Kuri A (2025)
Publication Language: English
Publication Type: Thesis
Publication year: 2025
Publisher: FAU University Press
City/Town: Erlangen-Nürnberg
ISBN: 978-3-96147-822-4
DOI: 10.25593/978-3-96147-823-1
The rising number of inverter-based resources (IBRs) deployed in power systems worldwide has led to redefining their roles with evolved service requirements. In the future, to maintain grid stability and reliability, IBRs will need to deliver some of the services synchronous machines still provide. Currently, almost all installed IBRs are grid-following, essentially following the system frequency and offering only a subset of benefits. However, envisioning a 100 % converter-operated system requires the ability of IBRs to participate in creating the grid voltage and, hence, the grid frequency. Thus, recent developments have led to Grid-Forming (GFM) controls, which combine power electronic equipment’s speed and flexibility with synchronous machines’ stabilizing characteristics. To tackle future power system challenges, a novel GFM converter control is conceptualized, implemented and validated as part of this dissertation.
GFM is well-established, particularly for microgrids and a necessary enabler for high integration of IBR-interfaced renewable resources with enhanced capability for stability. Understanding its impact on bulk power system is still in the early stages, but regulatory bodies have begun to define various tiers of requirements. The primary emphasis is on rapidly adjusting to system changes to maintain control stability, regulate active and reactive power, synchronize with the grid, and ensure compatibility with devices in the power system. Furthermore, to ensure uninterrupted support from the GFM resource during severe disturbances, it is required to characterize the converter operation during the current limiting mode. This mode is typically defined by a converter’s hardware capability and the ability to transition in and out of this restricted range. Thus, the GFM control should achieve current magnitude limitation with fault current contribution, have recovery capability to protect the semiconductor devices and sustain the grid.
Additionally, the technological transition necessitates new definitions of system demands and services, concise explanations of main characteristics, and utilization of services offered by GFM in bulk power systems. Due to the flexibility in design offered by these IBR controls, it is prudent to change perspective from inverter requirements to evolving system needs to derive GFM capabilities.
In a broader context, GFM assures global stability of a power system. The objective of this thesis is to propose a control strategy for GFM converters from a power system perspective that attains global stability. The control strategy is developed in three phases.
In Phase 1, a new concept called the ‘Phase Restoring Principle’ (PRP) is introduced, which preserves the essential phase-frequency relationship of voltage in extended power systems. PRP generates a nominal frequency via a novel angular transformation, ensuring exclusively phase-frequency stability. The power system frequency always returns to its nominal value with minimal order dynamics and no additional master control. Therefore, it presents a physical equivalence of an unlimited source and is analogous to an ideal slack. Further, the response is free of non-beneficial swing dynamics and attains the new operating point directly displaying the limited Vector Shift criteria proposed, for example, by National Grid (UK). PRP is the fundamental building block in developing the GFM control with slower control dynamics built over it. This concept prioritizes response to disturbances over response to setpoint changes from the control hierarchy.
The steady-state operating point, uniqueness, and boundary conditions of the system are derived analytically, as mandated by the ‘Existence Theorem’. The requirement of existence and uniqueness imposes a restriction on one parameter ratio of the overall system model. The generalized small-signal solution of the control principle is deduced and evidenced based on the linearization method. The proof of global stability of PRP is displayed based on the necessary boundary conditions and small-signal criteria indicating stable control behavior independent of variations in network parameters and topology (e.g., strong or weak grid).
Further, Phase 2 development addresses the need for steady-state active power limitation and relaxes the constant frequency response of PRP, though dynamic current overshoots are inherent. The Phase 1 control is enhanced with optimized active power control (APC). This allows the scheme to cooperate with other sources considering arbitrary active power dispatch via setpoints.
Finally, Phase 3 considers current limitation by adequate control of voltage magnitude. A current limiting control based on the voltage limiter concept with innovative usage of anti-windup methods is supplemented. The algorithm helps preserve the GFM properties while maintaining voltage source behavior and provides the best possible control response under severe faults.
The complete control concept and its three distinctive development stages are numerically investigated in a realistic ohmic-inductive test bench, and power system stability is analyzed under various network configurations. Simulation results for each phase are presented to support the mathematical hypothesis and visualize the controller’s response. The control scheme is extensively tested to ensure robustness under normal operating ranges, severe faults, and weak grid scenarios without other active sources (i.e., Short Circuit Ratio of zero). PRP responses are also compared with typical voltage sources and known GFM schemes. Investigation in both electromagnetic transients (EMT) and root mean square (RMS) simulation domains are conducted. The control scheme is further demonstrated and validated on a real-time simulator in a laboratory environment by adopting Power-Hardware-in-the-Loop test benches.
The above investigations reinforce the control performance, enabling interoperability with other converter controls and fulfilling ancillary service requirements. The complete control scheme retains global stability under a viable choice of parameters. The comprehensive development of the PRP control scheme reveals its capabilities and advantages against existing GFM schemes. The developed scheme is mature and viable for transfer to GFM field applications.
APA:
Kuri, A. (2025). A Grid-Forming Contr ol Concept from Power System Perspective (Dissertation).
MLA:
Kuri, Ananya. A Grid-Forming Contr ol Concept from Power System Perspective. Dissertation, Erlangen-Nürnberg: FAU University Press, 2025.
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