THREE-PHASE TRANSFORMER WINDING ARRANGEMENTS
Transformer is the key equipment for electrical energy transfer from one step to other step in power transmission and distribution system. Transformer may construct as single or three phases. Our discussion limited on three-phase transformer winding arrangements. There are three methods of arranging the windings of three-phase transformers are commonly used; such as star winding, delta winding and zig-zag winding which are presented in the diagram. Each method can be applied to either or both of the primary and secondary windings.
Star winding contains four terminals are used when a neutral connection is required for earthing. Normally in low voltage distribution transformer primary and secondary winding and primary distribution transformer secondary windings are used star arrangement.
Delta winding contains tree terminals are most frequently used on the high voltage winding, which is usually the winding connected to the supply. The delta connection also allows third harmonic currents to circulate which improves the waveforms of the line currents and voltages on both side of the transformer. Delta windings may be slightly more expensive because the insulation has to withstand the full line-to-line voltage.
The zig-zag winding requires each three-phase winding to be split in half. Each half is interconnected with a half-winding on another core limb. Zig-zag windings are used to suppress third harmonics or to provide a neutral connection as an earthing transformer as power rectifiers when high order harmonic can be trouble and have to be minimized.
As per standard IEC60076 some specified numbers and letters are used, such as for high voltage (HV) terminals have upper-case letters
The choice of letters and numbers depends of different nations or countries; like as-
The type of winding e.g. star, delta is given a letter, again upper case for HV and lower case for LV windings. The letters are,
Since a phase angle displacement can occur across the transformer due to its method of connection it is necessary to identify this displacement. The numbering system for this is based on the hands of a clock. Each ﬁve-minute position on a clock gives 30◦ phase displacement, such as
These are the commonly encountered displacements. (Note that the phasor rotation is anti- clockwise).
Hope the portrayed article on three-phase transformer winding arrangements will help you in your works.