Over the past two years, the technical committee responsible for IEEE 1547, Standard for Interconnecting Distributed Resources with Electric Power Systems, has added a section on power quality (PQ), including the parameters, indexes and limits on those for distributed energy resources (DERs) connecting to the electrical power system (EPS), including distribution and transmission systems. 


Within the IEEE Power and Energy Society (PES), the Power Quality Subcommittee creates PQ-related standards. Another group in the PES that spans multiple subcommittees is the Standards Coordinating Committee (SCC-22). As the name suggests, this group keeps a watch out for activities in numerous IEEE and international standards groups to prevent duplication or, worse, contradictions in standards.


These various efforts converged last summer at the IEEE PES general meeting, and the conversation continues with regard to the added PQ section and test section to “verify that an inverter (of a DER) that connects to the EPS complies with the DC injection limit specified in IEEE Std 1547.” 


Whereas the PQ subcommittee has been creating standards with more parameters and more ways to measure PQ phenomena, the 1547 committee wants a simple set of parameters with limits. Hundreds of emails have been exchanged between the various groups over one parameter in particular—total distortion.


A properly operating spinning-type electrical generator, such as those used in most utility generating stations, produces a pure, undistorted voltage sine wave. When connected to linear loads, it produces a pure, undistorted current sine wave. On the other side, wind- and solar-based DERs use inverters to convert direct current (DC) voltage to alternating current (AC) voltage that is synchronized to the fundamental frequency of the power grid, usually 60 hertz (Hz) in North America or 50 Hz in many other parts of the world. By nature of the design, these DERs produce distortion, just like nonlinear loads create harmonics when coupled to the grid.


The current standards on harmonics (IEEE 519 and IEC 61000-4-7) both use the 200-millisecond (msec) window of 10/12 cycles at 50 or 60 Hz and compute the harmonic and interharmonic values. There are different methods for grouping the 5 Hz of voltage or current frequency components that are derived using discrete or fast Fourier transforms on the 200 msec of waveforms. However, the common practice, which the IEEE adopted, is to group the actual harmonic component with the 5-Hz bin on either side together as a root mean square (rms) value representing that harmonic number.


For example, the third harmonic at 60 Hz is 180 Hz. The values at 175, 180 and 185 Hz are combined to be the third, while 235-, 240- and 245-Hz components make up the 4th harmonic. All of the 5-Hz bins in between those 190-to-230 Hz are grouped and combined using rms computations to be the 3–4 interharmonic. From there, compute total harmonic distortion as the rms of the harmonics divided by the fundamental value and, likewise, total interharmonic distortion as the rms of the interharmonics divided by the fundamental value.


But what if you wanted just a single index to represent all the distortion? What about the frequency components below the fundamental, often called subharmonics. What about the DC component becoming more prevalent in electrical systems, especially during times of high solar-flare activity? These components also create distortion of the sine wave and can have similar consequences to electromagnetic devices as the harmonics and interharmonics do.


The seemingly logical conclusion is to say that everything that isn’t the fundamental frequency component is distortion. But when doing the harmonics, the adjacent 5-Hz bins are included. Why not do that here, too? But the reason they are included at the harmonic level is because the mechanism that measures the waveforms can have a little bit of jitter in it, which means things aren’t, as the Brits would say, “spot on.” So give a little leeway to more accurately represent what is really occurring, rather than an artifact in the data acquisition.


That said, it is still a work in progress, though balloting on the new revision to IEEE 1547 is underway. Hopefully, the great minds at this month’s general meeting of the IEEE PES committees, along with IEEE 1547 committee members, can agree on just one thing—the single index that says it all.