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Three identical single-phase transformers are connected with high-voltage windings denoted A-B-C and low-voltage windings labeled a-b-c, representing their respective phases. This configuration creates a three-phase transformer system with terminals across all four winding types. The notation distinguishes between high and low voltage sides, enabling efficient three-phase power distribution and analysis.

Base quantities include a common base complex power and a voltage base ratio matching the ratio of rated line-to-line voltages. These base quantities ensure consistency and facilitate comparison across different components and systems. They simplify analysis by standardizing measurements, making it easier to evaluate transformer performance and behavior across the entire three-phase system.

In balanced three-phase transformers, neutral currents are zero because the three phase currents sum to zero. Consequently, there are no voltage drops across neutral impedances. This characteristic results in a per-unit equivalent circuit that resembles a single-phase ideal transformer, significantly simplifying analysis and understanding of the three-phase system.

Equations for single-phase three-winding transformers are derived by extending two-winding transformer equations. These relations are expressed in both actual and per-unit measures, ensuring they conform to the per-unit equivalent circuit model. This extension is crucial for accurately representing the practical behavior and performance of three-winding transformers in electrical systems.

Open-circuit tests evaluate external series impedances and shunt admittance branches in practical three-winding transformers. When one winding is open, the transformer behaves like a two-winding transformer. Standard short-circuit tests then determine per-unit leakage impedances, which represent series impedance elements in the equivalent circuits for practical transformers.

The equivalent circuit is simplified by neglecting shunt admittance branches and focusing on per-unit series impedances derived from leakage impedances obtained through short-circuit tests. This simplification provides a clearer understanding of transformer performance while maintaining accuracy for practical design and operation. The resulting model effectively represents both ideal and practical characteristics of the transformer.

When one winding of a three-winding transformer is open, the transformer behaves like a two-winding transformer. This characteristic is utilized during open-circuit testing to evaluate external series impedances and shunt admittance branches. Understanding this behavior is essential for proper testing procedures and accurate determination of transformer parameters.