The integration of renewable energy sources such as wind and photovoltaic systems necessitates upgrades to existing power transmission infrastructure. In Germany, multiple grid booster (GB) projects are underway to address this need. GBs enhance the utilization of existing transmission networks by providing curative congestion management, allowing grid utilization beyond the levels required by N-1 security standards without compromising stability or supply security. Often described as virtual transmission lines, GBs can also support voltage regulation, frequency stabilization, and harmonic mitigation. However, their high capital costs can limit widespread deployment.

Hybrid grid boosters (HGBs) present a pathway to reduce investment costs while adding flexibility that creates new revenue streams. An HGB combines two energy storage units: a fast but expensive battery energy storage system (BESS) and a slower but cost-effective storage unit. So HGB reduces the required energy storage capacity from the BESS. The objective is to leverage the advantages of both technologies while maintaining the performance of a pure BESS-based GB.

The HGB concept explored in this paper incorporates a BESS, an electrolyzer, a hydrogen-fired gas turbine, and gas storage. While GBs are typically activated by transmission system operators during congestion events, such occurrences are infrequent. Enabling GBs to provide ancillary services during non-congestion periods through (real-time) decentralized/centralized operational planning enhances their economic viability and grid value.

This paper presents a detailed and scalable dynamic model of the HGB system for stability analysis and compliance testing. The grid model, while generic, captures grid strength and the interactions between frequency and instantaneous supply-demand balance. The electrolyzer model incorporates coupled electrical and thermal dynamics, while the synchronous generator is represented using a dq-frame 2.2 model suitable for transient stability studies. Standard models for the gas turbine, governor, and exciter are employed.

The control strategy includes a mechanism for a smooth and seamless dynamic power allocation between the BESS and the gas turbine during congestion events. Various control methods, including grid-following and grid-forming approaches, are evaluated under different grid events, such as faults, voltage deviations, and frequency fluctuations.

Overall, the developed HGB model and control design provide a robust framework for analyzing system performance under real-time operational planning and compliance testing scenarios. This work supports the deployment of HGBs as a cost-effective and flexible solution to enhance renewable integration and grid reliability.