Dynamic Ancillary Services: From Grid Codes to Transfer Function-Based Converter Control

Conventional grid-code specifications for dynamic ancillary services provision such as fast frequency and voltage regulation are typically defined by means of piece-wise linear step-response capability curves in the time domain. However, although the specification of such time-domain curves is straightforward, their practical implementation in a converter-based generation system is not immediate, and no customary methods have been developed yet. In this paper, we thus propose a systematic approach for the practical implementation of piece-wise linear time-domain curves to provide dynamic ancillary services by converter-based generation systems, while ensuring grid-code and device-level requirements to be reliably satisfied. Namely, we translate the piece-wise linear time-domain curves for active and reactive power provision in response to a frequency and voltage step change into a desired rational parametric transfer function in the frequency domain, which defines a dynamic response behavior to be realized by the converter. The obtained transfer function can be easily implemented e.g. via a PI-based matching control in the power loop of standard converter control architectures. We demonstrate the performance of our method in numerical grid-code compliance tests, and reveal its superiority over classical droop and virtual inertia schemes which may not satisfy the grid codes due to their structural limitations.Comment: 7 pages, 9 figure

Quantitative Stability Conditions for Grid-Forming Converters With Complex Droop Control

In this paper, we study analytically the transient stability of grid-connected distributed generation systems with grid-forming (GFM) complex droop control, also known as dispatchable virtual oscillator control (dVOC). We prove theoretically that complex droop control, as a state-of-the-art GFM control, always possesses steady-state equilibria whereas classical droop control does not. We provide quantitative conditions for complex droop control maintaining transient stability (global asymptotic stability) under grid disturbances, which is beyond the well-established local (non-global) stability for classical droop control. For the transient instability of complex droop control, we reveal that the unstable trajectories are bounded, manifesting as limit cycle oscillations. Moreover, we extend our stability results from second-order GFM control dynamics to full-order system dynamics that additionally encompass both circuit electromagnetic transients and inner-loop dynamics. Our theoretical results contribute an insightful understanding of the transient stability and instability of complex droop control and offer practical guidelines for parameter tuning and stability guarantees

How Many Grid-Forming Converters do We Need? A Perspective From Small Signal Stability and Power Grid Strength

Grid-forming (GFM) control has been considered as a promising solution for accommodating large-scale power electronics converters into modern power grids thanks to its grid-friendly dynamics, in particular, voltage source behavior on the AC side. The voltage source behavior of GFM converters can provide voltage support for the power grid, and therefore enhance the power grid (voltage) strength. However, grid-following (GFL) converters can also perform constant AC voltage magnitude control by properly regulating its reactive current, which may also behave like a voltage source. Currently, it still remains unclear what are the essential difference between the voltage source behaviors of GFL and GFM converters, and which type of voltage source behavior can enhance the power grid strength. In this paper, we will demonstrate that only GFM converters can provide effective voltage source behavior and enhance the power grid strength in terms of small signal dynamics. Based on our analysis, we further study the problem of how to configure GFM converters in the grid and how many GFM converters we will need. We investigate how the capacity ratio between GFM and GFL converters affects the equivalent power grid strength and thus the small signal stability of the system. We give guidelines on how to choose this ratio to achieve a desired stability margin. We validate our analysis using high-fidelity simulations

Robust Adaptive Control of STATCOMs to Mitigate Inverter-Based-Resource (IBR)-Induced Oscillations

The interaction among inverter-based resources (IBRs) and power network may cause small-signal stability issues, especially in low short-circuit grids. Besides, the integration of static synchronous compensators (STATCOMs) in a multi-IBR system for voltage support can deteriorate small-signal stability. However, it is still challenging to understand the impact mechanism of STATCOMs on IBR-induced oscillation issues and to design STATCOMs' control for damping these oscillation issues in a multi-IBR system, due to complex system dynamics and varying operating conditions. To tackle these challenges, this paper proposes a novel method to reveal how STATCOMs influence IBR-induced oscillation issues in a multi-IBR system from the viewpoint of grid strength, which can consider varying operating conditions. Based on this proposed method, we find critical operating conditions, wherein the system tends to be most unstable; moreover, we demonstrate that robust small-signal stability issues of the multi-IBR system with STATCOMs can be simplified as that of multiple subsystems under critical operating conditions, which avoids traversing all operating conditions and establishing system's detailed models. On this basis, an adaptive control-parameter design method is proposed for STATCOMs to ensure system's robust small-signal stability under varying operating conditions. The efficacy of proposed methods is validated by a 39-node test system

Joint Oscillation Damping and Inertia Provision Service for Converter-Interfaced Generation

As renewable generation becomes more prevalent, traditional power systems dominated by synchronous generators are transitioning to systems dominated by converter-interfaced generation. These devices, with their weaker damping capabilities and lower inertia, compromise the system's ability to withstand disturbances, pose a threat to system stability, and lead to oscillations and poor frequency response performance. While some new converter-interfaced generations are capable of providing superior damping and fast frequency control, there is a lack of effective measures to incentivize manufacturers to adopt them. To address this gap, this paper defines the joint oscillation damping and inertia provision services at the system level, seeking to encourage converter-interfaced generation to provide enhanced damping and fast frequency response capabilities. Our approach is anchored in a novel convex parametric formulation that combines oscillation mode and frequency stability constraints. These constraints ensure a sufficient damping ratio for all oscillation modes and maintain transient frequency trajectories within acceptable limits. They are designed to integrate smoothly into various operational and planning optimization frameworks. Using this formulation, we introduce a joint service for oscillation damping and inertia provision based on a cost-minimization problem. This facilitates the optimal allocation of damping and virtual inertia to converters, achieving both small-signal stability and frequency stability. Furthermore, we investigate the economic effects of introducing this service into a new ancillary service market, assessing its impact on system operations and cost-efficiency. Numerical tests highlight the service's efficacy in ensuring both small-signal stability and frequency stability, and offer insights into potential economic benefits.Comment: Submitted for IEEE PES journal for possible publication