Stability and Frequency Regulation of Inverters with Capacitive Inertia

In this paper, we address the problem of stability and frequency regulation of a recently proposed inverter. In this type of inverter, the DC-side capacitor emulates the inertia of a synchronous generator. First, we remodel the dynamics from the electrical power perspective. Second, using this model, we show that the system is stable if connected to a constant power load, and the frequency can be regulated by a suitable choice of the controller. Next, and as the main focus of this paper, we analyze the stability of a network of these inverters, and show that frequency regulation can be achieved by using an appropriate controller design. Finally, a numerical example is provided which illustrates the effectiveness of the method

A Robust Consensus Algorithm for Current Sharing and Voltage Regulation in DC Microgrids

In this paper a novel distributed control algorithm for current sharing and voltage regulation in Direct Current (DC) microgrids is proposed. The DC microgrid is composed of several Distributed Generation units (DGUs), including Buck converters and current loads. The considered model permits an arbitrary network topology and is affected by unknown load demand and modelling uncertainties. The proposed control strategy exploits a communication network to achieve proportional current sharing using a consensus-like algorithm. Voltage regulation is achieved by constraining the system to a suitable manifold. Two robust control strategies of Sliding Mode (SM) type are developed to reach the desired manifold in a finite time. The proposed control scheme is formally analyzed, proving the achievement of proportional current sharing, while guaranteeing that the weighted average voltage of the microgrid is identical to the weighted average of the voltage references.Comment: 12 page

Voltage Control of DC Microgrids:Robustness for Unknown ZIP-Loads

In this letter we propose a new passivity-based control technique for Buck converter based DC microgrids comprising ZIP-loads, i.e., loads with the parallel combination of constant impedance (Z), current (I) and power (P). More precisely, we propose a novel passifying input and a storage function based on the mixed potential function introduced by Brayton and Moser, relaxing restrictive (sufficient) conditions on Z, P and the voltage reference, which are usually assumed to be satisfied in the literature. Consequently, we develop a new passivity-based controller that is robust with respect to uncertain ZIP-loads

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

Secondary frequency control in power systems with arbitrary communication delays

In this paper, we consider a Kron-reduced microgrid that consists solely of generator units. The frequency control we consider consists of primary droop control at the generator units, as well as distributed averaging integral (DAI) control that enforces both synchronization among the generator frequencies and optimal power dispatch. To any generating unit, a DAI controller is attached that exchanges information with neighboring nodes. In practice, this is subject to time delays, which impedes the stability analysis. By performing scattering transformation of the input and output variables at each end of the communication link between two DAI controllers, and trans -mitting the scattered variables instead of the controller output, stability of the nominal frequency is guaranteed. The time delays are not restricted to any bounds

Exponential convergence under distributed averaging integral frequency control

We investigate the performance and robustness of distributed averaging integral controllers used in the optimal frequency regulation of power networks. We construct a strict Lyapunov function that allows us to quantify the exponential convergence rate of the closed-loop system. As an application, we study the stability of the system in the presence of disruptions to the controllers' communication network, and investigate how the convergence rate is affected by these disruptions. (C) 2018 Elsevier Ltd. All rights reserved