The power transformer is a key device in the power grid systems. The mechanical degradation of windings represented by the clamping force looseness will cause the decline of the short circuit withstand ability, and cause further damages. This paper proposes a clamping force diagnosis method for operating windings based on the study of the vibration response. In the theory part, the influence of the load current on the natural frequency of windings is discussed, and the influence of the natural frequency change on the steady-state vibration response is studied to obtain the vibration feature related with the clamping force. The subspace method is used to fuse two vibration sequences with the same characteristics to eliminate the measurement error. In the experiment, the free vibration test was performed on a short circuit on-load winding structure. It was proved that the natural frequency change can be extracted from the relationship between the amplitude change of the 100 Hz component and the current change. In the field tests, the vibration sequences of two typical transformers were compared, and the results show that the vibration feature extracted from the relationship between the amplitude variation and the current change contains the structural information of windings.
The existing transformer short-circuit test equipment and technology have several limitations. The purpose of short-circuit test is to simulate the test of high-voltage equipment under the short-circuit fault condition of the power system, usually, a large capacity test such as transformer burst short-circuit test, circuit breaker dynamic heat stability test, to verify the reliability and stability of related equipment under extreme working conditions. At present, the large capacity test generally uses methods of power grid dedicated line power supply and short-circuit testing alternator group. However, these two methods have difficulties in construction approval, high investment costs, complex site requirements, a large impact on the power grid, and low control accuracy. Theoretical studies on equivalent simulation verification method of short-circuit withstand capability test of distribution transformer demonstrated the feasibility of using modern power electronics and advanced energy storage technology to replace conventional power grid dedicated line power supply, short-circuit testing alternator and other short-circuit impact test equipment. It could complete the transformer short-circuit impact test specified by (IEC60076-5) with lower cost and higher efficiency, verify the short-circuit impact resistance of the transformer, and improve the test capacity.
An analytical formulation for magnetic field penetration through a circular aperture in a thin perfect electric conductor (PEC) screen with finite-thickness is developed. Especially, we propose an analytical formula for the magnetic coupling between two circular loops separated by the screen and placed coaxially with the aperture. The formula is verified by finite element simulations and experimental data. The thickness effect is explained by the cut off attenuation of TE01 mode in the aperture. The influences of loopscreen-loop distance and loop radius on the coupling intensity are also investigated.
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