activity can be detected at an early stage, preventive maintenance measures may be taken. Due to the complex structure of the transformer, accurate determination of PD location is one of the major challenges in front of power utilities. This problem comes to be vital in open access systems. A hybrid transformer model and its application for locating partial discharge and studying its propagation along the transformer windings are proposed in this article. It is based on the structural data of the transformer for equivalent model parameters determination. The lumped parameter equivalent model for winding sections is constructed satisfying the quasi-static condition. Then an algorithm is developed to use the constructed matrices for PD propagation and location studies by application of the multi-conductor tr...

This paper investigates the performance of a recently proposed online transformer internal fault detection technique through detailed non-linear three-dimensional finite element modelling of the windings, magnetic core and transformer tank. The online technique considers correlation of transformer instantaneous input and output voltage difference and input current at the power frequency and uses the ellipse shape ΔV-I locus as the finger print of the transformer that could be measured every cycle to identify any incipient faults. The technique is simple, fast and suitable for online monitoring of in-service transformers. A detailed three-dimensional finite element model of single-phase transformer is developed and various physical winding deformations with different fault levels are simulated to assess their impacts on the online ΔV-I locus. As transformer field testing under various internal fault conditions cannot be easily conducted, the main contributions of this paper are accurate finite elem...

In the present paper a variable-bias digital differential relay for power transformers protection is proposed. The amount of bias is varied linearly with the value of through current (mean of the currents entering and leaving the transformer). To take care of mismatch between CT ratios on primary and secondary sides of the power transformer, secondary currents of CT are corrected using a precalculated correction factor for the power transformer tap in operation. In the event of an abnormal operation, a fault discriminant based on fundamental, second and fifth harmonic components of differential current is used. It provides discrimination between faults and magnetizing inrush /overexcitation copditions. Trip and indication signals arc issued in the former case, while only an indication is given in the latter case. The relay scheme is simulated on a personal computer. The advantage of the proposed scheme is its ability to detect even low-level internal faults, which may not be detected by a fixed-b...

(1999). A welding transformer with a rotating magnetic field. Welding International: Vol. 13, No. 10, pp. 835-836.

The paper reviews work on the transformer winding thermal problem, emphasising the increasing possibility of using numerical methods to solve it. The validity of the authors' own numerical analysis is established by comparison with published experimental results. In the disc-type winding, inequality of flow between parallel paths can cause local overheating. As one approach to this problem, results are given for oil velocities measured in an isothermal perspex model simulating hydraulically a vertical radial “slice” of winding.

The rectifier transformer's service differs from the power transformer's service because of the rectifier load. Under certain fault conditions, such as a commutation failure, d.c. magnetization may...

Synopis A formula is given for the voltage at a tap in the primary of a transformer, and it is shown that under given conditions this voltage will be independent of the load on the transformer. Impedance is defined as a tensor.