A different kind of harmony
The Stevie Wonder/Paul McCartney classic “Ebony and Ivory” asked why we can’t live together in harmony. Though I am pretty sure that song wasn’t the inspiration for this, three of the largest and most influential standards-making groups in the world have been trying to live in harmony for the last few years in the power quality realm.
Progress was slow at first, but we are beginning to see the fruits of those labors in the latest documents from the power quality related working groups and task forces of the IEEE (Institute of Electrical and Electronic Engineers), IEC (International Electrotechnical Commission) and the NFPA (National Fire Protection Association). As we are in a global economy that is becoming more interconnected every day, a brief update on the progress of these standards might be helpful.
This will also help sort through the marketing hype on new power quality monitors on the market, and what this all means to you, the users.
A hot topic
To capture the data and present the same data in the same manner between two power quality monitors has been a hot topic amongst the PQ standards groups for some time. We are now approaching such, primarily through the efforts of the IEEE 1159 task forces and Working Group 9 of the IEC.
The basic data acquisition and characterization of the data is covered in the recently approved IEC 61000-4-30 Testing and measurement techniques—power quality measurement methods. Close behind it will be the IEEE 1159.1
Recommended Practice For Power Quality Measurements in AC Power Supply Systems, which will go out for ballot this year. These documents define how to measure many of the power quality phenomena that were first enumerated back in 1995 with the approval of IEEE 1159 Recommended Practice on Monitoring Electric Power Quality. That document has the infamous Table 4.2, which categories the phenomena as below:
That table serves as the basis for the power quality chapter in NFPA 70B document. The IEC 61000-4-30 categorizes the phenomena in a similar fashion:
A new standard
The original 1159 standard did not tell how to determine the rms magnitude that is compared against the 90 percent of nominal limit and duration that normally categories a sag (or dip). The documents being worked on in the task forces now will do just that, as is the case in the 1000-4-30 document. These newer documents also define two categories of performance for the PQ monitors—Class A and Class B.
The accuracies and methods for Class A are defined such that two instruments that call themselves Class A instruments should produce the same magnitude and duration value when they both monitor the same waveforms. Note that many of the Class B methods are left up to the instrument manufacturer to define how they do it.
So, an instrument spec sheet that says 1000-4-30 compliant may actually be referring to Class B, which doesn’t make it much different than what was done in the past.
Many of the parameters in the aforementioned tables require complex algorithms to come up with the simple parameters that are used to characterize the data, such as for flicker.
Separate standards are referred to in the IEC 1000-4-30 document to define those methods, and for flicker, it is IEC 6100-4-15—Testing and measuring techniques, Flicker meter—Functional and design specifications. This defines how the parameters Pst (perceptibility short term) and Plt (perceptibility long term) are calculated.
Separate standards, such as IEC Standard 1000-3-3, “Limitation of Voltage Fluctuations and Flicker in Low-Voltage Supply Systems for Equipment with Rated Current less than 16A” and IEC Standard 1000-3-5, “Limitation of Voltage Fluctuation and Flicker in Low-Voltage Power Supply Systems with Rated Current greater than 16A” set the acceptable limits of these parameters.
Initially, 1000-4-15 only addressed 230V, 50Hz lighting. But thanks to the efforts of the IEEE 1453 Voltage Flicker task force, they eventually adopted Amendment 1, addressing 120V/60Hz lighting. It is this version that was incorporated in the IEEE 1453 standard, that is being balloted in the IEEE.
Together in harmony
Harmonics have a similar set of parallel documents. IEEE Standard 519 Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems is probably among the most known PQ standards in the United States. However, it doesn’t address how you calculate the harmonic parameters, such as THD (total harmonic distortion) from the individual harmonic values in such as way that two instruments would produce the same value in a variety of situations.
Along comes IEC 61000-4-7 Testing and measurement techniques Section 7: General guide on harmonics and interharmonics measurements and instrumentation, for power supply systems and equipment connected thereto, which goes that extra step to define the measurement process. You should expect to see a similar methodology in the next revision of IEEE 519.
The last major category to be mentioned here is transients. Whereas the IEEE 1159 defines different categories by duration as well as impulsive versus oscillatory, neither the 1159 document nor the 1000-4-30 document nor the pending 1159.1 document address the measurement methodologies in the normative sections.
Given that, be wary of ones claiming compliance to such, since it would appear right now that just about anyone could claim compliance to something that doesn’t exist.
Mains signalling are signals sent out on the power line as control functions for other utility equipment. These don’t come in to play in the United States. Another difference is in the calculation of unbalance.
In the United States, the formula most often used is the maximum deviation from average divided by the average voltage, whereas in the world of the International Electrotechnical Commission, it is the sum of the negative sequencing components divided by the positive sequencing components. Watch for a future article covering how these produce different values in different situations. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.