617 research outputs found
Synchronization of Nonlinear Circuits in Dynamic Electrical Networks with General Topologies
Sufficient conditions are derived for global asymptotic synchronization in a
system of identical nonlinear electrical circuits coupled through linear
time-invariant (LTI) electrical networks. In particular, the conditions we
derive apply to settings where: i) the nonlinear circuits are composed of a
parallel combination of passive LTI circuit elements and a nonlinear
voltage-dependent current source with finite gain; and ii) a collection of
these circuits are coupled through either uniform or homogeneous LTI electrical
networks. Uniform electrical networks have identical per-unit-length
impedances. Homogeneous electrical networks are characterized by having the
same effective impedance between any two terminals with the others open
circuited. Synchronization in these networks is guaranteed by ensuring the
stability of an equivalent coordinate-transformed differential system that
emphasizes signal differences. The applicability of the synchronization
conditions to this broad class of networks follows from leveraging recent
results on structural and spectral properties of Kron reduction---a
model-reduction procedure that isolates the interactions of the nonlinear
circuits in the network. The validity of the analytical results is demonstrated
with simulations in networks of coupled Chua's circuits
Output Impedance Diffusion into Lossy Power Lines
Output impedances are inherent elements of power sources in the electrical
grids. In this paper, we give an answer to the following question: What is the
effect of output impedances on the inductivity of the power network? To address
this question, we propose a measure to evaluate the inductivity of a power
grid, and we compute this measure for various types of output impedances.
Following this computation, it turns out that network inductivity highly
depends on the algebraic connectivity of the network. By exploiting the derived
expressions of the proposed measure, one can tune the output impedances in
order to enforce a desired level of inductivity on the power system.
Furthermore, the results show that the more "connected" the network is, the
more the output impedances diffuse into the network. Finally, using Kron
reduction, we provide examples that demonstrate the utility and validity of the
method
Voltage Stabilization in Microgrids via Quadratic Droop Control
We consider the problem of voltage stability and reactive power balancing in
islanded small-scale electrical networks outfitted with DC/AC inverters
("microgrids"). A droop-like voltage feedback controller is proposed which is
quadratic in the local voltage magnitude, allowing for the application of
circuit-theoretic analysis techniques to the closed-loop system. The operating
points of the closed-loop microgrid are in exact correspondence with the
solutions of a reduced power flow equation, and we provide explicit solutions
and small-signal stability analyses under several static and dynamic load
models. Controller optimality is characterized as follows: we show a one-to-one
correspondence between the high-voltage equilibrium of the microgrid under
quadratic droop control, and the solution of an optimization problem which
minimizes a trade-off between reactive power dissipation and voltage
deviations. Power sharing performance of the controller is characterized as a
function of the controller gains, network topology, and parameters. Perhaps
surprisingly, proportional sharing of the total load between inverters is
achieved in the low-gain limit, independent of the circuit topology or
reactances. All results hold for arbitrary grid topologies, with arbitrary
numbers of inverters and loads. Numerical results confirm the robustness of the
controller to unmodeled dynamics.Comment: 14 pages, 8 figure
Towards Optimal Kron-based Reduction Of Networks (Opti-KRON) for the Electric Power Grid
For fast timescales or long prediction horizons, the AC optimal power flow
(OPF) problem becomes a computational challenge for large-scale, realistic AC
networks. To overcome this challenge, this paper presents a novel network
reduction methodology that leverages an efficient mixed-integer linear
programming (MILP) formulation of a Kron-based reduction that is optimal in the
sense that it balances the degree of the reduction with resulting modeling
errors in the reduced network. The method takes as inputs the full AC network
and a pre-computed library of AC load flow data and uses the graph Laplacian to
constraint nodal reductions to only be feasible for neighbors of non-reduced
nodes. This results in a highly effective MILP formulation which is embedded
within an iterative scheme to successively improve the Kron-based network
reduction until convergence. The resulting optimal network reduction is, thus,
grounded in the physics of the full network. The accuracy of the network
reduction methodology is then explored for a 100+ node medium-voltage radial
distribution feeder example across a wide range of operating conditions. It is
finally shown that a network reduction of 25-85% can be achieved within seconds
and with worst-case voltage magnitude deviation errors within any super node
cluster of less than 0.01pu. These results illustrate that the proposed
optimization-based approach to Kron reduction of networks is viable for larger
networks and suitable for use within various power system applications
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