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Abstract
This paper will cover how to use a PMI Revolution recorder to verify compliance according to the IEEE 519-2014 harmonic standard. Recommended initialization settings and data analysis techniques are reviewed, along with some background on the Revolution’s capabilities with regards to the standard.
The IEEE 519-2014 standard covers definitions, monitoring recommendations, and limits for harmonic distortion on low, medium, and high voltage systems. The 2014 edition is significantly revised from the 1992 version. The most significant changes involve the “Point of Common Coupling”, using the IEC 61000-4-7 definitions for harmonics and interharmonics, specifying 3- second and 10- minute averaging, and revising the actual voltage and current harmonic limits. Measuring to this standard requires a sophisticated PQ recorder such as the Revolution, which is capable of measuring harmonics and interharmonics.
How to Set Up a Revolution in ProVision to Record Harmonics and Interharmonics:
Open the ProVision software and connect to the Revolution Recorder. Right- click on the unit serial number and select “Initialize”. When the window opens for choosing the initialization settings, click on the “Advanced” Tab.
Under Interval Graphs, select the V THD and I THD boxes. Then make sure ‘V Harmonics Magnitude’, ‘I Harmonics Magnitude’ are selected, then enter the desired harmonic range. The Revolution PQ recorder can record up to and including the 51st Harmonic. Then go to the Interharmonic section and select ‘Harmonic subgroups’, ‘THD Harmonic Subgroups’ and ‘THD Interharmonic Subgroups’. Also, if they are not already selected make sure ‘Voltage’ and ‘Current’ are also checked. These steps are shown in Figure 1.
Figure 1. Setting up ProVision to record harmonics and interharmonics
The IEEE 519 Standard describes the statistical profiling for voltage and current harmonics as follows:
Very short and short time harmonic values should be accumulated over periods of one day and one week, respectively. For very short time harmonic measurements, the 99th percentile value (i.e., the value that is exceeded for 1% of the measurement period) should be calculated for each 24-hour period for comparison with the recommended limits in Clause 5.
For short time harmonic measurements, the 95th and 99th percentile values (i.e., those values that are exceeded for 5% and 1% of the measurement period) should be calculated for each 7-day period for comparison with the recommended limits in Clause 5, the section covering recommended harmonic limits. These statistics should be used for both voltage and current harmonics with the exception that the 99th percentile short time value is not recommended for use with voltage harmonics.
The 95% and 99% percentiles are used with various multipliers in the harmonic limit tables.
The recommended measurement window for all systems has been set to 200 ms. This means the measurement will capture 12 cycles for a 60 Hz system and 10 cycles for a 50 HZ system. This method of sampling will provide values every 5 Hz. IEEE 519 now considers the Harmonic Component Magnitude to be the center frequency value combined with the 5 Hz leading and trailing that harmonic value. For example, the first harmonic magnitude consists of the 235, 240 and 245 Hz magnitudes, as shown in Figure 2, in the harmonic subgroup in blue. This harmonic definition is from IEC 61000-4-7 and corresponds to the harmonic subgroup definition. The Revolution computes harmonics and interharmonics with this method as per IEC 61000-4-7.
Figure 2: 3D Graph of Harmonic and Interharmonic Voltage Values over Time
When taking measurements for the very short time Harmonics, use a 3- second recording interval. Short time harmonic values are taken over a 10-minute interval based on 200 consecutive very short time values for a specific frequency component. Any PMI recorder may be set to the 3- second or 10- minute recording interval. It’s also possible to compute 10- minute data from recorded 3- second data by exporting the interval data to Excel.
Where Should you Place your PQ recorder?
The proper place to locate your PQ recorder, according to IEEE 519 is known as the Point of Common Coupling, or PCC. The previous 1992 version of the standard never truly defined the meaning of the Point of Common Coupling. The original description could be applied to any point, accessible by both the customer and the utility for direct measurement of the Harmonic Indices. Within any industrial plant, the PCC was defined as a point between the linear and the non-linear loads.
The release in 2014 eliminated much of the doubt about the location of the PCC. IEEE 519 now defines the Point of Common coupling as “the point in the power system closest to the user where the system owner or operator could offer service to another user. Frequently for service to industrial users (i.e., manufacturing plants) via a dedicated service transformer, the PCC is at the HV side of the transformer. For commercial users (office parks, shopping malls, etc.) supplied through a common service transformer, the PCC is commonly at the LV side of the service transformer.”
Retrieving Data Samples, Charts, & Graphs
Once the duration of your recording is complete, you may retrieve the recorder and download the file. Once downloaded, both voltage and current THD data can be read as either a chart or as a visual graph. Access to the charts is done by going to report, selecting interval and choosing either current or voltage THD. Each value can be read as a percentage to compare to the maximum value allowed charts contained in IEEE 519.
Analysis of Results
For a quick glimpse of your results, you can view the results of your recording as a 3D graph. This will provide you with an overall view of which harmonic, if any, is the source of the trouble.
It is important to remember that there are 3 levels of service; general systems, dedicated systems, and special purposes. These are shown in Figure 3.
Figure 3. Voltage THD limits for different systems
Using data from the chart, you can now compare that data to the VTHD limits set in the standard, as shown in Figure 4.
Figure 4. Individual harmonic limits at different voltage levels
A new category has been added to the Voltage distortion Limits table. The old range of V ≤ 69 kV has been broken into two categories, V ≤ 1.0 kV and 1.0 kV ≤ V ≤ 69 kV. The under 1.0 kV category is new, and especially important since this represents the secondary of the distribution transformer, which is now a possible PCC.
As an example, we will look at a recording made earlier for a test sample, shown in Figure 5 below.
Figure 5. Voltage THD stripchart with 3% IEEE limit annotated on the graph (averages shown)
In this example, 3% THD thresholds are used as per the “Special Applications” limit in Table 10 of of IEEE 519 (as shown in Figure 3). The 3% limit has been drawn using ProVision annotations directly represented on the graph. Only the average is shown, which is recommended over the readings of the maximum or the minimum. The table in Figure 6 shows a block of data from a VTHD recording.
Figure 6. Voltage THD data exported for percentile analysis
The annotation makes it easy to see that channel 1 has recorded values in excess of 3%. If this were a recording for a facility classified as “Special Purpose,” a fix would need to be made. The data from each voltage harmonic can be examined to find the specific harmonic (or harmonics) causing the problem. The data does indicate that the harmonic distortions are acceptable for the other 2 classes.
The graphic data can also be used to determine the acceptability of PQ. Note that at some point, all 3 channels exceed the 3% value needed for the “Special Purpose” category.
Provision can also be set to display Total Demand Distortion (ITDD) if you know the maximum load current. To view this, select Total Demand Distortion under the reports tab. The user will be prompted to enter the value for you maximum load current. With these 3 tools, compliance with IEEE 519 may be verified.
One important update to the Current distortion limits (Table 2 from IEEE Std. 519) are explicit limits for several harmonic ranges, dependent on the circuit stiffness (ratio of ISC - short circuit current to IL, the max load demand current).
It is important to remember that there are 3 levels of service; general systems, dedicated systems, and special purposes. The determination the acceptable level of distortion. ITHD similarly divides into 5 levels based on the ratio of ISC/IL, as shown in Figure 7.
Figure 7. Harmonic distortion limits
Conclusion
With a simplified version of IEEE 519 released in 2014, maintaining quality power throughout the electrical grid is now more straightforward. A clearer understanding of PCC along with a more consistent, standards-based approach to PQ will allow utilities to supply a better quality product to their customers. The powerful tools of a Revolution recorder and ProVision software are the perfect tool to help you meet the IEEE requirements.
Michael Hetmanski
Design Engineer
mhetmanski@powermonitors.com
http://www.powermonitors.com
(800) 296-4120
