45 research outputs found
Ensuring Disturbance Rejection Performance by Synthesizing Grid-Following and Grid-Forming Inverters in Power Systems
To satisfy dynamic requirements of power systems, it is imperative for
grid-tied inverters to ensure good disturbance rejection performance (DRP)
under variable grid conditions. This letter discovers and theoretically proves
that for general networks, synthesizing grid-following (GFL) inverters and
grid-forming (GFM) inverters can always more effectively ensure the DRP of
multiple inverters, as compared to homogeneous inverter-based systems that
solely utilize either GFL or GFM inverters. The synthesis of GFL inverters and
GFM inverters can concurrently increase the smallest eigenvalue and decrease
the largest eigenvalue of the network grounded Laplacian matrix. This can be
equivalent to rematching the proper short-circuit ratio (SCR) for GFL and GFM
inverters, thereby ensuring the DRP of inverters both in weak and strong grids.
The results reveal the necessity of synthesizing diverse inverter control
schemes from the network-based perspective. Sensitivity function-based tests
and real-time simulations validate our results.Comment: 6 page
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
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
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
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