Differences between IEC and IEEE design of transformers
In this post, we are going to explore some of the differences between IEC and IEEE design of transformers. Our remarks are not valid for VFD transformers only. Instead, most of them equally apply for distribution and power transformers as well. Some of them, however, are specific to VFD transformers (e.g. harmonics). But let’s start from the beginning…
IEC and IEEE transformer standards
Majority of transformer OEMs manufacture their products in accordance with international standards. In case of transformers, the two main families of standards are IEC and IEEE. While IEC as well as IEEE standards are used globally, there are some regional specifics and customer preferences. IEC standards are used around the globe. As a matter of fact, IEC is the predominant base in Europe. Almost all IEC standards are aligned with European Norms (EN) that are again harmonized with national standards in the EU countries. IEC standards are also widely used in Asia.
In North America, however, users often specify ANSI/IEEE standards and most North American manufacturers base their transformer design on ANSI/IEEE. Although IEC standards are often accepted as well, there is tendentiously a preference for IEEE.
Therefore, you may ask yourself: What is the difference between IEC and IEEE transformer design? That is a valid question which we try to address in this blog post.
IEC versus IEEE
Overall, one can say that both IEC and IEEE take a similar approach on most transformer related topics. Moreover, the individual committees certainly collaborate among one another and try to align certain aspects. Yet, there are some differences, especially when looking into details.
IEC world
In the “IEC world” the main standard for power transformers is IEC 60076. This standard consists of multiple parts focusing on specific topics. Same standard applies also for VFD transformers. Additionally, the topics related to converter duty operation are addressed in IEC 61378.
IEEE world
In the “IEEE world” the base standard for power transformers is ANSI/IEEE C57.12.00 for design and ANSI/IEEE C57.12.90 for testing. Requirements for semiconductor power rectifier transformers are then defined in ANSI/IEEE C57.18.10.
Insulation coordination / Test voltages
Both IEC and IEEE standards specify test voltages based on the system voltage.
In IEC 60076-3 the rated withstand voltages are provided based on the highest voltage for equipment (denoted Um, an RMS value). There is the rated lightning impulse withstand voltage (abbreviated as ‘LI’) and the rated short duration induced or separate source AC withstand voltage (abbreviated as ‘AC’).
IEEE C57.12.90 uses basically the same philosophy, but the specific test voltages naturally differ. Also, the naming is slightly different. In IEEE, the impulse test voltage is called basic lightning impulse insulation level, better known as BIL.
Construction
One possible difference in the construction is the transformer tank of liquid filled transformers. While the IC based design uses corrugated tank at lower power rating and conservator type of tank for higher ratings, ANSI/IEEE design also features the sealed tank option.
Examples of some differences
Temperature rise
In IEC, there are two values of temperature rise. One value indicates the temperature rise of top oil while other value is the average winding rise. For ‘standard’ ambient conditions (20°C yearly average, 30°C monthly average, 40°C max. temperature) the temperature rise is 60 K for top oil and 65 K for windings. Therefore, the temperature rise is denoted as 60 K / 65 K.
As per IEEE the temperature rise for top oil and for windings is the same value. For standard ambient temperatures the rise is 65 K.
When a lightning impulse test becomes a type test?
This is a good example to illustrate the comparison of IEC and IEEE transformer standards. In both relevant standards the lightning impulse test is a type test (design test) at lower system voltages and becomes a routine test for higher voltages.
Acc. to IEC 60076-3 a lightning impulse test is a routine test for Um > 72.5 kV. In IEEE Std. C57.12.90 lightning impulse test is a routine test for Um ≥ 115 kV.
Similar difference for the threshold applies also for the switching impulse test that is again a type test until certain voltage level and afterwards becomes a routine test.
Harmonics and their consideration
Talking specifically about VFD transformers there is a major difference how the standards approach harmonics.
Testing
The approach with regards to testing is generally quite similar in IEC and IEEE. These standards differentiate tests performed on each unit (routine tests), tests performed on one transformer of a type and finally special tests.
Terminology
Not surprising there are also differences in the terminology. It basically means that same or similar items are called a bit differently. These differences do not necessarily result from the standards but might have more historical background.
Examples:
Oil level indicator (European) – Oil level gauge (North American)
On-load tap changer (European) – Load tap changer (North American)
Terminal box (European) – Terminal chamber (North American)
Type tests (IEC) – Design tests (ANSI/IEEE)
Summary
IEC and IEEE standards are the most common standards the design and testing of transformers. In each “world” there are standards applicable for power transformers plus “add-on” standards covering the topics related to converter operation (rectifier duty). While IEC and IEEE are quite well aligned on power transformers, the particular aspects related to converter operation (e.g. harmonics) are handled a bit differently.
References
[1] VFD transformers – entire series, https://mb-drive-services.com/category/vfd_transformers/
[2] International Electrotechnical Commission (IEC), https://www.iec.ch/homepage
[3] Institute of Electrical and Electronics Engineers (IEEE), https://www.ieee.org/
