Published On: Tue, Mar 9th, 2021

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VFD tansforrmers some practical details and explanations

Classical winding connections are:

a) Star or wye, denoted as „Y“ or “y”.  The windings are connected phase to neutral, and

b) Delta denoted as „D“ or “d”. The windings are connected phase to phase,

which are used in power transformers creating vector groups such as e.g. Yy0, Yd1, Dy1, Dy5, Yz5 etc. Capital letter is used for primary winding (higher voltage side) and small letter for secondary winding (lower voltage side).

With these conventional winding connections a phase shift of is  30° or its multiple can be achieved.

Above two winding connections are used in VFD transformers mostly. However, the smallest phase shift between windings is  30°. This unit is called one hour in clock notation. Such phase shift is suitable for a 12-pulse rectifiers, i.e. a three – winding transformer with two secondary windings 30° phase shifted.  Examples of vector groups are such e.g. YY0d11, Dd0y11 etc. The Fig. 1. shows one practical  example Dd0y11.

If need that the rectifier achieve higher pulse number with less harmonic distortion, some more winding connections are needed. The connections for this purpose are:

a) zig-zag, denoted as „Z“,

b) extended delta,

c) polygon delta.

The standard star and delta connected transformer windings are shown in Fig. 2. Fig. 3. Standard star and delta connection of three – phase  transformer windings, two standard drawing forms each.

These types of winding connections allow to reach smaller phase shifts than 30⁰ even 10⁰ or  7,5⁰. Problems for small degree of phase shifts are manufacturing tolerances, cost and overall complexity set the practical limits.

Typical vector groups depending on pulse number of variable frequency drive input section shows table below.

In phase shifting transformers a 30⁰ phase displacement between secondary windings is used to achieve a 12 – pulse configuration. For higher pulse number a smaller phase displacement is required: less than  30⁰, i.e. less than one hour in clock number notation as noted in many literature articles. For such purpose modified winding connections of transformer windings are used:

Zig – Zag

The Zig – Zag (zigzag) is denoted as “Z” or “z”. It is modified star connection with each phase  consisting of two magnetically connected phases. Sometimes it is called “interconnected star”.

The circuit diagram is shown in Fig. 4. Fig. 4. Zig – Zag star connection of (or interconnected star) three – phase transformer winding s, two standard drawing forms.

Extended delta

Extended delta, together with polygon delta, are common winding configurations to achieve phase displacement les than 30°. Notation is usually same as delta and the phase displacement is added in brackets, e.g. d(-15°). The circuit diagram is shown in Fig. 5. Fig. 5. Simplified extended delta connection of three – phase  transformer windings, two standard drawing forms.

Polygon delta

The  polygon windings are characterized by hexagonal form of voltage vectors. The notation is usually same as extended delta, i.e. “d” with displacement angle in brackets.

The circuit diagram is shown in Fig. 6. 