As power systems accommodate higher shares of renewable generation, short-term power imbalances become more frequent and can manifest as pronounced voltage and frequency excursions under low-inertia conditions. E-STATCOMs (STATCOMs equipped with energy storage) offer a practical means to provide both voltage support and fast frequency assistance under grid-forming control. Among candidate implementations, double-star multilevel-converter (DS-MC)-based E-STATCOMs enable centralized energy-storage integration at the dc link, which improves thermal management and maintainability. Nevertheless, conventional dc-side power-based internal-energy regulation in DS-MCs can undesirably couple loss compensation to the energy-storage path, accelerating storage cycling and constraining operation when the storage is unavailable. This paper introduces a control strategy that assigns DS-MC total internal-energy regulation to the ac-side active-power path, while reserving dc-side storage power solely for frequency support. By decoupling internal-energy management from inertial-response provision, the proposed scheme enables flexible operation as either a STATCOM or an E-STATCOM according to storage availability and mitigates unnecessary storage cycling. The proposed strategy is verified through offline simulations and laboratory-scale experiments.
A S the penetration of renewable energy sources increases, renewable intermittency and limited predictability exacerbate short-term mismatches between power supply and demand. In conventional synchronous-generator-based systems, synchronous-machine rotational inertia provides an inherent energy buffer that mitigates such mismatches. However, power-electronic-based resources operating under gridfollowing (current-controlled) control do not contribute comparable inertial energy. Consequently, the resulting imbalance can induce voltage-magnitude and frequency fluctuations (corresponding author: Jae-Jung Jung and Shenghui Cui) Ki-Hyun Kim, Yeongung Kim and Jae-Jung Jung are with the School of Electronic and Electrical Engineering, Kyungpook National University, Daegu, 41566, South Korea (e-mail: rlarlgus5615@knu.ac.kr; free77a@knu.ac.kr; jj.jung@knu.ac.kr).
Shenghui Cui is with Department of Electrical and Computer Engineering, Seoul National University, Seoul, 08826, South Korea (e-mail: cuish@snu.ac.kr) and, under severe conditions, may ultimately compromise system stability [1], [2]. To address these challenges, the energy-storage-enhanced static synchronous compensator (E-STATCOM), which integrates a static synchronous compensator (STATCOM) with an energy storage (ES), such as batteries or supercapacitors, has emerged as a promising gridstabilization solution. Beyond the conventional STATCOM functionalities of voltage regulation and reactive power compensation at the point of common coupling (PCC), an E-STATCOM can adopt grid-forming (GFM) control to emulate inertial response and support frequency dynamics. Moreover, depending on the power and energy capabilities of the integrated storage, the E-STATCOM can provide sustained active power injection, thereby enabling fast primary frequency support. Consequently, E-STATCOMs can improve power quality and strengthen both voltage and frequency stability in modern power systems [3], [4], [5].
E-STATCOM topologies reported in the literature predominantly adopt multilevel converters [6], [7], [8]. As illustrated in Fig. 1, multilevel converters are composed of a series connection of submodules, each consisting of power semiconductor switches and a dc-link capacitor. Owing to this modular structure, multilevel converters offer scalable voltage levels and are well suited for high-voltage and high-power applications [9]. In addition, multilevel converters can achieve low outputcurrent total harmonic distortion (THD) and high-quality output-voltage waveforms even at low switching frequencies, while providing redundancy against submodule failures. These advantages make multilevel converters a particularly promising topology for E-STATCOM applications.
As illustrated in Fig. 1(a) and (b), E-STATCOMs implemented with single-star and single-delta multilevel converters typically adopt a distributed ES arrangement because a common dc-link is not available. The distributed ES is typically co-located with the power stage at the submodule level, and the resulting thermal proximity to loss-generating power semiconductor switches can expose the ES to harsh temperature conditions, potentially exceeding the recommended operating range [10], [11]. Such thermal-management issues can accelerate ES aging, reduce lifetime, and increase both cost and cooling-system complexity. Moreover, even though redundancy can maintain operation after an ES fault, shutdown of the entire E-STATCOM is often required to safely service and replace the faulty ES unit [10].
By contrast, as shown in Fig. 1(c), double-star multilevelconverter (DS-MC) based E-STATCOMs allow a centralized ES to be installed at the dc-link [12], [13]. This configuration decouples ES placement from the power stage, enabling independent installation and tighter temperature regulation, which extends ES lifetime and simplifies system management [14]. In addition, dc-link isolation using disconnectors permits the E-STATCOM to continue operating as a conventional STATCOM while ES maintenance is performed [4], [10]. Accordingly, Fig. 1(c) has attracted increasing interest as a practical DS-MC-based E-STATCOM topology.
Stable operation of a DS-MC requires regulation of the total internal energy. [15] The total internal energy of a DS-MC can be regulated through either the dc-side power exchange or the ac-side active power exchange. Under GFM operation, when total internal energy regulation is assigned to the ac side, two practical implementations are commonly used. The first employs a cascaded structure, where the totalenergy controller (TEC) generates an active power reference and the active power controller (APC) generates the converter frequency and angle commands from the active power error. In such a cascaded configuration, stable operation requires a clear separation of control bandwidths [16], [17]. If the TEC is tuned too slowly, rapid stabilization of the total internal energy becomes challenging. Conversely, if the APC is
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