![]() These four standard configurations are given as: Delta-Delta (Dd), Star-Star (Yy), Star-Delta (Yd), and Delta-Star (Dy). We now know that there are four different ways in which three single-phase transformers may be connected together between their primary and secondary three-phase circuits. Transformer Winding Identification Connection ![]() ![]() Then, Star-Star would be labelled Yy, Delta-Delta would be labelled Dd and interconnected star to interconnected star would be Zz for the same types of connected transformers. Symbols are generally used on a three phase transformer to indicate the type or types of connections used with upper case Y for star connected, D for delta connected and Z for interconnected star primary windings, with lower case y, d and z for their respective secondaries. Each winding has two ends normally labelled 1 and 2 so that, for example, the second winding of the primary has ends which will be labelled B1 and B2, while the third winding of the secondary will be labelled c1 and c2 as shown. The secondary windings are labelled with small (lower case) letters a, b and c. The standard method for marking three phase transformer windings is to label the three primary windings with capital (upper case) letters A, B and C, used to represent the three individual phases of RED, YELLOW and BLUE. With a single the three-phase transformer there are three magnetic flux’s in the core differing in time-phase by 120 degrees. With three single-phase transformers connected together, the magnetic flux’s in the three transformers differ in phase by 120 time-degrees. ![]() The three available voltages, which themselves are each displaced from the other by 120 electrical degrees, not only decided on the type of the electrical connections used on both the primary and secondary sides, but determine the flow of the transformers currents. Depending upon how these sets of windings are interconnected, determines whether the connection is a star or delta configuration. A three phase transformer has three sets of primary and secondary windings. Three Phase Transformer Star and Delta Configurationsīut what do we mean by “star” (also known as Wye) and “delta” (also known as Mesh) when dealing with three-phase transformer connections. When transformers are used to provide three or more phases they are generally referred to as a Polyphase Transformer. The combinations of the three windings may be with the primary delta-connected and the secondary star-connected, or star-delta, star-star or delta-delta, depending on the transformers use. In the case of three phase transformer windings, three forms of connection are possible: “star” (wye), “delta” (mesh) and “interconnected-star” (zig-zag). The primary and secondary windings of a transformer can be connected in different configuration as shown to meet practically any requirement. Three-phase supplies have many electrical advantages over single-phase power and when considering three-phase transformers we have to deal with three alternating voltages and currents differing in phase-time by 120 degrees as shown below. Three-phase, also written as 3-phase or 3φ supplies are used for electrical power generation, transmission, and distribution, as well as for all industrial uses. If we take three single-phase transformers and connect their primary windings to each other and their secondary windings to each other in a fixed configuration, we can use the transformers on a three-phase supply. But voltage transformers can also be constructed for connection to not only one single phase, but for two-phases, three-phases, six-phases and even elaborate combinations up to 24-phases for some DC rectification transformers. Thus far we have looked at the construction and operation of the single-phase, two winding voltage transformer which can be used increase or decrease its secondary voltage with respect to the primary supply voltage. Three-phase voltages (and currents) are raised or lowered by means of three phase transformers as the three phase transformer can have its windings connected in various ways. A three-phase electrical system in used to generate and transmit electric power over long distances for use by offices and industry.
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