VFD transformers and harmonics
Introduction
The next post in our series discusses VFD transformers and harmonics. We have already learned what harmonics are and how they impact the supply grid (Network harmonics – intro). In this series we look more specifically into harmonics and their impact on VFD transformer.
Motivation
Harmonics are crucial to be considered in VFD transformer design. The purpose is mainly to avoid overheating and local hot spots. The goal is to ensure reliable transformer operation. Today’s article presents the impact of harmonics on the transformer and the design considerations to be made. We also refer to guidelines in the standards how to deal with harmonics.
VFD transformers and harmonics: Before we start…
There is one consideration we shall do right at the beginning. It is fundamental for understanding the topic:
The network harmonics can be minimized by using VFD input section with higher pulse number and input transformer with suitable phase shifting. However, this principle applies for network harmonics, i.e. transformer primary side. The currents in the secondary windings (and partly also in the primary windings) are still exposed to high harmonics regardless of phase shift. This fact is illustrated in figure 2. The upper part shows the spectrum of harmonic currents at the point of common coupling. The distortion is very low – less than 1%. The lower part depicts the spectrum of current flowing in the secondary windings of the transformer. The harmonic distortion is much higher – over 27%. The distortion of current flowing in the primary winding(s) depends on the transformer design. You may refer to our previous post on multi-winding transformers: click here to read the article.
There are few exceptions, but those are seldom and more relevant for active front end VFD.
Harmonic spectrum
As explained in IEC 60146-1-2, the transformer manufacturer cannot know the exact spectrum of current. Therefore, it is the responsibility of the purchaser to specify the harmonic spectrum. In most cases the spectrum of current on the converter side will contain characteristic harmonics of a 6-pulse system, i.e. dominant 5th and 7th harmonic.
Note that the converter side windings are historically called valve windings in the international standards.
Finite element analysis
The consideration of harmonics is a complex problem. We believe that the transformer manufacturer shall simulate the transformer design using a finite element tool. This shall be done at least for the first prototype. The model can be reduced using axis symmetry. Finite element model allows to consider specific material selection and exact geometry of the transformer.
The harmonics create additional radial flux components that in turn cause additional losses and harmonic heating. Results from the simulation model shall be compared with measurements on a prototype. When done correctly, the simulations and measurements shall have good concordance.
Thermal effect of harmonics
Harmonics cause additional losses. The cooling concept must consider such additional losses. These must be properly evacuated in order to stay within the temperature class and to avoid hot spots. In case of forced cooling the flow rate of cooling medium must be increased. A multi-winding transformer often has secondary windings stacked on top of each other. In such configuration the upper winding will be the hottest one as the inlet temperature of cooling medium (air, oil, synthetic liquid,…) is the highest.
Remark: Harmonics do not affect the windings only. Losses are induced also into electrostatic shield etc.These losses might be independent on actual load to some extent.
Harmonic losses and efficiency
System efficiency is often part of technical evaluation. The transformer efficiency shall consider additional losses caused by harmonics. Transformer manufacturer usually provides only no load and full load losses based on sinusoidal supply. Therefore, it is the responsibility of the system designer to put provision for harmonic losses.
Temperature rise test
When performing a temperature rise test on rectifier duty transformer following options are possible:
1. Perform temperature rise test with sinusoidal current
In this case an equivalent thermal current shall be used for the test. The equivalent thermal current is higher than the nominal current to consider additional harmonic losses. For more details you may refer to IEC 61378-1. The same standard also mentions that this method does not provide the same local loss distribution as the real waveshape with harmonics.
2. Perform temperature rise test incl. specified harmonics
If the test facility allows to inject harmonics it can be used for temperature rise test. However, this approach is practically difficult. Harmonic generator is usually limited to several low order harmonics. Therefore, the whole harmonic spectrum most likely cannot be emulated. There might also be a limitation in magnitude of the harmonics being injected.
3. Perform temperature rise test using state of the art grid simulator.
Power electronic based grid simulator allows the emulation of specific waveform that can be easily customized per test. Such equipment provides wide range of flexibility. While it is quite new technology it might find broader usage in specialized test fields.
4. Perform temperature rise test as part of combined test or string test
During a drive system combined test or a string test with load machine the transformer is tested in “real conditions” including harmonics generated by the VFD. However, such test is very expensive. In higher power range an external test facility is usually needed. Testing is also logistically challenging.
DC component
Harmonics are usually understood as multiples of fundamental frequency. DC bias can be considered as special kind of harmonic (0 Hz component). The origin of d.c. component can be the operation mode of rectifier, control strategy, supply unbalance, effects from the load etc. Standard design can tolerate only very small dc component. For higher values the transformer must be dimensioned accordingly. It might lead to reduced flux density. Eventually there is a need for air gap in order to avoid transformer saturation.
Impact of harmonics on sound level
Some installations have strict requirements on maximum noise level. This is particularly the case for installations in or nearby urban areas. Every transformer creates certain noise wen energized. The source is the magnetic circuit and it is known as characteristic humming. For projects with VFD transformers the impact of harmonics on noise shall be analyzed. It shall be differentiated between current harmonics and voltage harmonics. DC component, mentioned in paragraph above, also has a significant impact on transformer sound level.
Summary
– Harmonics and their proper consideration is essential in VFD transformer design.
– Harmonics cause additional harmonic losses. This effect shall be considered in transformer design in order to avoid overheating and hot spots.
– In case of guaranteed drive system efficiency the true efficiency of the transformer incl. harmonic losses shall be considered.
– Harmonics affect transformer noise. This might be critical for transformers located outdoors and customers having strict requirements on noise emissions.
We hope that the post helped to explain the topic of VFD transformers and harmonics. You may refer to the series of network harmonics that partly overlaps with this article.
References
[1] Trasfor – custom built dry type transformers and reactors, Trasfor – Homepage
[2] M. Bruha et al., “Protection of VSD transformers”, 2017 Petroleum and Chemical Industry Conference Europe (PCIC Europe), Vienna, May 2017
