Magnetizing Inrush Current Research Articles

Modeling and analysis of a single-phase core-type transformer under inrush current and nonlinear load conditions

In this paper, a transformer model based on the equivalent electrical circuit considering the nonlinearity of the iron core has been developed. The nonlinear behavior of the iron core was taken into account by using the inverted Jiles–Atherton hysteresis model. The single-phase core-type transformer was analyzed under different energization, remnant flux for inrush current and nonlinear load conditions. Illustrative and remarkable simulation and experimental results related to inrush current along with the primary and secondary currents under nonlinear load conditions were demonstrated. In addition, magnetic flux in the iron core and the hysteresis curve as a consequence of the relation of the magnetic flux and magnetizing inrush current were demonstrated. Operation of the transformer under various nonlinear load conditions have been demonstrated with simulation and experimental results. Variation of peak value of the inrush current for different levels of remnant flux in the iron core and different switching-on angles of the voltage applied to the primary was further indicated.

Novel Method for Discrimination of Transformers Faults from Magnetizing Inrush Currents Using Wavelet Transform

Discrimination of power transformer inrush from fault current is most important to proper operation of differential relays. Most differential relays discriminate the inrush current from short circuit fault according to high content of second harmonic of inrush. Unfortunately, in some power system operation, it has been seen that this algorithm is not accurate enough due to the presence of some kinds of loads. On the other hand, current transformer (CT) saturation due to large inrush or fault current also can cause the discrimination algorithms mal-operation. This paper presents a novel algorithm for power transformer differential protection which differentiates internal faults from those of magnetizing inrush currents in the presence of CT saturation. In order to eliminate the impact of CT saturation on the performance of the proposed technique, at first the currents are compensated by a CT saturation compensation method. Furthermore, a technique based on wavelet transform is used for discrimination of transformer internal fault from magnetizing inrush currents. Using discrete wavelet transform (DWT) features of primary and secondary differential currents and those of flux measured at transformer primary winding, an algorithm was proposed. Discrimination criterion is based on synchronization of maximum or minimum points between wavelet coefficient of differential current and flux of the transformer. The efficiency of the algorithm was evaluated using different scenarios of inrush and internal fault current of a power transformer simulated in MATLAB/Simulink software environment.

Current signal processing-based methods to discriminate internal faults from magnetizing inrush current

Two new methods, current change ratio (CCR) and percentage area difference (PAD) were proposed to solve a problem of how to distinguish between internal faults and inrush condition when transformer is switched on. This problem may delay operation or may mal-operate some protection schemes like deferential protection. The methods were concluded after observing and analyzing the behavior and shape of large number of both inrush and internal fault signals that had been obtained using a model transformer in a laboratory. The methods were practically tested on a three-phase transformer with rated power of 20 kVA at Cardiff University’s laboratory and the data were processed using LabVIEW and MATLAB programs. The results showed that internal faults can be correctly distinguished from inrush condition within a short time (from 5 to 10 ms), particularly the minor internal faults such as the interturn fault which is submerged to inrush current and make it is too difficult to be detected. The advantages of these algorithms are simple in design and faster than the second harmonic method which is the most popular method used for solving this problem.

Residual Magnetic Flux of On‐Load Transformer for Controlled Switching

When a transformer is being energized, a large magnetic inrush current flows depending upon the closing phase angle of the circuit breaker and the residual magnetic flux in the iron core. One of the suppressing methods of the magnetic inrush current is to control the closing phase angle of the circuit breaker. Controlled switching has been used for no‐load transformer energization until now. However, the transformer may be switched under an on‐load state. In this case, a large magnetic inrush current will flow. To suppress the inrush current by using controlled switching, the information of the residual magnetic flux in the iron core is needed. The residual magnetic flux for no‐load transformers has been investigated. However, the residual magnetic flux for on‐load transformers was not investigated. In this paper, the residual magnetic flux for an on‐load transformer was investigated by experiments and calculations. © 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Simulink model of transformer differential protection using phase angle difference based algorithm

<p class="p1">An application of phase-angle-difference based algorithm with percentage differential relays is presented in this paper. In the situation where the transformer differential relay is under magnetizing inrush current, the algorithm will be utilized to block the process. In this study, the technique is modeled and implemented using Simulink integrated with MATLAB. The real circuit model of power transformer and current transformers are considered in the simulation model. The results confirmed the effectiveness of the technique in different operation modes; such as, magnetizing inrush currents, current transformers saturation and internal transformer faults.</p>

EXPERIMENTAL INVESTIGATION OF POWER TRANSFORMERS PROTECTION USING WAVELETS AND PATTERN RECOGNITION TECHNIQUES

A few papers have been interested in the experimental investigation for the transformer protection; in this paper a real time laboratory investigation for power transformer protection is presented. A transformer of (5 KVA, 220/110 V) is used to represent a power transformer. The current transducers are used to transfer the primary and secondary current signals of the tested transformer to the data acquisition system connected to pc by sampling frequency of 10 kHz. Lab- View package is used as software. The protection algorithm based on wavelet transform and pattern recognition (support vector machine) as described in [21]. The experimental results indicate that it are compatible with the simulated results and the proposed technique is stable, reliable, and fast during the discrimination between internal and external faults, magnetizing inrush currents, and switching on internal faults, so, the proposed technique can be used in the real time applications.

A New Practical Approach for Discrimination between Inrush Currents and Internal Faults in Power Transformers

The power transformers are protected by differential relays. These relays use the second harmonic blocking to distinguish inrush current from internal fault current. In recent years, the performance of the second harmonic blocking has decreased and therefore new methods are needed to distinguish inrush current from internal fault current. The aim of this study is to present a new method for discrimination between magnetizing inrush currents and internal faults in differential protection of power transformers. The proposed scheme is based on calculating the dq0 transformation of current signals in the abc phases of transformer terminals for different inrush current and internal fault signals. Creating new waveforms using mathematical calculations, the signals are identifiable by their characteristics. The accurate diagnosis is based on creating a classification pattern for the discrimination algorithm. To substantiate the preciseness of proposed methodology, different states of inrush and fault currents are simulated in PSCADTM/EMTDCTM. Consequently, calculations and the distinction process are carried-out in MATLAB environment. The discrimination procedure needs only current signal data of less than a quarter of power frequency cycle and uses very simple classification rules. Eventually, both simulation and experimental results show that the accuracy of proposed method is high. The results of this study confirm that simple and accurate methods which use patterns can be developed for distinguishing the internal faulty current of transformer from inrush and healthy current.

Adaptive ANN based differential protective relay for reliable power transformer protection operation during energisation

Power transformer is the most expensive equipment in electrical power system that needs continuous monitoring and fast protection response. Differential relay is usually used in power transformer protection scheme. This protection compares the difference of currents between transformer primary and secondary sides, with which a tripping signal to the circuit breaker is asserted. However, when power transformers are energized, the magnetizing inrush current is present and due to its high magnitude, the relay mal-operates. To prevent mal-operation, methods revolving around the fact that the relay should be able to discriminate between the magnetizing inrush current and the fault current must be studied. This paper presents an Artificial Neural Network(ANN) based differential relay that is designed to enable the differential relay to correct its mal-operation during energization by training the ANN and testing it with harmonic current as the restraining element. The MATLAB software is used to implement and evaluate the proposed differential relay. It is shown that the ANN based differential relay is indeed an adaptive relay when it is appropriately trained using the Network Fitting Tool. The improved differential relay models also include a reset part which enables automatic reset of the relays. Using the techniques of 2nd harmonic restraint and ANN to design a differential relay thus illustrates that the latter can successfully differentiate between magnetizing inrush and internal fault currents. With the new adaptive ANN-based differential relay, there is no mal-operation of the relay during energization. The ANN based differential relay shows better performance in terms of its ability to differentiate fault against energization current. Amazingly, the response time, when there is an internal fault, is 1 ms compared to 4.5 ms of the conventional 2nd harmonic restraint based relay.

The Impact of Zero-Mode Inrush Current of T-Hin on Zero-Sequence Overcurrent Protection and an Improved Protection with the Second Harmonic Restraint

In recent years, the zero-mode inrush current of high-impedance transformer with built-in high-voltage winding (T-Hin), which has large amplitude and decays slowly, causes the misoperation of zero-sequence overcurrent protection. Compared with magnetizing inrush current, the waveform of zero-mode inrush current is inconsistent and irregular, and few researches have proposed the mathematical analysis as well as the improved protection using waveform characteristics. In this paper, the mathematical expression of transformer zero-mode inrush current is derived. Further considering the parameter differences, the zero-mode inrush current of T-Hin is larger, which tends to cause the misoperation. The mathematical waveforms fit well with the recorded waveforms. Both recorded waveforms and mathematical waveforms in various conditions prove that the second harmonic ratio (the ratio between the second harmonic and first harmonic) of zero-mode inrush current is significant. Based on the above analysis, a criterion based on the second harmonic ratio restraint of zero-mode inrush current is proposed. If the second harmonic ratio exceeds the setting value, it is considered that the inrush current is generated and sends a signal to restrain the protection. The theoretical setting value of the proposed criterion and the practical engineering method for determining the setting value are obtained.

An Adaptive Reclosing Switch Based on Shunt Resistance

In order to solve problems that the low coincidence success rate of the automatic reclosure of distribution network and short-circuit current has a large impact on the system when it coincides with the permanent fault, a shunt resistance type of adaptive reclosing switch is proposed. In order to improve the success rate of reconfiguration of the distribution network. By selecting the appropriate shunt resistance, the switch can not only suppress the magnetizing inrush current, limit the short-circuit current, reduce the system impact, but also use the current characteristics flowing through the shunt resistance to determine whether there is a permanent fault in the downstream, and realize the adaptive reclosing. The shunt resistance type adaptive reclosing switch can be separately applied to the power distribution switch to replace the original reclosing mode, and as a switch with protection function, the safety of the fault handling process is improved.

Multiscale multivariate fuzzy entropy‐based technique to distinguish transformer magnetising from fault currents

As vectors' similarity is defined on the basis of the hard boundary of Heaviside function in multiscale multivariate sample entropy, the two families of statistics show high sensitivity to the similarity tolerance. Multiscale multivariate fuzzy entropy (MMFE), which applied fuzzy membership function, overcomes the problem. Hence, the visualisation inrush current discrimination algorithm based on MMFE is proposed. The proposed algorithm divides the MMFE-τ plane into two parts: active and braking zones. The ratio of the current entropy area to the given braking zone area is designated as the entropy area ratio. The magnetising inrush and fault current of the transformer are identified on the basis of the numerical value comparison between the entropy area ratio and the fixed value of 1. Through the ATP/EMTP platform and dynamic testing system, the transformer currents under different operating conditions are analysed. Results demonstrate that the proposed algorithm can effectively distinguish the magnetising inrush from the fault current of the transformer and is superior to other methods.

A MEMD-MMFE-based discrimination technique to identify magnetizing current of interconnecting transformer of wind farm

This paper studies the time-frequency characteristics of short-circuit currents in a wind farm during the low-voltage ride-through (LVRT) process. The short-circuit current contains low-order harmonics that may affect second harmonic restraint component of transformer differential protection, while the second harmonic restraint is a classical way to block the differential relay during inrush events for the transformer. A maloperation case of the differential protection employed by the interconnecting transformer of wind farm under the LVRT process is investigated in this paper. It indicates that the second harmonic restraint cannot identify the inrush events reliably of the transformer in a wind farm. A visualization algorithm is proposed to identify the magnetizing inrush current of interconnecting transformer in wind farm based on multivariate empirical mode decomposition and multivariate multiscale fuzzy entropy (MMFE). The current is firstly decomposed into several aligned intrinsic mode functions for the signal preprocessing to reduce the effects of low-order harmonics from the short-circuit current of wind farm. Then the MMFE method is used to calculate the entropy area of the preprocessed current signal. By comparing the entropy area of the transformer current with a given restraint area, the inrush current can be discriminated. The simulation results of a practical wind farm verify the accuracy and effectiveness of the proposed algorithm.

A New Algorithm for Three-Phase Power Transformer Differential Protection Considering Effect of Ultra-Saturation Phenomenon Based on Discrete Wavelet Transform

Abstract One of transient phenomena that lead to the false trip of the power transformer differential protection during the energization of a loaded power transformer is the ultra-saturation phenomenon. This paper presents, at first, a new algorithm for three-phase power transformer differential protection considering effect of the ultra-saturation phenomenon based on Discrete Wavelet Transform (DWT). To model the ultra-saturation phenomenon, the nonlinear characteristic of the transformer core and the effect of the saturation of the current transformers are taken into account. It is assumed that the load of the transformer is a resistive and inductive load. In this algorithm, the ultra-saturation phenomenon, the external and internal faults of power transformer and the magnetic inrush current are simulated. To distinguish between these phenomena, appropriate criteria using DWT by the use of standard deviation of coefficients are presented. Also, one of the most important criteria for the digital relays is the time for making a decision. Thus, to determine the time of decision, the experimental results will be presented.

Comparison and Analysis of Magnetizing Inrush and Fault Condition for Power Transformer

This paper presents the second harmonics present in the primary current of a power transformer at different conditions using Fast Fourier Transform and Total Harmonic Distortion techniques to analyze the inrush condition and to distinguish it with fault condition of a power transformer. Result shows that the 2nd harmonic content is pre-dominant in inrush condition of primary current of the power transformer. It is observed that there are significant differences amongst the parameters found during inrush condition, normal condition and internal fault condition which are useful in the identification of magnetizing inrush current of power transformer. The simulation is done in MATLAB/SIMULINK.

Power transformer differential protection using current and voltage ratios

The main challenge in transformer protection is to find a fast and efficient differential relay algorithm that isolates the transformer from the system causing least damage. Algorithm should also avoid mal-operation while differentiating between the operating conditions. This paper presents an improved differential protection scheme for power transformer. The proposed scheme is based on the ratio of the absolute difference and absolute sum of the primary and secondary currents of each phase, supplemented by the ratio of the absolute difference and absolute sum of the primary and secondary terminal voltages of each phase. The proposed algorithm aims at avoiding mal-operation, possible with the conventional three-phase transformers differential protection scheme due to transient phenomena, including the magnetic inrush current, simultaneous inrush with internal fault, and faults with current transformer saturation. Investigation of the proposed differential protection scheme using both current and voltage ratios shows that it can provide fast, accurate, secure and dependable relay for power transformers.

Power transformer protection using fuzzy logic based controller

The Power Transformer is one of the most expensive and important electrical equipment of a Power Transmission System, the loss of this equipment through catastrophic failure can be very costly. To avoid this, methods and systems for monitoring and protecting of Power Transformers in service are being developed. Since many mal-operation cases of transformer differential protection are caused by inrush current problems and thus, the rapid discrimination of inrush current from internal fault is most essential to avoid the inadvertent operation of the protective relay. Therefore, measures must be taken to ensure that the Power transformer differential relay operates precisely to differentiate between fault and non-fault conditions to ensure service reliability. This differential relay must restrain from tripping during magnetizing inrush and rapidly operate the tripping during internal faults. This paper presents a new relaying algorithm to enhance the fault detection sensitivities of conventional techniques by using fuzzy logic approach of MATLAB/Simulink. The proposed fuzzy-based relaying relies on the principle of harmoniccurrent restraint, where the magnetizing-inrush current is characterized by large second harmonic contents that are not noticeably present in fault currents

A Method for Identification of Transformer Inrush Current Based on Box Dimension

Magnetizing inrush current can lead to the maloperation of transformer differential protection. To overcome such an issue, a method is proposed to distinguish inrush current from inner fault current based on box dimension. According to the fundamental difference in waveform between the two, the algorithm can extract the three-phase current and calculate its box dimensions. If the box dimension value is smaller than the setting value, it is the inrush current; otherwise, it is inner fault current. Using PSACD and MATLAB, the simulation has been performed to prove the efficiency reliability of the presented algorithm in distinguishing inrush current and fault current.

Method to secure the performance of the differential protection in presence of fault current limiter applied into the neutral line

Phase-to-ground fault is the most probable fault type in power systems, which is generally limited through neutral grounding resistor. The grounding resistor may have negative impacts on the system such as decrease in sensitivity of the differential protection. Fault current limiter (FCL) is a suitable alternative for the grounding resistor. However, the input signals of differential protections may be adversely affected by the FCL, resulting mal-operation of this protection. In this study, effects of FCL installed in neutral line of power transformers on the relevant differential protection algorithms are analysed. Performance of some well-known differential protection algorithms for discrimination between internal fault and magnetising inrush currents in presence of the neutral FCL is evaluated in detail. Then, a supplementary method based on the least error squares technique is proposed to rectify the deformed portion of the current waveform due to presence of the FCL. The reconstructed signal is considered as a proper input for the discrimination algorithms. Various simulations and experiments are carried out to validate the performance of the suggested technique. The results indicate the consistency of the suggested technique for different power systems, operating conditions, and real applications.

Study on the Unusual Misoperation of Differential Protection During Transformer Energization and its Countermeasure

The differential protection of the transformer, generator, or transmission line under normal operating conditions may misoperate when the adjacent transformer is switched-on. To research the cause of the misoperations, impacting factors, such as magnetizing inrush, sympathetic inrush, and current-transformer saturation are analyzed based on the field-recorded fault data and the simulation results in this paper. It is disclosed that the CT transient or CT local transient saturation, which are caused by the long-lasting dc component from the magnetizing inrush current, will lead to misoperation. To solve this problem, a new CT saturation detection algorithm based on the dc component in secondary current is proposed to prevent this sort of misoperation, and the effectiveness of the proposed algorithm is proved by the field-recorded fault data and simulation results obtained under different conditions.

Detection of high impedance faults using current transformers for sensing and identification based on features extracted using wavelet transform

High impedance fault (HIF) has long been a challenging problem in network protection due to its random behaviour and low magnitude. The conventional protection devices cannot be utilised since the HIF does not draw enough current to cause tripping. The feature of wavelet transform (WT) which decomposes a signal into different frequency bands and locations in time can be utilised to extract HIF features and detect its occurrence. In the study, HIF arcing currents associated with different types of contact surface are produced by experiments set up in the laboratory. Features of the fault currents are extracted and a new detection criterion is developed based on WT coefficients. Transformer magnetising inrush currents and capacitor switching transients are also produced by experiment and simulation, respectively, to represent transient disturbances commonly occurred in the distribution network. The detection criterion is used to discriminate the HIF from other normal non-fault transient events. Three different types of wavelet are tested and the Daubechies wavelet Db4 gives the best performance based on its detection and discrimination rates. The efficacy of utilising existing conventional current transformers for HIF application is also tested and verified by experiment.