42 research outputs found
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
Power Sharing Method Based on Droop Control for Three-Phase UPS Systems
The main objective of this paper is to improve the power sharing capability and to achieve synchronization between three-phase Uninterruptible power supply (UPS) units in the presence of load interruption. This paper presents a droop-controlled scheme in such a way that the computation of the instantaneous value of the active power and the reactive power are taken as feedback signals to the frequency and voltage restoration control system. The restored frequency and voltage are introduced to voltage controller circuit, which produces a suitable control signal to sinusoidal pulse width modulation circuit (SPWM). Thus producing a suitable trigger pulses to the inverter gate in order to guarantee synchronization between three-phase UPS units. Simulation of two UPS units with the same ratings (4 KW) are carried out using MATLAB. The results show the effectiveness of the proposed control system in achieving synchronization and improving the power sharing capability in the presence of load interruption. Keywords: Uninterruptible power supply; power sharing; parallel operation; droop control. DOI: 10.7176/JETP/9-4-02 Publication date: April 30th 201
Reverse and Forward Engineering of Local Voltage Control in Distribution Networks
The increasing penetration of renewable and distributed energy resources in
distribution networks calls for real-time and distributed voltage control. In
this paper we investigate local Volt/VAR control with a general class of
control functions, and show that the power system dynamics with non-incremental
local voltage control can be seen as distributed algorithm for solving a
well-defined optimization problem (reverse engineering). The reverse
engineering further reveals a fundamental limitation of the non-incremental
voltage control: the convergence condition is restrictive and prevents better
voltage regulation at equilibrium. This motivates us to design two incremental
local voltage control schemes based on the subgradient and pseudo-gradient
algorithms respectively for solving the same optimization problem (forward
engineering). The new control schemes decouple the dynamical property from the
equilibrium property, and have much less restrictive convergence conditions.
This work presents another step towards developing a new foundation -- network
dynamics as optimization algorithms -- for distributed realtime control and
optimization of future power networks
Reverse and Forward Engineering of Local Voltage Control in Distribution Networks
The increasing penetration of renewable and distributed energy resources in distribution networks calls for real-time and distributed voltage control. In this paper we investigate local Volt/VAR control with a general class of control functions, and show that the power system dynamics with non-incremental local voltage control can be seen as a distributed algorithm for solving a well-defined optimization problem (reverse engineering). The reverse engineering further reveals a fundamental limitation of the non-incremental voltage control: the convergence condition is restrictive and prevents better voltage regulation at equilibrium. This motivates us to design two incremental local voltage control schemes based on the subgradient and pseudo-gradient algorithms respectively for solving the same optimization problem (forward engineering). The new control schemes decouple the dynamical property from the equilibrium property, and have much less restrictive convergence conditions. This work presents another step towards developing a new foundation - network dynamics as optimization algorithms - for distributed real-time control and optimization of future power networks