Variable power transformer with voltage and phase shift control
Puoskari, Lauri (2025)
2025
Sähkötekniikan DI-ohjelma - Master's Programme in Electrical Engineering
Informaatioteknologian ja viestinnän tiedekunta - Faculty of Information Technology and Communication Sciences
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Hyväksymispäivämäärä
2025-08-18
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202508188309
https://urn.fi/URN:NBN:fi:tuni-202508188309
Tiivistelmä
Voltage and phase shift control are essential in modern electric power systems. Traditionally voltage control has been implemented with power transformers equipped with on-load tap chang ers, and phase shift control with phase shifting transformers. These devices however have their own disadvantages such as mechanical fatigue and discrete step-wise operation. This thesis presents a novel power transformer design that can achieve continuous voltage and phase angle control by controlling the magentic flux inside the core structures of the transformer.
The transformer presented in this thesis utilizes the nonlinear magnetic characteristics of its core structures to control the magnetic flux paths in them. The core structures are saturated locally by feeding a direct current (DC) through auxiliary DC biasing windings, restricting the flow of magnetic flux through said part of the core. The goal of the transformer is to be able to produce an operating area of 0.9–1.1 p.u. secondary voltage and -60–60°phase shift.
The transformer was designed for a 50 Hz three-phase power system with 110 kV primary voltage and 20 kV secondary voltage. The transformer consists of three separate five-limbed cores for each primary phase with a coil on each limb. The primary coil of each phase is on the middle limb and to its left are the secondary coils of the same phase and to its right are the secondary coils of the next phase. The DC biasing windings are on both top and bottom yokes of the core structures between each limb.
An analytical model for the transformer was defined, and it was used for determining the ap propriate numbers of turns for the coils. The resultant numbers of turns however did not produce the sought-after operating area, and the numbers of turns were therefore manually adjusted. The results of the analytical model were then used to determine the practical dimensions of the power transformer. A two-dimensional model with a mesh structure was then produced for finite element analysis (FEA).
Through static FEA, it was found that thin air gaps have to be added in the two limbs around the middle limb of the core structure to have the magnetic flux divide more evenly in the unsaturated scenario, since the voltages produced by the transformer without the air gaps were way too low for the intended operation. Through dynamic time-stepping FEA, it was in turn found that the voltage waveforms have increasing harmonic distortion with increasing absolute value of phase shift. The effect was smaller for the transformer without the air gaps, but still significant. The FEAs were carried out in no load conditions.
An operating area of 0.9–1.1 p.u. secondary voltage and -58–58°was achieved, which is fairly close to the original goal. The harmonic distortion in the secondary voltages, however, presents a new problem. In addition to reducing the harmonic distortion in the voltages, the transformer should also be tested in loaded conditions in the future.
The transformer presented in this thesis utilizes the nonlinear magnetic characteristics of its core structures to control the magnetic flux paths in them. The core structures are saturated locally by feeding a direct current (DC) through auxiliary DC biasing windings, restricting the flow of magnetic flux through said part of the core. The goal of the transformer is to be able to produce an operating area of 0.9–1.1 p.u. secondary voltage and -60–60°phase shift.
The transformer was designed for a 50 Hz three-phase power system with 110 kV primary voltage and 20 kV secondary voltage. The transformer consists of three separate five-limbed cores for each primary phase with a coil on each limb. The primary coil of each phase is on the middle limb and to its left are the secondary coils of the same phase and to its right are the secondary coils of the next phase. The DC biasing windings are on both top and bottom yokes of the core structures between each limb.
An analytical model for the transformer was defined, and it was used for determining the ap propriate numbers of turns for the coils. The resultant numbers of turns however did not produce the sought-after operating area, and the numbers of turns were therefore manually adjusted. The results of the analytical model were then used to determine the practical dimensions of the power transformer. A two-dimensional model with a mesh structure was then produced for finite element analysis (FEA).
Through static FEA, it was found that thin air gaps have to be added in the two limbs around the middle limb of the core structure to have the magnetic flux divide more evenly in the unsaturated scenario, since the voltages produced by the transformer without the air gaps were way too low for the intended operation. Through dynamic time-stepping FEA, it was in turn found that the voltage waveforms have increasing harmonic distortion with increasing absolute value of phase shift. The effect was smaller for the transformer without the air gaps, but still significant. The FEAs were carried out in no load conditions.
An operating area of 0.9–1.1 p.u. secondary voltage and -58–58°was achieved, which is fairly close to the original goal. The harmonic distortion in the secondary voltages, however, presents a new problem. In addition to reducing the harmonic distortion in the voltages, the transformer should also be tested in loaded conditions in the future.