Case Study: Short Circuit Analysis of Motor VFDs in Bypass Mode Versus Normal Mode and Its Impact on Calculated Short Circuit Currents & Duty Ratings
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Electrical Engineers, when conducting an Arc Flash Analysis, do you model Variable Frequency Drives (VFD) in bypass mode (if equipped) to assess the worst-case scenario? What is the expected magnitude of differences in the calculated short circuit values when comparing VFDs in bypass mode versus operating normally?
When conducting an Arc Flash Analysis, I typically model all Variable Frequency Drives (VFDs) in bypass mode, if they are equipped with this feature. This approach is taken unless the VFD is a regenerative type, as a rectifier cannot reverse power flow. VFDs, in general, do not allow fault current to be contributed from the motor to upstream devices, except when in bypass mode.
In a recent project involving a large electrical system modeled using SKM software, I analyzed a system with over 700 buses. In this system, 88 motors have VFD’s which total 2,988 HP. Each VFD in this project had bypass capabilities. As mentioned earlier, I usually model all VFDs in bypass mode during an Arc Flash Analysis to consider the worst-case scenario. However, I acknowledge that the likelihood of every VFD being in bypass mode simultaneously is extremely low, though not impossible.
With this consideration, I conducted the analysis by running the model with VFDs in bypass mode, exporting the results, and then running the model with VFDs in normal mode and exporting those results. I compared the calculated short circuit current at each piece of equipment. Additionally, I identified instances where equipment was determined to be overdutied (meaning the available fault current is not greater than the rating of the equipment) due to VFDs being in bypass mode, which would not have been the case if the VFDs were modeled in normal mode. The results were quite surprising.
The most unexpected finding was that there was only one instance where the calculated short circuit current exceeded the panel's rating when the VFD bypass was engaged. Conversely, when the VFD was not bypassed, the calculated short circuit current remained within the panel's rating.
With all VFDs operating in normal mode (not bypassed), the most significant change in calculated short circuit current was an increase of 13.54%, equivalent to 2,769A. Given that this specific panel is rated for 42k, whether the VFD was bypassed or not did not affect its duty rating.
By no means am I a statistician, however here are my findings:
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The histogram above shows the distribution of the differences in fault current. Most differences are relatively small, with a few larger differences, indicating that in most cases, the fault current changes modestly between the VFD bypass on and off states.
The histogram shows a right-skewed distribution. This means that while most of the differences are small, there are some large differences on the higher end. The mean difference is 150A, which suggests that, on average, changing the VFD bypass state results in an increase of 150A units in the fault current. The median (50th percentile) is 36.49, indicating that half of the changes in fault current are below this value, and half are above. The median being much lower than the mean confirms the skewness of the distribution. The standard deviation is 339A, indicating high variability in the differences. This suggests that while many changes are small, some are significantly larger.
The presence of outliers (differences as high as 2776A) suggests that for certain locations, changing the VFD bypass state can have a significant impact on the fault current. For the majority of locations, the differences are relatively modest, clustered around smaller values. This indicates that for many panels, the impact of changing the VFD bypass state might not be very substantial. However, the variability and the presence of large differences for some panels highlight that the significance of changing the VFD bypass state can vary widely depending on specific conditions or panel configurations.
Overall, while many locations experience only minor changes, the significant differences observed for some locations imply that the VFD bypass state can be an important consideration in specific contexts. It would be advisable to identify and focus on those locations where the difference is most pronounced.
I hope this gives some insight on Motor VFDs in Bypass Mode Versus Normal Mode and Its Impact on Calculated Short Circuit Currents & Duty Ratings.
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