Biblio

The core looseness fault is an important part of transformer fault. The state of the core can be obtained by analyzing the vibration signal. Vibration analysis method has been used in transformer condition monitoring and fault diagnosis for many years, while different methods produce different results. In order to select the correct method in engineering application, five kinds of joint time-frequency analysis methods, such as short-time Fourier transform, Wigner-Ville distribution, S transform, wavelet transform and empirical mode decomposition are compared, and the advantages and disadvantages of these methods for dealing with the vibration signal of transformer core are analyzed in this paper. It indicates that wavelet transform and empirical mode decomposition have more advantages in the diagnosis of core looseness fault. The conclusions have referential significance for the diagnosis of transformer faults in engineering.

In order to improve the reliability of power supply in adjacent hydropower stations, the auxiliary power systems of the two stations are connected through a contact transformer. The magnetizing inrush current generated by the connecting transformer of a hydropower station has the characteristics of high frequency, strong energy, and multi-coupling. The harm caused by the connecting transformer is huge. In order to prevent misoperation during the closing process of the connecting transformer, this article aims at the problem of setting the switching current of the connecting transformer of the two hydropower stations, and establishes the analysis model of the excitation inrush current with SimPowerSystem software, and carries out the quantitative simulation calculation of the excitation inrush current of the connecting transformer. A setting strategy for overcurrent protection of tie transformers to suppress the excitation inrush current is proposed. Under the conditions of changing switch closing time, generator load, auxiliary transformer load, tie transformer core remanence, the maximum amplitude of the excitation inrush current is comprehensively judged Value, and then achieve the suppression of the excitation inrush current, and accurately determine the protection setting of the switch.

This research paper presented a design that will address the challenges brought by remanence in ground-fault current interrupter devices (gfci). Remanence or residual magnetism is the magnetization left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed. Remanence will make the gfci devices less accurate and less reliable in tripping the current above threshold in just five (5) years. It affects the performance of the device in terms of efficiency, accuracy, and response time. In this research, the problems caused by remanence were alleviated by using two identical transformers in detecting residual current both for hot and neutral wires. The difference of the current detected by the two transformers will be the basis of the signal threshold in tripping the device. By doing so, the problems caused by remanence phenomenon will be solved without compromising the response time of the circuit which is around 16 mS. The design will extend the life span of GFCI devices up to 15 years.

Influence of remanent flux on hysteresis curve of the transformer core is addressed in this paper. In addition, its significant negative effect on transformer diagnostic tests is quantified based on experimental studies. Furthermore, a novel approach is proposed to efficiently and quickly eliminate the remanent flux. This approach is evaluated based on simulation studies on a 230/63-kV power transformer. Meanwhile, experimental studies are performed on both 0.2/0.2 and 20/0.4 kV transformers. These studies reveal that the approach not only is well able to eliminate the remanent flux, but also it has various advantages over the commonly used method. In addition, this approach is equally applicable for various power, distribution, and instrument transformer types.

In this paper, a transformer model has been developed. The model is based on the equivalent electrical circuit used in transient simulation studies which considers the non-linearity of the iron core. The non-linear ferromagnetic behavior of the iron core was obtained by using the Jiles-Atherton hysteresis model. The magnetizing inrush current of a core type single-phase transformer was analyzed under four different energization conditions. The primary winding of the transformer was connected to the supply at various instants while there was either some level of remanent flux or no remanent flux in the iron core. Corresponding simulation results are presented and discussed.

An improved current transformer model (CT) developed in DIgSILENT™ is presented in this paper. The hysteresis characteristics of magnetic material described by Jiles-Atheron theory are included. The results show that model can represent the saturation and remanence characteristics of CT core accurately. The model accuracy is verified by comparing the simulation results with PSCAD/EMTDC™.

This paper design three distribution devices for the strong and smart grid, respectively are novel transformer with function of dc bias restraining, energy-saving contactor and controllable reactor with adjustable intrinsic magnetic state based on nanocomposite magnetic material core. The magnetic performance of this material was analyzed and the relationship between the remanence and coercivity was determined. The magnetization and demagnetization circuit for the nanocomposite core has been designed based on three-phase rectification circuit combined with a capacitor charging circuit. The remanence of the nanocomposite core can neutralize the dc bias flux occurred in transformer main core, can pull in the movable core of the contactor instead of the traditional fixed core and adjust the saturation degree of the reactor core. The electromagnetic design of the three distribution devices was conducted and the simulation, experiment results verify correctness of the design which provides intelligent and energy-saving power equipment for the smart power grids safe operation.