The Art and Science of Transformer Ratio Measurement

The principle which allows one to measure voltage ratio and convert this reading to transformer turns ratio sounds very simple. In practice, however, the conversion is influenced by practical issues, such as cores whose permeability and loss changes with applied voltage and causes conversion errors. This is especially troublesome when testing three-phase transformers using single-phase excitation. Three-phase transformers use cores more efficiently than do single-phase transformers, thus the loss and permeability effects are reduced, provided three-phase excitation is employed. The advantages of using three-phase excitation for ratio measurement of three-phase transformers include: * reduced dependence on test voltage. * ability to identify transformer's configuration. * ability to measure directly the ratio and phase shift of phase shifting transformers. * ability to provide additional information regarding the balance of the phases, shorted turns and excessive core loss.