# Low-Latency, Three-Phase PMU Algorithms: Review and Performance Comparison
###### Abstract  
Pnasor Measurement Units are the most advanced instruments for power network monitoring, since they allow phasors, frequency and rate of change of frequency (ROCOF) to be measured in predetermined time instants with respect to an absolute time reference. The employed estimation algorithm plays a key role in overall performance under off-nominal conditions; the challenge to be faced is combining high steady-state accuracy with fast responsiveness in dynamic conditions, small reporting latency and reduced computational burden. Under regular operation, AC power networks are weakly unbalanced three-phase systems. Based on this consideration, the recent literature has proposed native three-phase estimation algorithms that effectively exploit this property to accurately identify the positive sequence synchrophasor, frequency and ROCOF. In this respect, the present paper describes three among the most promising three-phase algorithms based on the Space Vector transformation. By means of numerical simulations, it compares the achieved performance in terms of response time and estimation error both under steady-state and dynamic conditions. All the considered approaches enable a flexible design that allows balancing accuracy and responsiveness. For this analysis, the reporting latency has been limited to about one and half nominal cycles, i.e., 30 ms at 50 Hz; the P-class algorithm suggested by IEC/IEEE Std 60255-118-1 has also been included as comparison benchmark. Pnasor Measurement Unit (PMU); three-phase systems; synchrophasor; frequency; Rate Of Change Of Frequency (ROCOF); Space Vector; Interpolated Discrete Fourier Transform (IpDFT); Talor-Fourier Transform (FFT) +
Footnote †: journal: Applied Sciences  
In such challenging scenario, Phasor Measurement Units (PMUs) might represent a promising solution as they provide estimates of synchrophasors, frequency and rate of change of frequency (ROCOF) associated to the fundamental component in remote nodes of the grid, which are synchronized to Coordinated Universal Time (UTC) [6]. The recent IEEE/IEC Std 60255-118-1-2018 [7] (denoted as IEC Std for the sake of brevity), defines the operative requirements for PMUs in terms of measurement accuracy, latency and transient performance. The measurement accuracy is quantified by means of three indices, namely Total Vector Error (TVE), Frequency Error (FE), and ROCOF Error (RFE). The reporting latency, instead, represents the time delay after which measurement data is available at the PMU output with respect to the corresponding reporting instant (timestamp). Transient performance is measured by the response time required by the algorithm output to settle down after fast variations in amplitude or phase and by such response shape parameters. In particular, the IEC Std introduces two performance classes: M for high-accuracy monitoring applications, and P for low-latency applications where high responsiveness is recommended, such as relaying and fast control. The requirements of the IEC Std, as well as the superseded IEEE standards [8; 9], have pushed the development of highly sophisticated algorithms for synchrophasor, frequency and ROCOF estimation, capable of rejecting spurious contributions as well as accounting for the dynamics of the fundamental component [10; 11; 12]. In particular, the traditional definition of phasor has been extended to the novel concept of dynamic synchrophasor, whose amplitude and phase angle may vary as functions of time [13; 14]. In practice, though, an effective employment of PMU measurements requires complying with stringent latency limits [15]. Most of the existing approaches rely on segmenting the acquired signal and processing each segment via Discrete Fourier Transform (DFT) [16] or Taylor-Fourier Transform (TFT) [17], in order to extract the parameters associated to the fundamental component.