An Optimal Approach for Load-Frequency Control of Islanded Microgrids Based on Nonlinear Model

Parallel operation of inverter modules is the solution to increase the reliability, efficiency, and redundancy of inverters in microgrids. Load sharing among inverters in distributed generators (DGs) is a key issue. This study investigates the feasibility of power-sharing among parallel DGs using a dual control strategy in islanded mode of a microgrid. PQ control and droop control techniques are established to control the microgrid operation. P-f and Q-E droop control is used to attain real and reactive power sharing. The frequency variation caused by load change is an issue in droop control strategy whereas the tracking error of inverter power in PQ control is also a challenge. To address these issues, two DGs are interfaced with two parallel inverters in an islanded AC microgrid. PQ control is investigated for controlling the output real and reactive power of the DGs by assigning their references. The inverter under enhanced droop control implements power reallocation to restore the frequency among the distributed generators with predefined droop characteristics. A dual control strategy is proposed for the AC microgrid under islanded operation without communication link. Simulation studies are carried out using MATLAB/SIMULINK and the results show the validity and effective power-sharing performance of the system while maintaining a stable operation when the microgrid is in islanding mode.

As a kind of generation and distribution system with autonomous control, AC–DC hybrid microgrid plays a pivotal role in the transformation and development of modern power grids. To address the problems of microgrid system instability that occur in islanding mode, the study proposes a coordinated control strategy for hybrid AC/DC microgrid in islanding mode. First, the bidirectional AC/DC interconnection converter is modelled and parametrically designed; second, the control strategy of the interconnection converter in islanding mode is raised, including both active power control with double-loop control and seamless switching of operation modes with pre-synchronous control, and in the end, the effectiveness of the strategy is examined. The data display that under this strategy, the real and reactive power output gaps between the micro sources in the hybrid microgrid do not exceed 0.01 kW and 0.03 kVAR, respectively, and the current during islanding switching is more stable up to 32 A. This shows that the strategy effectively improves the shortcomings of the traditional regulation method, realizes the accurate distribution of real and reactive power, improves the power sharing between DC and AC subgrids, strengthens the stability and reliability of the hybrid microgrid, and helps to promote the technological progress of the hybrid microgrid and escorts the efficient and green development of the smart grid.

A hierarchical power management scheme is proposed in this paper for a typical DC Microgrid. Different from other Microgrids, the DC Microgrid can interface to the distribution system by Solid-State transformer (SST). The hierarchical power management strategy includes three control levels: 1) primary control for DC Microgrid to implement distributed operation 2) secondary control for the DC Microgrid bus voltage recovery to achieve seamless mode switch 3) tertiary control to manage the battery charge and discharge. The DC Microgrid can operate in islanding mode, including the individual control for distributed renewable energy source (DRER) and distributed energy storage device (DESD). In addition, the DC microgrid can operate in SST-enabled mode to interface to the distribution system. The DC Micorgrid can seamlessly switch between islanding mode and SST-enable mode. The consideration of state of charge (SOC) for battery is also involved into the tertiary control. To this end, a lab test-bed is constructed to verify the system performance. Lastly, several typical case studies are carried out and the experimental results verify the proposed power management strategy.

Coordinated Control Strategy for Microgrid in Grid-Connected and Islanded Operation

In this paper, an adaptive fuzzy output feedback control approach is investigated for a class of stochastic nonlinear strict-feedback systems without the requirement of states measurement. The stochastic nonlinear system addressed in this paper is assumed to possess unstructured uncertainties (unknown nonlinear functions) and, in the presence of unmodeled dynamics, dynamics disturbances. Fuzzy logic systems are used to approximate the unstructured uncertainties, and a fuzzy state observer is designed to estimate the unmeasured states. By combining the backstepping design technique with the stochastic small-gain approach, a new adaptive fuzzy output feedback control approach is developed. It is proved that the proposed control approach can guarantee that the closed-loop system is input-state-practically stability (ISpS) in probability, and the observer errors and the output of the system converge to a small neighborhood of the origin by appropriate choice of the design parameters. Simulation results are included to indicate that the proposed adaptive fuzzy control approach has a satisfactory control performance. In addition, the simulation comparisons with the previous methods show that the proposed adaptive fuzzy control approach has robustness to the dynamical uncertainties.

This article studied the load frequency control (LFC) of a multi-source microgrid with the presence of renewable energy sources. To maintain a sustainable power supply, the frequency of the system must be kept constant. A Proportional–Integral–Derivative (PID) controller is presented as a secondary controller to control the frequency of the microgrid in island mode, and the integral of squared time multiplied by error squared (ISTES) is used as a performance index. The use of the Craziness-Based Particle Swarm Optimization (CRPSO), which is an improved version of Particle Swarm Optimization (PSO), improves the convergence speed in optimizing the nonlinear problem of load and frequency controller design. The test microgrid is composed of the load and distributed generation units such as diesel generators, photovoltaics and wind turbines. The proposed controller provided the desired response to adjusting the microgrid frequency, achieving the final response after a short time and making it more stable and less oscillatory compared with the conventional system.

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Microgrid (MG) control in off-grid mode of operation is one of the main challenges in MGs. Under different loading conditions, voltage and frequency deviate from their nominal limits. The droop control method that applied with the current control and voltage control loops can restore the MG voltage and frequency to their nominal limits. However, it can’t regulate these parameters to their specified values. Additionally, the droop control method can’t do well when the system supplies either nonlinear or unbalanced loads. This paper proposes a control technique for adjusting the voltage and frequency of an MG in islanded mode under different loading conditions. The proposed method is based on applying the H-infinity ($$H\infty$$) robust control as a secondary controller with the droop control method to improve its performance. The proposed method is composed of four stages including; droop control loop, voltage control loop, current control loop and control loop for inductance–capacitance–inductance (LCL) filter and coupling circuit. $$H\infty$$ control has an internal model that can control the frequency and improve the system power quality by decreasing the disturbance frequencies. Moreover, it has two weighted parameters $$\xi$$ and $$\mu$$ that must be correctly adjusted to improve the stability of the system. For a proper adjusting of these parameters, the Harmony Search (HS) optimization method is applied. To prove the effectiveness and superiority of the proposed method, it is applied on a test system with three different loading scenarios using MATLAB/Simulink program. The results are compared with those of droop controller and they confirm that the proposed control method is more effective in controlling the microgrids under islanded mode operation.

Robust fuzzy model predictive control of discrete nonlinear systems is investigated in this paper. A recently developed Takagi–Sugeno (T–S) fuzzy approach which uses nonlinear local models is adopted to approximate the nonlinear systems. A critical issue that restricts the practical application of classical model predictive control is the online computational cost. For model predictive control of T–S fuzzy systems, the online computational burden is even worse. Especially for complex systems with severe nonlinearities, parametric uncertainties, and disturbances, existing model predictive control of T–S fuzzy systems usually leads to a very conservative solution or even no solution in some occasions. However, more relaxed results can be achieved by the proposed fuzzy model predictive control approach which adopts T–S systems with nonlinear local models. Another advantage is that online computational cost of the optimization problem through solving matrix inequalities can be significantly reduced at the same time. Simulations on a numerical example and a two-tank system are presented to verify the effectiveness and advantages of the proposed method. Comparisons among several T–S fuzzy approaches are illustrated and show that the best settling time is achieved via the proposed method.

The distributed generation (DG) should supply continuous power to local critical load, even in the condition of utility outage. This paper proposes a novel seamless transfer control strategy, which combines indirect current control and droop control. A microgrid structure of multi-masters and multi-slaves is also proposed, which based on the master-slave control and droop control. The master DGs are controlled by the novel control strategy and the slave DGs are controlled as conventional current sources based on PQ control. The advantages of this proposed strategy and microgrid structure are as follows. Firstly, the quality of critical load voltage can be maintained when utility outage occurs, Secondly, the reliability of microgrid in islanding mode can be improved because of the existence of multiple master DGs. The simulation results are provided to verify the effectiveness of this proposed control method.

In the case of sensitive and critical load, the distributed generation (DG) should supply continuous power, even in the condition of utility outage. This paper proposes a novel seamless transfer control strategy, which based on special master and slave DGs. The master DG unit is controlled by a novel control strategy, which combines indirect current control and droop control. A part of slave DGs are controlled as conventional current sources based on PQ control, which are named as Slave-Type1. Another part of slave DGs are controlled based on PQ control and droop control, which are named as Slave-Type2. When the islanding is confirmed, the control block of Slave-Type2 DGs will transfer from PQ control to droop control. The advantages of this proposed strategy and microgrid structure are as follows. Firstly, the quality of critical load voltage can be maintained when utility outage occurs, Secondly, the reliability of microgrid in islanding mode can be improved because of the existence of multiple master DGs. The simulation results are provided to verify the effectiveness of this proposed control method.

Integration of sub-gradient based coordinate for multiple renewable generators in microgrid

Pre-synchronization is necessary to reconnect microgrid to utility grid in order to avoid inrush current. The droop control is widely adopted for the operation of microgrid in islanded mode as it ensures power sharing in a decentralized way without critical communication, but during pre-synchronization it needs communication with every inverter to avoid large power sharing error, while in master-slave control scheme only the master inverter needs communication. This paper proposes a new pre-synchronization strategy for three-phase microgrid, in which the microgrid transfers seamlessly from droop control to temporary master-slave scheme after the start of the pre-synchronization, so the need of communication is greatly reduced and the power sharing during the pre-synchronization is ensured. An islanding detection method is incorporated, which facilitates the mode transfers. Besides, simple synchronization criteria are proposed which take into consideration differences of amplitude, phase and frequency without using PLL or arctangent function. Simulation and experimental results are provided to show the effectiveness of the proposed method.

This paper investigates microgrid systems characterized by the coexistence of discrete events and continuous events, a typical hybrid system. By selecting the charging and discharging processes of the energy storage unit as logical variables, a mixed logical dynamic (MLD) model for the microgrid in islanded mode is established. Based on this model, model predictive control (MPC) theory is employed to optimize the energy management strategy, aiming to stabilize the DC bus voltage of the photovoltaic (PV) unit and minimize the switching frequency of the energy storage unit’s charging and discharging processes during system operation.

This paper focuses on the active and reactive power balance problem of a new breed of microgrids called multifrequency microgrids (MFMG). MFMG has numerous advantages over traditional microgrids as it has several operational frequencies on the multifrequency bus. Orthogonal power flow theory and frequency selectivity criteria are the two main concepts behind the formation of MFMG. Due to frequency selectivity criteria at the load side, different active and reactive power imbalance cases appear in this microgrid. In this paper, a source side control technique is introduced to achieve proper power balance between different frequency sources and loads. A new three phase DC/MF (multifrequency) converter is proposed as the building block of three phase MFMG and simulated as grid forming, grid feeding, and load side converter.