The three phases of transformer windings can be arranged in star - , delta - or zigzag -connection. The vector group (star: Y or y; delta: D or d; zigzag; Z or z) is indicated on the transformer ’ s nameplate, with a capital letter for the highest voltage side and small letters for the windings on the other voltage levels. The neutral connection, if brought out of the transformer tank, is indicated by the letters N and n. The respective number of the vector group indicates the phase - shift as multiples of 30 ° of the phase angle of the line - to - earth voltages of the lower voltage side relative to the related line - to - earth voltages of the higher voltage side (mathematically positive sense). Table shows common vector groups of two - winding transformers.
The arrangement of the windings, whether in star - , delta - or zigzag - connection, has a significant influence on the impedance of the zero - sequence component and on the permissible loading of the transformer neutral. Star - or zigzag – connected windings have a defined zero - sequence impedance if the neutral is earthed. Delta - connected windings and ungrounded star - or zigzag - connections have an infinite zero – sequence impedance .
The maximal permissible loading of transformer neutral is defined as the capability to carry the rated current of the earthed winding through the neutral, expressed as the ratio of the current through earth to the transformer rated current . A high maximal permissible loading of the neutral is necessary in case of low - impedance earthing of the power system or if the system is operated with resonance earthing through a Petersen coil.
Transformers with vector groups Yd, Dy and Yyd (with additional compensation winding in delta - connection) as well as low - voltage transformers Yz have 100% neutral loading capability. The relation of zero - sequence impedance to positive - sequence impedance of these transformer types is in the range of 0.1 (zero - sequence impedance of z - winding) to 2.4 (Yy with compensation winding in delta - connection). The ratio X 0 / X 1 of a transformer depends on the detail of its construction. For transformers of vector group Yy, Yz and with three - leg core, the ratio is X 0 / X 1 = 3 – 10; with a five - leg core or for three single - phase transformers the ratio is X 0 / X 1 = 10 – 100. Low - voltage transformers in general have a ratio of zero - sequence impedance to positive - sequence impedance of X 0 / X 1 = 1. Table gives an overview of the maximal permissible loading of transformer neutrals for different vector groups and construction types.