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

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.Comment: Manuscript IM-18-20133R. Accepted for publication on IEEE Transactions on Instrumentation and Measurement (acceptance date: 9 March 2019

Iterative-Interpolated DFT for Synchrophasor Estimation: A Single Algorithm for P- and M-Class Compliant PMUs

We present a single synchrophasor estimation (SE) algorithm that is simultaneously compliant with both P and M phasor measurement unit (PMU) performance classes. The method, called iterative-interpolated discrete Fourier transform (i-IpDFT), iteratively estimates and compensates the effects of the spectral interference produced by both a generic interfering tone, harmonic or interharmonic, and the negative image of the fundamental tone. We define the three-point i-IpDFT technique for cosine and Hanning window functions and we propose a procedure to select the i-IpDFT parameters. We assess the performance of the i-IpDFT with respect to all the operating conditions defined in the IEEE Std. C37.118 for P- and M-class PMUs. We demonstrate that the proposed SE method is simultaneously compliant with all the accuracy requirements of both PMU performance classes

Iterative-Interpolated DFT for Synchrophasor Estimation: A Single Algorithm for P- and M-Class Compliant PMUs

We present a single synchrophasor estimation (SE) algorithm that is simultaneously compliant with both P and M phasor measurement unit (PMU) performance classes. The method, called iterative-interpolated discrete Fourier transform (i-IpDFT), iteratively estimates and compensates the effects of the spectral interference produced by both a generic interfering tone, harmonic or interharmonic, and the negative image of the fundamental tone. We define the three-point i-IpDFT technique for cosine and Hanning window functions and we propose a procedure to select the i-IpDFT parameters. We assess the performance of the i-IpDFT with respect to all the operating conditions defined in the IEEE Std. C37.118 for P- and M-class PMUs. We demonstrate that the proposed SE method is simultaneously compliant with all the accuracy requirements of both PMU performance classes

Architecture and Experimental Validation of a Low-Latency Phasor Data Concentrator

The paper presents the design principles of a Phasor Data Concentrator (PDC) that implements both the absolute and relative time data pushing logics together with a third one that aims at minimizing the latency introduced by the PDC without increasing the data incompleteness, as suggested in the IEEE Guide C37.244-2013. The performance of the aforementioned logics are assessed and compared in terms of reliability, determinism and reduction of the overall latency in two real Phasor Measurement Unit (PMU) installations adopting different telecom infrastructures. The first one is based on optical fiber links that transmit synchrophasor data measured by 15 PMUs installed in the sub-transmission network of the city of Lausanne, Switzerland. The second one adopts a 4G LTE wireless infrastructure to support the data streaming of 10 PMUs installed in a distribution network supplying the city of Huissen, in the Netherlands. The experimental results show that the proposed logic is characterized by the lowest latency, whereas the absolute time logic better mitigates the synchrophasor data latency variations

Synchronized Sensing for Wide-Area Situational Awareness of Electrical Grids in Non-Stationary Operating Conditions

Situational-awareness methods and technologies are crucial elements to ensure power systems reliability and security as they are used by automated decision-making processes of mission-critical applications. Inadequate system monitoring results in incorrect or delayed actions that may potentially lead to unsafe and unstable evolution of the grid state. The distributed and time-synchronized sensing of the so-called synchrophasors is becoming increasingly adopted by systems operators especially when they plan the refurbishment of their electrical grids with Phasor Measurement Units (PMU) that provide highly accurate, low latency, and high refresh-rate estimates of voltage and current phasors. However, the concept of synchrophasor is based on a static signal model and, therefore, it provides a reliable estimation of the monitored power system signal only for quasi-steady-state conditions. Given the massive integration of inverter-connected renewable energy resources that, as such, do not provide any inertia to the system, modern electrical grids are expected to experience larger dynamics, high shares of harmonic and inter-harmonic pollution, and unprecedented electromechanical transients. Processing tools able to correctly and timely track these conditions, are certainly needed as they may enhance the overall power system situational awareness and its associated security. Within this context, this Thesis proposes advanced synchrophasor networks for the monitoring and control of power grids operating in close-to-stationary conditions. More specifically, enhanced processing tools able to accurately and timely estimate the synchrophasors using extremely short observation intervals are proposed. The methods are based on the discrete Fourier transform and on the Hilbert transform. The compliance of the proposed methods with respect to international standards for measurement and protection applications is verified (IEEE Std. C37.118). The integration of the proposed algorithms into hardware platforms demonstrates their prospective deployability into a new PMU prototype. Further, the developed PMU is synchronized with respect to time using cutting-edge time dissemination technologies like the one provided by the White Rabbit protocol. The superior accuracy in estimating the synchrophasors provided by the algorithms and the PMU devices developed in this Thesis, calls for a calibration process of exceptional performance. In this view, the hardware and software architecture of an advanced validation platform for PMU type-testing is presented and metrologically characterized. Finally, the seamless streaming of synchrophasor data over the underlying communication infrastructure is investigated by considering wired and wireless physical layers and by proposing a time-deterministic phasor data concentrator. For power systems operating in non-stationary operating conditions, the Thesis proposes an innovative approach for the modeling of reduced-inertia electrical grids that goes beyond the concept of static phasor. The study is inspired by the theory on analytic signals and is based on the Hilbert transform that enables the modeling of dynamic signals. A theoretical formulation is outlined and validated over real-world datasets, demonstrating the effectiveness of Hilbert transform-based methods. These results may contribute to the development of novel sensing technologies and may yield to a disruptive innovation in the field of power systems modeling

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

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)

Iterative-interpolated DFT for Synchrophasor Estimation in M-class Compliant PMUs

The Interpolated Discrete Fourier Transform (IpDFT) represents the current state-of-the-art about DFT-based synchrophasor estimation (SE) algorithms for P-class Phasor Measurement Units (PMUs). However, the IpDFT is not robust against the interference produced by interharomonics (i.e., components characterized by frequencies that are not integer multiples of the fundamental one) and, therefore, it is not suitable for M-class PMUs. We propose an iterative-IpDFT (i-IpDFT) SE algorithm that iteratively compensates the effects of the spectral interference produced by both the interharmonic tone and the negative image of the fundamental tone. We assess the performance of the i-IpDFT with respect to all the operating conditions defined by the IEEE Std. C37.118-2011 for M-class PMUs, and demonstrate that the method is compliant with all the accuracy requirements

A white rabbit synchronized PMU

Within the context of time dissemination techniques for power systems applications, the paper illustrates the use of the White Rabbit (WR) technology with respect to its integration in Phasor Measurement Units (PMUs) networks. In particular, the paper refers to a WR network composed of a WR switch and WR modules integrated into a specifically-designed WR-PMU. The performance of the developed WR-PMU is assessed by means of a PMU calibrator, with a focus on the synchrophasor phase estimation in steady state conditions. The results show that the WR-PMU exhibits similar/better performance when compared to an equivalent PMU synchronized using the GPS. To further evaluate the developed WR-PMU, its performance is assessed in a WR network composed by two WR-PMUs connected to a single WR switch. The objective of this last experiment is to analyze the relative phase error between two PMUs when fed by the same reference signal provided by the calibrator

Beyond Phasors: Modeling of Power System Signals Using the Hilbert Transform

Modern power systems are at risk of largely reducing the inertia of generation assets and prone to experience extreme dynamics. The consequence is that, during electromechanical transients triggered by large contingencies, transmission of electrical power may take place in a wide spectrum well beyond the single fundamental component. Traditional modeling approaches rely on the phasor representation derived from the Fourier Transform (FT) of the signal under analysis. During large transients, though, FT-based analysis may fail to accurately identify the fundamental component parameters, in terms of amplitude, frequency and phase. Taking inspiration from the theory on analytic signals, this paper proposes a different approach to model signals of power systems electromechanical transients based on the Hilbert transform (HT). We compare FT- and HT-based approaches during representative operating conditions, i.e., amplitude modulations, frequency ramps and step changes, in synthetic and real-world datasets. We further validate the approaches using a contingency analysis on the IEEE 39-bus

Reduced Leakage Synchrophasor Estimation: Hilbert Transform Plus Interpolated DFT

Synchrophasor estimation is typically performed by means of spectral analysis based on the discrete Fourier transform (DFT). Traditional DFT approaches, though, suffer from several uncertainty contributions due to the stationarity assumption, spectral leakage effects, and the finite-grid resolution. This paper addresses these limitations, by proposing a joint application of the Hilbert transform (HT) and the interpolated DFT (IpDFT) technique. Specifically, the HT enables the suppression of the spectral leakage generated by the negative image of the tones under analysis, whereas the IpDFT limits the effects of spectrum granularity. In order to relax the constraint in terms of measurement reporting latency, the proposed estimator can adopt a window length of 40 ms and yet provides a noticeable estimation accuracy with a worst-case total vector error and frequency error equal to 0.02% and 4 mHz, respectively, in steady-state conditions. In this context, this paper discusses the most suitable setting of the algorithm parameters and their effect on spurious component rejection. Moreover, a thorough metrological characterization of the algorithm estimation accuracy and responsiveness with respect to the IEEE Std. C37.118.1 is carried out in order to detect the main uncertainty sources as well as possible room for enhancement. Finally, a comparison with two consolidated IpDFT approaches shows the actual performance enhancement provided by the proposed algorithm