A power consensus algorithm for DC microgrids

A novel power consensus algorithm for DC microgrids is proposed and analyzed. DC microgrids are networks composed of DC sources, loads, and interconnecting lines. They are represented by differential-algebraic equations connected over an undirected weighted graph that models the electrical circuit. A second graph represents the communication network over which the source nodes exchange information about the instantaneous powers, which is used to adjust the injected current accordingly. This give rise to a nonlinear consensus-like system of differential-algebraic equations that is analyzed via Lyapunov functions inspired by the physics of the system. We establish convergence to the set of equilibria consisting of weighted consensus power vectors as well as preservation of the weighted geometric mean of the source voltages. The results apply to networks with constant impedance, constant current and constant power loads.Comment: Abridged version submitted to the 20th IFAC World Congress, Toulouse, Franc

Multiplex PI-Control for Consensus in Networks of Heterogeneous Linear Agents

In this paper, we propose a multiplex proportional-integral approach, for solving consensus problems in networks of heterogeneous nodes dynamics affected by constant disturbances. The proportional and integral actions are deployed on two different layers across the network, each with its own topology. Sufficient conditions for convergence are derived that depend upon the structure of the network, the parameters characterizing the control layers and the node dynamics. The effectiveness of the theoretical results is illustrated using a power network model as a representative example.Comment: 13 pages, 6 Figures, Preprint submitted to Automatic

Multilayer proportional-integral consensus of heterogeneous multi-agent systems

A distributed proportional-integral multilayer strategy is proposed, to achieve consensus in networks of heterogeneous first-order linear systems. The closed-loop network can be seen as an instance of so-called multiplex networks currently studied in network science. The strategy is able to guarantee consensus, even in the presence of constant disturbances and heterogeneous node dynamics. Contrary to previous approaches in the literature, the proportional and integral actions are deployed here on two different layers across the network, each with its own topology. Explicit expressions for the consensus values are obtained together with sufficient conditions guaranteeing convergence. The effectiveness of the theoretical results are illustrated via numerical simulations using a power network example

Multilayer proportional-integral consensus of heterogeneous multi-agent systems

A distributed proportional-integral multilayer strategy is proposed, to achieve consensus in networks of heterogeneous first-order linear systems. The closed-loop network can be seen as an instance of so-called multiplex networks currently studied in network science. The strategy is able to guarantee consensus, even in the presence of constant disturbances and heterogeneous node dynamics. Contrary to previous approaches in the literature, the proportional and integral actions are deployed here on two different layers across the network, each with its own topology. Explicit expressions for the consensus values are obtained together with sufficient conditions guaranteeing convergence. The effectiveness of the theoretical results are illustrated via numerical simulations using a power network example

Control strategy for direct voltage and frequency stabilityenhancement in HVAC/HVDC grids

Direct voltage fluctuations due to the presence of relatively large DC reactors (as an essen-tial part of HVDC breakers), lack of inertia, and unwanted frequency fluctuations in theAC side of HVDC grids, have major consequences on the stability of HVAC/HVDC grids.The use of the DC Power System Stabilizer (DC-PSS) can damp and eliminate voltageoscillations caused by the presence of the DC reactors. However, DC-PSS cannot addressthe issues of inertia and unwanted frequency fluctuations. A method to improve inertiais proposed here that can operate well with the droop controller, and DC-PSS does notinterfere with power-sharing and does not interact with any of these elements. Since thepresence of a droop controller in HVAC/HVDC grids associates with power and directvoltage, the method proposed here can improve direct voltage fluctuations by eliminatingsevere power peaks. Moreover, this method does not change the voltage level of the entiresystem, so there is no need to change the set-points of controllers. In addition, all param-eters of the controllers are tuned by an intelligent algorithm, and the Participation factor(PF) scheme is used to find the proper placement of the proposed controller

Distributed Controllers for Multi-Terminal HVDC Transmission Systems

High-voltage direct current (HVDC) is an increasingly commonly used technology for long-distance electric power transmission, mainly due to its low resistive losses. In this paper the voltage-droop method (VDM) is reviewed, and three novel distributed controllers for multi-terminal HVDC (MTDC) transmission systems are proposed. Sufficient conditions for when the proposed controllers render the equilibrium of the closed-loop system asymptotically stable are provided. These conditions give insight into suitable controller architecture, e.g., that the communication graph should be identical with the graph of the MTDC system, including edge weights. Provided that the equilibria of the closed-loop systems are asymptotically stable, it is shown that the voltages asymptotically converge to within predefined bounds. Furthermore, a quadratic cost of the injected currents is asymptotically minimized. The proposed controllers are evaluated on a four-bus MTDC system.Comment: arXiv admin note: substantial text overlap with arXiv:1406.5839, arXiv:1311.514

Cooperative frequency control with a multi-terminal high-voltage DC network

We consider frequency control in power systems made of several non-synchronous AC areas connected by a multi-terminal high-voltage direct current (HVDC) grid. We propose two HVDC control schemes to make the areas collectively react to power imbalances, so that individual areas can schedule smaller power reserves. The first scheme modifies the power injected by each area into the DC grid as a function of frequency deviations of neighboring AC areas. The second scheme changes the DC voltage of each converter as a function of its own area's frequency only, relying on the physical network to obtain a collective reaction. For both schemes, we prove convergence of the closed-loop system with heterogeneous AC areas

Enhancement of Grid Dynamic Performance using VSC-based Multi-terminal HVDC Systems in Multilevel Modular Converter Topology

Modern power systems have expanded, both in size and complexity. More challenges will emerge with the integration of an increasing number of renewable generation sources to the existing power systems, which are driven to operate closer to their technical limits under pressure from economic objectives in deregulated markets and environment impacts. The transmission systems must be strengthened to transmit a larger amount of power from the remote renewable generation sources to load centers while ensuring a higher degree of flexibility and stability in operating the power systems. The Voltage-Source Converter (VSC) in Modular-Multilevel Converter (MMC) topology for High-Voltage Direct Current (HVDC) application has been recently developed and has become an attractive solution to address the new challenges to the existing power system. This thesis deals with the utilizations of MMC-VSC-HVDC systems in Multi-Terminal Direct Current (MTDC) configuration to enhance the dynamic performance of AC power systems. In this framework, several supplementary controllers are integrated into the standard VSC station controller to exploit the distinct advantages in the areas of controllability and flexibility of the MMC-VSC-MTDC systems. The integrated supplementary controllers are developed to address most of the dynamic stability aspects in the power systems: small-signal stability (SSS), transient stability including frequency and voltage stabilities. The main contributions of this work include: the development of a generic RMS model of the MMC-VSC-MTDC system and its corresponding linearization; the development of a novel frequency controller which enables the MMC-VSC-MTDC system to effectively support the power flow in primary frequency control; the investigation of several major factors influencing the contribution of the VSC-MTDC system to the damping of system oscillations and the demonstration of the capability of supporting system voltage during symmetrical grid faults. The thesis also proposed the design as well as appropriate methodologies for selecting parameters of the supplementary controllers. The controllers are firstly investigated in individual studies under consideration of several influence factors to explore their main features. Furthermore, possible interactions between the investigated supplementary controllers which may influence their effectiveness’s are identified. Based on the investigation, proper counter-measures are proposed to mitigate the interactions

Advanced control of multi-microgrids for grid integration

Thanks to tremendous growing interest, the significant number of microgrids form a system called Multi-Microgrid, where multiple microgrids are interconnected to support local loads and exchange power to or from grid. Industry demands for advanced control and optimal coordination among microgrids with consideration of high penetration of renewable energy and complex system architectures. This thesis focuses on different key aspects of power systems and microgrids to develop novel approaches targeting the problem. Firstly, different topologies of microgrids are studied from the literature review and most popular system architectures are considered in the study for proposing advanced control techniques. Distributed control systems with nested formation in the microgrids are proposed for improved power sharing strategy. The distributed control is designed to achieve self-healing capability of multi-microgrids during any contingency event. Local controllers of the inverters in each microgrid are interconnected through the nested formation. A nested optimization algorithm is designed to achieve power exchange between different microgrids. Multi-terminal HVDC network based multi-microgrids have been proposed for advanced control strategy due to its widespread application in power system. Adaptive droop control has been proposed based on consensus algorithm and matrix-based solutions to provide frequency support and power sharing between AC microgrids through the HVDC network. The proposed adaptive droop algorithm is featured to maintain frequency and voltage during contingency events and ensure efficient power sharing. Distributed hierarchical control system is proposed as well for multi-microgrids with nested formation-based optimization techniques to ensure proper power sharing in four-level based multi-microgrid topologies. The algorithm features energy management within the multi-microgrid through virtual controllers of primary and secondary frequency control. In addition, to the energy management issue, low system strength of grid has been considered to offer a wide range of areas under the advanced control of multi-microgrid. In that regard, single machine infinity bus model has been considered to implement control of grid forming inverters for integration with weak grid. Novel grid resynchronization and virtual synchronous generator control has been proposed to achieve multi-microgrids integration with weak grids. Then, various simulation studies are performed to test the effectiveness of the proposed controls. The time domain simulations are performed on EMT power system tool PSCAD under different operating conditions, such as loading variations, N-1 contingency events, grid frequency change disturbance, islanding conditions etc. In addition to the time domain simulation studies, stability analysis of the proposed control has been carried out. In the stability analysis, pole-zero map, Nyquist plots and Bode plots have been demonstrated to analyse the stable conditions of the proposed control. The optimization algorithms results are also included in the simulation studies to reflect the performance of the control. Finally, the advanced control solutions outcomes through time domain and stability results are compared with conventional control. It has been demonstrated that all proposed solutions perform better than conventional approaches and reflect significant improvement on the multi-microgrids. Furthermore, industry standards have been considered in the weak grid integration study and case studies are carried out based on power industry practices, including industry regulatory grid codes according to the power industry in Australia. The results indicate that the proposed controls are able to satisfy industry grid codes