از نقطه نظر اقتصادی مصرف مس در ترانسهای اتصال ستاره بیشتر از مثلث است چون جریان سیم پیچی در اتصال ستاره همان جریان خط است ولی در اتصال مثلث به نسبت 1.73 کوچکتر است. در اتصال ستاره زمین نشده بارگیری نامتعادل از ترانس سبب تشدید عدم تعادل ولتاژی شده و عملاً امکان بهره برداری از آن بعنوان ترانس توزیع وجود ندارد. در صورت زمین نمودن اتصال ستاره نیز جریانهای هارمونیک 3 و ضرایب آن به سیستم زمین راه یافته و موجب تداخل در سیستمهای مخابراتی می شوند. به هر حال مگر در مواردی که اجتناب از تغییر فاز اولیه و ثانویه مطلوب نظر نباشد این اتصال توصیه نمی گردد. در هندبوکهای مختلف به مزایا و معایب اتصالات ترانسفورمالتورها نسبت به هم پرداخته می شود. در زیر نمونه ای از آنها را می بینید.
Wye–Wye Connections of Transformers
Joining together the terminals of similar windings with the same polarity derives the neutral of the wye connection. This neutral point is available and can be brought out for any desired purpose, such as grounding or zero-sequence current measurements and protection. For high-voltage transmission systems, the use of the wye-connected transformer is more economical because the voltage across the phase of each winding is a factor of 1.73 less than the voltage between the lines. If the neutral point is grounded, it is not necessary to insulate it for the line voltage.
If the neutral is not grounded, the fault current during a system line to ground fault is insignificant because of the absence of a zero-sequence current path. If the neutral is grounded in the wye–wye transformer and the transformer is made with a three-limb core, the zero-sequence impedance is still high. As a result, the fault currents during a system line-to-ground fault are relatively low. For wye–wye transformers made of three single-phase units or with a shell-type or five-limb core-type, the zerosequence impedance is approximately equal to the positive-sequence impedance. The fault current during a system ground fault for this case is usually the limiting factor in the design of the transformer. In all types of wye-wye transformer connections, only the transformer positive-sequence impedance limits the fault current during a system three-phase system fault.
With the wye connection, the voltages are symmetrical as far as the lines are concerned, but they introduce third-harmonic (or multiples of the third harmonic) voltage and current dissymmetry between lines and neutral. The third-harmonic voltage is a zero-sequence phenomenon and thereby is exhibited in the same direction on all phases. If the transformer and generator neutrals are grounded, thirdharmonic currents will flow that can create interference in telephone circuits. If the transformer neutral is not grounded, the third-harmonic voltage at the neutral point will be additive for all three phases, and the neutral voltage will oscillate around the zero point at three times the system frequency. Thirdharmonic voltages are also created on the lines, which can subject the power system to dangerous overvoltages due to resonance with the line capacitance. This is particularly true for shell-type threephase transformers, five-limb core-type transformers, and three-phase banks of single-phase transformers.
For any three-phase connection of three-limb core-form transformers, the impedance to thirdharmonic flux is relatively high on account of the magnetic coupling between the three phases, resulting in a more stabilized neutral voltage. A delta tertiary winding can be added on wye–wye transformers to provide a path for third-harmonic and zero-sequence currents and to stabilize the neutral voltage. The tertiary in this application will be required to carry all of the zero-sequence fault current during a system line-to-ground fault.
The most common way to supply unbalanced loads is to use a four-wire wye-connected circuit.
However, the primary windings of the transformer bank cannot be wye-connected unless the primary neutral is joined to the neutral of the generator. In this case, a third-harmonic voltage exists from each secondary line-to-neutral voltage because the generator supplies a sinusoidal excitation current. The third-harmonic currents created by the third-harmonic voltages can be a source of telephone interference.
If the primary neutral is not connected to the generator, single-phase or unbalanced three-phase loads in the secondary cannot be supplied, since the primary current has to flow through the high impedance of the other primary windings.