Star-delta transformer

Reversitig DOL starter Star-delta starter Auto transformer starter Primary resistance starter Dual-speed starter Three stage stator-rotor starter Schematic for panel space heater and internal illutnination Schematic for instrutnent wiritig... 

Residual voltage transformer (star/open delta transformer)... 

Star/delta or auto-transformer switching of a motor For motors compensated individually and having a star/ delta or auto-transformer switching, the changeover from star to delta windings or from one step of the auto-... 

These transformers are three-phase and may be connected for zig-zag or star/delta connections (Section 20.9.1). The delta may also be made open type by inserting a resistor across it to help adjust the zero-sequence impedance, if required. 

Depending on the value of the resistor, the motor will, in general, draw a heavier current then when started using star-delta or auto-transformer methods. The resistors must be rated to carry the limited motor starting current for the time they are in circuit. 

Three methods of arranging the windings of three-phase transformers are commonly encountered star, delta and zig-zag. Each method can be applied to either or both of the primary and secondary windings, Figure 6.6 shows the three forms. 

This method is applicable to single-star or delta-connected capacitor banks. Unbalance can be detected through the use of an RVT (residual voltage transformer) (Section 15.4.3). See Figure 26.4. The theory of operation is that any unbalance, of the system or the capacitor bank, will shift the neutral and reflect as the residual voltage across the open delta and can be used for the protective scheme. The unbalance voltage across the open delta in the event of failure of a unit in any series group can be expressed by... 

For example a transformer has a delta HV winding a star LV winding and a 4-30° displacement. It is described by letters and numbers as a Dyll trausformer. 

The upper bridge is fed by a Dyll delta-star transformer which has a 30° phase shift between the primary and secondary line currents. The lower transformer Ti has zero phase shift. See sub-section 6.4 for an explanation of phase shifts in transformer windings. 

In order to obtain the full benefit of harmonic cancellation the two bridges must be controlled in a common manner. The control system will enable the fundamental current in both supply lines of the same phase to be in-phase, i.e. the star primary line current must be in-phase with the delta primary line current. See Reference 12, Chapter 3 which emphasises this aspect. The controlled firing of the delta-star bridge T cancels the 30° degree phase shift of the transformer. From the Fourier analysis point of view this can be achieved by adding a + 30° phase shift to the delta primary line current. 

The magnitude of the two parts is divided by 3 to obtain the primary line current of the delta-star transformer. The result is then added to the hue current of the star-star transformer. The total magnitude of the supply line harmonic coefficient nsum is given by. 

For the larger rated Medium Voltage hoists paralleled power units can be used. This provides a further method of reducing harmonics by phase shifting the harmonics generated by each power stack which results in cancellation at certain frequencies. For cancellation at lower frequencies such as around the 6th harmonic a phase shift of 30deg can easily be achieved by using a combination of star and delta wound supply transformers to the power units. 

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