I am preparing for a job interview and have been given a Delta-wye transformer schematic and have been asked to discuss the phase shift between the primary and secondary windings. I know that there is a phase shift of 30 degrees, but in this particular drawing there is a neutral line on the delta side with no clear connection point marked. I haven't dealt with transformers in a while and am wondering specifically where the neutral is connected to in this schematic? I am assuming I will be asked more about this transformer configuration and would like to completely understand what is going on here with that neutral.
Any help is greatly appreciated!
I might add that normally these 3 transformers would be built as one chunk of steel with 3 sections, one for each phase. The floating Vn wire is just to show that the primary is delta only, neutral is not used. Each floor has its own 3-phase step-down transformer to convert vac delta to vac wye vac delta to power the outlets and lights on each floor from a 3-phase 42 breaker panel. There is plenty of heavy machinery that uses single or 3-phase for power such as machine shops, ultrasonic welders, etc.
This is typical of what I did as an electrician so it is easy to write about. In large buildings it is mostly a distribution panel for all the sub-panels on each floor. It is not unusual to have a double-neutral feed to such a building, to account for imbalances in the loading of the phases. For more details about the 30 degree phase shift between phase windings, please see this article: The phasor diagram at the lower right shows the geometric relationships between the high voltage circuit and low voltage circuit currents , and the equations at the bottom center show those relationships mathematically.
As the loading on a delta—delta transformer becomes unbalanced, high currents can circulate in the delta windings leading to a voltage imbalance. Balanced loading requires the selection of three transformers with equal voltage ratios and identical impedances. Also, the amount of single-phase load should be kept low because the center-tapped transformer must supply most of the single-phase load.
As the single-phase load is increased, the center-tapped transformer will increase its loading more than the other two transformers and will eventually overload. If one of the single-phase transformers in the delta—delta bank fails, the bank can be operated with only two transformers forming an open delta configuration. The kVA rating of the bank is reduced, but three-phase power is still supplied to the load. Wye—wye transformers, as shown in Figure 3, can serve both three-phase and single-phase loads. The single-phase load should be distributed as evenly as possible between each of the three phases and neutral.
Figure 4 illustrates the wye—wye connection, either as three single-phase transformers or as a single three-phase unit. Both bushing labels and polarity dots are shown. One problem inherent to wye—wye transformers is the propagation of third- harmonic currents and voltages.
These harmonics can cause interference in nearby communication circuits as well as other assorted power quality problems. The delta—wye connection is the most commonly used three-phase transformer connection. The wye-connected secondary allows single-phase load to be distributed among the three phases to neutral instead of being placed all on one winding as with a four-wire delta secondary. If one of the single-phase transformers in the delta—wye bank fails, the entire bank becomes inoperative.
Figure 6 illustrates the delta—wye connection, either as three single-phase transformers or as a single three-phase unit. Analyzing the delta—wye transformer illustrates many important concepts regarding the operation of polyphase transformers. The analysis can be done on either a voltage or a current basis. Since voltage potential difference or the subtraction of two phasor quantities is rather abstract and difficult to visualize, current or the flow of charge will be used as the basis for analysis, since current is easy to conceptualize.
The currents owing in the windings of a delta—wye transformer are shown in Figure 7. Note that the arrows indicate instantaneous directions of the AC current and are consistent with the dot convention. The analysis must begin in one of the two electric circuits, either the delta- connected high voltage circuit or the wye-connected low voltage circuit. Since current is being used as the basis for analysis, the wye-connected circuit is selected as the starting point, since in a wye-connected circuit, the line currents leaving the transformer and the phase currents owing in the transformer windings are equal.
This relationship between line and phase currents simplifies the analysis. The analysis starts by labeling all line and phase currents. This is shown in Figure 8. Note that lower-case subscripts indicate line currents in the low voltage circuit, and upper-case subscripts indicate line currents in the high voltage circuit. In the low voltage circuit, the phase currents are identical to the corresponding line currents, so they also are labeled I a , I b , and I c.
When the transformer windings are drawn, a particular high voltage winding corresponds to the low voltage winding drawn parallel to it.
Delta and Wye 3-Phase Circuits
Equations above express the high voltage circuit line currents in terms of the low voltage circuit line currents. At this point, numerical values can be substituted for I a , I b , and I c. Keeping in mind that I a , I b , and I c represent a balanced set of phasors , arbitrary per-unit values are selected to represent a-b-c phase sequencing:.
No reference is made to primary or secondary. The primary windings of a transformer are those windings to which voltage is applied.
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The secondary windings have an induced voltage impressed across them. Usually, the primary windings are the high voltage windings, but this is not always the case. A good example of an exception is a generator step-up transformer.
GE Transformer delta-star nameplate. Key points As primary in delta connected: One problem associated with this connection is that the secondary voltage is shifted by 30 0 with respect to the primary voltage.
Delta-Star Connection of Transformer
This can cause problems when paralleling 3-phase transformers since transformers secondary voltages must be in-phase to be paralleled. Therefore, we must pay attention to these shifts. Premium Membership Premium membership gives you an access to specialized technical articles and extra premium content electrical guides and software. Jignesh Parmar Jignesh Parmar has completed M. Tech Power System Control ,B. He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India.
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Delta-wye transformer - Wikipedia
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