VFD transformers: Multi-winding design
Today we look at the design of multi-winding VFD transformer feeding variable frequency drives. These VFD transformers are often classified in the category of ‘special transformers’. What does it mean? Why are they special? If you want to get these answers and a little insight into the design, keep reading this article.
In our previous post [1] we have briefly explained the differences between ordinary power transformer and a VFD transformer. If you missed that part, you can read the article here: VFD transformers – Introduction.
Having the requirements on VFD transformers in mind we will discuss the impact on the design.
Purpose of multi-winding design
One purpose of multi-winding VFD transformer is to minimize the harmonics of VFD injected into the grid. This is achieved by using rectifiers with higher pulse number (most common 12-pulse, 18-pulse, 24-pulse, 30-pulse or 36-pulse) in combination with corresponding phase shifting input transformer. It means that the VFD transformer has multiple secondary windings with relative phase displacement. Such phase shifting helps to eliminate certain harmonics from the spectrum seen on the primary side of the transformer. Generally the higher the rectifer pulse number the more low order harmonics can be eliminated or minimized.
Principle of harmonic cancellation
Let’s explain the mechanism on the simplest multi-winding transformer:
A three-winding transformer has one primary winding and two secondary windings. The secondary windings have 30 degree phase shift. Note that this phase shift is valid for the fundamental component (sometimes called power frequency, i.e. the grid frequency). Simple two-winding transformer would connect a 6-pulse rectifier bridge to the grid. Such 6-pulse rectifier has a voltage or current ripple on the dc side with 6th harmonic. Now we go back to our three-winding transformer. The 6th harmonic is shifted by 6 * 30 degree = 180 degree. It means that the 6th harmonic components associated with each secondary winding and corresponding rectifier bridge are in opposite phase and cancel each other. The 6th harmonic on the DC side is eliminated and therefore also corresponding 5th and 7th harmonics on the AC side disappear.
Winding connections
Classical winding connections are:
– star (wye), denoted as ‘Y’,
and
– delta, denoted as ‘D’.
These two connections are used in distribution and power transformers creating vector groups such as e.g. Yy0, Yd1, Dy1 etc. Capital letter is used for the winding with higher voltage and small letter for the winding with lower voltage.
Above two winding connections are used in VFD transformers as well. However, the smallest phase shift between windings is 30 degree. This unit is called one hour in clock notation. Such phase shift is suitable for a 12-pulse system, i.e. a three-winding transformer with two secondary windings 30 degree phase shifted. Examples of vector groups: Yy0d1, Dd0y11 etc.
Shall the rectifier achieve higher pulse number with less harmonic distortion, some more winding connections are necessary. These windings connections are:
– zig-zag, denoted as ‘Z’,
– extended delta,
– polygon delta.
These winding connections allow to reach smaller phase shifts than 30 degree. Theoretically an arbitrary small phase shift can be achieved, but factors such as manufacturing tolerances, cost and overall complexity set the practical limits.
Vector group / connection group
The vector group informs about the winding connection of each transformer winding and their relative phase shift. Five above mentioned connections are being used: star, delta, zig-zag, extended delta and polygon delta. The side with higher nominal voltage(s) is marked with capital letter, the side with lower nominal voltage(s) with small letter.
Typical vector groups depending on pulse number of VFD input section are shown in Table 1:
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. One hour corresponds to 30° (full circle is 360°). The hour is further divided into 60 minutes (60′).
As example, extended delta winding with -20 degree relative phase shift can be written as d11.33 or d(-20°).
Refer to [3] for more details.
Transformer configurations
People often talk about “5-winding transformer” for 24-pulse input section of VFD or “7-winding transformer” for 36-pulse input rectifier etc. They simply mean that the transformer has one primary winding and n secondary windings (n = pulse number / 6). However, this is certain simplification and might not be 100% true. For different reasons the primary winding may be internally split into several parallel windings. Therefore, 24-pulse transformer might have one, two or four primary windings as depicted in figure below. The dash line shows the transformer unit and shall explain that in all three cases it is one single transformer. Another alternative would be using e.g. two 12-pulse transformers in parallel with corresponding phase shifts.
Note that the primary windings are paralleled internally so the exact configuration is not obvious when looking at the transformer from outside. In all cases there will be only 3 phases (3 bushings) to connect to.
Remark 1
Primary side is often marked as ‘HV’, secondary side as ‘LV’. These abbreviations associate high voltage and low voltage. The nominal voltages of converter side windings in medium voltage VFDs are often above 1 kV and therefore the term ‘LV’ might not sound 100% correct. However, it is a common practice to use HV/LV notation.
Uniformity of impedances
Phase shift is not the only requirement in multi-winding transformers. Important is also the uniformity of short circuit impedances. What does it mean?
Multi-winding VFD transformers do not have just one single short circuit impedance value. Instead, the transformer can be described by impedance matrix. Let’s take single core 24-pulse transformer with primary winding P and secondary windings S1 to S4. The impedance matrix consists of individual impedances:
P – (S1…S4), P – S1, P – S2, P – S3, P – S4
S1 – S2, S1 – S3, S1 – S4, S2 – S3, S2 – S4, S3 – S4
One winding is always shorted and the other one is supplied with voltage. Plus there is the case where all secondary windings are shorted simultaneously. When the impedances P – S1, P – S2 and so on have similar values, we talk about uniformity of impedances. Design with uniform impedances is exposed to lower dynamic forces in some fault conditions and the mechanical integrity is much easier. In case of one secondary winding shorted the dynamic forces can be significantly higher than when all secondary windings are shorted. The reason are unbalanced ampere-turns. This is not intuitive, but very important for mechanical integrity.
Remark 2
When specifying an impedance value of a multi-winding VFD transformer it shall be clear whether it is the value when one individual secondary winding is shorted or when all secondary windings are shorted.
Loosely coupled windings
Multi-winding VFD transformers shall normally have loosely coupled secondary windings. What does it mean and why is it important?
The mutual impedance shall be as small as possible. It helps to avoid interactions, e.g. during commutations in rectifier groups. Otherwise one rectifier group could disturb the other one. The harmonic spectrum could be affected as well. Concentric winding structure inherently leads to loose coupling. IEC 61378-1 describes winding arrangements for closely coupled, loosely coupled and decoupled converter side windings. The standard uses the term ‘valve’ as explained earlier in VFD transformers – Introduction.
The loosely coupled and decoupled winding arrangement is shown in figure 3 that is adopted from IEC 61378-1.
International standards
Following international standards provide information on power and distribution transformers and some of them also address specific topics on multi-winding VFD transformers.
IEC 60076
IEC 61378-1
IEC 61378-3
IEEE Std. C57.18.10
References
[1] VFD transformers: Introduction, https://mb-drive-services.com/vfd_transformer_intro/
[2] Trasfor – custom built dry-type transformers and reactors, http://trasfor.com/
[3] VFD transformers: Clock number and vector group, https://mb-drive-services.com/clock-number-and-vector-group/
