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The three-phase supply of distribution transformers sometimes changes unexpectedly to two-phase supply, due to external reasons, e.g. a burned fuse or interruption of one of the phases. In this case, secondary voltages result, which are no longer equal to the nominal phase voltage in all phases, i.e. one voltage is still equal to the nominal voltage, while the other two are not, but their vector sum is also equal to the nominal voltage, since it is obtained from the same primary polar voltage. This occurs for all connections except the star-to-star connection, for which none of the secondary voltages is equal to the nominal phase voltage. The magnitude of each voltage depends on the ampere-turns, the core dimensions and the magnetic permeability that appears in each core branch (limb). These voltages are calculated from the equivalent magnetic circuit and the magnetising curve of the core material.