Cailing Huo

The electromagnetic acoustic transducer (EMAT), which uses a flexible dual-coil configuration can be easily applied for non-contact ultrasonic testing of curved structures and objects with narrow inspection space. In this paper, the high-temperature performance of a flexible dual-coil EMAT on ferromagnetic steel measurement is analyzed. The flexible dual-coil EMAT consists of an electromagnetic coil, an eddy-current coil, and a test specimen. Firstly, the working principle and transduction mechanism in ferromagnetic steel by the designed dual-coil EMAT are analyzed. Then, a finite element model of the designed dual-coil EMAT is established, and the magnetic field and acoustic field distribution of the electromagnetic coil are simulated. Finally, a high-temperature experimental platform of flexible dual-coil EMAT is built, and the relationship between the designed EMAT’s received signal waveform and the temperature is analyzed. The results show that the designed EMAT allows the measurement of ferromagnetic steels between 25°C to 770°C without physical coupling or active cooling.

This paper proposes a residual flux detection method based on the different polarities of response currents. When a positive–negative alternating DC voltage is loaded, the direction of the residual flux can be determined by the difference between the various polarities of the response current waveforms. As a result, residual flux value can be calculated using the empirical formula that relates residual flux to the responce current. In this formula, unknown parameters can be obtained using the field-circuit coupling method. Finally, this paper employs a closed iron core as an example to obtain the corresponding formula and then verifies its accuracy through experiments. The results show that the accuracy of the proposed method is less than 5%, which is higher than that of other existing methods. The method presented in this paper not only accurately detects the residual flux of a transformer but also requires no additional energy from the transformer.

When the power transformer is out of operation, due to the inherent hysteresis properties of the iron core material, the residual magnetism will be appeared in the iron core and then a large inrush current may be generated, which will lead to the closing failure of the transformer. Therefore, the study of accurate residual magnetism detection methods can avoid the harm of inrush current to the power grid. This paper first analyzes the formation mechanism and detection principle of residual magnetism. Then, the existing detection methods are summarized, mainly including empirical estimation method, voltage integration method, pre-magnetization method, DC and AC indirect detection methods. On the basis of summarizing the advantages and disadvantages of existing methods, their accuracy, directionality, and applicability are compared and analyzed in this paper. Finally, the problems and development trends of these methods are summarized and prospected. This research in this paper can not only provide reference for the demagnetization of transformer cores, but also provide a certain theoretical basis for solving the problem of inrush current.

Residual magnetism is one of the prime reasons contributing to the inrush current problem in power transformers, so effective detection of residual magnetism will help solve the inrush current problem. In this article, a residual magnetism detection method of power transformer cores based on magnetization time is proposed, and then efficient demagnetization of iron cores is realized. First, when positive and negative alternating small excitations are applied, the polarity of residual magnetism is discriminated by analyzing the positive and negative magnetization times. Second, the residual magnetism value is determined by analyzing the relations between the residual magnetism and the positive magnetization time using the finite element method, and experiments verify that the accuracy of the presented method is within 5%. Finally, based on the proposed detection method, the demagnetization excitation under known residual magnetism is determined, and the experimental results show that the demagnetization effect is better, which further proves the accuracy of the presented detection method. The research in this article will be used to test the residual magnetism of power transformers without any requirements for transformers, which has important scientific theoretical significance and engineering application value.

When the power transformer is reconnected to the power grid, and if there is a large residual flux in the iron core, an inrush current may result. To suppress inrush current, the detailed information of the residual flux needs to be known. In this article, a residual flux density measurement method for a single-phase transformer core based on energy changes is proposed. First, when positive and negative dc voltages (referred to as the direction along or opposite to the initial residual flux density) are applied, the positive and negative energy changes of the power source can be obtained by measured response currents. Then, the direction of the residual flux density is determined by comparing the gained energy changes. The magnitude of residual flux density can be calculated by the empirical formula between residual flux density and positive energy. The square iron core is investigated in this work to establish the corresponding empirical formula by finite element method. Finally, the accuracy of the empirical formula is verified on the experimental platform. The results show that the relative error of the proposed method is within 5%, which has higher accuracy than existing methods.

When a power transformer is reconnected to a power grid, if the residual flux in its iron core is large, significant inrush current may be generated and result in closing failure. Therefore, accurate residual flux measurement is necessary to avoid the harmful effects of inrush current. This work proposes a residual flux density measurement method for the power transformer core based on magnetising inductance. Firstly, when positive and negative DC voltages are applied along or opposite to the direction of the initial residual flux density, the measured positive magnetising inductance is smaller than the negative so that the direction of residual flux density can be determined by comparing their values. Secondly, the magnitude of residual flux density can calculated by analysing the empirical formula between residual flux density and positive magnetising inductance using the finite element method. Finally, this work takes the square iron core as the research object, establishes the corresponding empirical formula, and verifies its accuracy through experiments. The experimental results show that the proposed method has higher accuracy compared with the voltage integration method widely used in this field.

Power transformer is the main part of the power grid. After some tests and operations of transformers, a certain residual flux density ( $B_{r}$ ) is left in the iron core due to the ferromagnetic material hysteresis phenomenon. When the transformer is switched on with no load, $B_{r}$ will lead to inrush current. Therefore, research on measuring $B_{r}$ is of great significance for transformer protection. In this article, a $B_{r}$ measurement method based on applied dc voltage is proposed. First, combined with the magnetic circuit and transient circuit analysis, the relationship between $B_{r}$ and the extremum value of the transient current difference is obtained. Then, the finite element method (FEM) is used to calculate the transient current under different $B_{r}$ and test circuit parameters; the suitable test circuit parameters will be determined, and then the empirical formula for calculating $B_{r}$ is obtained. Finally, the experimental results verify the correctness of the simulation analysis.

Residual flux density in the single-phase transformer core can dramatically increase inrush current when the transformer is energized. To reduce inrush current, it is necessary to study residual flux density measurement. This paper proposes a new residual flux density measurement method based on time constant. Firstly, the generation principle of residual flux density is analyzed under different magnetization states, and it is found that the positive relative differential permeability is smaller than the negative at different residual flux density. To obtain the relative differential permeability, when an appropriate DC excitation is applied, the measurement circuit is equivalent to a first-order RL circuit. Then, combining magnetic circuit and transient circuit analysis, the relationship between time constant and relative differential permeability is obtained. It is conclusion that the positive time constant is less than the negative. Residual flux density direction is determined by comparing the positive and negative time constant, and the magnitude of residual flux density is calculated by the relationship between residual flux density and the difference of the positive and negative time constant. Finally, the empirical formula between residual flux density and time constant difference of the square core is obtained in finite element method, and then verified on the experimental platform. Compared with other measurement methods, the relative error of proposed empirical formula is within 4.58 %, and it has higher accuracy in this paper. The proposed method in this paper can provide a reference for selecting the demagnetization voltage, which improves the effectiveness of demagnetization.

The residual flux measurement is one of the difficulties in the transformer research field. This paper presents a residual flux density (Br) measurement method considering the relative differential permeability (μrd) of the core material. Firstly, the basic relationship between μrd and Br is analyzed on the basis of the magnetic domain theory. Then, combining the experimentally measured relationship between Br and μrd, using μrd as an intermediate variable, the relationship between Br and time constant is derived. Furthermore, a square iron core model is established by the finite element method, and the empirical formula for calculating Br is fitted. Finally, the feasibility of this method and the accuracy of the proposed empirical formula are verified through experiments.

When the no-load transformer is connected to the power grid, an inrush current may be resulted as the existed residual flux in the transformer core. To suppress this inrush current, the detail in-formation of the residual flux needs to be known. In this paper, a transient current method for obtaining the residual flux density in the single-phase transformer is proposed. Firstly, two DC volt-ages with opposite polarities are sequentially applied in the meas-uring winding, and then two corresponding transient response currents can be detected. Based on above information, an empiri-cal equation in regarding of the residual flux density and the transient current difference will be obtained. To make sure the in-itial residual flux density does not change greatly, the magnitude of the DC voltage has been determined as well. Finally, the pro-posed method is verified by the experiment results.