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The paper presents the analysis of magnetic field that surrounds the power transformer after it has been switched off. The purpose of this paper is to determine the possibility of defining the residual fluxes in the legs of the transformer based on the measurement of this field. It was also intended to determine the type and the location of magnetic sensors.

Numerical analysis of the magnetic field was performed. A three-dimensional model of the transformer’s magnetic core was created in the Flux 3D simulation program. The analysis was concerned with an oil-filled transformer and a dry transformer. The magnetic field of Earth was taken into account.

The research has shown that magnetic induction of the leakage field produced by residual magnetization of the core is comparable to the magnetic induction of the Earth’s field. It was also found that the measurement of the magnetic induction should be performed as close as possible to the core. The interior of the tank turned out to be a convenient space for the placement of the sensors.

The influence of external ferromagnetic objects, and devices generating magnetic field, on the measurement was not considered. It should be taken into account in the future work.

On the basis of the analysis, it was proposed to measure the magnetic induction vector of the leakage field at three points. The sensors should be placed in front of the columns at a position that is half of their height. The measurement can be performed with satisfactory accuracy by sensors located on the surface of the windings.

The paper presents two new algorithms of controlled switching the power transformer. The purpose of this paper is to obtain formulas that determine the moments of closing of the circuit breaker poles. The study contains projects of control systems for both algorithms.

Mathematical formulas for the time instants of the breaker poles closing were developed on the basis of electric circuit theory and magnetic circuit theory. The presented systems were simulated using a model created in the Alternative Transients Program/Electromagnetic Transients Program software.

Numerical simulations have proved that the shown systems properly perform the controlled switching carried out in accordance with the proposed algorithms. The times of the poles closing were correctly determined and the inrush currents were reduced to a level of the current of unloaded transformer.

The results achieved are better than those shown in the literature. The solutions presented in the literature provide a reduction of inrush current to a value comparable to the rated current of the transformer, which is ten times greater than the no-load current. Additional achievement of the work is the development of analytical formulas that determine the times of the breaker poles closing.