Dominic Groß

This work develops a constraint-aware grid-forming (GFM) control that explicitly accounts for current limits and modulation limits within the GFM oscillator dynamics generating the GFM voltage reference (i.e., phase angle and magnitude). Broadly speaking, the voltage reference generated by the constraint-aware GFM control minimizes the deviation from conventional unconstrained GFM droop control, while respecting current and modulation limits. The resulting GFM control achieves fast current limiting while preserving transient stability, e.g., exhibiting infinite critical clearing time. To develop the control, we first characterize and analyze the set of converter voltages that do not result in constraint violations. Next, an efficient algorithm for projecting voltages onto the feasible set is developed. Subsequently, these results are used to restrict the dynamics of GFM droop control to the set of feasible voltages. Finally, detailed simulation studies and hardware experiments are used to illustrate and validate the response to short-circuit faults and phase jumps.

Automatic generation control (AGC) adjusts generator set-points through communication to maintain frequency stability and scheduled power transfer between neighboring areas of power systems. Due to cyber-vulnerabilities in communication networks, the AGC signal is susceptible to attacks, potentially leading to abnormal frequency deviations. This paper analyzes an attack strategy targeting AGC and the factors influencing its success. First, the paper outlines the attacker model and briefly reviews the feedback attack policy. Then, it examines the impact of power system parameters on the likelihood of a successful attack, using a decision tree algorithm, establishing guidelines to identify worst-case scenarios from a system operator’s perspective. Adhering to these guidelines can increase the probability of a successful attack, which is verified using phasor simulation with detailed turbine/governor and protection models. Additionally, the paper examines the attacker threat model, which identifies the attack surface required to steer the system frequency to an unsafe state while avoiding the activation of the remedial action scheme (RAS). If an attacker gains access, they could prevent the tripping of generator circuit breakers, thereby increasing the risk of a system-wide collapse and/or blackout.

In this paper, we study a constrained network flow problem and associated networked dynamics that resemble but are distinct from the well-known primal-dual dynamics of the constrained flow problem. Crucially, under a change of coordinates, the networked dynamics coincide with primal-dual dynamics associated with the constrained flow problem in edge coordinates. Next, we show that, under mild feasibility assumptions, the networked dynamics are globally asymptotically stable with respect to the set of optimizers of its associated constrained flow problem in nodal coordinates. Subsequently, we apply our stability results to establish frequency stability of power-limiting grid-forming droop control. Compared to conventional grid-forming droop control, power-limiting droop control explicitly accounts for active power limits of the generation (e.g., renewables) interfaced by the converter. While power-limiting droop control has been demonstrated to work well in simulation and experiment, analytical results are not readily available. Moreover, we (i) show that the converter frequencies synchronize to a common synchronous frequency for each grid-forming converter, (ii) characterize the synchronous frequency in the case of converters operating at their power limit, and (iii) establish that power-limiting droop control exhibits power-sharing properties similar to conventional unconstrained droop control. Finally, the analytical results are illustrated using an Electromagnetic transient (EMT) simulation.

Interconnecting power converters (IPCs) are the main elements enabling the interconnection of multiple high-voltage alternating current (HVAC) and high-voltage direct current (HVDC) subgrids. To ensure stable operation of the resulting hybrid ac/dc systems, grid-following (GFL) and grid-forming (GFM) controls need to be carefully assigned to individual IPC terminals when using common IPC controls. In contrast, dual-port GFM control imposes a stable voltage on the ac and dc terminals and can be deployed on all IPCs regardless of the network configuration. In this work, we use hybrid ac/dc admittance models, eigenvalue sensitivities, and case studies to analyze and quantify the underlying properties of AC-GFM control, AC-GFL, and dual-port GFM control. Compared to common AC-GFM and AC-GFL controls, dual-port GFM control (i) renders IPCs dissipative over a much wider range of frequencies and operating points, (ii) significantly reduces the sensitivity of IPC small-signal dynamics to operating point changes, and (iii) exhibits an improved dynamic response to severe contingencies. Finally, the results are illustrated and validated in an experimental scaled-down point-to-point HVDC system.

We analyze a dual-port grid-forming (GFM) control for power systems containing ac and dc transmission, converter-interfaced generation and energy storage, and legacy generation. To operate such a system and provide standard services, state-of-the-art control architectures i) require assigning grid-following (GFL) and GFM controls to different converters, and ii) result in highly complex system dynamics. In contrast, dual-port GFM control (i) subsumes common functions of GFM and GFL controls in a simple controller, ii) can be applied to a wide range of emerging technologies independently of the network configuration, and iii) significantly reduces system complexity. In this work, we provide i) an end-to-end modeling framework that allows to model complex topologies through composition of reduced-order device models, ii) an in-depth discussion of universal dual-port GFM control for emerging power systems, and iii) end-to-end stability conditions that cover a wide range of network topologies, emerging technologies, and legacy technologies. Finally, we validate our findings in detailed case studies.

The continuous implementation of new High Voltage Direct Current (HVDC) systems in the existing power system arises different challenges and opportunities. One important decision to be made by transmission systems operators is the control role assignment of the Interconnecting Power Converters (IPCs) conforming the HVDC system. The control role is understood as the converter operation both on the AC side (grid-following and grid-forming) and the DC side (grid-following and grid-forming). In general, the control role of these IPCs is static, i.e., changing the control role of the IPC is typically not considered in the power system operation. This paper considers the control role of the IPCs as dynamic, allowing the operators to have improved performance, stability enhancement, and flexibility. A methodology to optimally assign the control role of the IPC for a day-ahead scheduled generation and demand profile is proposed. This methodology considers (i) small-signal stability, (ii) system dynamic performance, and (iii) steady-state performance. Finally, an optimization problem is formulated to determine the best converter control role configuration to operate in the considered power flow scenarios. The proposed techniques are validated in a system with high renewable penetration and highly variable power flows.

Electrical power systems are transitioning from fuel-based generation to renewable generation and transmission interfaced by power electronics. This transition challenges standard power system modeling, analysis, and control paradigms across timescales from milliseconds to seasons. This tutorial focuses on frequency stability on timescales of milliseconds to seconds. We first review basic results for grid-following (GFL) and grid-forming (GFM) control of voltage source converters (VSCs), typical renewable generation, and high voltage direct current (HVdc) transmission. In this context, it becomes apparent that GFL and GFM control functions are needed to operate emerging power systems. However, combining GFL resources, GFM resources, and legacy generation on the same system results in highly complex dynamics that are a significant obstacle to stability analysis. The remainder of the tutorial provides an overview of recent developments in universal GFM controls that bridge the gap between GFL and GFM control and provide a pathway to a coherent control and analysis framework accounting for power generation, power conversion, and power transmission.