Microgrids consist of multiple parallel-connected distributed generation (DG) units with coordinated control strategies, which are able to operate in both grid-connected and islanded modes Microgrids are attracting considerable attention since they can alleviate the stress of main transmission systems, reduce feeder losses, and improve system power quality When the islanded microgrids are concerned, it is important to maintain system stability and achieve load power sharing among the multiple parallel-connected DG units However, the poor active and reactive power sharing problems due to the influence of impedance mismatch of the DG feeders and the different ratings of the DG units are inevitable when the conventional droop control scheme is adopted Therefore, the adaptive/improved droop control, network-based control methods, and cost-based droop schemes are compared and summarized in this paper for active power sharing Moreover, nonlinear and unbalanced loads could further affect the reactive power sharing when regulating the active power, and it is difficult to share the reactive power accurately only by using the enhanced virtual impedance method Therefore, the hierarchical control strategies are utilized as supplements of the conventional droop controls and virtual impedance methods The improved hierarchical control approaches such as the algorithms based on graph theory, multi-agent system, the gain scheduling method, and predictive control have been proposed to achieve proper reactive power sharing for islanded microgrids and eliminate the effect of the communication delays on hierarchical control Finally, the future research trends on islanded microgrids are also discussed in this paper

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Review of Active and Reactive Power Sharing Strategies in Hierarchical Controlled Microgrids

Optimal operation of hybrid AC/DC microgrids under uncertainty of renewable energy resources: A comprehensive review

Microgrids (MGs) are seen today as the solution to the challenge of meeting the ever-increasing demand of energy which is clean. A variety of challenges need to be overcome for these MGs to be successful varying from stability, power quality to protection, and management. Since the various control strategies are actively being researched and emerging challenges still being addressed, there has been a lot of interest related to the aspect of testing. The ever-increasing complexity of control mechanisms invokes the need for safer, faster yet reliable testing methods, which do not compromise on the degree of detail and at the same time are economical and require the minimum testing time. Off-line simulation, real-time (RT) simulation, hardware in the loop simulation, RT emulation, hardware test beds, pilot plants, and other integrated approaches are the major methods for testing in MGs. This paper gives an overview of the testing techniques and discusses the trends in the various technologies employed for testing MGs. First, the various methods are briefly discussed and a comparison is provided. Next, the various research efforts carried out on the aspect of MG testing are categorized and presented one-by-one. Furthermore, new methodologies and emerging approaches for the testing have also been highlighted, and the challenges and future research avenues have been presented.

Real-Time Testing Approaches for Microgrids

This work presents an extensive review of hierarchical control strategies that provide effective and robust control for a DC microgrid. DC microgrid is an efficient, scalable and reliable solution for electrification in remote areas and needs a reliable control scheme such as hierarchical control. The hierarchical control strategy is divided into three layers namely primary, secondary and tertiary based on their functionality. In this study, different methods of primary control for current and voltage regulation, secondary control for error-correction in voltage and current, power sharing in a microgrid and microgrid clusters and tertiary control for power and energy management with a primary focus on minimal power loss and operational cost in a DC microgrid system are reviewed in-depth. Along with this, the advantages and limitations of various control structures like centralised, decentralised, distributed are discussed in this study. After a comparative study of all control strategies, the optimum control schemes from the author's point of view are also presented.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-rpg.2019.1136

Review of hierarchical control strategies for DC microgrid

A comprehensive survey on control strategies of distributed generation power systems under normal and abnormal conditions

In this study, a microgrid with two voltage source inverters (VSIs), operating in islanded or grid-connected mode is analysed. The active power control by droop coefficients and reference frequencies is demonstrated. In addition, the inductive and resistive droop strategies are compared by considering the complex line impedance. By measuring the line impedance in the prototype, it was found that, even for the low-voltage microgrid, the line impedance was not predominantly resistive, but complex. For this line impedance condition, an investigation to determine the best droop control law considering the partial derivatives. Observing the partial derivatives and the experimental results obtained it is demonstrated that inductive droop control is better to control the active power for complex line impedance. For a scenario with resistive unbalanced loads and different power sharing between the inverters, the proposed control, implemented in the a–b–c reference frame, was validated. A novel absolute damping factor is developed to improve the transient response and reduce the reactive power flow. The VSIs are connected to the grid without transformers to reduce the connection costs. Simulation and experimental results are presented to confirm the improvements achieved using the implemented control method.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-rpg.2014.0250

Power sharing control strategies for a three-phase microgrid in different operating condition with droop control and damping factor investigation

The power electronics interfaces of distributed generation units in microgrids usually employ droop control for defining their active and reactive power injections. The voltage and current control loops can be implemented in different ways, presenting different degrees of complexity and resulting performance, in terms of harmonic distortion and voltage regulation in steady-state and transient conditions. The microgrid load is usually assumed to be balanced, what is seldom the case in real systems. This paper discusses the application of two control methods in a − b − c reference frame for two three-phase microgrid units transformer-less connected and working in the islanded and grid-connected modes. The first is based in the common assumption that the system is balanced, while the second allows the droop based reference voltages of the three phases to be different but keeps the reference frequency the same and the reference angles shifted by 120°. The choice of resistive or inductive droop and the relevance of a damping factor loop is discussed. The performance of the two control methods with balanced and unbalanced loads as well as under transient and steady state conditions are compared based on experimental results obtained in a laboratory set-up.

Analysis of a microgrid with unbalanced load comparing three-phase and per-phase voltage droop control

This paper shows the experimental results of two three-phase microgrids working in islanded mode. Mi-crogrids were connected without isolation transformer and implemented in only one dSPACE hardware (DS1103), that is linked with simulation MATLAB/Simulink environment by Real-Time Interface. It is shown as two three-phase PWM pulses are generated and as all the necessary system control signals are measured using one DS1103 unit. The control system for each microgeneration unit is independent. It based in the droop control method and voltage and current loops with PI controller per phase.

Wanderson Ferreira de Souza

Papers

Power sharing control strategies for a three-phase microgrid in different operating condition with droop control and damping factor investigation

Analysis of a microgrid with unbalanced load comparing three-phase and per-phase voltage droop control

Microgrid units in the islanded operation mode implemented in the dSPACE DS1103