11,396 research outputs found
A Framework for Robust Assessment of Power Grid Stability and Resiliency
Security assessment of large-scale, strongly nonlinear power grids containing
thousands to millions of interacting components is a computationally expensive
task. Targeting at reducing the computational cost, this paper introduces a
framework for constructing a robust assessment toolbox that can provide
mathematically rigorous certificates for the grids' stability in the presence
of variations in power injections, and for the grids' ability to withstand a
bunch sources of faults. By this toolbox we can "off-line" screen a wide range
of contingencies or power injection profiles, without reassessing the system
stability on a regular basis. In particular, we formulate and solve two novel
robust stability and resiliency assessment problems of power grids subject to
the uncertainty in equilibrium points and uncertainty in fault-on dynamics.
Furthermore, we bring in the quadratic Lyapunov functions approach to transient
stability assessment, offering real-time construction of stability/resiliency
certificates and real-time stability assessment. The effectiveness of the
proposed techniques is numerically illustrated on a number of IEEE test cases
Transient growth in the flow past a three-dimensional smooth roughness element
This work provides a global optimization analysis, looking for perturbations inducing the largest energy growth at a finite time in a boundary-layer flow in the presence of smooth three-dimensional roughness elements. Amplification mechanisms are described which can bypass the asymptotical growth of Tollmien–Schlichting waves. Smooth axisymmetric roughness elements of different height have been studied, at different Reynolds numbers. The results show that even very small roughness elements, inducing only a weak deformation of the base flow, can localize the optimal disturbance characterizing the Blasius boundary-layer flow. Moreover, for large enough bump heights and Reynolds numbers, a strong amplification mechanism has been recovered, inducing an increase of several orders of magnitude of the energy gain with respect to the Blasius case. In particular, the highest value of the energy gain is obtained for an initial varicose perturbation, differently to what found for a streaky parallel flow. Optimal varicose perturbations grow very rapidly by transporting the strong wall-normal shear of the base flow, which is localized in the wake of the bump. Such optimal disturbances are found to lead to transition for initial energies and amplitudes considerably smaller than sinuous optimal ones, inducing hairpin vortices downstream of the roughness element
A decentralized scalable approach to voltage control of DC islanded microgrids
We propose a new decentralized control scheme for DC Islanded microGrids
(ImGs) composed by several Distributed Generation Units (DGUs) with a general
interconnection topology. Each local controller regulates to a reference value
the voltage of the Point of Common Coupling (PCC) of the corresponding DGU.
Notably, off-line control design is conducted in a Plug-and-Play (PnP) fashion
meaning that (i) the possibility of adding/removing a DGU without spoiling
stability of the overall ImG is checked through an optimization problem; (ii)
when a DGU is plugged in or out at most neighbouring DGUs have to update their
controllers and (iii) the synthesis of a local controller uses only information
on the corresponding DGU and lines connected to it. This guarantee total
scalability of control synthesis as the ImG size grows or DGU gets replaced.
Yes, under mild approximations of line dynamics, we formally guarantee
stability of the overall closed-loop ImG. The performance of the proposed
controllers is analyzed simulating different scenarios in PSCAD.Comment: arXiv admin note: text overlap with arXiv:1405.242
Robust Decentralized Secondary Frequency Control in Power Systems: Merits and Trade-Offs
Frequency restoration in power systems is conventionally performed by
broadcasting a centralized signal to local controllers. As a result of the
energy transition, technological advances, and the scientific interest in
distributed control and optimization methods, a plethora of distributed
frequency control strategies have been proposed recently that rely on
communication amongst local controllers.
In this paper we propose a fully decentralized leaky integral controller for
frequency restoration that is derived from a classic lag element. We study
steady-state, asymptotic optimality, nominal stability, input-to-state
stability, noise rejection, transient performance, and robustness properties of
this controller in closed loop with a nonlinear and multivariable power system
model. We demonstrate that the leaky integral controller can strike an
acceptable trade-off between performance and robustness as well as between
asymptotic disturbance rejection and transient convergence rate by tuning its
DC gain and time constant. We compare our findings to conventional
decentralized integral control and distributed-averaging-based integral control
in theory and simulations
Plug-and-play and coordinated control for bus-connected AC islanded microgrids
This paper presents a distributed control architecture for voltage and
frequency stabilization in AC islanded microgrids. In the primary control
layer, each generation unit is equipped with a local controller acting on the
corresponding voltage-source converter. Following the plug-and-play design
approach previously proposed by some of the authors, whenever the
addition/removal of a distributed generation unit is required, feasibility of
the operation is automatically checked by designing local controllers through
convex optimization. The update of the voltage-control layer, when units plug
-in/-out, is therefore automatized and stability of the microgrid is always
preserved. Moreover, local control design is based only on the knowledge of
parameters of power lines and it does not require to store a global microgrid
model. In this work, we focus on bus-connected microgrid topologies and enhance
the primary plug-and-play layer with local virtual impedance loops and
secondary coordinated controllers ensuring bus voltage tracking and reactive
power sharing. In particular, the secondary control architecture is
distributed, hence mirroring the modularity of the primary control layer. We
validate primary and secondary controllers by performing experiments with
balanced, unbalanced and nonlinear loads, on a setup composed of three
bus-connected distributed generation units. Most importantly, the stability of
the microgrid after the addition/removal of distributed generation units is
assessed. Overall, the experimental results show the feasibility of the
proposed modular control design framework, where generation units can be
added/removed on the fly, thus enabling the deployment of virtual power plants
that can be resized over time
Fixed-Time Gradient Flows for Solving Constrained Optimization: A Unified Approach
The accelerated method in solving optimization problems has always been an
absorbing topic. Based on the fixed-time (FxT) stability of nonlinear dynamical
systems, we provide a unified approach for designing FxT gradient flows
(FxTGFs). First, a general class of nonlinear functions in designing FxTGFs is
provided. A unified method for designing first-order FxTGFs is shown under
PolyakL jasiewicz inequality assumption, a weaker condition than strong
convexity. When there exist both bounded and vanishing disturbances in the
gradient flow, a specific class of nonsmooth robust FxTGFs with disturbance
rejection is presented. Under the strict convexity assumption, Newton-based
FxTGFs is given and further extended to solve time-varying optimization.
Besides, the proposed FxTGFs are further used for solving equation-constrained
optimization. Moreover, an FxT proximal gradient flow with a wide range of
parameters is provided for solving nonsmooth composite optimization. To show
the effectiveness of various FxTGFs, the static regret analysis for several
typical FxTGFs are also provided in detail. Finally, the proposed FxTGFs are
applied to solve two network problems, i.e., the network consensus problem and
solving a system linear equations, respectively, from the respective of
optimization. Particularly, by choosing component-wisely sign-preserving
functions, these problems can be solved in a distributed way, which extends the
existing results. The accelerated convergence and robustness of the proposed
FxTGFs are validated in several numerical examples stemming from practical
applications
Estimating Dynamic Load Parameters from Ambient PMU Measurements
In this paper, a novel method to estimate dynamic load parameters via ambient
PMU measurements is proposed. Unlike conventional parameter identification
methods, the proposed algorithm does not require the existence of large
disturbance to power systems, and is able to provide up-to-date dynamic load
parameters consistently and continuously. The accuracy and robustness of the
method are demonstrated through numerical simulations.Comment: The paper has been accepted by IEEE PES general meeting 201
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