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Abstract
On August 12th, 2014, the IEEE Standards Association updated their 519 standard for Harmonic control in electrical power systems. One of the first things you will notice about the updated version is that much of the extraneous material was removed from this version. Sections 4 -9 of the 1992 release were removed from 2014 in order to simplify the Standard. Another important reference material was relocated to the appendices of 2014, such as interharmonics, flicker, TIF (Telephone Influence factor) and SCR notch depth limits. This paper will cover the changes that affect the supply side of the power grid system.
Updates and Clarifications
- The PCC is defined in greater detail
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.”
- The meaning of TDD is clarified
Total demand distortion (TDD). The total root-sum-square harmonic current distortion, in percent of the maximum demand load current (15 or 30 min demand).
New 2014 definition:
Total demand distortion (TDD): The ratio of the root mean square of the harmonic content, considering Harmonic components up to the 50th order and specifically excluding interharmonics, expressed as a percent of the maximum demand current. Harmonic components of order greater than 50 may be included when necessary.
- A new method of measuring and recording harmonic content is described with an emphasis on statistical sampling
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 recommend 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 1, in the harmonic subgroup in blue. This harmonic definition is from IEC 61000-7 and corresponds to the harmonic subgroup definition.
Figure 1. Harmonic magnitude
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 recorder from Power Monitors may be set to the 3 second or 10-minute interval. Revolutions can record harmonic subgroups for precise adherence to the Standard. For example, when initializing one of PMI’s Revolution Recorders, the values are done by selecting the harmonic subgroup (Figure 2). With other recorders, interharmonic energy is included in the harmonic calculation as per the 1992 Standard and may result in conservative limits.
Select “V Harmonics Magnitude,” “I Harmonics Magnitude”
Enter desired harmonic range
Check “Harmonic Subgroups”
Check “THD Harmonic Subgroups”
This will record old-style harmonics, as well as IEC harmonics as stripcharts.
Figure 2. Harmonic interval graph setup
The new standard only gives interharmonic limits for voltage, and only low frequencies where interharmonics equate to light flicker. It’s more straightforward to record flicker directly rather than using interharmonics.
- The current distortion limits have been updated
The most 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).
Table 1 from IEEE Std. 519: Voltage distortion units (above)
Table 2 from IEEE Std. 519: Current distortion limits for systems rated 120V through 69 kV (above)
For the weekly short time (10 min) harmonic currents measurements, 95% of the readings should be less than the values given in the appropriate current range table.
In addition to this, the standard now specifies the amount of distortion allowed beyond the limits in Table 2 for brief amounts of time. For daily recordings using the very short time interval (3 s), harmonic currents may be up to 2.0 times the limit, up to 1% of the time. For the weekly short time (10 min) harmonic currents measurements, 99% of the readings should be less than 1.5 times the limits. Keep in mind that these limits are in percent of IL, the maximum demand current for the circuit, not the actual load current during the measurement.
- Voltage distortion limits are updated
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. In addition to this added range, the time frame for short time values (1.5 times permissible table value) is reduced from 1 hour down to 10 minutes. Very short time value is now defined as 3 seconds.
- The recommendations for increasing harmonic current limits put active and passive filters on equal footing with phase shifted multipulse drives
In the standard developed in 1992, the limits for any characteristic harmonic order could receive an increase by means of a multiplier (Section 10.4) if phase shift transformers or converters with pulse number higher than 6 are used.
In the new standard, Table 5 of section 5.5 gives multiplier values to apply if the given harmonic orders are limited to 25% of the values given in the other 3 current distortion tables.
Table 5 from IEEE Std. 519: Recommended multipliers for increases in harmonic current limits
Per the 2014 standard, Section 5.5: “It is recommended that the values are given in Table 2, Table 3, and Table 4 be increased by a multiplying factor when actions are taken by a user to reduce lower-order harmonics. The multipliers given in the second column of Table 5 are applicable when steps are taken to reduce the harmonic orders given in the first column.”
Conclusion
By removing some of the more theoretical dissertations on power quality from the new standard, IEEE has made the standard a more user-friendly document. That fact, combined with more specific information on the data gathering process, means that the IEEE 519 standard will be utilized more fully throughout the industry. There will certainly be much less guesswork involved for this to be properly implemented and applied.
Mike Hetmanski
Design Engineer
mhetmanski@powermonitors.com
http://www.powermonitors.com
(800) 296-4120
