TRANSFORMER THEORY Transformers TABLE 1:   Voltage and Current Ratings of Transformers Transformer Connection (Primary to Secondary) Primary Secondary Line Phase Line Phase Volt. Current Volt. Current Volt. * Current Volt. Current D-D V I V I 3 V a aI V a aI 3 Y-Y V I V 3 I V a aI V 3 a aI Y-D V I V 3 I V 3 a 3 aI V 3 a aI D-Y V I V I 3 3 V a aI 3 V a aI 3 *a = N₁/N₂; 3 1.73 Example 1: If  line  voltage  is  440  V  to  a  3f  transformer  bank,  find  the  voltage  across  each primary winding for all four types of transformer connections. D-D:   primary voltage = V = 440 volts Y-Y:   primary voltage = V 3 440 1.73 254.3  volts Y-D:   primary voltage = V 3 440 1.73 254.3  volts D-Y:   primary voltage = V = 440 volts Example 2: If line current is 10.4 A in a 3f  transformer connection, find the primary phase current. D-D:   primary phase current = I 3 10.4 1.73 6  amps Y-Y:   primary phase current = I = 10.4 amps Y-D:   primary phase current = I = 10.4 amps ES-13 Page 10 Rev. 0