Distributed Event-Triggered Secondary Recovery Control for Islanded Microgrids

This paper presents primary and secondary control of islanded microgrid. In conventional primary droop control, there is a problem of poor voltage regulation of microgrid. The drawback of conventional primary droop control is improved by voltage shifting based primary control. Voltage shifting based primary control is not completely satisfied by the voltage regulation of microgrid. Also, there is current sharing problem in both types of primary control. So, in secondary control, both current sharing and voltage regulation are achieved by slope adjusting and voltage shifting simultaneously. In this secondary control method, average value of current, voltage, and droop resistance of two neighboring converters is calculated then controlled by additional layer of distributed secondary control over both primary controls. By adjusting droop coefficient of different converters making same value of output impedance of converter, current sharing and voltage regulation are achieved. Also, in transient state, current sharing is achieved by using this secondary control technique. The performance of proposed microgrid is verified by MATLAB simulation.

In islanded microgrids, secondary frequency control is often employed to compensate for frequency deviations caused by droop control, thereby enhancing the frequency security of the system. However, distributed generators (DGs) such as renewable energy sources and energy storage systems in microgrids have inherent power output constraints. Ignoring these constraints may lead to the system frequency failing to meet expectations or even exceeding safety limits, posing security risks. To address this issue, this paper proposes a distributed secondary frequency control strategy that considers power constraints. First, a model of the islanded microgrid system is established. Second, a distributed secondary frequency controller incorporating power constraints is designed, and the stability of the proposed control strategy is analyzed using Lyapunov theory. Finally, a simulation model is established in Simulink to verify the feasibility of the proposed control strategy through simulation.

In today’s world, where the grid structure becomes smarter day by day, microgrid, one of the fundamental methods for the integration of renewable energy, are taking firm steps to be building blocks of the modern electrical grid. Along with its advantages, the microgrid brings extra challenges which attracts the attention of many researchers in these days. This paper focuses on addressing two of the main challenges: voltage stability and reactive power sharing in islanded mode. Contents of this paper include: first, elaboration of general hierarchical control structures of inverter-based distributed generators (DGs), second, review of primary droop control methods and analysis of their drawbacks, third, review and comparison of current secondary reactive power control methods to find the gap of more practical secondary control method. In the paper, a centralized secondary controller based on intermittence level, effective-impedance to load and severity of voltage deviation is proposed to overcome the disadvantage of current secondary reactive power control methods. The proposed method takes practical aspects into consideration and prioritizes DGs in reactive power balancing and voltage stability. Performance of proposed method is illustrated by a simulation study.

The hierarchical control strategy enables seamless integration of power electronics-based distributed generators (DGs) into microgrids (MGs). This work presents a distributed coordinated secondary voltage and frequency control strategy of islanded MGs, utilizing distributed model predictive control (DMPC) approach. The Proposed approach is entirely distributed in nature, with plug and play features and requires minimal communication network with information exchange between immediate neighbors. This technique facilitates efficient information exchange and obviates the need for centralized control. Droop control is the primary controller for all the DGs, which leads to the deviations in DG voltage and operating frequency from standard nominal values. Secondary control enables the restoration of DG voltage and frequency to their nominal values. Input–output feedback linearization (IOFL) is used to transform nonlinear dynamics of inverter-interfaced DGs into first-order linear dynamics between input and output. Compared to some published works, the single-integrator dynamics eliminates the need to compute the derivative of the output voltage. Furthermore, DMPC has been used for all DG units as secondary controller, to achieve voltage and frequency regulation. This approach takes advantage of distributed architecture with optimal control actions and enhances reliability and flexibility of MG operations. Moreover, the proposed approach ensures that voltage and frequency of overall system are restored to its reference value while maintaining accuracy in real power sharing. Effectiveness of proposed control scheme is validated with simulations conducted on islanded MG test system using MATLAB/Simulink toolbox.

In order to achieve the flexible and efficient utilization of distributed energy resources, microgrids (MGs) can enhance the self-healing capability of distribution systems. Conventional primary droop control in microgrids exhibits deviations in voltage and frequency and lacks research on voltage–frequency control during network reconfiguration. Therefore, this paper investigates the control strategy of secondary control for voltage and frequency during the process of reconstructing distribution networks to operate in the form of microgrids after faults. Firstly, the mathematical model of three-phase voltage-source inverters with droop control is analyzed. Then, inspired by the generator’s secondary frequency control strategy, an improved secondary control method based on droop control is proposed, and the effectiveness of the improved droop control strategy is verified. Finally, simulation examples of distribution networks with various dynamic boundary topology changes are designed to simulate a relatively stable control result, validating the proposed strategy.

This paper presents a distributed voltage unbalance compensation method for an islanded microgrid (MG) system. By using hierarchical control concept, we design a distributed compensation scheme in the secondary control layer, while the traditional inverter control methods including voltage and current PR control, droop control, virtual impedance are implemented in the primary layer. Different from most existing centralized compensation methods, no specified individual centralized secondary controller exists. We decompose the traditional centralized controller into several local secondary controllers. A novel distributed PI controller with negative sequence current feedback is designed in each secondary controller. By allowing them to exchange information with their neighboring controllers respectively, the unbalanced voltage in the sensitive load bus (SLB) can be compensated by all the distributed generators (DGs) cooperatively. In addition, the compensation effort of each DG is designed to be proportional to their power ratings respectively. An islanded microgrid system consisting of 4 DGs is built in MATLAB to validate the proposed method.

Lyapunov-based distributed secondary frequency and voltage control for distributed energy resources in islanded microgrids with expected dynamic performance improvement

A voltage-shifting-based state-of-charge balancing control for distributed energy storage systems in islanded DC microgrids

A new robust decentralized approach based on adaptive fuzzy backstepping method for microgrid secondary voltage control

In an islanded DC microgrid composed of droop-controlled parallel distributed generators, the DC bus voltage endures derivation as load changes. Several secondary voltage control (SVC) methods have been proposed to compensate for bus voltage derivation, divided into centralized and distributed SVC methods. However, the existing SVC methods have many limitations, including low reliability, high system complexity, poor bus voltage regulation, and sometimes even weakened load sharing performance under primary droop control. In this paper, a small-AC-signal (SACS)-injection-based SVC method in a communication-less manner is proposed to overcome these deficiencies. Firstly, the proposed SACS-injection-based SVC method is explained in detail. Then, comprehensive design procedures for essential control parameters are provided via the steady-state and dynamic analysis of the system. Finally, experiment and simulation results demonstrate the effectiveness and advantages of the proposed SACS-injection-based SVC method.

This paper describes a control method for distributed generation (DG) units to implement active power sharing and frequency recovery simultaneously in an islanded microgrid. Conventional active power–frequency ( P – f ) droop control is used for the DG controller, and the frequency deviation is recovered by the DG itself via self-frequency recovery control, without requiring secondary frequency control. Because the electrical distance (impedance) from each DG unit to a point where the load demand changes differs among DG units, the instantaneous frequency deviations may differ between DG units. These differences are fed into the integrators of the self-frequency recovery control and may result in errors in active power sharing. To solve this problem and share active power more accurately, a compensation control method is developed for active power sharing, which considers the droop coefficients of each of the DG units. Simulation results show that the proposed control method is effective.

Because the output power of the loads and distributed generation(DG) units is uncertainty in islanded microgrid, the traditional power-frequency(p-f) droop control will lead the frequency to exceed the limit. Aiming at this problem, an improved secondary frequency control strategy without communication is proposed. According to the one-to-one correspondence between the power and the frequency of the droop curve, the power is taken as the control variable to dynamically adjust the droop coefficient. Then small signal model was set up to verify the stability of the system by the static working point, and analyzing the stability of the system when the parameters change by the root-locus method. Finally, the simulation and experiment results verify the feasibility and effectiveness of the proposed strategy.

Performance analysis is challenging in modern power distribution networks due to the increasing penetration of distributed energy resources (DERs) interfaced by voltage source converters (VSCs). Parameter uncertainty, black-box models of the components, and low inertia are some of the issues that must be addressed. The latter can represent high vulnerabilities to sudden load changes in a microgrid (MG). Additionally, the islanded operation represents a challenge for inverter-based (DERs), where secondary control must regulate the microgrid frequency and voltage to its nominal values. When a secondary control strategy is implemented, it is necessary to validate its performance under several conditions. Most existing research papers focus on the microgrid’s small-signal and transient stability. At the same time, little has been done on the influence of the load model on voltage stability. This type of analysis is required to ensure an adequate transition between the grid-connected and stand-alone modes. This paper provides a voltage stability analysis of a microgrid, considering secondary control based on receding horizon and frequency-dependent load models. Simulation results demonstrate the robust performance of the secondary control and validate the importance of considering or adapting voltage stability indices to ensure adequate microgrid performance.

Chapter 2 - Distributed voltage restoration and power allocation control in islanded DC microgrids

This paper presents a robust distributed secondary control (DSC) scheme for inverter-based microgrids (MGs) in a distribution sparse network with uncertain communication links. By using the iterative learning mechanics, two discrete-time DSC controllers are designed, which enable all the distributed energy resources (DERs) in an MG to achieve the voltage/frequency restoration and active power sharing accuracy, respectively. In special, the secondary control inputs are merely updated at the end of each round of iteration, and thus, each DER only needs to share information with its neighbors intermittently in a low-bandwidth communication manner. This way, the communication costs are greatly reduced, and some sufficient conditions on the system stability and robustness to the uncertainties are also derived by using the tools of Lyapunov stability theory, algebraic graph theory, and matrix inequality theory. The proposed controllers are implemented on local DERs, and thus, no central controller is required. Moreover, the desired control objective can also be guaranteed even if all DERs are subject to internal uncertainties and external noises including initial voltage and/or frequency resetting errors and measurement disturbances, which then improves the system reliability and robustness. The effectiveness of the proposed DSC scheme is verified by the simulation of an islanded MG in MATLAB/SimPowerSystems.