VFD transformers: Differential protection
Differential protection of VFD transformers might be a controversial topic. The more a reason for us to bring it up, explain the principle, describe the advantages and drawbacks and provide our view and recommendation for the users. We hope that this article will be useful source of information for you. As always, we appreciate your feedback through our email, social media or by using the contact form at the bottom.
Introduction
Protection of VFD transformers was already discussed in [1] along with highlighting the specific aspects. We have explained the challenges related to overcurrent protection of multi-winding transformers in light load condition and touched other topics as well. Today we look specifically into differential protection of VFD transformers.
Differential protection in general
Differential protection is very popular protection of distribution and power transformers. Many experts consider it as primary protection while having overcurrent protection as secondary or backup. Differential protection is sensitive and acts very fast. Therefore it is considered as reliable. Applying differential protection to a conventional 2-winding transformer is quite simple as it only requires a set of current transformers (CT) on each side, i.e. six (6) CTs. Standard transformer protection relays [2, 3] available on the market support differential protection of such transformers. Therefore, implementation is easy and straight-forward.
The “IEEE/ANSI code” for differential protection relay is 87 as defined in IEEE Std C37.2. According to the definition it is “a device that operates on a percentage, phase angle, or other quantitative difference of two or more
currents or other electrical quantities”.
Implementation for multi-winding VFD transformers
For multi-winding transformers the application of differential protection becomes more cumbersome. First of all, as the number of transformer secondary windings goes up, the amount of CTs increases as well. Typical 24-pulse rectifier transformer would require 3 CTs on the primary side and 12 CTs on the secondary windings.
And that is not all. Another challenge in realization of differential protection of multi-winding transformers is that the protection relays normally do not support the special vector groups being used. The currents of secondary mounted CTs cannot be simply summed up as the currents have a phase displacement. Arithmetic summation would be incorrect. As the differential protection is meant to be very sensitive, incorrect summation would cause false trips. Alternatively, the differential protection would be parameterized less sensitive (tolerating larger error). That is not an option either as it downgrades the basic idea of differential protection. Therefore, additional sum CTs need to be used to compensate the relative phase displacements of the secondary windings. Such additional components mean higher cost and complexity.
We can immediately notice increased complexity and high number of components. It is assumed that the transformer protection relay supports differential protection of a three-winding transformer and can internally handle the 30° phase shifting. For phase shifts smaller than 30°, i.e. fractions of one clock, additional phase shifting transformers are required. Also, sum CTs are needed. The quantity of components is summarized in Figure 3:
Figures 2 and 3 show two possible implementations. On the left hand side there is a solution with single primary winding. There is only one protection relay needed, but additional phase shifting transformers and sum CTs are required as already explained.
The solution on the right hand side is based on a transformer with primary winding internally split into two or having two parallel 12-pulse three-winding transformers with relative phase shift on the primary side. Two identical protection relays are needed, but phase shifting transformers instrument transformers and sum CTs can be eliminated.
Note that the solution with two primary windings requires the CTs on primary side to be installed in the transformer. Solution with just one primary winding can have primary current measurement placed e.g. in the switchgear (and protect also the line between switchgear and transformer).
Having 30-pulse or 36-pulse transformer, the amount of CTs for measurement, phase shifting instrument transformers and sum CTs further increases.
Differential protection of VFD transformers and harmonics
Differential protection is aimed to be a sensitive protection. Therefore, care shall be given to the harmonics. It is especially important for multi-winding transformers. While there is very low harmonic distortion on the primary side, the secondary windings are loaded with highly distorted currents. Note that harmonic cancellation happens inside the transformer, but currents in the secondary windings have the typical 6-pulse spectrum with THDi up to 30% or even higher. A proper filtering of the signal is required. It shall be done in a way to minimize impact on the differential protection itself.
Differential protection zone
VFD systems are from protection point of view divided into several protection zones. Moreover, differential protection is a classical zone protection where the zone is determined by location of the two (or multiple) current measurements. When already implementing a complex differential protection of a multi-winding transformer some users desire to protection the connections between transformer and VFD as well (cables or busbars). Therefore, the CTs on the secondary side are not mounted in the transformer itself, but rather at the rectifier input terminals inside the VFD. Mechanical adaptions of the VFD are required to accommodate larger number of CTs inside the VFD enclosure. When thinking about this concept and its pros and cons it is just a step to a combined transformer + VFD differential protection.
Combined differential protection
As described above, a differential protection of input transformer plus transformer connections to the VFD is linked with extra effort and requires larger amount of extra components. Why not extend the protection zone and protect the area from transformer input side to the VFD output towards the electric motor?
Of course, there are certain considerations to be taken care of, but overall this concept offers several advantages that cannot be ignored:
1. Only one set of CTs on transformer primary side is required (and this measurement is normally already available for the purpose of overcurrent protection).
2. Current measurement at the inverter output is standard part of every VFD. These sensors are inherently designed to handle variable output frequency. They feature high accuracy as the current measurement is used for VFD control and protection.
3. Differential protection zone is extended to cover input transformer and VFD plus the connections between both components. It can detect any failure happening within this zone.
4. Transformer protection relay is not needed as the functionality can be implemented as protection function directly in VFD firmware.
Some considerations about the concept of extended differential protection zone were published at the PCIC conference back in 2017 [4]. Of course, it requires some smart algorithms to make the concept work despite of different power factor on each side, variable frequency at inverter output etc. However, all these obstacles can be solved.
Transformer differential protection: DFE versus AFE drives
DFE
Diode front end (DFE) drive topologies feature a mutli-winding VFD transformer feeding multi-pulse rectifier. 12-pulse is suitable for lower power range or connection to rather strong grid. 18-pulse and 24-pulse rectifiers are quite common and 36-pulse DFE is not unusual, either. For 12-pulse rectifier supplied from a 3-winding transformer the transformer differential protection is doable. For higher number of windings the realization becomes more complex.
AFE
Active front end (AFE) topologies usually feature more simple transformers with less windings (2-winding or 3-winding) as the harmonics are cancelled by combination of modulation strategy (selective harmonic elimination) and transformer phase shifting. Therefore, differential protection is generally less complex to realize. On the other hand, AFE drives always have current and voltage measurement on the input side and any fault on the transformer is detected by the rectifier anyway.
Summary
A wrap up on differential protection of VFD transformers:
– Transformer differential protection – easily done for a 2-winding or 3-winding transformer.
– As the number of windings increases, the complexity of such protection function increases as well.
– Large number of current transformers (CT) is required. For a 36-pulse transformer with 6 secondary windings it means 3 CTs on primary side and 18 CTs on the secondary side. For a transformer with 15 secondary windings as much as 45 CTs would be required.
– Besides that the phase displacement has to be considered as well. This might require additional summing CTs and a protection relay that supports such functionality.
– DFE transformers, especially in higher power range, feature multiple windings (often 24-pulse or 36-pulse rectifiers meaning 4 or 6 secondary windings) ⇒ Transformer differential protection becomes rather complex.
– AFE transformers are usually more simple. Implementation of differential protection somewhat easier. However, AFE rectifiers inherently measure the input currents for purpose of control and protection ⇒ Transformer differential protection is less important.
Conclusion
Differential protection is fast and reliable way to protect distribution and especially larger power transformers. For 2-winding or 3-winding transformers the amount of components is kept small, implementation is easy and parameterization directly supported by the protection relay.
In case of VFD multi-winding transformers the basic principle is still the same, but the complexity increases exponentially with the number of windings. Not only the amount of current transformers (CT) increases, but there are additional elements required to compensate the phase displacement between the secondary windings. Generally the protection can be realized, but the specific considerations related to VFD applications need some compromises that in the end reduce the sensitivity of this protection method.
A novel differential protection combining transformer and VFD into one common differential protection zone is an interesting alternative. It requires practically no additional components. What needs to be added are several lines of code in the VFD control and protection firmware.
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References
[1] Protection of VFD input transformers, https://mb-drive-services.com/protection_of_vfd_transformers/
[2] RET670 – transformer protection relay, https://new.abb.com/substation-automation/products/protection-control/transformer-protection/ret670
[3] RET 670 – Transmission transformer protection, https://www.hitachiabb-powergrids.com/au/en/offering/product-and-system/substation-automation–protection—control/products/protection-and-control/transformer-protection/ret670
[4] M. Bruha, M. Visser, J. Von Sebo, E. Virtanen, P. Tallinen, “Protection of VSD transformers”, 2017 Petroleum and Chemical Industry Conference Europe (PCIC Europe), Vienna, May 2017
[5] L. Sevov, S. Kennedy, R. Paes, P. Ostojic, “Differential protection for medium voltage pulse transformers”, 2014 IEEE Petroleum and Chemical Industry Technical Conference (PCIC), pp. 173-184
