Published On: Tue, Mar 9th, 2021

Multi-winding variable frequency drive transformers

Author: Radoje Jankovic

Multi-winding variable frequency drive transformers

Multi-winding  variable frequency drive (VFD) transformer suppzing variable frequency drives. These variable frequency drive transformers are often classified in the category of „special transformers“. 

The Figure 1 shows a single-line schematic-graphic symbol of an 18-pulse converter transformer. The transformer is four-winding; one primary and three secondary windings. Phase sequence labels from left to right. Clock number counterclockwise.

Fig. 1. Single-line schematic-graphic symbol of an 18-pulse converter transformer. Phase sequence markers from left right.

The Figure 2. shows a  single-line schematic-graphic symbol of an 18-pulse converter transformer. The transformer is four-winding; one primary and three secondary windings. Phase sequence markers from right to left.  For this connection the phase shift is 20⁰, and the vector group is Yd(-20)dOd(+20).

Fig. 2. Single-line schematic-graphic symbol of an 18-pulse converter transformer.  Phase sequence markers from right to left. 

The Figure 3.  shows a typical single – line circuit (graphic symbol) in single – line (one – line) circuit diagrams of power 36 – pulse converter transformer, two primary windings and three secondary winding each. Phase sequence labels from right to left. For this connection the phase shift is 10⁰, and the vector group is Yd11:20d11:40y0d0:20d0:40,  Yd11:20d11:40yd0:20d0:40d1.

Fig. 3. Typical single – line circuit (graphic symbol) in single – line (one-line)   circuit diagrams of power 36 – pulse converter transformer. Phase sequence labels from right to left.

One of purposes of multi – winding VFD transformers is to minimize the harmonics of VFD injected into the network. This is achieved by using rectifiers with higher pulse number;

a) 12-pulse,

b) 18-pulse,

c) 24-pulse,

d) 30-pulses,

e) 36-pulses,

in combination with corresponding phase shifting input transformer. It means  that the VFD transformer has multiple secondary winding with relative phase displacement. Such phase shifting helps to eliminate certain harmonics from the spectrum seen on the primary side of transformer. Generally speaking the higher the rectifier pulse number the more low order harmonics can be eliminated or minimized.  

Table 1: Examples of transformer connection groups for common pulse number

Clock notation

The clock notation is such as a „normal clock“, i.e. the circle is divided into 12 hours. 1 hr = 30⁰ while full circle is 360⁰. The hour is further divided into 60 minutes, 1 hr = 60′.

As example, extended delta winding with -20⁰  relative phase shift can be written as d11.33 of d(-20⁰).

To be continued.

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Multi-winding variable frequency drive transformers