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This paper presents an approach based on the use of transformer no-load and light load current harmonic analysis to detect the presence of an interturn fault at the incipient stage. The presence of fault is detected by performing a trend analysis of specific harmonic components and primary current magnitude. Transformer testing methods suffer from a common drawback of offline nature and require an expert opinion for accurate condition assessment. This paper presents a method that transforms no-load current harmonic analysis, which is a special test for transformer testing, into an online method at light load conditions. Thus, the necessity of complete shutdown and dependency on expert opinion is eliminated since it only requires one reference signature per phase. The proposed approach is verified for different vector groups of the three-phase core-type transformer and three independent units connected in star-star and for a fault on the low-voltage or high-voltage winding of the transformer. The presented experimentation shows that the proposed approach can detect the presence of a fault at an incipient stage involving less than 2% of turns and fault current up to 1 p.u. A case study is presented that validates the method for the transformer of a higher power rating. [ABSTRACT FROM AUTHOR]

Power transformer, as one of the most important apparatus in power system, must be protected against the turn-to-turn faults. Although the various methods based on the terminal currents and/or voltages have been presented for transformer protection, the flux-based methods can be used to achieve the more accurate, sensitive, and secure results. Since the symmetrical form of the magnetic flux distribution will be disturbed due to any fault occurrence in the transformer windings, it can be considered as an appropriate criterion to achieve a suitable protection algorithm, which can detect the fault occurrence and identify the faulty phase/region. In this paper, a new leakage flux sensor is introduced and then a flux-based technique is proposed to detect and identify the faulty phase in transformers to apply in online conditions. By using the proposed technique, the faulty region can be identified in offline conditions, as well. The experimental results (which are done on a distribution transformer) show that this technique is secure in the face of overfluxing and transformer energizing conditions and can detect turn-to-turn faults with high sensitivity. Moreover, tap changer operation, imbalanced loads, and/or unsymmetrical voltage sources cannot influence the proposed technique security. [ABSTRACT FROM PUBLISHER]

This paper investigates the performance of a recently proposed online transformer internal fault detection technique and examines the impact of harmonics through detailed nonlinear simulation of a transformer using 3-D finite-element modelling. The proposed online technique is based on considering the correlation between the instantaneous input and output voltage difference (\Delta \bf V) and the input current of a particular phase as a fingerprint of the transformer that could be measured every cycle to identify any incipient mechanical deformation within power transformers. To precisely emulate real transformer operation under various winding mechanical deformations, a detailed 3-D finite-element model is developed. Detailed simulations with (non)sinusoidal excitation are performed and analyzed to demonstrate the unique impact of each fault on the \Delta \bf V\-\bf I locus. Impacts on harmonic order, magnitude, and phase angle are also investigated. Furthermore, practical measurements have been performed to validate the effect of a winding short-circuit fault on the proposed \Delta \bf V\-\bf I locus without and with the impact of system harmonics. [ABSTRACT FROM PUBLISHER]

This paper investigates the behavior of power transformers under the occurrence of permanent or intermittent winding insulation faults. For the study of these phenomena, a simple and efficient permeance-based electromagnetic transformer model is proposed, which is based on the simultaneous consideration of magnetic and electric equivalent circuits. To incorporate the internal faults in this model, a suitable equivalent circuit of the faulty winding is described. With the aid of this transformer model, the onload exciting current Park's Vector Approach will be applied for diagnosing the occurrence of permanent and intermittent winding faults. Experimental and simulation tests results are presented in this paper, which demonstrate not only the adequacy of the digital transformer model for winding fault studies, but also the effectiveness of the proposed technique for detecting winding interturn insulation faults in operating three-phase transformers. [ABSTRACT FROM AUTHOR]

Owing to several advantages over traditional phase shifters, applications of hexagonal phase-shifting transformers (PSTs) keep expanding. These transformers are built with unique connections of their windings, neither delta nor wye. Short-circuit modeling and analysis methods are not yet fully established for hexagonal phase shifters. This work is divided into two parts. In this part, a digital model of a hexagonal PST is developed and validated. This original material is of assistance when performing short-circuit system studies, relay setting calculations, as well as when modeling internal faults and developing protection techniques for the transformer itself. This paper explains construction of hexagonal PSTs, deriving their sequence impedances under varying taps and compares the model with manufacturer test data and a physical made-to-scale model developed specifically for this study. The transient model introduced in this paper can easily be integrated into the commonly used Alternate Transients Program and real-time digital simulator modeling environments. Part II of this work presents original protection methods for hexagonal PSTs. [ABSTRACT FROM AUTHOR]

The transfer function (IT) of a transformer winding (or its natural frequencies) is known to be dependent on many factors, like the type of winding excited, terminal configuration and type of secondary winding, arrangement of windings, clearances, tap and its positions, phase position, etc. Further, it was well established about half a century ago that the natural frequencies are influenced due to the complex interactions that exist between windings. Strangely, all publications dealing with TF until now have, so far, ignored this interaction. However, to date, what is not clearly understood is the extent of contribution from the individual windings, influence of the type of winding (disc/layer, interleaved, etc.), and effect of the terminal condition of the secondary winding in defining the shape of the resultant TF. This paper attempts to pro- vide quantitative answers to many of these questions by considering a two-winding transformer. Further, it is demonstrated how knowledge about the TF of individual windings, when considered in isolation, could be utilized to identify the contributions from each winding to the pole structure of a two-winding TF. This feature is a novelty being reported for the first time, and it is conjectured that this finding could extend the diagnosing capability of TF method from fault detection to fault location. The results presented are based on extensive analytical calculations, circuit simulations, and experiments on model coils for different terminal conditions and winding types. It is believed that the contributions of this paper resolve some issues existing in TF interpretation and impart new impetus for further research. [ABSTRACT FROM AUTHOR]

This paper describes a new wireless differential protection system for detection of turn-to-turn faults in air-core inductors. The proposed system is composed of a pair of floating magnetic field sensors placed at opposite ends of an air-core inductor, which wirelessly communicate their measurements to a remote receiver. The receiver compares the magnetic field measurements from the sensors and triggers an alarm or circuit interruption system if the mean-squared field difference exceeds a specified threshold. The main contributions of this paper are a description of a specific realization of this differential air-core inductor protection system with several novel features and experimental results that demonstrate the sensitivity and latency of the proposed protection system. Our results suggest that deployment of the proposed system in shunt-connected inductors could help to prevent catastrophic damage resulting from turn-to-turn faults by detecting and arresting these faults in their incipient stages. [ABSTRACT FROM PUBLISHER]

Obtaining information on the voltage dips that occur in a network is one of the most important tasks of a power quality monitoring program. This paper focuses on two types of voltage dips that although mentioned in the literature, their characteristics have not been presented before. These types are: voltage dips due to transformer saturation and multistage voltage dips due to faults (dips that present more than one stages of voltage magnitude before the fault clearing operation). Simulations and real measurements are used to show the features of these types of phenomena. Overall, the paper offers the means for a systematic processing of the measurements obtained by power quality monitors. [ABSTRACT FROM AUTHOR]

The detection of failures within power transformers is considered an important issue since these components are of critical importance for power system reliability; moreover, their replacement cost is extremely high. In monitoring the transformer condition along its useful life, frequency-response analysis (FRA) has gained great interest due to its sensitivity to failures in the windings and the iron core. These failures can be detected by evaluating transfer function changes by means of statistical and mathematical indices and classified according the frequency band in which these changes take place. However, this procedure involves evaluation inaccuracies due to disturbances or minor changes during FRA measurements. The new methodology is based on the decomposition of the original responses in several levels of decomposition (filtering) using the discrete wavelet transform, and the subsequent comparison using smooth versions of the responses. Fault detection is further supported with statistical indices calculated using the frequency band where abnormal differences appear. This procedure gives more robustness to the method and reduces the possible influence of disturbances during measurement in the diagnosis result. The methodology has been tested using different failure cases and two of them are used for validation purposes in this paper. [ABSTRACT FROM AUTHOR]

Power transformers can experience large inrush currents upon energization, the severity of which depends on the source strength, the leakage impedance and residual flux of the transformer, and the angle of the applied voltage at energization. A novel inrush current reduction strategy has been implemented which involves setting a single-phase transformer's residual flux to a known polarity after the transformer has been de-energized, a process called “prefluxing,” and controlling the instant of transformer energization based on the flux polarity, seeking not to eliminate inrush current but to substantially reduce it. Unlike a popular suggested solution, this strategy does not require prior knowledge of the transformer's flux. The device used for prefluxing is simple in construction and operates at substantially lower voltage levels when compared to the transformer's rated voltage. The presented strategy has been successfully implemented on an 18-kVA laboratory transformer with inrush current levels reduced below the rated current of the transformer even when accounting for typical breaker deviations. This paper describes the operation of the reduction strategy, including theory, device sizing, and implementation, and presents the successful laboratory results, all of which provide the basis for implementing inrush current reduction in three-phase transformers using a three-pole circuit breaker. [ABSTRACT FROM PUBLISHER]

Interturn faults are a significant problem in power transformers that can eventually burgeon into catastrophic faults and likely result in an overall network failure. The main problem with SFRA as one of the well-recognized diagnostic tools for detecting winding faults is its restriction to the domain of offline testing since the method requires injection of a test signal into the transformer windings through the high-voltage bushings. To address the difficulties arising from this issue, in this contribution, a new technique for online transfer function monitoring of the power transformers windings through a quite simple, economic, and noninvasive capacitive sensor installed on the surface of the transformer bushing is presented. Interturn faults with various levels of severity were imposed on the winding of a 35-kV/400-V, 100-kVA oil-immersed distribution transformer to evaluate the feasibility and sensitivity of the method. The experimental results proved that the newly developed online monitoring system is feasible and sensitive to detect unambiguously interturn faults even down to 0.2% shorted turns along the winding. As such, the proposed approach in this paper enables timely warning of a rising failure and serves a better purpose in assessing the health of a faulty transformer. [ABSTRACT FROM AUTHOR]

This paper compares the performances of an optical voltage transformer (OVT) and a conventional voltage transformer (VT) in a laboratory for steady state and transients. It also presents a developed equivalent circuit model for OVT. In addition, OVT impulse response has been investigated, and field records have been evaluated to confirm that optical systems can satisfactorily replace magnetic transformers. Both experimental results and field records show that the transient performance of an OVT is better than that of a conventional one, and the performance of OVT is comparable to that of the magnetic VT. Differences are found within experimental errors. A transfer function for OVT is derived by using the obtained frequency response characteristics. A circuit model is developed from the derived transfer function by using a network synthesis method. The model is verified through the frequency response results and tested with PSPICE simulation program. The results show that the developed model can be used as a legitimate OVT model. [ABSTRACT FROM AUTHOR]

This paper presents an analysis method based on explicit dynamic analysis, enabling detailed and representative investigations of the mechanical effects of low-impedance faults in power equipment. To illustrate the effectiveness of the proposed method, a given transformer design having caused the projection of a heavy chimney/bushing assembly over a significant distance is used as a case study. A comparison about the observed projection distance of this assembly shows that the method provides representative results, thus offering the significant advantage of replacing costly and unpractical tests. The method is then used to study the retrofit measure proposed in this case to retain the chimney, further exemplifying its power to effectively study different design and retrofitting options. [ABSTRACT FROM AUTHOR]

When fault currents contain decaying dc components, current transformers (CTs) face the risk of becoming saturated. Traditionally, fault current waveform or CT model analyses were used to detect CT saturation. This paper analyzes the decaying dc components in fault currents to detect CT saturation. The decaying dc component in fault currents is first estimated using phasor-based computations. Such a component is then used to de- fine a detection index. The proposed detection index can be easily used to detect CT saturation because its value in an unsaturated period is within a pre-known range. After CT saturation has been detected, the current samples and phasors in the latest unsaturated period are used to correct the saturated current samples. The proposed algorithm was tested using MATLAB/SIMULINK simulator and realized on a DSP Starter Kit to demonstrate its effectiveness and applicability. [ABSTRACT FROM AUTHOR]

Frequency-response analysis (FRA) has been accepted as one of the most sensitive tools to detect mechanical faults of power transformers. Correct interpretation of FRA responses is crucial when assessing the integrity of a transformer. Transformer FRA responses have distinctive frequency regions which are predominated by core, windings, and measuring setup. This paper addresses one of the major factors that affect the FRA responses: the winding structure itself. In terms of the structures of single winding, they can be categorized into windings with either higher low-series capacitance in proportion to the shunt capacitance. Correspondingly, the FRA responses of windings of high series capacitance exhibit the increasing trend of magnitude, while windings of low series capacitance display a steady magnitude trend with the features of resonances and quasi-antiresonances or antiresonances. For a winding made of two concentric winding coils, the pattern of alternating resonances and antiresonances would be exhibited on its FRA response. [ABSTRACT FROM AUTHOR]

Single-phase adaptive reclosure (SPAR) schemes applied to transmission lines have been an effective method to improve the stability of power system. Attractive techniques have been proposed for SPAR schemes based on the tripped fault-phase voltage. But for transmission lines with shunt reactors, the voltage is so minor that the error of voltage transformer limits the applications of voltage-based SPAR schemes. To overcome this disadvantage, a new scheme using differential current between the calculated current based on transient fault model and the actual measured current from the fault phase of shunt reactor is proposed in this paper. The calculated current is obtained by using the transient fault model whether there is a transient or permanent fault. In the case of transient fault, the calculated current is close to the measured current due to the actual fault model being close to the calculated model. As for the permanent fault, the calculated current is quite different from the measured current. Therefore, the differential current of the fault phase can be employed to distinguish permanent fault from transient fault. At last, the Alternative Transients Program simulation results show that the developed algorithm can identify the permanent fault correctly and reliably, and is promised to be applied to SPAR for transmission lines with shunt reactors. [ABSTRACT FROM AUTHOR]

Abstract-This paper proposes an efficient compensation algorithm based on an adaptive network-based fuzzy inference system capable of converting a sampled current waveform that is distorted by current-transformer (CT) saturation to a compensated current waveform. Quick response time, no cumulative estimation error, desired sample-by-sample output, no dependency of CT parameters/characteristics and secondary burdens, and simplicity are some attractive features of the proposed method. The accuracy and robustness of the introduced compensation algorithm are demonstrated through extensive test cases reflecting a wide range of variations in fault conditions and CT parameters. The proposed algorithm has also been implemented and tested on a digital signal processor board. [ABSTRACT FROM AUTHOR]

Abstract-Wavelet networks (WNs) are an efficient model of nonlinear signal processing developed in recent years. This paper presents a comparative study of WN efficiency for the detection of incipient faults of power transformers. After 700 groups of training and testing gases-in-oil samples are processed by fuzzy technology, we compare and analyze the network training process and simulation results of five WNs which include two types of WNs with two different activation functions and evolving WN. A lot of diagnostic examples show that the diagnostic accuracy and efficiency of the proposed five WN approaches prevail those of the conventional back-propagation neural-network method and are suitable for faults diagnosis of power transformers, especially with the evolving WN achieving superior performance. [ABSTRACT FROM AUTHOR]

Several cases of mal-operation of transformer differential protection with second-harmonic blocking after clearance of external fault are reported. These mal-operations all occurred at the nonrestraint region of percentage restraint plane. The previous theory cannot be utilized directly to analyze this phenomenon. Therefore, a mathematical model for analyzing the transient course of external fault inception and removal, together with the CT model involving the magnetic hysteresis effect, is proposed in this paper. It is proved that the magnetic linkage of one CT core can be pushed into the region nearby the saturation point by the high fault current with aperiodic component. As soon as the external fault is removed, the magnetic linkage formed by the primary current with low amplitude is not high enough to pull the operating point of the magnetic linkage back to the linear region. This phenomenon is named as CT local transient saturation, which results in the big measuring angle error and relative smooth waveform. In this case, the transformer differential protection using second harmonic blocking inevitably mal-operates. This point of view is verified with the simulation tests. [ABSTRACT FROM AUTHOR]

The subject of this paper is to construct a current-transformer (CT) model based on the Jiles-Atherton (J-A) theory of ferromagnetic hysteresis using transient analysis of control systems (TACS) functions within the Electromagnetic Transients Program (EMTP) in a closed-loop manner. The J-A theory of ferromagnetic hysteresis is presented by the nonlinear first-order ordinary differential equations and inclusive of five parameters that can be found by measurements of hysteresis characteristic. We can use TACS functions within EMTP to describe the J-A theory of ferromagnetic hysteresis. The powerful advantage of this way is the interaction between the electrical network and TACS in EMTP. Compared with the CT model based on the type 96 nonlinear hysteresis inductor, the simulated result of the CT model based on the J-A theory of ferromagnetic hysteresis is almost the same in steady-state situations, but apparently different in a fault transient situation. The analysis and simulation of the actual measured fault profiles in the Taiwan power system is an example of the validation of this proposed model. Therefore, this proposed CT model based on the J-A theory of ferromagnetic hysteresis has the analysis capability of the transient response. The flexibility of TACS makes the simulation of any other situation easier and is another advantage. [ABSTRACT FROM AUTHOR]

This paper presents a novel scheme based on error estimation for identification of magnetizing inrush and internal fault conditions in transformers. Because the inrush current is a peaked wave with a dead angle, we can discriminate internal fault current from inrush current by comparing the similarity between the actual wave and two reference waves under two different frequency conditions per half cycle. Experimental results indicate that this technique is effective under different fault conditions, and an operating time of 10-13 ms can be achieved for internal faults in transformers. [ABSTRACT FROM AUTHOR]

In this paper, we propose a fault diagnosis method for a group of distribution transformers using the graphical information of frequency responses. For a group of transformers with similar capacities and ratings, the patterns of transformer's magnitude plots appear to be almost identical with minor variations of amplitudes and locations of existing peaks and valleys in the plots. Therefore, it is reasonable to establish prototypes composed of several peaks and valleys for magnitude plots and to investigate their changes for the purpose of diagnosis. We select six most significant regions for check regions, and their movements are encoded into a ternary string which is then compared with strings already constructed for various types of faults. Each string-by-string comparison is quantified into correlation values by computing Hamming distances and weighting selected bits. The fault type whose correlation values is largest is determined as a most probable fault in a test transformer. The diagnosis performance of the proposed method is proven to be reliable via experiment data on several types of faults. [ABSTRACT FROM AUTHOR]

Such an optimization analysis of the operating conditions of the ground-fault protection excited by the third harmonic voltage for the two-state system of the generator-transformer unit transverse parameters is presented in this paper. The analysis was made for a unit in which a change in its transverse branch parameters occurs as a result of generator breaker switching operations. The optimization analysis has shown that effective ground-fault protection of the generator stator windings is possible, regardless of the operating conditions of the generator breaker. [ABSTRACT FROM AUTHOR]

This paper presents a high impedance fault (HIF) detection method based on decision trees (DTs). The features of HIF, which are the inputs of DTs, are those well-known ones, including current [in root mean square (rms)], magnitudes of the second, third, and fifth harmonics, and the phase of the third harmonics. The only measurements needed in the proposed method are the current signals sampled at 1920 Hz. It will reduce the cost of hardware compared with methods that use high sampling rates. A new HIF model is also used. The data of current signals are from the simulation of Electromagnetic Transients Program (EMTP). The DT algorithm trained can successfully distinguish the HIFs from most normal operations on simulation data, including switching loads, switching shunt capacitors, and load transformer inrush currents. Testing on experimental data is recommended for future work. [ABSTRACT FROM AUTHOR]

The paper investigates three different ways of using the transfer function method for detecting mechanical winding displacements in power transformers. The most reliable approach is time-based comparison, which requires finger print data from a previous measurement. Such information is, however, usually not available. For multilegged transformers without zigzag-connected windings the results of separately tested legs can be used as mutual references (construction-based comparison). A third approach is to compare the transfer functions with those obtained from an identically constructed transformer (type-based comparison). However, for a transformer with given nominal specification data, the winding design may over time undergo changes which causes changes to the transfer function. It is proposed to solve this problem by calculating tolerance bands using transfer functions from a big group of the same-type transformers. A novel statistical algorithm for this purpose is presented. The approach is demonstrated for a set of 28 specified identically 200-MVA power transformers. [ABSTRACT FROM AUTHOR]

This paper proposes genetic algorithm tuned wavelet networks (GAWNs) for data mining of dissolved-gas-analysis (DGA) records and incipient fault detection of oil-insulated power transformers. The genetic algorithm-based (GA) optimization process automatically tunes the parameters of wavelet networks: translation and dilation of the wavelet nodes, and the weighting values of the weighting nodes. The GAWNs can identify the complex relations between the dissolved gas content of transformer oil and corresponding fault types. The proposed GAWNs have been tested on the Taipower Company's diagnostic records, using four diagnosis criteria, and compared with artificial neural networks (ANNs) and conventional methods. Experimental results demonstrate that the GAWNs have remarkable diagnosis accuracy and require far less learning time than ANNs for different diagnosis criteria. [ABSTRACT FROM AUTHOR]

Dissolved gas analysis (DGA) is one of the most useful techniques to detect incipient faults in power transformers. However, the identification of the faulted location by the traditional method is not always an easy task due to the variability of gas data and operational variables. In this paper, a novel extension method is presented for fault diagnosis of power transformers, which is based on the matter-element model and extended relation functions. Thus, incipient faults in power transformers can be directly identified by the degree of relation. The application of this new method to some transformers has yielded promising results. [ABSTRACT FROM AUTHOR]

When an electric arc is generated in transformer oil, a strong overpressure may rapidly develop in the tank, especially for low-impedance faults. This paper presents the test method and results obtained in a project aimed at assessing the human and environmental risks associated with this type of fault. Limit energy and pressure values were defined in terms of different safety levels. The best method of limiting the energy and pressure at a safety level in the transformer tank during internal faults of greater than 1.7 kA is to protect the transformer with a current limiting fuse. Other ways to increase the mechanical withstand of transformer tanks under such faults are also discussed. [ABSTRACT FROM AUTHOR]

This paper presents the multipliers to shift the through-fault protection curve for various connections of distribution transformers. Cases with a secondary center tap and with secondary open delta were analyzed as well as the effect of using single-phase units with different short-circuit impedances. [ABSTRACT FROM AUTHOR]