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Page 109

| Figure 7-2 Connection for testing polarity of single-phase, two-winding transformer (subtractive polarity shown). |

| voltage across the open leads is greater than that applied across the H or high side winding, the polarity is additive, if less, the polarity is subtractive. If there are additional windings, the third is marked with the letter Y and the fourth with Z, the letter H for the highest-voltage winding, X for the next highest, etc. |

| Three-phase Transformers |

| Polarity for three-phase transformers is somewhat more complex. In addition to the direction of the winding, there may exist an angular displacement between the secondary voltages, depending on how the high voltage windings are connected to the three-phase primary; these will be described below. |

| To simplify the connection of three-phase transformers, the H1 lead is brought out on the right-hand side facing the high-voltage side of the transformer case. The X1 lead is brought out on the left-hand side facing the low-voltage side. The remaining H and X leads are brought out numbered as shown in Table 7-1. Winding leads marked Y and Z, if any, are brought out and numbered in the same manner as the X leads. |

| A single-phase check of the three-phase transformers can be made by testing each phase, one at a time. Referring to one of the connections in Table 7-1 (e.g., the delta-delta connection), a low ac voltage may be applied between H1 and H2 and the voltage measured between H2 and X2 should be equal to the difference of the voltages measured between H1 and H2 and between X1 and X2. Similarly, a low ac voltage may be applied between H1 and H3 and the voltage measured between H3 and X3 should be equal to the difference of the voltages measured between H1 and H3 and between X1 and X3. Again, applying the low ac voltage between H2 and H3, the voltage measured between H2 and X2 should be equal to the difference of the voltage measured between H2 and H3 and between X2 and X3. |

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