The transformer vector group show the phase difference between the primary and secondary sides of the transformer. It also determines the high voltage and low voltage windings arrangement of three phase transformers. The three phase transformer is connected in various ways.
On the basis of connection, the vector group of the transformer is determined. Three phase transformer is divided into four main groups according to the phase difference between the corresponding line voltage on the high voltage side and the low voltage sides. The phase difference is the angle by which the low voltage line lags the high line voltage, and is measured in units of 30° in clockwise direction.
Types of Vector group
Group number 1 – no phase displacement
Group number 2 – 180° phase displacement
Group number 3 – (-30°) phase displacement
Group number 4 – (+ 30°) phase displacement
The connection Y d 11 gives the following information – Y indicates that the high voltage is connected to star and d indicates the low voltage is connected in delta. The 11 indicates that the low line voltage lag, high line voltage by 11 Χ 30° = 330° measured from higher voltage phasor in a clockwise direction.
The phasor differences can also be measured by using the clock methods. Consider the minute hand of the clock shown the high voltage and the low voltage winding is represented by the hour hand. The angle of 30° is the angle between two adjacent figures on the clock dial and is taken as the unit of dial shift.
When the hour hand of the clock is at 12, then the phase displacement is zero. When the hour hand is at 1 then the phase shift -30° degree. At 6 the phase shift is 6 Χ 30º = 180º. Similarly, when the hour hand is at 11 the phase shift is 11 Χ 30º =330º.
The number 0, 6, 1, and 11 in the group reference number indicates the primary to secondary phase shift regarding the hours of the clock. The connection designated by Dy11 is the delta-star transformer in which the low voltage line phasor is at 11 and is a phase advanced of +30° on the corresponding line voltage on the high voltage side.
The only transformer in the same group may be connected in parallel. For example, a star-star, 3-phase transformers can be parallel with another three phase transformer whose windings are either connected in Y-Y or ∆-∆. The ∆-∆ transformer cannot be parallel with Y ∆ transformer.
Phase Shift in Star Delta Transformer
The phase shift is the angular displacement between the maximum positive value of the alternating quantities having the same frequency. In other words, the angular displacement between high voltage and the low voltage terminal and the corresponding neutral points (real or imaginary), expressed with reference to high voltage side is known as phase displacement (or shift) of the transformer.
There is no phase shift between the star-star and delta-delta 3-phase transformer. Most of the power transformer is either star-delta or delta star connected. In such type of transformers, even in the normal operating condition, the phase to phase voltages and phase to neutral voltage of high voltage side are displaced from the corresponding voltage of low voltage side. Similarly the current on the two sides are displaced.
Considered 3-phase star-delta transformer with primary side Y-connected and secondary with delta-connected as shown in the figure below. The polarity markings are indicated on each phase. The dots on the windings indicate the terminals which are positive at the same time on the un-dotted terminals.
The phases on the star sides are marked as A, B, C and the phases on the delta side are marked as a, b, c. The labelling is indicated to the diagram corresponding to + 90º connections in which the positive sequences on the delta side is lead by 90º corresponding to star side.
Thus, the lines currents flow through the phase a and A. The alternating way is to label delta as b→a, c→b and a→c; Thus we get a standard yd , -30º connection. If the polarities on the delta side are also reversed, we have standard yd , 30º connection.
They are used for line to line voltage, and yd are used for line currents and phase (line to neutral) voltage. Line to line transformation ratio is being taken to be unity. The positive and negative sequence voltages on the primary (star) and secondary (delta) sides of the transformer are shown below. When the power flow is reversed, and the delta of the transformer acts as primary and star act as secondary, then the voltage phasor does not change, but all the current phasor are reverse.
Thus, the magnitude of phase shift is same for positive and negative sequence component. The direction of phase shift is in the case of negative phase sequence is the reverse of that applies to the positive phase sequence component (due to reverse phase sequence).
The magnitude and direction of phase shift depend on the transformer group and allocation of phase references. The phase shift of zero sequence quantities needs not to be considered in star-delta transformer because the zero sequence currents do not flow in lines on the delta connected side.
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