# RMS Modeling and Control of a Grid-Forming E-STATCOM for Power System Stability in Isolated Grids
## 1 Introduction  
The increasing penetration of renewable energy sources (RESs) in power systems, particularly in weak grids, has introduced new stability challenges. The transition from conventional synchronous generators (SGs) to inverter-based resources (IBRs) such as wind farms, solar PV, energy storage systems (ESSs), and hybrid plants is reshaping the electric grid at an accelerated pace [1]. In conventional power systems, SGs and their auxiliary control elements provide essential grid services, including inertial response, which slows down the initial rate of change of frequency (RoCoF) after generation trips; fast frequency response (FFR) for frequency stabilization; voltage regulation; power oscillation damping; and negative sequence current injection [2]. However, as IBRs replace SGs, power systems are experiencing higher RoCoF and lower short-circuit current contributions, potentially compromising system strength and stability [3]. To address the stability challenges posed by the increasing penetration of IBRs, synchronous condensers (SynCons) have traditionally been deployed as a well-establishedsolution for grid strength enhancement. These rotating machines help stabilize voltage and provide inertial response, making them effective in weak grids. However, their high capital cost and long deployment time limit their feasibility for widespread adoption [4]. As an alternative, power electronics-based solutions such as E-STATCOMs have gained attention [5; 6; 7]. Unlike conventional STATCOMs, which primarily provide reactive power support in a grid-following mode, the E-STATCOM extends this functionality by dynamically injecting and absorbing active power through energy storage systems such as supercapacitors (SCs), electrochemical batteries, or flywheels. The letter \"E\" in E-STATCOM represents \"energy\", emphasizing its additional capability to support frequency stability alongside dynamic voltage control. While traditional STATCOMs enhance voltage stability, they do not contribute to inertial response, making them less effective in power systems with high renewable energy penetration. In contrast, an E-STATCOM with grid-forming (GFM) control can emulate the behavior of an SG, providing both inertial response and fast frequency regulation, making it a valuable asset for modern power grids. Transmission system operators (TSOs) have recognized E-STATCOMs as a promising solution for short-term load fluctuation compensation and inertial response provision. In December 2020, the four German TSOs outlined the need for grid-forming (GFM) E-STATCOM plants [8]. In 2022, a \(\pm\)300 Mvar E-STATCOM was commissioned in Germany as a pilot project, delivering inertia support for 10 s. Other initiatives, such as the ViSync project in Spain, aim to integrate SCs and Li-ion batteries into an E-STATCOM for high-voltage transmission grid support in the Lanzarote-Fuerteventura system (Canary Islands, Spain) [9]. Similar developments are underway in Australia [10] and Hawaii, USA [11]. Recently, the E-STATCOM has emerged as a promising alternative solution to compensate for wind power plants [12], offering black-start capability and meeting islanded operation requirements, among other services. Beyond energy storage integration, the E-STATCOM's distinguishing feature is its grid-forming (GFM) control strategy. Unlike grid-following converters, which require a stable external voltage reference, GFM converters actively regulate voltage and frequency, making them crucial for weak grids and isolated systems. These devices are designed to maintain a constant or nearly constant internal voltage phasor within sub-transient to transient time frames [13]. An extensive literature on the control of GFM converters is available, with a good summary provided in references [14; 15].