What Are the Limits Here? Rough guide for PQ limits

in 2008, I presented “Effects of the Feudal Society on Power Quality” at a conference in Germany. It was a humorous look into why European countries tend to have legally enforceable limits for the permissible levels of most PQ parameters (harmonics, interharmonics, flicker, frequency, etc.). European limits apply to the susceptibility and emission of such by equipment powered from the electrical systems, as well as those levels that utilities must maintain on the distribution and transmission systems.

In North America, we tend to have “recommended practices” with few legislated limits, with the noted exception of ANSI C84.1 regarding nominal voltage levels at service entrances. IEEE Standard 1547-2018, “Interconnection and Interoperability of DERs and Associated Electric Power Systems Interfaces,” covers limits of a few of the typical PQ phenomena. While nationally applicable, it is a voluntary standard that should improve how distributed energy resources interact with and function on the electric distribution system. It still requires state regulators to get behind it and write it into their rules and regulations.

Many IEEE documents specifically address power quality, most of which (but not all) fall under the jurisdiction of the PQ Subcommittee under the T&D Committee in the Power & Energy Society. These “standards” are predominately “recommended practices,” with newer exploratory documents referred to as “guides.” Several standards do specify limits for the particular PQ parameter, such as IEEE 519-2014, “IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems,” which covers harmonics and interharmonics, and IEEE 1453.1-2012, “IEEE Guide—Adoption of IEC/TR 61000-3-7: 2008, Electromagnetic compatibility (EMC)—Limits—Assessment of emission limits for the connection of fluctuating installation to MV, HV and EHV power systems,” which has limits covering voltage fluctuations that result in light flicker.

IEEE 519 is probably the best known IEEE PQ standards. Since its 1992 origin, it is also probably one of the most abused and confused, even after a major revision in 2014. Its basis is simple. In most situations, it is the current distortion resulting from equipment in a facility that consumes electricity in a nonlinear manner that results in a voltage distortion that the electric utility supplier passes along to consumers.

The standard set limits on the voltage distortion by the utilities and the current distortion of the facility, both measured at the point of common coupling (PCC), often called the service entrance. The limits are contained in Table 1 for voltage harmonics and Table 2 for current harmonics. The latter requires that the user of the standard determines the maximum short-circuit current at PCC and the maximum demand-load current (fundamental frequency component) at the PCC under normal load operating conditions. Most facility managers and electrical contractors tasked with determining compliance can’t easily access these numbers, which can lead to assumptions and to the wrong limits.

A statement in the interharmonic limits caused some consternation for those trying to apply the standard: “For interharmonic current components with frequencies that are not integer multiples of the power frequency, users should limit the components to sufficiently low levels so as to not produce undesirable effects on the power system and connected equipment.”

The latest work is trying to better address the limits for interharmonics, as recent data has found significant effects from certain interharmonics on some LED lighting devices.

The standards also address signals above the normal frequency range, the 50th harmonic, from some of the newer technology power supplies used in adjustable-speed drives and other electronic loads that are causing problems. The standards committees are still trying to quantify how to measure such and the effects the type of equipment before determining limits.

Addressing LEDs

Speaking of LEDs, the 1453 standard with regard to voltage fluctuations resulting in light flicker has undergone considerable revision to address newer lighting types. It was written to address the flicker issues with 60W incandescent light bulbs. The flicker meter defined in the standard produces a number to be used as a limit. The perceptibility short-term parameter comes from an intensive mathematical process that is concealed from the user. If it is 1 or higher, most people will perceive a flicker problem, but not so much lower. It sounded great and was adopted in the international and IEEE standards. Unfortunately, the algorithm used doesn’t work well with LEDs and other new lighting types. There is more work to be done.

For now, it seems we will continue on the PQ path of using phrases such as “generally acceptable levels” and “working cooperatively to keep below objectionable levels.”

About the Author

Richard P. Bingham

Power Quality Columnist

Richard P. Bingham, a contributing editor for power quality, can be reached at 732.248.4393.

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