# Low-Latency, Three-Phase PMU Algorithms: Review and Performance Comparison
###### Abstract
In general, processing a longer segment enables better disturbance rejection and wider measurement bandwidth, but it also results in increased computational cost, higher latency and slower response. Therefore, an optimal tradeoff between accuracy and responsiveness is hard to find and a practical limit of two nominal cycles has been achieved through a combined application of Hilbert transform and TFT, at the expense though of higher computational complexity and sensitivity to spurious interferences [18]. A detailed review of the main synchrophasor estimation approaches is beyond the scope of this paper and can be found in [6; 19]. The Forcescue transformation is often used for an effective analysis of three-phase AC networks: it allows expressing a sinusoidal three-phase signal in terms of its symmetrical components [20]. A peculiar characteristic of real-world power systems is that they are weakly unbalanced during regular operation: in this case, the positive sequence component alone contains most of the information associated to the fundamental term of a three-phase quantity. In the recent literature, several PMU algorithms exploits this symmetry property by using the Clarke/Park transformation [21] or Principal Component Analysis and Maximum Likelihood estimators [22]. In this context, the Space Vector (SV) -based approach [23] can be also adopted for reducing computational burden without sacrificing performance. Instead of dealing with each phase separately, the SV with respect to a reference frame rotating at the system nominal frequency is computed. Digital filters whose frequency responses can be customized to reach specific performance goals [24] are employed to extract the contribution of the positive sequence component from the complex-valued SV signal. It is worth noting that the SV approach can be also used in conjunction with other well-known methods for estimating synchrophasor, frequency and ROCOF such as interpolated DFT (lpDFT) [25] or Taylor-Fourier (TF) filtering [26], thus leveraging the three-phase symmetry of power systems for reduced computational cost and improved accuracy. In this paper, we carry out a rigorous and extensive performance assessment among three SV-based estimation algorithms, specifically designed for high responsiveness applications, with maximum latency set to two nominal cycles. To the best of Authors'knowledge, this review represents the first attempt of a comparative analysis for highlighting peculiarities and strengths of the different methods. For this purpose, we analyze the algorithms' performance in terms of response time and measurement accuracy, both under steady-state and dynamic conditions; the P-class reference algorithm provided by the IEC Std is considered as benchmark. In this framework, the paper main contributions consist in: (i) a reasoned analysis of the most suitable setting for an optimal tradeoff between accuracy and latency, (ii) a thorough characterization of the algorithms' performance not only in the entire IEC Std test set, but also under other conditions including unbalance and harmonic distortion at off-nominal frequency. The paper is organized as follows. In Section 2, we introduce the considered measurement algorithms. Section 3 presents the results of the performance comparison. In Section 4, we provide some closing remarks and outline the future steps of the research activity.