Solution of Topical Issues in Theory of Transformers

In the article, the authors summarized their experience in solving some controversial issues in the theory of transformers, and briefly outlined the essence of their developments. For power transformers of the TM series, the presence of an excess of the active resistance of the primary winding relative to the reduced active re-sistance of the secondary winding was established. The obtained correction factors, taking into account the electrical skin effect in the windings and their heating, can be used to calculate short-circuit currents and heat distribution in other types of transformers. An expression is given that makes it possible to correct the transfor-mation ratio of one of the transformers connected to parallel operation in such a way that it becomes possible to fully load both devices, despite their different short-circuit voltages. Expressions are presented that make it possible to reveal the ratio of hysteresis and eddy-current losses in the magnetic circuit of a particular trans-former, as well as the exponent with which the losses in the magnetic circuit depend on the frequency of mag-netization reversal. A method is proposed for determining the three components of losses in a magnetic circuit, based on an idle experiment at three frequencies. A method is described for determining the thickness of sheets of a magnetic circuit, at which the total losses in it are minimal, and a method for determining the heating time constant of dry transformers is presented, which makes it possible to accelerate their thermal tests.

Download Full-text

Two-Phase Short Circuit in the Power Grid Powered by a Transformer with a Y/Δ-11 Winding Connection

Elektrotekhnologii i elektrooborudovanie v APK ◽

10.22314/2658-4859-2020-67-3-25-30 ◽

2020 ◽

pp. 25-30

Author(s):

Aleksandr S. Serebryakov ◽

Vladimir L. Osokin ◽

Sergey A. Kapustkin

Keyword(s):

High Speed ◽

Short Circuit ◽

Relay Protection ◽

Electric Circuit ◽

Operational Reliability ◽

Research Purpose ◽

Industrial Complex ◽

Two Phase ◽

Short Circuit Currents ◽

Secondary Winding

The article describes main provisions and relations for calculating short-circuit currents and phase currents in a three-phase traction transformer with a star-triangle-11 connection of windings, which feeds two single-phase loads in AC traction networks with a nominal voltage of 25 kilovolts. These transformers provide power to the enterprises of the agro-industrial complex located along the railway line. (Research purpose) The research purpose is in substantiating theoretical equations for digital intelligent relay protection in two-phase short circuits. (Materials and methods) It was found that since the sum of instantaneous currents in each phase is zero, each phase of the transformer works independently. We found that this significantly simplifies the task of analyzing processes with a two-phase short circuit. In this case, the problem of calculating short-circuit currents in the traction network can be simplified by reducing it to the calculation of an ordinary electric circuit with three unknown currents. (Results and discussion) The article describes equations for calculating short-circuit resistances for one phase of the transformer when connecting the secondary winding as a star or a triangle. The currents in the phases of the transformer winding at short circuit for the star-triangle-11 and star-star-with-ground schemes are compared. It was found that when calculating short-circuit currents, there is no need to convert the secondary winding of the traction transformer from a triangle to a star. (Conclusions) It was found that the results of the research can be used in the transition of relay protection systems from electromagnetic relays to modern high-speed digital devices, which will increase the operational reliability of power supply systems for traction and non-traction power consumers.

Download Full-text