A Control Strategy for Islanded Microgrids With DC-Link Voltage Control (original) (raw)
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IEEE Transactions on Power Delivery, 2012
For islanded microgrids, droop-based control methods are often used to achieve a reliable energy supply. However, in case of resistive microgrids, these control strategies can be rather different to what conventional grid control is accustomed to. Therefore in this paper, the theoretical analogy between conventional grid control by means of synchronous generators (SGs) and the control of converter-interfaced distributed generation (CIDG) units in microgrids is studied. The conventional grid control is based on the frequency as a global parameter showing differences between mechanical power and ac power. The SGs act on changes of frequency through their P /f droop controller, without inter-unit communication. For CIDG units, a difference between dc-side power and ac-side power is visible in the dc-link voltage of each unit. Opposed to grid frequency, this is not a global parameter. Thus, in order to make a theoretical analogy, a global measure of the dc-link voltages is required. A control strategy based on this global voltage is presented and the analogy with the conventional grid control is studied, with the emphasis on the need for interunit communication to achieve this analogy. A known control strategy in resistive microgrids, called the voltage-based droop control for CIDG units, approximates this analogy closely, but avoids inter-unit communication. Therefore, this control strategy is straightforward for implementation as it is close to what control engineers are used to. Also, it has some specific advantages for the integration of renewables in the network.
Journal of Operation and Automation in Power Engineering, 2023
The widespread adoption of microgrids in electric power systems has brought numerous advantages such as decentralized control, reliability, cost-effectiveness, and environmental benefits. However, one of the most critical challenges faced by islanded microgrids is ensuring frequency and voltage stability. This paper addresses these stability issues that arise when microgrids operate independently, disconnected from the main network through the point of common coupling (PCC). These microgrids rely on renewable resources like photovoltaic (PV) systems, wind turbines, and energy storage systems, which often require DC to AC conversion through inverters to simulate synchronous generators. To overcome the frequency and voltage stability challenges, this research utilizes the droop control technique to regulate the active and reactive power of distribution generators (DGs). The droop control technique is implemented and simulated using MATLAB software, specifically employing a multi-DC bus-based inverter. The simulation results demonstrate that the DGs successfully supply the required total power to meet load demands while maintaining frequency and voltage stability. Through the droop control technique, active and reactive power sharing is achieved, ensuring stability at nominal values. The DGs can effectively maintain a constant power profile at desired values, even in the presence of static and dynamic loads.
An overview of control approaches of inverter-based microgrids in islanding mode of operation
Renewable and Sustainable Energy Reviews, 2017
Increased penetration of distributed generation (DG) into the power systems has created fundamental challenges from the viewpoints of control and reliable operation of systems. Microgrids (an aggregation of DG units, loads, and storage elements) with proper control strategies can be a good solution for removing or facilitating these challenges. The introduction of inverter-based microgrid in a distribution network has facilitated the utilization of renewable energy resources, distributed generations, and storage resources; furthermore, it has improved power quality and reduced losses, thus improving the efficiency and the reliability of the system. As most DG units are connected via a power electronic interface to the grid, special control strategies have been developed for inverter interfaces of DG units in islanded microgrids. This paper presents an overview of advanced control methods for microgrids, especially the islanded and inverter-based. Moreover, various control methods are compared and categorized in terms of their respective features. It also summarizes microgrid control objectives with their most problematic solutions as well as their potential advantages and/or disadvantages. Finally, some suggestions are put forward for the future research.
Decentralized Control of Low-Voltage Islanded DC Microgrid Using Power Management Strategies
2017
This paper intended to control a DC microgrid in islanded operation mode using decentralized power management strategies. The DC microgrid under study included a wind turbine generator (WTG), photovoltaic (PV), battery energy storage system (BESS) and dc constant power load. According to the newly proposed strategy, each of distributed generation sources of energy and battery energy storage system can be deployed independently within any controlled microgrid through the droop method. Proposed I/V characteristic curve could be regulated locally and in real-time based on the available power of DGs and the battery state of charge (SOC), to synchronize the module performances independently and establish the power balance in the DC microgrid. Proposed strategy for the battery enables the system to supply independently the power required for the load demand when the DGs are not capable of supplying the required power to the load. This can maintain the common bus voltage within the allowab...
Control Strategies of a DC Microgrid for Grid Connected and Islanded Operations
—This paper proposes an algorithm for coordinated control of the distributed generators integrated to a dc micro-grid (DCMG), in islanded and grid connected modes of operation. The proposed DCMG connects various types of nonconventional energy sources, storage system to the dc, and three-phase, as well as single-phase ac loads. A control strategy for three-phase voltage source inverter to integrate the three-phase load, as well as utility grid into the DCMG, under various operating scenarios, has also been proposed. The proposed control strategy uses a combination of the feedback and feed-forward control loops. Dual proportional integral controllers for ac voltage regulation and inner current control have been suggested in two rotating direct-and quadrature-axis synchronous reference frames for controlling the respective positive and negative sequence components. Simulations are carried out to verify the robustness of the proposed algorithm and control strategy under different operating conditions including fault scenario and its effectiveness in maintaining the dc voltage of the microgrid.
Control strategies of DC-bus voltage in islanded operation of microgrid
International Conference on Electric Utility Deregulation and Restructuring and Power Technologies, 2011
This paper focuses on the energy management system and stability of DC bus in both grid-connected and islanded operations in microgrid system. The microgrid system consists of wind turbines, photovoltaic panels, batteries and super-capacitors, also includes both AC and DC zones. Voltage of DC bus must be kept stable especially in islanded operation. In grid-connected operation voltage of DC bus is controlled by inverter. Real power from renewable energy generations and storage system can be transferred to AC zone through DC bus. In islanded operation, inverter must be controlled to keep magnitude and frequency of AC bus stable, so storage system is used to regulate voltage of DC bus. Simulation results in the paper show that voltage of DC bus can be kept steady and power can be kept balance with the strategy in microgrid system.
Transition From Islanded to Grid-Connected Mode of Microgrids With Voltage-Based Droop Control
Microgrids are able to provide a coordinated integration of the increasing share of distributed generation (DG) units in the network. The primary control of the DG units is generally performed by droop-based control algorithms that avoid communication. The voltage-based droop (VBD) control is developed for islanded low-voltage microgrids with a high share of renewable energy sources. With VBD control, both dispatchable and less-dispatchable units will contribute in the power sharing and balancing. The priority for power changes is automatically set dependent on the terminal voltages. In this way, the renewables change their output power in more extreme voltage conditions compared to the dispatchable units, hence, only when necessary for the reliability of the network. This facilitates the integration of renewable units and improves the reliability of the network. This paper focusses on modifying the VBD control strategy to enable a smooth transition between the islanded and the grid-connected mode of the microgrid. The VBD control can operate in both modes. Therefore, for islanding, no specific measures are required. To reconnect the microgrid to the utility network, the modified VBD control synchronizes the voltage of a specified DG unit with the utility voltage. It is shown that this synchronization procedure significantly limits the switching transient and enables a smooth mode transfer.
Advanced Islanded-Mode Control of Microgrids
2011
This thesis is focused on modeling, control, stability, and power management of electronically interfaced Distributed Energy Resource (DER) units for microgrids. Voltage amplitude and frequency regulation in an islanded microgrid is one of the main control requirements. To that end, first a mathematical model is developed for an islanded DER system and then, based on the developed model, amplitude and frequency control schemes are proposed for (i) balanced and linear loads and (ii) unbalanced and nonlinear loads. The proposed control strategy for unbalanced and nonlinear loads, utilizes repetitive control scheme to reject the effects of unbalanced and/or distorted load currents. Moreover, a new approach is proposed to maintain the effectiveness of the repetitive control under variable-frequency operational scenarios. The thesis also presents an adaptive feedforward compensation strategy to enhance the stability and robustness of the droop-controlled microgrids to droop coefficients and network uncertainties. The proposed feedforward strategy preserves the steady-state characteristics that the conventional droop control strategy exhibits and, therefore, does not compromise the steadystate power shares of the DER systems or the voltage/frequency regulation of the microgrid. Finally, a unified control strategy is proposed to enable islanded and grid-connected operation of DER systems, with no need to detect the microgrid mode of operation or to switch between different controllers, simplifying the control of the host microgrid. The effectiveness of the proposed control strategies are demonstrated through time-domain simulation studies conducted in the PSCAD/EMTDC software environment.
Improved Droop Controller for Distributed Generation Inverters in Islanded AC Microgrids
Periodica polytechnica. Electrical engineering and computer science /, 2024
Stability in island microgrids is crucial for efficient power distribution among distributed generation (DG) inverters. Conventional droop control, while effective in power sharing, poses challenges with voltage stability due to frequency and voltage deviations resulting from changing load power. Such deviations can lead to system instability, impacting power flows within each inverter. Therefore, this paper introduces a proposed droop control approach that effectively tackles the issues of frequency and voltage deviation, aiming to restore them to their rated values and significantly enhance transient response in power flows among inverters. The novel method incorporates integrating controllers for frequency and voltage, coupled with the utilization of virtual impedances. These virtual impedances, comprising virtual positive/negative-sequence impedance (VPI/VNI) loops at the fundamental frequency and a virtual harmonic impedance (VHI) loop at harmonic frequencies, play a crucial role in overcoming mismatched line impedance conditions, ultimately improving overall system performance. Simulation results demonstrate the effectiveness and outstanding performance of inverters operating in parallel within an island AC microgrid. The proposed approach ensures stable voltage and frequency levels in all operational states, regardless of varying load conditions.
Microgrid (MG) is a relatively new concept for the integration of distributed generation (DG) along with the loads in a distribution system. Islanded microgrid can be considered as a weak grid that has less inertia compared with the conventional power system. This reality makes the microgrid vulnerable to contingencies. Towards a flexible, safe and secure operation of an islanded MG, researchers have introduced a hierarchical control structure comprising tertiary, secondary and primary control. The primary control plays an important role in maintaining the voltage and frequency stability by sharing the loads among the DGs. This paper reviews and categorizes various primary control methods that have been introduced to control the voltage and frequency of inverter-based microgrids. Moreover, the reviewed methods in terms of their potential advantages and disadvantages are compared. Finally, the future trends are presented. (J.P.S. Catalão).