VFD transformers: Need for a tap changer

Tap changer: Yes or No?

Shall a VFD transformer be equipped with a tap changer? What are the pros and cons of a tap changer on converter duty transformer? What kind of tap changer shall be used? How many taps does the VFD transformer have? This and more in our article Need for a tap changer.

Note that we will mainly talk about tap changers on VFD input transformers. Special category are output transformers with tap changers, used e.g. in test stand applications. There the tap changer might be used to adapt the output voltage in a wide range (e.g. multiple nominal output voltages of a test bench). These applications are mostly tailor made and not the core of this article.

Purpose of a tap changer

The basic purpose of a tap changer is the ability to change the turn ratio of a transformer. By doing that one can impact the output voltage in two ways (whereas the principle is still the same):

(i) Keep the output voltage at a constant value, resp. in a narrow range

(ii) Adjust the output voltage according to the needs

(i) Keeping output voltage (almost) constant

In our first case the tap changer aims to compensate voltage fluctuation on the primary side by trying to keep the secondary voltage within a narrow range. Usage can be e.g. in weaker grids that tend to have larger voltage variation around the nominal value.

Especially in a diode front end (DFE) based drives all the voltage variations sum up and it can be more challenging to find the right balance. Voltage design of DFE VFD shall consider:

– voltage variation of supply grid

– voltage drop across the VFD input isolation transformer

– voltage fluctuation of dc link

It might be challenging to find a suitable design that can handle the sum of voltage variation in all possible modes of operation. The situation is schematically illustrated in fig. 1 with typical grid voltage tolerance of +/-10% (common industrial MV grid), input isolation transformer with 3-6% voltage drop (full load versus no-load voltage) and certain inherent voltage ripple in dc link (few % depending on VFD topology and design).

As the DFE rectifier is passive there is no way to actively compensate the voltage variation by the VFD itself. The only option remains the tap changer mounted on the VFD transformer. Despite that, a good compromise in voltage design can often be found without the need for a tap changer.

(ii) Adjusting the output voltage

This scenario uses the tap changer in an opposite way. Changing the tap position is used to adjust the transformer secondary voltage. It is less common operation mode, but there can be specific reasons to do so. For example:

— Optimization of a particular operation point of a drive system

— Test stands and other applications requiring a versatile drive system covering a range of nominal voltages

The latter case would typically require a different implementation of a tap changer.

Types of a tap changer

Power transformers utilize two basic types of tap changers:

♦ De-Energized Tap Changers (DETC)

♦ On-Load Tap Changers (OLTC)

1. De-Energized Tap Changer

This is the basic type of tap changer. It is also called Off-Circuit Tap Changer (OCTC) or No-Load Tap Changer (NLTC). Many people call it “off-load” tap changer. However, it leads to the same abbreviation as the other type of tap changer and is therefore a bit unlucky.

DETC is a very common device used in distribution and power transformers. The tap position (and therefore the turn ratio) can only be changed when transformer is de-energized. The system is not designed to change the tap under load. It shall be interlocked against such attempt. The amount of taps and the step size can be selected arbitrarily. However, most common configurations frequently used are:

± 2 x 2.5%

– tap changer has 5 positions: -5%, -2.5%, 0%, +2.5% and + 5%

– step size is 2.5% of the nominal voltage

± 5%

– tap changer has 3 positions: -5%, 0% and + 5%

– step size is 5% of the nominal voltage

The middle tap is called the “principal tap”. Although most tap changers are symmetrical (same number of steps in “plus” and “minus” direction) it is not a must and a tap changer can be asymmetrical as well.

Off-circuit tap changer of oil-insulated transformers is traditionally realized as a mechanical switch that is operated either manually (most of them – by means of a “wheel”) or it can optionally be motorized. Dry type transformers usually have suitable leads for corresponding tap positions.

Besides above examples of linear tap changers the regulation can also be reversing or a coarse/fine combination. Such solutions might be preferred to cover wider range of taps or for purpose of high accuracy.

DETC in VFD transformers

De-energized tap changer is also used in VFD transformers. Main purpose is a small adjustment of a transformer turn ratio. However, the experience shows that tap changer is set in a position during commissioning and usually left in that position “forever” unless there is a very strong reason to change. Note that DETC is not meant to compensate voltage variation “online”. Therefore, the question arises how essential such DETC tap changer really is? There is no general answer to this. If the drive system can be dimensioned to cope with the expected voltage variation without the need of a tap changer then it is usually the recommended way. A tap changer presents additional cost of the transformer (material cost, labor cost during assembly and testing etc). Moreover, it also requires periodic maintenance. From our long experience it is good to avoid tap changer when the system performance can be met without DETC. However, there are certainly cases where the system design becomes too cumbersome and DETC might be a reasonable choice.

2. On-Load Tap Changer

As the name indicates the tap position can be changed while the transformer is in normal operation, i.e. energized and transmitting power. OLTC is a complex electro-mechanical device allowing to change the tap position under full load condition. For a long life time of the tap changer itself and the whole transformer it is important to minimize any arcing between the contacts.

OLTC is available in conventional design or as a vacuum based OLTC.

OLTC is VFD transformers

OLTC is hardly ever used in VFD transformers. There are multiple reasons for it. First is the price tag of a transformer equipped with an OLTC. Another reason is the implementation. VFD transformers are often much more complex than distribution or power transformers from winding connection point of view. OLTC is controlled by a voltage controller that could interact with the VFD control in adverse manner. And finally there is a concern related to the reliability of a VFD transformer with OLTC.

Exception to above explanation might be larger modular multilevel converters (MMC) where an OLTC might be beneficial in case of loss of one cell (n-1 operation). Note that this type of MMC converters utilizes a simpler two-winding transformer so the situation is a bit different compared to multi-winding transformer designs used in majority of VFD topologies-

Location of a tap changer

A tap changer, no matter if DETC or OLTC, can be mounted on the primary or secondary side of the transformer. The choice is purely technical depending on currents and voltages. For power transformers the tap changer is often installed on the star-connected winding (wye) close to the star point. Obvious advantage is the reduced insulation level.

In VFD transformers the tap changer is mounted on the primary (grid) side – at least we have never seen anything else. The selection is rather straight-forward, particularly in multi-winding transformers. Mounting the tap changer on converter side windings would require a dedicated tap changer for each secondary winding. Moreover, converter side windings are exposed to much higher harmonic distortion than the grid side windings. Finally, converter side windings are also exposed to a common mode voltage: Another good reason to keep the tap changer on the primary side.

Pros and cons of a tap changer

– Pros’

Advantage of a tap changer is certain flexibility in adjusting the system voltage. Especially in drive systems with a passive diode rectifier such option can be useful. In particular, tap changer can be used in case of longer periods of grid under- or overvoltage. Tap changer can also be used for fine adjustment or voltage error due to tolerances of components and system uncertainties. Voltage adjustment via tap changer can improve the voltage utilization, compensate power derating, avoid overloading of system components or marginally improve efficiency. A simple de-energized tap changer (DETC) is relatively inexpensive although it can get more costly for certain multi-winding transformer designs.

– Cons’

There are also certain drawbacks of a solution with tap changer.

First of all, tap changer presents additional cost. While it is relatively inexpensive device (talking about DETC) in most cases, in some converter duty transformer designs the complexity and associated cost increases. It is because for certain multi-winding transformers a set of mechanically interlocked tap changers might be necessary instead of just one design. Another cost driver is the fact that a tap changer increases the dimensioning power of the transformer (not to be mistaken with nameplate power). Moreover, the testing of transformer gets more demanding as certain tests shall be performed at various tap positions. Last but not least, tap changer also requires regular maintenance – typically once per year. Always keep in mind that the DETC cannot be operated while the transformer is energized. It is therefore not suitable means to compensate short term fluctuation of grid voltage. It is only used for corrections of long term deviations from the nominal voltage. Based on out extensive experience, the tap changer is typically set into a specific position during commissioning of a drive system and left in that specific position unless there is a very strong reason to change it (which is seldom the case).

Therefore, as an alternative to a de-energized tap changer the system engineers and users can consider options such as using active front end (AFE) drive that inherently adjusts the voltage on-line or using a diode front end (DFE) based VFD designed to cope with the voltage fluctuation.

Summary

Tap changer is a popular means to adjust the turn ratio of a transformer. The device is used in distribution and power transformers and can be utilized in VFD transformers as well. However, as the VFD transformers generally tend to be more complex, so is also the implementation of a tap changer. If necessary, a de-energized tap changer is being used. On-load tap changer is extremely rare. OLTC might be used in utility applications, but in industrial applications it usually isn’t justified.

When using a tap changer is VFD applications a care shall be taken to ensure reliable and safe operation. The maintenance intervals recommended by the manufacturer shall be respected.

References

[1] VFD transformers: Introduction, https://mb-drive-services.com/vfd-transformers-introduction/

[2] VFD transformers: Multi-winding design, https://mb-drive-services.com/vfd_transformer_design/

[3] VFD transformers: Harmonics, https://mb-drive-services.com/vfd-transformers-harmonics/

[4] De-energized tap changers and switches (Hitachi ABB), https://www.hitachiabb-powergrids.com/offering/product-and-system/transformer/tap-changers-and-switches/de-energized-tap-changers-and-switches

[5] Hitachi ABB US: De-energized tap changers, http://alamo.abbus.com/_derived/PowerComponents/TapChangers/de-energized.html

[6] Trasfor – custom built dry-type transformers and reactors, http://trasfor.com/