,
Abstract
IEEE 519-2014 specifies measurements, analysis methods, and limits for harmonic voltage and current. A full IEEE 519 study is a straightforward, but rigorous analysis that can require multiple recordings and a statistical analysis. However in many, if not most cases the utility voltage is well within compliance. For those situations, a simplified method is shown here to demonstrate compatibility with the IEEE 519 specs without requiring the full analysis.
Review of IEEE 519 Limits
The IEEE 519-2014 standard gives limits for voltage and current harmonics. The voltage portion is the responsibility of the utility, and in many investigations the only question is whether the delivered voltage at the point of common coupling (PCC) is within limits or not, regardless of the level of current distortion. Thus, only voltage harmonics are discussed in this simplified approach.
The standard calls for measuring each harmonic to the 50th on two timescales - 3 seconds (“very short time”, or vs), and 10 minutes (“short time”, or sh). The recording length for the 3 second readings is at least one day, and for 10 minutes, at least one week. Once these recordings are in hand, 95th and 99th percentiles for each harmonic and THD are computed. To fully check compliance, the different thresholds for each 3 second and 10 minute harmonic must be check, in addition to separate thresholds for 3 second and 10 second THD.
The actual limits are shown for 1000V and under:
The limits are lowest (strictest) for the 10 minute readings, and higher for the 3 second readings. This fact, along with the observations below, can be used to streamline the analysis if harmonics are low overall.
Theory Behind The Simplified Approach
The relationship between THD, 3 second, and 10 minute readings allows a simpler approach if the harmonic levels are relatively low. There are three key observations that enable this approach. First, the THD of a set of harmonics is greater than any individual per-unit harmonic:
using the notation from IEEE 519, where “vs” indicates “very short term” (3 second) values, and Fn,vs is the per-unit magnitude of the nth very short term harmonic. This is also true for the 10 minute short time harmonics and its THD. Because the THD is always higher than each specific harmonic, the 519 limits are higher for THD than for individual harmonics. However, if the THD is checked against the lower individual harmonic limit, and it’s under that limit throughout the recording, then mathematically each harmonic must also be under that limit. Of course, if the THD is under the harmonic limit, it will also be under the higher THD limit too. Thus, if the THD is low enough, it isn’t necessary to check each individual harmonic.
The second observation is that the 3 second harmonics are always higher than the 10 minute harmonics. This follows from the fact that the 10 minute values are an RMS average of the 3 second values. Since an RMS average of a collection, like an arithmetic average, is always less than or equal to the maximum value in the collection, a 10 minute harmonic magnitude is always less than or equal to the maximum 3 second harmonic reading in that 10 minute period:
using the notation of IEEE 519. For this reason, the harmonic limits for 10 second values are lower (stricter) than the 3 second limits in IEEE 519. However, if the 3 second values are under the lower 10 minute thresholds for the entire recording, then it must also be true that the 10 minute readings are under those thresholds too.
The third observation relates to percentiles. IEEE 519 adds a complication - the raw values aren’t compared to threshold, but instead 99th or 95th percentiles are used. However, a percentile of a collection is always less than the maximum value in that collection. Thus, if every individual value in a recording is under a limit, then any percentile from that measurement type is also under the limit.
These three results, combined with the fact that in reality, voltage distortion at the PCC is often very low, allows for a simplified procedure for checking IEEE 519 compliance that works most of the time. If the largest measurement type (3 second THD) is lower than the lowest threshold (10 minute individual harmonic limit), then mathematically all other readings are also in compliance, and don’t need to be explicitly checked. If not, there are still simplifications that may be possible, which leads to a procedure that checks the simplest, but most likely case first, and if necessary leads to a more complex full IEEE 519 analysis.
Simplified Procedure
The flowchart for the simplified IEEE 519 analysis is shown in Figure 1. Two custom graphs are helpful, these are available by downloading the graph package from here. One graph is used for Step 3, and one for Steps 4 and 5. Steps 1 and 2 involve collecting and preparing the data. Step 3 is a simple analysis of THD on a single graph, and in many cases is the only step necessary to demonstrate full IEEE 519 voltage compliance. If Step 3 fails, proceed to Step 4. Here, another THD check and a harmonic level check is performed. Many failures from Step 3 will end here, again demonstrating IEEE 519 compliance. If not, proceed to Step 5. This step is a little more complex, and it’s less likely that compliance will be shown - Step 5 only catches situations where the harmonics are roughly between the 3 second and 10 minute thresholds. However, it’s still much simpler than a full 519 analysis. A failure in Step 5 is either an obvious out of tolerance situation with high harmonics, or an ambiguous one that requires the full IEEE 519 computation. Fortunately this outcome is uncommon.
Figure 1. Simplified IEEE 519 compliance procedure
Step 1
The first step is to record the data itself. Because the 3 second harmonic values are always higher or equal to the 10 minute values, choose a 3 second interval for the recording, over a 1 week period, as shown in Figure 2. The key values are circled in orange. At least voltage magnitude should be selected for harmonics, and in Figure 2 the odd harmonics to the 31st are enabled. THD is also needed. Optional but useful to have are Pst flicker (for very low interharmonics), and periodic waveform capture (for interharmonic calculations or spot checks for even harmonics). These may be needed if the full IEEE 519 process is necessary.
Figure 2. Recorder setup to gather IEEE 519 voltage data. Enable harmonics, THD, and 3 second interval
Step 2
Once the data is collected the harmonics should be scaled to per unit values. Use the ProVision scale factor settings to scale the raw voltage magnitudes, as detailed in the white paper Per-Unit Analysis with ProVision. For convenience in comparing with the IEEE limits as percentages, multiply the per-unit factor by 100. For example, if the nominal is 120V, the ProVision scale factor is 100 x (1/120) = 0.83333. Choose Tools, Scale Factor from the ProVision menu to set these.
Step 3
A full IEEE 519 analysis would start with comparing the 3 second THD with its limit of 12%. Instead, compare the THD to the lowest voltage limit-- the 5% limit for 10 minute individual harmonics. In most real-world cases, the 3 second THD will be under 5% for the entire recording. If so, then the 10 minute THD must also be under 5% for the entire time. Since the 3 second THD is higher than any individual 3 second harmonic, then all 3 second harmonics must also be under 5%, and thus in compliance. Finally, since each 10 minute harmonic is less than each corresponding 3 second harmonic, they also must be under 5%. And if all individual 3 second readings are good, then the 95% or 99% percentiles must also be good.
Therefore, if the max 3 second THD is under 5% this location is in compliance with IEEE 519 voltage limits, with no further analysis required.
With a 3 second stripchart interval, the recorder will log a min, ave, and max THD every 3 seconds for each channel. The min and max THDs are 1 second readings, but for this analysis only the 3 second average is needed. The Voltage THD graph from the graph package shows just the average for each phase on a single plot. Figure 3 shows this graph for a typical recording. A red dashed line annotation shows the 5% threshold, and a blue dashed line the 8% threshold. Here all three phases are well under 5% for the entire recording (over 1 week). Thus, this location is IEEE 519 voltage compliant without any further analysis. If there are excursions past 5%, proceed to Step 4.
Figure 3. Step 3 - check 3 second THD - if under 5% everywhere, data is fully compliant with no further analysis
Step 4
Next, check to see if the max THD on every channel is under 8% for the entire recording. This may be checked visually with the same graph as Step 3. If the THD for any phase is under 8%, but above 5%, then check to see if all 3 second harmonics are under 5%. If they are, then the analysis is done-- the location is IEEE 519 voltage compliant. No 10 minute readings are needed since the thresholds checked here are the 10 minute limits, but applied to the 3 second values. If the 3 second values (which are always >= 10 minute values) are under the 10 minute limits, both sets are within limits.
To check the individual 3 second harmonics, use the Voltage Harmonic Magnitudes graph from the package. Figure 4 shows this graph with an example file. Here, the 5% threshold has been marked with green dashed lines. There is a separate plot on the graph for phase A, B, and C. Within each plot, all harmonics are displayed. As long as every trace is under the limit at all times, the data shows IEEE compliance. It’s not necessary to distinguish which harmonic is which unless there is an excursion past the threshold. In this example, each harmonic is under the limit for the entire week, and thus the location IEEE compliant. If there is an excursion for either THD or a specific harmonic, proceed to Step 5.
Figure 4. Checking individual harmonics to the 5% limit. All Phase A harmonics are in top plot (black), phase B in middle (red), Phase C in bottom (blue)
Step 5
In most cases, IEEE compliance will be demonstrated in Step 3 or 4. If not, there is still hope for a simpler analysis than the full 519 breakdown. Here we take advantage of the percentile nature of the true 519 limits. The 3 second limits are based on the 99th percentile, which is roughly 14 minutes over a 24 hour day. The 10 minute limits are based on a 95th percentile, which is roughly 50 minutes over a week. In Step 5, brief excursions past the raw limits are allowed, as long as they are within 14 minutes per day or 50 minutes per week (7.2 minutes per day), depending on the threshold.
First, examine the 3 second THD traces. At least one channel must exceed 8% to get to Step 5. If the total time per week over 8% is under 50 minutes, and the total time over 12% is under 14 minutes per day, proceed to then check the 3 second individual harmonics. If the THD is not with those limits, a full IEEE 519 analysis is required. For the individual harmonics, check to see if 7.5% is exceeded by any harmonic for more than 15 minutes per day, and 5% for more than 50 minutes per week. If the THD and individual harmonics are under those limits, then the location is IEEE 519 voltage compliant.
If Step 5 fails, the location may still be within the IEEE 519 specification. If the results are close to the limits, a detailed analysis with true 10 minute computations and percentiles will be needed to confirm a pass or fail. The approach in Steps 3 and 4 is conservative, designed for a quick analysis in the common situation of very low harmonics. A significant exceedance with those tests may still be within limits, but it may take the detailed analysis to reveal it. An example that fails the tests in Step 5 is shown in Figure 5. The 7.5% threshold for individual harmonics is exceeded for a short amount of time (less than 14 minutes total). The 5% level is exceeded for much too long. The harmonics shown are 3 second values, and the 5% level is intended for 10 minute values, so in some cases the data could actually be in compliance. However, here the 5% level is exceeded continuously for very long stretches (almost an entire day in some cases), so there’s no chance of the 10 minute smoothed harmonic values staying under 5% for 95% of the time. The offending harmonic can be read from the point table at the right (circled in orange) - the phase A 3rd. The other harmonics may be ignored in the detailed analysis, since they pass the Step 5 test.
Figure 5. Checking harmonics in Step 5. Phase A 3rd harmonic is over the 5% limit for too long
Conclusion
IEEE 519-2014 presents a new, more rigorous method for analyzing voltage harmonic levels. A full analysis requires 3 second and 10 minute THD and individual harmonics, along with a percentile analysis. In most cases, the actual voltage distortion is well within the strictest of limits in the standard. For those cases, the mathematical relationship between THD and the harmonics, the 3 second and 10 minute values, and percentiles allows for a much simpler process to validate compliance without a lengthy analysis. In the few cases where the simple process fails, the simplified results are a starting point for the more detailed process.
Chris Mullins
VP of Operations and Engineering
cmullins@powermonitors.com
https://www.powermonitors.com
(800) 296-4120