0885-8950 (c) 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TPWRS.2020.2968422, IEEE Transactions on Power Systems Improving Small-Signal Stability of Power Systems with Significant Converter-Interfaced Generation Juan Manuel Mauricio, Member, IEEE, and Andres E. Leon, Senior Member, IEEE Abstract—This work presents a hierarchical control strategy to improve the stability of electrical networks with significant converter-interfaced generation (CIG). Due to the lack of inertia of CIG systems, these networks can undergo a high rate of change of frequency, compromising the frequency stability. In a first level control, a local controller based on the virtual synchronous generator (VSG) concept is used to emulate iner- tia and provide short-term frequency regulation. However, the inclusion of significant VSG units can have a negative impact on the damping of inter-area oscillations. Therefore, in a second level control, a centralized controller is proposed to damp these low-frequency electromechanical oscillations affected by VSGs. Several practical issues such as the identification of a system model for the control design, the compensation of communication delays, and the discrete-time implementation of the controller are particularly analyzed. The introduced supplementary controls allow increasing the penetration of renewable energy sources without jeopardizing the frequency and small-signal stability. Eigenvalue analysis and nonlinear hybrid simulations combining DIgSILENT and Python are performed to validate the proposed control strategy. Index Terms—Communication time delay, inertia emulation, low-frequency oscillation damping, system identification, virtual synchronous machine (VSM), wide-area measurements. I. I NTRODUCTION F REQUENCY measurements indicate that the continuous growth of nonconventional renewable energies is reduc- ing the system inertia, consequently degrading the frequency stability [1]. To deal with this situation, system operators are demanding converter-interfaced generation (CIG) systems able to emulate inertia and support the short-term frequency regulation [2]. In the last years, several techniques have been proposed to provide such supplementary services. They can be broadly classified into two approaches. In the first approach, the converter is synchronized with the grid using a phase- locked loop and exchanging an additional power proportional to the variation and time derivative of the frequency [3]; in the second approach, the converter is controlled to emulate Manuscript received June 1, 2019; revised October 23, 2019 and December 21, 2019; accepted January 17, 2020. Date of publication xxxxx xx, xxxx; date of current version xxxxx xx, xxxx. This work was supported by the Spanish Ministry of Economy and Competitiveness (grant number ENE2017-84813- R), the European Union Horizon 2020 Program (grant number 764090), and the Consejo Nacional de Investigaciones Cient´ ıficas y T´ ecnicas (CONICET). Paper no. TPWRS-00769-2019. J. Mauricio is with the Department of Electrical Engineering, Universidad de Sevilla, Sevilla 41092, Spain (e-mail: jmmauricio@us.es). A. Leon is with the Instituto de Investigaciones en Ingenier´ ıa El´ ectrica (IIIE) “Alfredo Desages”, Universidad Nacional del Sur (UNS)-CONICET, Bah´ ıa Blanca, Argentina (e-mail: aleon@iiie-conicet.gob.ar). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier xxxx/TPWRS.xxxx the dynamic behavior of a synchronous generator (e.g., vir- tual synchronous generators (VSGs) [4], synchronverters [5], power-synchronization control [6], and synchronous power control [7]–[9]). In weak systems, the second approach has shown to be very effective to improve the frequency control in terms of both the rate of change of frequency (RoCoF) and the frequency nadir [10]. The emulation of a synchronous generator entails some challenges, similar to the installation of a new con- ventional generator in a multi-machine power system [11]. A VSG introduces an electromechanical mode [9], and the virtual rotating mass affects existing inter-area modes. The creation and interconnection of areas based on VSGs modify the small- signal characteristics of the system and change the oscillation frequencies in which power system stabilizers were originally tuned. As a consequence, the higher the virtual inertia units are dispatched, the higher the risk of adverse effects on the damp- ing of power system electromechanical oscillations. Additional control loops based on local measurements have been proposed in [12]–[14] to improve the stability and damping properties of VSGs. However, the damping term of these schemes mainly impacts on the local electromechanical mode of the VSG, and it is not fully effective to mitigate inter-area oscillations. Different decentralized control approaches using local mea- surements have also been considered in [15]–[19] to improve the damping of power system oscillations. Controls based on local measurements can damp certain modes, but they lack sufficient observability to properly damp inter-area oscilla- tions [20]–[22]. On the other hand, the increasing number of phasor measurement units (PMUs) in power systems and the advances in wide-area measurement systems have enabled to implement control strategies using remote measurements in nearly real time [22]–[24]. Remote measurements transmit information related to the overall network dynamics and have a much higher observability of inter-area modes. In addition, controls based on remote measurements achieve the damping effect with lower control actions [20] and are less prone to destabilize local modes than controls based on local measure- ments [25]. Although current communication networks face several challenges such as data packet dropouts, random com- munication delays, and false data injection attacks, different solutions are being proposed to deal with these issues (e.g., see [26]–[28]), making remote communications more reliable and failure resilient. The study of effective strategies to damp inter-area oscillations (e.g., those based on global information) will enable the large-scale integration of renewable energy sources and a better use of their limited energy storage for frequency support services. In this paper, a hierarchical control strategy is proposed Authorized licensed use limited to: Universidad de Sevilla. Downloaded on March 06,2020 at 15:29:23 UTC from IEEE Xplore. Restrictions apply.