Intermodulation

Summary

Intermodulation (IM) or intermodulation distortion (IMD) is the amplitude modulation of signals containing two or more different frequencies, caused by nonlinearities or time variance in a system. The intermodulation between frequency components will form additional components at frequencies that are not just at harmonic frequencies (integer multiples) of either, like harmonic distortion, but also at the sum and difference frequencies of the original frequencies and at sums and differences of multiples of those frequencies. Intermodulation is caused by non-linear behaviour of the signal processing (physical equipment or even algorithms) being used. The theoretical outcome of these non-linearities can be calculated by generating a Volterra series of the characteristic, or more approximately by a Taylor series. Practically all audio equipment has some non-linearity, so it will exhibit some amount of IMD, which however may be low enough to be imperceptible

Official source

https://en.wikipedia.org/wiki/Intermodulation

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PMU-Based ROCOF Measurements: Uncertainty Limits and Metrological Significance in Power System Applications

Asja Derviskadic, Guglielmo Frigo, Mario Paolone, Yihui Zuo

In modern power systems, the Rate-of-Change-of-Frequency (ROCOF) may be largely employed in Wide Area Monitoring, Protection and Control (WAMPAC) applications. However, a standard approach towards ROCOF measurements is still missing. In this paper, we investigate the feasibility of Phasor Measurement Units (PMUs) deployment in ROCOF-based applications, with a specific focus on Under-Frequency Load-Shedding (UFLS). For this analysis, we select three state-of-the-art window-based synchrophasor estimation algorithms and compare different signal models, ROCOF estimation techniques and window lengths in datasets inspired by real-world acquisitions. In this sense, we are able to carry out a sensitivity analysis of the behavior of a PMU-based UFLS control scheme. Based on the proposed results, PMUs prove to be accurate ROCOF meters, as long as the harmonic and inter-harmonic distortion within the measurement pass-bandwidth is scarce. In the presence of transient events, the synchrophasor model looses its appropriateness as the signal energy spreads over the entire spectrum and cannot be approximated as a sequence of narrow-band components. Finally, we validate the actual feasibility of PMU-based UFLS in a real-time simulated scenario where we compare two different ROCOF estimation techniques with a frequency-based control scheme and we show their impact on the successful grid restoration.

2019

Synchrophasor-Based ROCOF Measurements: Feasibility in Real-World Scenarios

Asja Derviskadic, Guglielmo Frigo, Mario Paolone

In modern power systems, Rate of Change of Frequency (ROCOF) is employed in monitoring and control applications. However, a standard approach towards ROCOF measurements is still missing. In this paper, we investigate the feasibility of Phasor Measurement Units (PMUs) deployment in ROCOF -based applications. For this analysis, we select three representative synchrophasor estimation algorithms and compare their accuracy in datasets inspired by real-world acquisitions. Based on the proposed results, PMUs prove to be accurate ROCOF meters, as long as the harmonic and inter-harmonic distortion within the measurement pass-band is reduced. In the presence of transient events, the synchrophasor model looses its appropriateness, as the signal energy spreads over the entire spectrum and cannot be approximated as a sequence of narrowband components. For these reasons, a re-definition of the IEEE Std. C37.118.1 requirements is recommended to suitably define ROCOF measurements based on real-world operating conditions.

IEEE2018

Design and experimental validation of an FPGA-based PMU simultaneously compliant with P and M performance classes

Asja Derviskadic, Mario Paolone

Low-inertia grids are characterized by high shares of harmonic and inter-harmonic distortion, produced by the inverters that interface non-conventional generation assets to the electrical grid. These interfering tones largely compromise synchrophasor estimation and may jeopardize protection and control strategies based on Phasor Measurement Units (PMU). Indeed, the IEEE Std. C37.118 does not require resiliency against inter-harmonic tones for protection PMUs. In view of increasing synchrophasor technology reliability, the paper presents the design and the experimental validation of a PMU that is simultaneously compliant with both protection (P) and measurement (M) class of PMU performance defined in the IEEE Std. C37.118. The synchrophasor estimator is based on the interpolated DFT and iteratively estimates and compensates the effects of the spectral interference produced by a generic interfering tone and the negative image of the fundamental tone. The proposed hardware architecture is based on a Field Programmable Gate Array (FPGA).

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