Because I'm looking at an existing setup and the transformer is Delta doesn't say delta, but has the triangle on it for primary, so that means delta right? The voltage is to ground on each phase. Is this an incorrect installation? The transformer primary windings are probably configured in a delta fashion - it does not care if the supply originated from a wye or delta secondary it only wants to see volts input and preferably at degree phase angles which either system will be. One more very important point not covered above, which is subtle but has a large potential effect: If the secondary is wired in delta, the primary must also be wired with only three wires whether the actual source is wye or delta.
Because wye to delta transformers are very uncommon, the problem comes up most often when reversing a delta to wye step down transformer to get a step up transformer. You must not make a connection to the wye point neutral on the input side when you do that. If you are interested in the physics behind this, let us know. A phase is connected to each corner of the delta. Although delta windings are often operated ungrounded, a leg of the delta can be center tapped and grounded, or a corner of the delta can be grounded.
In a wye configuration, one end of each of the three windings is connected to form a neutral.
A phase is connected to the other end of the three windings. The neutral is usually grounded. The following paragraphs describe three-phase transformers which utilize the delta and wye connections. Next part of this article will discuss three-phase transformers using the open-delta and open-wye connections, where one of the single-phase transformers making up the three-phase bank is omitted.
Delta-wye transformer - Wikipedia
The leg of the transformer with the missing transformer is referred to as the phantom leg. Delta—delta transformers, as shown in Figure 1, often are used to supply loads that are primarily three phase but may have a small single-phase component. The three-phase load is typically motor load while the single-phase component is often lighting and low voltage power.
The single-phase load can be fed by grounding a center tap on one of the legs of the delta secondary, then connecting the single-phase load between one of the phases on the grounded leg and this grounded neutral. The connection diagram on the left shows how a delta—delta connection can be made, either with three single-phase transformers or with one three-phase transformer. The dashed lines indicate the transformer outlines. The three single-phase transformer implementation can be seen by disregarding the outer dashed outline and the bushing labels shown at that outline, and concentrating on the three smaller single-phase transformer outlines.
The schematic diagram at the upper right is perhaps easier to analyze, as the delta connections can clearly be seen. 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.
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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.
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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.
Three-phase voltage transformations
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. 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: