Decoupled Internal Energy Regulation and Inertial Response Provision for Grid-Forming Multilevel-Converter-Based E-STATCOMs

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.

💡 Research Summary

As renewable penetration grows, power systems experience frequent short‑term imbalances that cause pronounced voltage and frequency excursions, especially under low‑inertia conditions. Energy‑storage‑enhanced STATCOMs (E‑STATCOMs) have emerged as a promising solution because they can provide both voltage support and fast frequency assistance when operated in grid‑forming (GFM) mode. Among the various topologies, the double‑star multilevel converter (DS‑MC) is attractive: it centralizes the energy‑storage (ES) device at the dc‑link, improving thermal management, maintainability, and allowing the converter to continue operating as a conventional STATCOM during ES maintenance.

Existing DS‑MC‑based E‑STATCOMs typically regulate the total internal energy of the converter by using the dc‑side power exchange. In this scheme the ES continuously compensates the converter’s internal losses, which forces the ES into persistent charge‑discharge cycles even when no frequency‑support event occurs. This accelerates degradation, raises RMS currents, and prevents the device from falling back to pure STATCOM operation when the ES is unavailable.

The paper compares two internal‑energy‑regulation schemes. Scheme I uses the ac‑side active‑power path, while Scheme II uses the dc‑side power. Scheme I suffers from a fundamental bandwidth‑separation issue: the total‑energy controller (TEC) and the active‑power controller (APC) share the same power loop, so the TEC must be tuned much slower than the APC to avoid destabilizing the frequency‑angle dynamics. If the TEC is made faster, the phase margin collapses and the system becomes unstable; if the APC is made faster, the converter loses its “slow voltage‑source” characteristic, degrading its inertial response. Scheme II avoids the bandwidth problem but forces the ES to continuously supply loss‑compensation power, leading to the aforementioned cycling and loss of flexibility.

To overcome these drawbacks, the authors propose a novel GFM control strategy that (1) assigns total internal‑energy regulation to the ac‑side active‑power degree of freedom, (2) reserves the dc‑side ES power exclusively for inertial response, and (3) decouples the TEC from the APC by enforcing current‑source behavior on the TEC. Specifically, the TEC output power reference (P_{\text{ref}}) is transformed into a d‑axis current reference (i_{d,\text{ref}}) using the measured PCC voltage components, and this current reference is fed forward to the inner‑loop current controller. Consequently, the TEC can act almost instantaneously to correct internal‑energy errors, while the APC only sees a small residual error and can be tuned with a low bandwidth, preserving GFM dynamics. The inertial contribution is modeled as a voltage source that drives the dc‑side current controller, ensuring that the ES supplies power only during frequency‑support events.

The overall control architecture includes outer‑loop APC and reactive‑power controller (RPC), an inner‑loop virtual‑admittance current controller, arm‑energy‑balancing and circulating‑current controllers, and the two feed‑forward blocks (current‑reference and inertial‑power mapping). A super‑capacitor ES is used in the experimental validation because of its high power density and long cycle life; the ES is deliberately limited to inertial support to minimize required capacity.

Simulation results demonstrate rapid convergence of the internal‑energy error under the proposed scheme, even during large load steps and voltage sags, while maintaining stable frequency‑angle dynamics. Experimental tests on a laboratory‑scale DS‑MC prototype confirm that (i) the converter can operate as a pure STATCOM when the ES is disconnected, (ii) when the ES is connected it provides fast inertial power without participating in loss compensation, and (iii) the internal‑energy regulation remains fast and stable thanks to the current‑feed‑forward term.

In summary, the paper delivers a control solution that (a) accelerates internal‑energy regulation without compromising GFM performance, (b) eliminates unnecessary ES cycling, thereby extending ES lifetime and reducing operational cost, and (c) offers seamless mode transition between STATCOM and E‑STATCOM operation. These advantages make the proposed strategy highly relevant for future low‑inertia grids with high renewable penetration.