The Need for Probabilistic Limits of Harmonics: Proposal for IEEE Std 519 Revision Paulo Ribeiro Calvin College / BWX Technologies, Inc Guide Carpinelli University of Cassino, Italy Porto Alegre, Brasil Agosto 2001
THD V THDV 1.2% 0.8% 0.4% 0.0% 2/2 2/3 2/4 2/5 2/6 2/7 2/8 Time-Varying Nature of Harmonics An Inevitable Reality The causes of variations are the continuous changes in system configurations, linear load demands and operating modes of non linear loads
Time-Varying Nature of Harmonics “The variations generally have a random character and the only way one can describe the behavior of such characteristics is in statistical terms”
Harmonic Indices Summation Laws Harmonic Impedances Probability Functions Statistical measures Time-dependent Limits Aspects to be Considered
Usual indices are referred only to the harmonic amplitude Indices neglecting the harmonic phase angles
Harmonic phases angles are randomly varying:
0.4 fX 0.3 0.2 0.1 X 0 -3 -2 -1 0 1 2 3 4 -4 Time-Varying Nature of Harmonics • Marginal probability density function
Time-Varying Nature of Harmonics • Time ranges involved (hours, day, etc.) • Time interval between readings and the window width • Application to estimate the thermal effects • Multimodal patterns
Time-Varying Nature of Harmonics Application to estimate the thermal effects of harmonics The time factor is completely lost and the same pdf can correspond to different time varying harmonics, with consequent different thermal responses of electrical components
Time-Varying Nature of Harmonics If new statistical data is used properly, a more accurate and reliable prediction of thermal effects should be possible
ð Time-Varying Nature of Harmonics Conditional probability density functions Example: pdf of the 5th Harmonic Voltage given that its amplitude is greater than or equal to 6% • to reduce the volume of data to be analyzed • to represent extreme events • to introduce intriguing new statistical measures
Statistical Measures • average value m • standard deviation s • 95% probability value PC95% • 99% probability value PC99% • maximum value MAX The IEC 1000-3-6 and EN 50160 refer to PC95% and MAX
x x 0.95 0.95 m = 0.70 s = 0.27 m = 0.27 s = 0.16 Statistical Measures Harmonic pdfs with the same PC95% and MAX can cause different effects
Statistical Numerical Measures Expected value, probabilities calculated from the conditional pdf can be useful numerical measures
Summation Laws Two semi-empirical approaches have been proposed in literature in order to allow easy and fast evaluation in practical applications such as standards αh = summation exponent dependent on harmonic order khj = diversity factor
Summation Laws The laws should be continuously verified to take into account: • new electronic components • the presence of unbalanced conditions • the influence of the network supplying the harmonic sources
Harmonic Impedances Usually, the network harmonic impedances are assigned with reference to deterministic values and without taking into account the correlation with disturbing currents The network harmonic impedance has statistical behavior due to: • network component uncertainties • load variability (including power factor correction capacitors) • supply system variability
THDV Equipment Time-Dependent Limits Up to now the studies on harmonic effects mainly (but not only) refer to static distortions Studies are needed on harmonic effects when time-varying distortion is applied
Proposition For IEEE 519 IEEE 519-1992 introduced the concept of probability distribution plots to characterise variations in harmonic levels. A reasonable method of using these types of plots to evaluate harmonic levels would be to compare the steady state harmonic limits with the measured harmonic level that is not exceeded 95% of the time (the 95% probability point). This is consistent with the evaluation of compatibility levels in IEC standards. In order to further develop this topic the IEEE Probabilistic Aspect of Harmonics Task Force is preparing a proposal for the Std. 519 in which the time-varying nature of harmonics voltages are considered in the establishment of the limits. Any limits on the short duration harmonic levels should be based on the possible impacts of these harmonic levels. Effects such as metering error and equipment ageing are the accumulated result of harmonic levels over time. Other effects include the sensitive electronics to short burst of high harmonic levels or certain types of waveform distortions.
Proposition For IEEE 519 Recommendations are being prepared, with suggested procedures and values to be discussed by the members of the Std. 519 Revision Task Force. These recommendations will be consistent with the 519 Application Guide. Probability distributions and short-term disturbance limits shall be established. The “continuous limits” could be taken as, for example, the 95% probability limit. The values will be established based on the impact of short-term harmonics on different equipment.
THD V THDV 1.2% 0.8% 0.4% 0.0% 2/2 2/3 2/4 2/5 2/6 2/7 2/8 Proposition For IEEE 519
Observations The harmonic limits have to be assigned on the basis of harmonic effects, which differ substantially depending on the characteristic of the affected equipment. There are long-term effects such as the component aging which is the accumulated result of harmonic levels over time, and “instantaneous” effects such as the malfunction of electronic equipment caused by only one short burst of high harmonic level. The probabilistic nature of harmonics thus requires the establishment of time-dependent limits. Several standards recognize this need, but have not comprehensively dealt with the issue if at all. For instance, the IEC 1000-3-6 includes the time variation by setting limits in terms of percentile and accepting that the maximum weekly value of the voltage harmonic over very short 3 s periods should not exceed an A times the assigned planning/design continuous limits.
Observations At present, time-dependent limits can only be implemented by common sense engineering judgement. Previous studies have treated harmonic effects from a steady-state / static distortion point of view. The IEEE Task Force on Probabilistic Aspects of Harmonics is working on the preparation of a proposal to be presented to the IEEE Std 519 Review Task Force with some suggestions for establishing time-varying harmonic limits.
Conclusions • Due to the high number of probabilistic aspects to be taken into account, the IEEE 519 should be explicitly consider this problem.n • The time-varying nature of harmonics requires the adoption of probabilistic Standards • Efforts should be made to simplify their practical application, by introducing global indices which are able to capture more than one aspect of the Power Quality