Root cause of VFD transfomer failures
Before we dive into the topic let us stress the fact that transformers belong to very reliable electric equipment and their failure rate is very low. This statement is generally valid also for VFD transformers. But no matter how reliable a certain device is, failure can never be completely excluded. Indeed, also VFD transformer may experience a failure. This article touches some of the root causes of VFD transformer failures.
We intentionally focus on VFD transformers and their possible failure modes. Most VFD transformers differentiate from typical distribution transformers. If you are our regular reader, you shall be quite familiar with that topic. If not, we have listed here some relevant articles published earlier:
VFD transformers differ from power transformers by the design requirements and also the way they are operated. Both of these factors affect the protection scheme and possible failure mechanism. As the failure of a VFD input transformer has a fatal consequences on the availability of the entire drive system, the issue shall be taken very seriously.
If the transformer experiences severe damages beyond the repair, a new transformer needs to be manufactured. That may take several months. Current shortages in the commodity market further extend the lead time.
The good news is that with a deep understanding of the most common transformer failures you are in best position to mitigate them.
Impact of environment
It may sound surprising but the environmental factors have a dramatic impact on the reliability of a transformer. Dust, corrosion, humidity or animals may cause a transfomer failure. What initially develops as a slow deterioration of insulation usually ends up in a sudden breakdown.
Oil immersed transformers are generally quite well protected as the active part is submersed in oil providing the insulation, cooling and environmental protection function. However, the oil quality shall be monitored. Moreover, components such as bushings are still fully exposed to environmental conditions.
Dry type transformers may be more sensitive to environmental conditions. It all comes to the type of enclosure (IP rating), way of cooling (natural/forced, single loop/double loop), installation (indoor/outdoor) etc.
The issue is often not the transformer as such but the selection of a suitable design fitting the environmental conditions.
Well, the impact of environment affects all transformers in a similar way, no matter whether they are used in VFD applications or not.
Rectifier faults as a root cause
This failure mode is already very specific for converter transformers, among them also VFD transformers. As we will explain, this issue is particularly important for diode front end (DFE) type of rectifiers for at least two reasons.
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Fast transient overvoltages
Very fast transient overvoltages put the insulation system of a transformer at great risk. The conventional overvoltage protection, such as e.g. use of surge arresters, may not be sufficient. Although the transient overvoltage phenomenon affects all transformers, in VFD aplications the root cause may be somewhat different and the severity is higher.
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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: Protection, https://mb-drive-services.com/protection_of_vfd_transformers/
