When the operating company of a mining site in Western Australia decided to make the processing facility bigger, the electrical engineer faced an issue he was used to tackling. The major transformer used at the site was an ordinary transformer because its three-wire phases meant that neutral was unavailable. The entity needed neutral for a procedure of current protection of the crushing equipment as well as for 240-volt power supply for the factory’s electricity and water supply. The correct decision was not to replace the transformer, which was a 10 MVA unit functioning well for fifteen years. The company decided to opt for a delta transformer, which was a delta wye transformer with an artificial neutral, which was able to deliver line protection as well as supply with power auxiliary power needed by the site. With the delta transformer being installed, the neutral was established, thus the factory started its operations on time. How to select the right delta transformer depends on the winding structure applied: delta-delta connection or completely different combination.
Zusammenfassung: Delta transformers are those in which one of the windings or both are connected in delta form. Delta refers to a closed triangle configuration, where each phase winding gets connected at its end with the next one. In delta transformers, the voltage of the line equals the voltage of the phase.
What a Delta Connection Is — and Why It Is Used
In delta connection, the transformer three-phase windings are connected together end-to-end to create a closed loop that looks like a delta. There isn’t a neutral point, which means that there are no neutral conductors, and the line voltages are the same as the phase voltages. And the currents flowing through lines will be equal to the vector sum of wires currents. Delta connection has many electrical features that prove it effective for certain applications and give it some advantages over wye (star) connection. The absence of a neutral gives the possibility to connect loads only between the lines but not between a line and a neutral conductor, which means that the load has to have higher voltage ratings (230V or 120V appliances cannot be connected to higher voltage line in delta connection). The delta connection works to keeps triplen harmonic currents (third, sixth, and ninth harmonics) inside the winding delta connection, thus preventing them from going either on the supply side or the load side. Thus, this is the major benefit of a delta connection in distribution transformer working with nonlinear loads – computers, variable frequency drives, LED lighting, and UPS systems will not make nonlinearity currents to go to the primary side, thus avoiding the heating of the transformers, cables, turbines, and motors. Moreover, delta connection is capable of working with unbalanced loads, thus if any phase on delta-connected device is more loaded than the rest phases, the circulating current within the delta will make sure that the loads will be distributed evenly.
The physical and economic characteristics of a delta connection should also be taken into consideration. The design of the winding in the case of delta connection uses copper conductors that are not thicker that copper wires in the case of wye connection, and the insulation is needed for the line voltage.For an introduction to the full range of transformer winding configurations and their applications, see the product catalogue at Subian Electric, which includes delta‑delta, delta‑wye, and wye‑delta transformers, as well as grounding transformers, isolation transformers, and specialty power transformers for the global market.
The Three Main Delta Transformer Configurations — and Where Each One Belongs
The table below summarises the three principal transformer configurations that involve a delta winding, their defining electrical characteristics, and the applications where each is the correct choice.
| Konfiguration | Primary Connection | Secondary Connection | Key Electrical Characteristics | Am besten geeignet für |
|---|---|---|---|---|
| Delta‑Delta (Δ‑Δ) | Delta | Delta | There is no neutral point on either windings; the unbalanced load is efficiently handled because of the circulating currents in the delta. It also filters the triplen harmonics on both sides. The single-phase supply can be taken across any two lines at full line voltage. In case of the fault of any one phase of a delta-delta transformer, the transformer can be used in an open-delta (V-V) mode at a reduced capacity; thus, providing this much-needed fault tolerance in industrial applications where the costly stoppage of a production process is involved. | Three-phase delta connections are suitable for industrial motor drives, resistance heating, welding apparatus, etc. where neutrals are unnecessary. The exception handling property of the three-phase delta connection is important in continuous process industries like coal mining industry, oil and gas industry, and chemical processing. |
| Delta‑Wye (Δ‑Y) | Delta | Wye (Star) | For single-phase loads and for grounding the system, there is a neutral on the secondary side. The delta winding on the primary side prevents the flowof third harmonics upstream from the supply side. The wye winding provides a reference to earth for the load. Along with the voltage transformation coming with a 30-degree phase difference between primary and secondary voltages, this deserves careful consideration when connecting transformers in parallel. | Standard step-down transformers divide current into three-phase and single-phase types. They receive power from the same secondary line, for instance, three-phase motors powered with 480V, and 277V lighting, from a secondary line carrying 480Y/277V in North America. Similarly, in IEC markets, step-down power transformers supply either 400V motors or 230V loads from 400Y/230V secondary. These transformers are the most frequently applied layout in commercial and industrial sectors. |
| Wye‑Delta (Y‑Δ) | Wye (Star) | Delta | It supplies the neutral on the primary side, which permits grounding for protectiveness in a system. The delta in the secondary side traps load-induced harmonics, thereby stopping them from getting to the primary. The purpose is to work with a generator of a wye connection when stepping up to a transmission line of a delta connection. The wye side can be grounded to maintain stable system voltage and to direct ground fault current. | Step-up transformers are used to connect a wye-connected generator with a delta-connected transmission line. They are an example of a grounding transformer that establishes a neutral point of an ungrounded delta system. This type of extra grounding is popular in industries supplied via delta-connected source but needing a neutral for protection against short-circuit current. |
Harmonics, Grounding, and the Open‑Delta Option: Three Practical Considerations
Aside from the fundamental winding configuration, three major factors help finalize the suitable delta transformer for any case.
The first one is harmonics. Non-linear applications like variable frequency drives, uninterruptible power supplies, LED lights, and electronic ballasts create harmonic currents that flow into the transformer. The third harmonic, that is 180 Hz on a 60 Hz system and 150 Hz on a 50 Hz system, and its odd multiples (triplen harmonics) can greatly disrupt because it will combine in the neutral conductor of a delta-wye system, which can lead to overheating. Delta connections trap these triplen harmonics, with them circulating inside delta and converted into the heat. Delta-delta is capable of trapping harmonics on both sides of its operation. Delta-wye will trap good only on the side of the primary. Wye-wye has no capability to trap harmonics and is rarely used in the industry due to that. In case, the loads produce too many harmonics, then the delta transformer with at least one delta winding (especially the one that faces the harmonic source) should be used.
The second consideration is the grounding and protection from ground fault. Delta will never have a neutral so there is no natural grounding point to use.When the utility transformer is delta-connected, the system is ungrounded unless a grounding transformer is utilized. Such an ungrounded system possesses its specific dangers: when a ground fault occurs, the breaker does not trip, and there is an overvoltage at the unfaulted phases, and there can be a second ground fault causing a fault between the phases. The use of the wye-delta grounding transformer, or the zig-zag grounding transformer allows the grounding of the system that is required for proper operation with the usage of the ground fault. The grounding transformer connects to delta bus, whereas its neutral (which is either grounded directly or with the help of the resistor limiting the fault current as much as needed for the process of the protection relay operation) allows to have a controlled behavior of the ground fault.
The third type of the connection is the open-delta (V-V) configuration. When the delta-delta transformer fails in one of its phases (a turn or a phase), the rest of its phases can work in the open-delta connection mode thus allowing to supply a certain three-phase load. In the configuration of the open-delta, the capacity is only 57,7% of the capacity of delta-delta connection (rather than 66,7%), as it happens due to 60 degrees phase shift between two windings instead of 120 degrees, thus making it impossible to provide the current value above the rating for the each of the windings. The connection of the open-delta is used in emergency situations — it allows a critical process to continue functioning while the new transformer is obtained — but it is not used for normal operation of the delta-delta transformer. The voltage regulation of the open-delta mode is worse than that of the delta-delta one, as well as the phase balance. In case the transformer failure leads to huge losses for the manufacturing plant, the open-delta option served as a temporary reliable solution.For a deeper look at how transformer configurations integrate with the broader electrical distribution system, our article on the beste grundlegende Käuferleitfaden für elektrische Transformatoren einen umfassenden Rahmen für die Spezifizierung und Bewertung eines jeden Transformator-Kaufs.
How to Specify a Delta Transformer: The Parameters That Matter
The preparation of a purchase specification for a delta transformer entails that the buyer stipulates the electrical specifications that the transformer should fulfill, and provides a high level of details, so that each bidder will offer a similar product. The key parameters are kVA rating (which has to be defined through the load calculation applying the 80% continuous‑loading rule for the thermal margin), primary and secondary voltage and winding configuration (e.g. when it comes to a delta-wye step-down transformer — “13.8 kV Delta primary, 480Y/277 V secondary” or in the case of a delta-delta isolation transformer — “480 V Delta primary, 480 V Delta secondary”), impedance (expressed as a percentage of the transformer’s kVA base andusually ranges from 5% to 6% for distribution transformers; impedance that is too low leads to lower voltage quality, but higher fault currents, and it should be coordinated with protection means and equipment short circuit rating), insulation class and heat rise (for a dry transformer Class F or Class H, where the rise can range from 80 to 150 degrees Celsius, or for oil transformers the temperature rise is usually around 65 degrees Celsius above 30 degrees Celsius), method of cooling (ONAN for oil transformers, AN or AF for dry ones; forced cooling allows to increase the kVA rating of a dry transformer up to 25-33%. Therefore, the buyer should specify if the extra capacities are required), taps (off-circuit taps are standard and usually of 2.5% steps up and down the nominal primary voltage to adjust for the actual supply voltage; on-load tap changer is more expensive, but could be applied for critical applications and if the primary voltage varies a lot), enclosure (NEMA 1 for dry indoor conditions, NEMA 3R for outside, NEMA 4X for wash-down, or NEMA 12 for dust-tight industry), and standard (IEC 60076, IEEE C57.12, or standard of the local national, because a standard determines the way tests should be conducted and the methods that the equipment needs to be documented in). A good specification makes the supplier provide answers to all of the above and vague specification gives the manufacturer a chance to make his own judgment.
Häufig gestellte Fragen
What is the difference between a delta and a wye transformer?
A delta winding consists of a closed-loop triangle without a neutral. The line voltage of a delta connected system is equal to the phase voltage; however, the line current is √3 times the phase current in this configuration. A wye winding has a neutral point where the three windings intersect, which makes it possible for the line voltage to be √3 times of the phase voltage, while the line current is equal to the phase current. Moreover, due to the advantages of wye connection, such as neutrality for single phase loads and possibility to ground the system, many users use it in conjunction with delta windings.
When should I use a delta‑delta transformer?
Try using a delta-delta transformer under a load condition that is usually characterized by the use of three-phase or three-wire systems, that primarily includes motors, heating devices, or welding machines, noting that there is no need for a neutral wire in either transformers. Besides, a delta-delta transformer can easily operate under unbalanced load conditions, trap the harmonics on both sides of these transformers, and can be also utilized in the case of the failure of a single phase in an opened delta connection. Thus, a delta-delta transformer is a perfect solution for industrial applications that imply loading heavy motors, for instance.
Can a delta transformer supply single‑phase loads?
Indeed, a delta-connected secondary may be used for supplying single-phase loads linked between any two lines and capable of withstanding the full value of line voltage. However, single-phase loads should be built to handle line voltage of the particular delta system used — i.e., 480-volt equipment for 480-volt delta systems — and balanced on all three phases so that excessive imbalance does not occur. A delta secondary cannot be applied to loads requiring a neutral circuit for operation at low voltage, such as 120-volt or 277-volt loads.
What is an open‑delta transformer connection?
The open-delta connection consists of two single-phase transformers connected in two of the three phases to serve a three-phase load. The set-up utilizes 57.7% of the capacity of a transformer in a full three-phase delta connection. This type of connection is used in the case of failure of one of the phases in a full three-phase system or as an inexpensive and permanent solution in some light load applications. voltage regulation is poorer and also the phase balance is not stable as that of the full three-phase system.
Referenzen
- IEEE C57.12.00 — Standard für allgemeine Anforderungen an flüssigkeitsgetauchte Verteilungs-, Leistungs- und Regeltransformatoren. The foundational IEEE standard for transformer design, testing, and performance, including the delta and wye winding configurations and the associated electrical characteristics.
- IEC 60076 — Leistungstransformatoren. The international standard series that defines the design, testing, and performance requirements for power and distribution transformers, including the vector group designations — Dyn11, YNd1, and others — that specify the winding configuration and the phase relationship.
- Eaton — Transformer Winding Configurations and Application Guide. Manufacturer of delta‑delta, delta‑wye, and wye‑delta transformers, with technical documentation covering the selection, the installation, and the grounding requirements for each configuration.
- Schneider Electric — Transformer Vector Groups and Winding Connections. Manufacturer guidance on the specification and the application of delta and wye transformer winding configurations for commercial and industrial power systems.
The process of selecting the ideal delta transformer entails identifying a winding design that corresponds to the type of electrical system and its traits such as voltage, grounding, harmonics, and neutral. A delta-delta transformer is used for the three-phase and three-wire industrial load and has the necessary harmonic mitigation and safety elements for continuous processing plants. A delta-wye transformer steps down voltage and creates neutral for a mixed load of both single-phase and three-phase systems and has capabilities to mitigate harmonics from input supply. A wye-delta transformer elevates generator voltage for electricity transmission or creates neutral for a delta system to enhance safety. Each configuration of delta transformers serves its purpose and a knowledgeable buyer knows which transformer is needed for the application instead of choosing the one the seller happens to have in stock. Co. Subian is a maker of delta transformers in all three configurations and is capable of providing transformers with necessary kVA range and voltage class, impedance, and certifications that are demanded by the market. It is essential to use correctly made transformers for operation of factories, mines, or processing facilities where the transformers must be appropriately designed for the electrical system, making the configuration process extremely important.