Research
Snapshots
My research is focused on enabling the once-in-a-generation transition of the electric power infrastructure from being dominated by fossil-fuel-driven synchronous generators to one that is supported by power-electronics circuits processing energy from renewable resources of energy. Achieving a seamless transition will be central to realize societal-scale aspirations to decarbonize and ensure a sustainable energy future. Research to enable this effort spans several broad areas covering and intersecting power systems and power electronics. Snapshots below illustrate some thematic areas of contemporary interest across theory and application.
Grid-forming Inverter Technology
Large-scale integration of inverter-based resources (IBRs) with the power grid has sparked several concerns spanning stability, security, and protection. A majority of IBRs interfaced with the grid today are of the grid-following (GFL) type, wherein, the inverter synchronizes to (and follows) the grid voltage using a phase-locked loop and injects a specified amount of active and reactive power. A growing body of work has recognized that power grids with a high penetration of GFL inverters can be faced with small-signal stability issues. As a solution to a wide body of such concerns surrounding GFL IBRs, consensus is forming towards the adoption of grid-forming (GFM) inverter technology. In the GFM paradigm, IBRs do not follow the grid, rather, they form it and offer voltage and frequency regulation much alike conventional synchronous generators. Several primary-control methods have been demonstrated to offer GFM capability; of these, recent literature has focused dominantly on droop control, virtual synchronous machine (VSM) control, and virtual oscillator control (VOC).
Stakeholder Engagement across Academia and Industry
Recognizing the growing importance of GFM technologies across scales and industries, the US Department of Energy recently funded the Universal Interoperability for Grid-Forming Inverters (UNIFI) Consortium, to be led by the National Renewable Energy Laboratory (NREL). The objective of UNIFI is to create an ecosystem that will enable researchers, industry partners (inverter manufacturers, software vendors, developers, consultants), utilities, system operators, and other stakeholders collaboratively pursue advances in a broad range of GFM technologies and applications; Benjamin Kroposki from NREL will serve as the Organizational Director of UNIFI. The Consortium will define system-level guidelines and unit-level functions to prove vendor-agnostic interoperability of GFM technologies; conduct and coordinate research, development, and demonstration; and formulate mechanisms for workforce training.
Modeling
Synchronous generators and inverter-based resources are complex systems with dynamics that cut across multiple intertwined physical domains and control loops. Modeling individual generators and inverters is, in itself, a very involved activity and has attracted dedicated attention from power engineers and control theorists over the years. Control and stability challenges associated with increasing penetration of grid-following inverters have generated tremendous interest in grid-forming inverter technology. The envisioned coexistence of inverter technologies alongside rotating machines call for modeling frameworks that can accurately describe networked dynamics of interconnected generators and inverters across timescales. We are working towards developing integrated system models for such a setting by: i) adopting a combination of circuit- and system-theoretic constructs, ii) unifying representations of three-phase signals across reference-frame transformations and phasor types, and iii) leveraging domain-level knowledge, engineering insights, and reasonable approximations. A running theme through our effort is to offer a clear distinction between physics-based models and the task of modeling. Among several insights spanning the spectrum from analytical to practical, we highlight how differential-algebraic-equation models and algebraic power-flow phasor models fall out of the detailed originating electromagnetic transient models.
accuracy, complexity, rigor, and prevalence of the different formulations across the literature are shown in the north-east corner
assumptions and simplifications that facilitate traversing across the problems are also listed alongside; problem (P1) pertains to dynamic operating conditions, while (P2)-(P4) apply to steady-state operation
Optimization
Managing energy in electric networks with IBRs is a challenging undertaking. This has to be accomplished while acknowledging dynamics cutting across multiple timescales, in the face of uncertainty, and potentially with competing interests from operators and owners. Literature in energy management for synchronous-generator-based resources is a mature topic; that devoted to IBRs is growing, albeit scattered. We are particularly interested in rigorously tying together problem formulations across timescales, quashing boundaries between optimization and control problems with the aim of improving efficiency, and benchmarking complexity involved in different algorithms.
Control
Controlling collections of networked IBRs offers several technical challenges, since volatile dynamics in sources and loads are not automatically filtered by rotating inertia of large machines, as is the case in the bulk power system that serves us. To this end, a key technical contribution we have made is the development of a nonlinear synchronization-enforcing control scheme. In this approach, power-electronics inverters are controlled to emulate the dynamics of nonlinear oscillator circuits. Given the inherent electrical coupling between the (virtual) oscillators, the inverters synchronize and a stable power system can be obtained without communication. Stability and robustness can be guaranteed leveraging notions from coupled-oscillator, circuit, and network theory. The efforts of my group in this line of research have resulted in several theoretical contributions. For instance, we have demonstrated that classical droop control laws are embedded in the nonlinear dynamics of Lienard-type oscillators; and unified circuit-, network-, and control-theoretic notions to develop synchronization conditions for a wide class of nonlinear circuits. From an application standpoint, we have validated our findings in laboratory prototypes, and recently explored how splay synchronization (a phenomenon in which the phases of the nonlinear oscillators spread out uniformly on the unit circle) can be leveraged for decentralized power-quality improvement in dc-dc converters.
