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The power transformer is the major electrical equipment in the power system and should be able to resist the damage that caused by the abnormal conditions, like the influence of the inrush current and the electromagnetic force. However, the residual flux, which is the most common influence factor of the inrush, can result in large current magnitude...
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... Factorii care influențează magnitudinea curentului de intrare sunt [1][2][3][4][5][6][7]: ...
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... Differential relay should cater Vector Group of Transformer [14,15], Zero Sequence Current Flows [16], On Load Tap Changers [17], and must not initiate tripping during normal operation of transformer including energization of transformer (Inrush Current [18,19]), over fluxing [20], and tap changer operation. • Differential relay should remain stable during through fault even if the CT saturation of one or more CTs occur [21,22]. ...
In power systems, the programmable numerical differential relays are widely used for the protection of generators, bus bars, transformers, shunt reactors, and transmission lines. Retrofitting of relays is the need of the hour because lack of proper testing techniques and misunderstanding of vital procedures may result in under performance of the overall protection system. Lack of relay’s proper testing provokes an unpredictability in its behavior, that may prompt tripping of a healthy power system. Therefore, the main contribution of the paper is to prepare a step-by-step comprehensive procedural guideline for practical implementation of relay testing procedures and a detailed insight analysis of relay’s settings for the protection of an Extra High Voltage (EHV) auto transformer. The experimental results are scrutinized to document a detailed theoretical and technical analysis. Moreover, the paper also covers shortcomings of existing literature by documenting specialized literature that covers all aspects of protection relays, i.e., from basics of electromechanical domain to the technicalities of the numerical differential relay covering its detailed testing from different reputed manufacturers. A secondary injection relay test set is used for detailed testing of differential relay under test, and the S1 Agile software is used for protection relay settings, configuration modification, and detailed analysis.
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... The shock produced by the magnitude of this current can lead to its deenergization or to the generation of failure situations. The factors influencing the magnitude of the input current are: the residual flux in the core of the transformer, the nonlinear magnetization characteristics of the transformer core, the phase angle of the supply voltage at the moment of the transformer energization, the impedance and the short-circuit power of the power supply [1][2][3][4][5][6][7]. ...
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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 transformer is one of the major pieces of equipment in the electric power system, continued and stable operation of the transformers is very important for reliable power delivery. Electromagnetic forces and stresses have been a growing concern among electric power system operators and researchers. The higher electromagnetic forces and stresses not only damage the transformer but also lead to power system-related issues in the electric power system. In this study, the electromagnetic forces are investigated for the 1500-kVA tap winding transformer with three different primary winding voltage levels. Axial and radial electromagnetic forces are obtained by employing the transformer’s finite element models. Electromagnetic force characteristics of the five different tapping positions are investigated in this work, including (1) lower, (2) middle, (3) upper, (4) side, and (5) upper-lower tappings of the primary voltage winding using finite element analysis. The lowest electromagnetic forces are calculated in the side tapping and upper-lower tapping. The electromagnetic forces are higher in the upper tapping and lower tapping. The short-circuit experimental test is also performed to verify the accuracy of the numerical method.
When the power transformer is connected to the power grid, a surge of inrush current may occur as a result of residual flux. In order to mitigate this effect, measurement of residual flux is necessary. In this paper, a method for measuring the residual flux of ferromagnets was proposed, utilizing magnetic sensors and a finite element model. Residual flux originated from residual magnetization, which could be obtained from a matrix of spatial magnetic flux density measured by sensors and the conversion matrix calculated from the finite element model. By combining the residual magnetization with the background magnetic field, the residual flux could be determined. Experimental results confirmed the accuracy of the proposed method, which was particularly suited for measuring non-uniform residual flux, and also enabled determination of residual flux direction. In comparison to existing methods, this approach offered significant advantages.
When the power transformer is reconnected to the power grid, if there is a large residual flux in the iron core, an inrush current may be resulted. To suppress inrush current, the detail information of the residual flux needs to be known. In this paper, a residual flux density measurement method for single-phase transformer core based on energy changes is proposed. Firstly, when positive and negative DC voltages (referred as the direction along or opposite to the initial residual flux density) are applied, the positive and negative energy changes of 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 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.
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.
