Question from Dan <omitted>: On some of our 3-Phase Cabinet PDU's, they are reporting an 'Out-of-Balance' condition (ie. >20%). Can you please have someone explain how the percentage is being calculated so that I can better understand what's going on here?
Engineer Reply: The average of the three loads is calculated
first. If the average is less than ½ Amp, the loads are considered
balanced. Otherwise, the maximum deviation of the three loads from the
average is calculated. If the maximum deviation is less than ½ Amp, the
loads are considered balanced. Otherwise, the maximum deviation is
calculated as a percentage of the average. If this percentage is greater
than the configured "3-Phase Load Out-Of-Balance Threshold", then the
loads are considered to be out-of-balance. A more detailed explanation follows:
Out of the box, our 3-Phase PDU units that measure Input
Current contain a system configuration option called "3-Phase Load
Out-Of-Balance Threshold". The default is 20%. When any load
is out-of-balance by 20% or more, a red asterisk (*)
will appear next to every wattage figure that is reported and the message "*Unbalanced 3-phase loads – value suspect"
will appear on-screen.
The default threshold of 20% is what our engineers have
determined to be acceptable with respect to the wattage information
deviation. If desired, it can be changed to a percentage of the user's
choosing. The feature determines the percentage of the worst imbalance
from the mean (average) of all three load measurements, and then checks it
against the configured percentage around the mean, thus making it a plus or
minus range. Here are the calculations:
Calculate average line current: IAVG = (IX
+ IY + IZ) / 3
Calculate maximum imbalance: M = abs (IX – IAVG)
If abs (IY – IAVG) > M
M = abs (IY – IAVG)
If abs (IZ – IAVG) > M
M = abs (IZ – IAVG)
Calculate imbalance percentage: B = (M / IAVG)
x 100
Assuming 208V Delta and a 1.0 power factor, here is an example that illustrates the minor deviation when loads are imbalanced at just under the 20% mark. Specifically, in order to illustrate the worst case while being just under the 20% mark, note that the numbers I've chosen below result in one measured load being +19.95% from mean and another being -19.95% from mean. These Branch loads (which are not well matched)… XY = 19.8A@208V = 4118.4W YZ = 11.1A@208V = 2308.8WXZ = 27.0A@208V = 5616.0W …will result in these Line loads: X = 40.64A Y = 27.13A Z = 33.91A Total power as determined from branch load values is 12043W. Reported in the PDU interface after calculations using the Input Current values will be 12211W. Deviation with this OOB scenario for Power (Watts) reporting with this OOB situation will be less than +1.4%.
My thanks to Bruce for the detailed explanation.
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