Search Results (52)

Residual magnetism in the core of a power transformer can lead to an increased inrush current during closing, which may trigger relay protection malfunctions and cause equipment aging. Accurate detection of residual magnetism is crucial for grid safety. Traditional offline detection requires interrupting operation, while online methods are susceptible to interference and have limited accuracy. This paper proposes a method for detecting residual magnetism in power transformers based on the hysteresis area of magnetization. First, the magnetic flux distribution of the transformer is analyzed through finite element simulation, revealing that low-frequency excitation can make the core’s magnetic flux density distribution more uniform, and that the leakage flux and the flux inside the core have similar characteristics, which helps to determine the optimal position for flux detection. Next, the relationship between the small hysteresis loop area and residual magnetism is studied, revealing a monotonic mapping relationship between the normalized area of the negative hysteresis loop under current pulse excitation and the residual magnetism. Finally, experimental verification shows that this method effectively detects residual magnetism under different levels and operational conditions. The method is non-invasive, real-time, and highly resistant to interference, offering a new approach for residual magnetism detection in power transformers. Full article

To address the security and stability requirements of renewable energy clusters under extreme weather conditions, this study investigates the coordinated voltage support mechanisms between grid-following and grid-forming converters. This paper proposes an emergency control strategy suitable for such scenarios. First, a reactive power-voltage control architecture for new energy units is constructed to clarify the information interaction process. A mode-based coordinated strategy is designed: during steady-state voltage support, grid-following units adopt reactive power-voltage droop control for voltage regulation, while grid-forming units achieve autonomous support based on the virtual synchronous generator algorithm. During low-voltage ride-through, both types of units are controlled to output corresponding reactive power according to the depth of voltage drop until the voltage is restored. Hardware-in-the-loop simulation verification shows that under steady-state conditions, the strategy meets the voltage control accuracy requirements, and partial grid-forming transformation can reduce voltage overshoot and accelerate stabilization. During low-voltage ride-through, grid-forming transformation can reduce voltage fluctuations, shorten adjustment time, and mitigate reactive inrush current, effectively enhancing the voltage support capability of renewable energy plants. Full article

Accurate classification between magnetizing inrush currents and internal faults is essential for reliable transformer protection and stable power system operation. Because their transient waveforms are so similar, conventional differential protection and harmonic restraint techniques often fail under dynamic conditions. This study presents a two-stage classification model that combines a self-organizing map (SOM) and a convolutional neural network (CNN) to enhance robustness and accuracy in distinguishing between inrush currents and internal faults in power transformers. In the first stage, an unsupervised SOM identifies topologically structured event clusters without the need for labeled data or predefined thresholds. Seven features are extracted from differential current signals to form fixed-length input vectors. These vectors are projected onto a two-dimensional SOM grid to capture inrush and fault distributions. In the second stage, the SOM’s activation maps are converted to grayscale images and classified by a CNN, thereby merging the interpretability of clustering with the performance of deep learning. Simulation data from a 154 kV MATLAB/Simulink transformer model includes inrush, internal fault, and overlapping events. Results show that after one cycle following fault inception, the proposed method improves accuracy (AC), precision (PR), recall (RC), and F1-score (F1s) by up to 3% compared with a conventional CNN model, demonstrating its suitability for real-time transformer protection. Full article

The 2 × 25 kV flexible traction power supply system (FTPSS), using a three-phase-single-phase converter as its power source, effectively addresses the challenges of neutral section transitions and power quality issues inherent in traditional power supply systems (TPSSs). However, the bidirectional fault current and low short-circuit current characteristics degrade the effectiveness of traditional TPSS protection schemes. This paper analyzes the fault characteristics of FTPSS and proposes a fault identification method based on empirical wavelet transform (EWT) and 1-sequence faulty energy. First, a composite sequence network model is developed to reveal the characteristics of three typical fault types, including ground faults and inter-line short circuits. The 1-sequence differential faulty energy is then calculated. Since the 1-sequence component is unaffected by the leakage impedance of autotransformers (ATs), the proposed method uses this feature to distinguish the TPSS faults from disturbances caused by electric multiple units (EMUs). Second, EWT is used to decompose the 1-sequence faulty energy, and relevant components are selected by permutation entropy. The fault variance derived from these components enables reliable identification of TPSS faults, effectively avoiding misjudgment caused by AT excitation inrush or harmonic disturbances from EMUs. Finally, real-time digital simulator experimental results verify the effectiveness of the proposed method. The fault identification method possesses high tolerance to transition impedance performance and does not require synchronized current measurements from both sides of the TPSS. Full article

To circumvent the computational bottlenecks associated with the intermediate steps (e.g., least squares fitting) in conventional sine wave similarity principles and directly acquire the energy metrics required for stabilized sinusoidal waveform characterization, this study leverages time domain probability distribution theory. From a complementary advantage perspective, a novel transformer inrush current identification criterion is developed using the Wasserstein distance metric. The methodology employs feature discretization to extract target/template signals, transforming them into state vectors for sample labelling. By quantifying inter-signal energy distribution disparities through this framework, it achieves a precise waveform similarity assessment in sinusoidal regimes. The theoretical analysis and simulations demonstrate that the approach eliminates frequency domain computations while maintaining implementation simplicity. Compared with conventional sine wave similarity methods, the solution streamlines protection logic and significantly enhances practical applicability with accelerated response times. Furthermore, tests conducted on field-recorded circuit breaker closing waveforms using MATLAB R2022a confirm the effectiveness of the proposed method in improving transformer protection performance. Full article

An inrush current will be generated inside a converter transformer during no-load switching on or off. The inrush current will cause severe vibration of the winding and other components of the converter transformer. However, the current research on the vibration characteristics caused by the inrush current is insufficient, and the influence of the converter transformer components cannot be effectively evaluated. Therefore, this paper discusses the building of an electromagnetic transient model of no-load closing of a conventional DC converter station, and analyzes the time–frequency characteristics of the inrush current under different working conditions. The finite element model based on the actual converter transformer was built and verified. The vibration characteristics of some converter transformer components under excitation of the inrush current were studied. The research results can monitor the vibration of a converter transformer under different working conditions, and can avoid the damage of converter transformer components caused by an excessive inrush current. Full article

The residual magnetism of the iron core of power transformers can cause an excitation inrush current, posing a threat to the safe and stable operation of the power grid. This paper proposes a transformer remanence measurement method based on a demagnetization hysteresis loop to address the problems of large errors, complex operation, and poor universality in existing remanence measurement methods. This method is designed for off-grid transformers to avoid potential interference to the power grid caused by current pulses during the measurement process. This method constructs an RLC oscillation circuit that utilizes capacitor energy storage and iron core magnetic field energy conversion, combined with the dynamic characteristics of hysteresis loops, to achieve accurate measurement of residual magnetism and synchronous demagnetization. The effectiveness of this method has been verified through residual magnetism measurement experiments on ring transformers and large converter transformers, and it can be applied in specific engineering practice operations. Theoretical analysis shows that the charging range of energy storage capacitors is affected by the hysteresis characteristics of the iron core and the saturation magnetic flux, and the residual magnetization value can be directly calculated based on the difference in the intersection point of the longitudinal axis of the demagnetization hysteresis loop. Simulation and experimental results show that the measurement error of the proposed method is less than 5%—significantly better than traditional methods. This method does not require complex control strategies, has high precision and efficiency, and can provide reliable technical support for residual magnetism detection and suppression of off-grid power transformers. Full article

Circuit breakers are effectively utilized for the controlled switching technique to mitigate inrush current when energizing an unloaded transformer. The core of the controlled switching technique is to obtain the appropriate closing angle based on the residual flux after opening. For the prediction of residual flux, the voltage integration method faces the difficult problem of determining the integration upper limit, while the Jiles- Atherton (J-A) model has the advantages of clear physical meaning of parameters, accurate calculation, and the ability to iteratively solve residual magnetism. It has low dependence on the initial conditions and greatly avoids the influence of DC offset and noise on measurement results. Firstly, an improved particle-swarm optimization algorithm is proposed in this paper to address the problem of slow convergence speed and susceptibility to local optima in current particle-swarm optimization algorithms for extracting J-A model parameters. The problem of slow convergence speed and susceptibility to local optima in traditional particle-swarm optimization algorithms is solved by optimizing the velocity and position-update formulas of particles in this algorithm. This new algorithm not only accelerates convergence speed, but also balances the overall and local search capabilities. Then, based on the J-A model, residual flux prediction of the transformer is carried out, and a transformer no-load energization experimental platform is built. A simulation model combining the J-A model and classical transformer is constructed using PSCAD/EMTDC to predict the residual flux of the transformer at different closing angles. Finally, by combining simulation with actual experimental waveform data, the accuracy of residual flux prediction was verified by comparing the peak values of the inrush current. Full article

In practical engineering, due to the residual magnetism in the iron core of the converter transformer, the inrush current is generated in the case of no-load closing, and the inrush current leads to an oil flow surge inside the converter transformer. Under the influence of oil flow, the gas relay plate will be deflected, and when the deflection angle is too large, the gas relay will malfunction. Because of the lacking research on the influence of the misoperation of the gas relay under the excitation inrush of the converter transformer, it is difficult to effectively suppress the misoperation of gas relay under inrush current. Therefore, the finite element model of the electromagnetic hydraulic coupling of the converter transformer is established in this paper to obtain the oil flow velocity through the gas relay under the inrush current. A fluid–solid coupling model was established inside the gas relay to study the deflection characteristics of the baffle of the gas relay under the inrush current, and the relationship between the inrush current amplitude and the deflection angle of the baffle was mathematically fitted to effectively predict whether the gas relay has the risk of misoperation. From the experimental results, it can be seen that when the amplitude of the inrush current exceeds 6.37 kA, the gas relay will have the risk of misoperation. Finally, the influence of oil flow impact on the gas relay baffle under multiple cycles is considered. The results can provide an effective reference for analyzing the diagnosis of gas relay misoperation under the effect of inrush current. Full article

During operation, converter transformers enter a saturation state, leading to phenomena such as magnetising inrush currents. Accurately measuring the excitation characteristic curve of an iron core under deep-saturation conditions is essential for analysing low-frequency transient phenomena in transformers. This paper presents a method for calculating the excitation characteristics of a converter transformer under deep iron core saturation. The method involves establishing an improved T model for the converter transformer and conducting open-circuit experiments in the linear working region to obtain the excitation characteristic curve and knee point parameters. AC-DC hybrid excitation is used to achieve deep saturation, and measurements of saturated inductance at different levels of saturation at the transformer terminals are taken. The mathematical relationship between saturated inductance and magnetic impedance is derived, allowing deduction of the magnetising characteristic curve of the converter transformer under deep-saturation conditions based on measured saturated inductance values. A finite element simulation analysis was performed on a single-phase four-column converter transformer with a capacity of 250 MVA. Additionally, a test platform for toroidal transformers and dry-type transformers has been set up to carry out excitation characteristic measurement and verification. Experimental results demonstrate that errors are maintained within 10% or less, validating this approach’s effectiveness. Full article

Inrush current is still a persistent problem affecting the quality of the power system. This paper presents the theoretical aspects of this transient phenomenon, the simulation of the phenomenon for prediction purposes and its measurement in a test laboratory. In the operation of power transformers, there may also be cases where electrical phenomena affecting the operation of transformers may overlap. When power transformers are in operation, there may also be cases where electrical phenomena affecting their operation overlap. This paper includes a study describing such a situation, where the maximum value of the inrush current is amplified by the fact that the series resonance condition has been met. The intersection of these electrical phenomena resulted in internal and external electrical discharges that led to the overhaul of a 440 MVA transformer; the description of this situation is based on field data. The paper focuses on real information and situations that support the development of a maintenance management schedule based on accurate and up-to-date data. The information presented in this paper will be particularly useful to personnel specializing in power transformer design and/or those monitoring/operating high power transformers in the energy sector, but can also be a teaching aid for students interested in such transient phenomena. Full article

This article presents the measurement verification of a novel strategy for inrush current reduction in an unloaded three-phase Dy transformer. The strategy combines appropriate pre-magnetization of transformer cores with an original control switching system using initial phase values of the supply voltage as control variables. Measurements were recorded for primary voltages and currents as well as secondary voltages during transient states at start-up under no-load conditions. Various inrush scenarios were examined across the full angular spectrum of initial phase angles, both polarities, and in regard to different pre-magnetization current values. A detailed analysis of the inrush currents was performed using proprietary automated software based on the recorded data. A comparative study with a nonlinear mathematical model of the transformer was also conducted. Additionally, key technical aspects of the designed system for implementing the proposed pre-magnetization strategy with controlled voltage energization are discussed. Full article

Grid-forming inverters (GFMIs) have been identified as critical assets in ensuring modern power system reliability. Their ability to synthesize an internal voltage reference while emulating synthetic inertia has sparked extensive research. These characteristics have recently piqued interest in their capacity to provide blackstart ancillary services. The blackstart of a bulk power system poses significant challenges, namely the large transients from the energization of unloaded transformers, rotational motor loads, and long transmission cables, which have been effectively studied using conventional synchronous generators (SGs). The concept of an inverter-based resource (IBR)-based blackstart continues to be an open research area necessitating further investigations due to the known limitations of IBRs such as low short-circuit current capabilities. This paper presents a blackstart case study of a bulk power system investigating the performances of a conventional SG to a GFMI when utilizing hard switching methods. The paper qualitatively investigates the transient inrush currents from the transformer and rotational load energization sequences. Additional examinations into the significance of the GFMI’s current-limiting schemes and voltage control loop compensator gains are presented. Furthermore, the harmonic distortions from the transformer energization sequence are also evaluated. Finally, a full network energization case is presented to demonstrate how both sources can provide blackstart provisioning services. The models are developed in EMTDC/PSCAD using real-world transmission planning data. Full article

Three LED drivers which can be used for illumination, but whose main task is the transmission of information (data) via the light of the LEDs, are explored in this paper. The converter circuits need no capacitors for the energy transformation and avoid an inrush current. The lack of necessity of electrolytic capacitors reduces cost and space. Dimming the illumination is also easy to achieve. The control concept of the converters and the generation of pulsing of the LEDs for transmitting the information (data) are explained. The converters can also be expanded to more stages to drive more LEDs with different types of information. All three converters are explained in detail; all presented circuits are built up and simulated with LTSpice. Several data transmission concepts are applied and demonstrated through simulations. Full article

This article presents the results of an experimental verification of three-phase Dy transformer dynamics under no-load conditions. This study is motivated by previous ferroresonance analyses where the occurrence of inrush currents has been observed. The measurements covered all available electrical quantities in a transient state (12 measured and 3 additionally computed waveforms) during the device’s start-up under no-load conditions, as well as in a long-term steady state. A detailed analytical analysis is carried out for the obtained comprehensive set of measurement results. As a result of the conducted research, the mathematical model of the pre-magnetized three-phase Dy transformer is modified. Particular attention is paid to the issue of residual magnetism of the transformer core and its consideration in further research. The original strategy for energizing a three-column three-phase Dy transformer with a suitable pre-magnetization of its columns and original control switching system with a given/set value of the initial phase in the supply voltage is put to the test. The evolution of the induced inrush phenomenon up to the quasi-steady state under given (forced) conditions is documented (currents, voltages and the dynamics of changes taking place in the core (hysteresis loops)). This article represents a continuation of ongoing work on the study of transient states (dynamics of transformer inrush currents). At present, the Dy three-phase transformer is analyzed because of the requirements of industrial operators. Full article