Parallel Inverters In Microgrid Research Articles

Parallel Operation Strategy of Inverters Based on an Improved Adaptive Droop Control and Equivalent Input Disturbance Approach

The operation of parallel inverters in microgrids is an important way to expand system capacity, but there are problems of circulating current fluctuations and power sharing errors in parallel inverters’ operation. In this paper, a parallel operation strategy for inverters based on improved adaptive droop control and Equivalent Input Disturbance (EID) is proposed. Firstly, the model and control topology of parallel inverters are presented. Secondly, in order to solve the power sharing error caused by impedance difference and the voltage drop caused by virtual impedance, a reference voltage generator based on improved adaptive droop control is proposed. Then, considering the circulating current fluctuation as a disturbance, a voltage controller based on an equivalent input disturbance approach is proposed to reduce the disturbance. Finally, simulation in MATLAB/Simulink is carried out to verify the effectiveness of the parallel operation strategy.

Frequency and Voltage Control Techniques through Inverter-Interfaced Distributed Energy Resources in Microgrids: A Review

Microgrids (MG) are small-scale electric grids with local voltage control and power management systems to facilitate the high penetration and grid integration of renewable energy resources (RES). The distributed generation units (DGs), including RESs, are connected to (micro) grids through power electronics-based inverters. Therefore, new paradigms are required for voltage and frequency regulation by inverter-interfaced DGs (IIDGs). Notably, employing effective voltage and frequency regulation methods for establishing power-sharing among parallel inverters in MGs is the most critical issue. This paper provides a comprehensive study, comparison, and classification of control methods including communication-based, decentralized, and construction and compensation control techniques. The development of inverter-dominated MGs has caused limitations in employing classical control techniques due to their defective performance in handling non-linear models of IIDGs. To this end, this article reviews and illustrates advanced controllers that can deal with the challenges that are created due to the uncertain and arbitrary impedance characteristics of IIDGs in dynamics/transients.

Operation of Parallel Inverters in Microgrid Using New Adaptive PI Controllers Based on Least Mean Fourth Technique

This paper shows the operation of the microgrid using a new adaptive PI controller based operational (control) scheme. The core of the proposed control scheme is the suggested adaptive PI controller. The parameters of the PI controller are adaptively tuned using a variable step-size least mean fourth algorithm with no need for any system model to operate this adaptive controller. The main merit of the proposed scheme is that it stabilizes the magnitude and frequency of the voltage at any loading condition such as variable balanced loads, variable unbalanced loads, and nonlinear loads. The proposed scheme has a simple structure and accurate performance. In addition, the structure of proposed scheme provides a seamless transition toward any loss or reconnection of any inverter in the microgrid. Furthermore, the suggested operational scheme is flexible enough to enable the microgrid to be operative in a grid-connected mode and to transfer from the voltage control mode to power control mode with a smooth transitional procedure. To validate the meritorious performance of the suggested scheme, its performance is compared to similar schemes based on a linear controller (regular PI controller), single-neuron PI controller (adaptive PI controller), recursive least square-support vector machine based PI controller (another adaptive PI controller), and nonlinear controller (sliding mode controller) for different operations of the microgrid.

Control Power Sharing of Parallel Inverters in Microgrid with Consideration of Line Impedance Effect

This paper presents a power sharing control method for use between paralleled three-phase inverters in an islanded microgrid. In this study, the mismatch of power sharing when the line impedances have significant differences for inverters connected to a microgrid has been solved, the accuracy of power sharing in an islanded microgrid is improved, the voltage droop slope is tuned to compensate for the mismatch in the voltage drops across line impedances by using communication links. The method will ensure in accurate power sharing even if the communication is interrupted. If the load changes while the communication is interrupted, the accuracy of power sharing is reduced but the proposed method is better than the conventional droop control method. In addition, the accuracy of power sharing base on the proposed method is not affected by the time delay in the communication channel and local loads at the output of the inverters. The control model has been simulated in Matlab/Simulink with two or three inverters are connected in parallel. Simulation results demonstrate the accuracy of the proposed control method. Futhermore, in order to validate the theoretical analysis and simulation results, an experimental setup was built in the laboratory. Results obtained from the experimental setup verify the effectiveness of the proposed method.

Enhanced Virtual Synchronous Generator Control with Fuzzy Logic Controller for Parallel Inverters in Microgrids

Objective: To reduce the power oscillations and also to provide accurate active power and reactive power sharing. Methods: In this paper enhanced Virtual Synchronous Generator (VSG) control strategy along with Fuzzy Logic Controller (FLC) is employed for parallel inverters in microgrids. It deals with design of the system and simulation results shows the improvement obtained through it. Findings: By using enhanced VSG control, the oscillation damping and proper transient active power sharing are achieved by adjusting the virtual stator reactance and also accurate reactive power sharing is achieved based on inversed voltage droop control. Furthermore, FLC is used as suitable controllable mechanism (which was PI in basic VSG) for controlling the parallel inverters in microgrids with which power oscillations are further damped and accurate active and reactive power sharing are achieved. The simulation results prove that the proposed method is better than the basic VSG for parallel inverters in microgrids. Application: This scheme is used for control of inverters in distributed source environments. This is essential for the operation of large AC systems, where distances between inverters make communication impractical. Use of multilevel inverters or an adaptive neuro-fuzzy inference system (ANFIS) can be considered as improvement for this paper. Keywords: Distributed Power Generation, Droop Control, Fuzzy logic controller, Microgrids; Reactive Power Control, Virtual Synchronous Generator Control1.

Combining droop and direct current vector control for control of parallel inverters in microgrid

Traditionally, active and reactive power sharing between parallel inverters based on droop control method is considered as a high priority for operating an islanded microgrid. An issue in droop control of parallel inverters in the islanding mode is the voltage drop associated with reactive power sharing within a microgrid. However, maintaining microgrid voltage stability while sharing active power between parallel inverters should be the primary objective for a microgrid. This study proposes to integrate droop and direct-current vector control (DCVC) techniques for active power sharing and bus voltage control within a microgrid. The proposed approach consists of a droop-only controlled unit whose function is similar to a traditional slack bus generator and the rest inverters controlled by using a combined droop-DCVC technique. The droop-DCVC controlled units provide active power sharing among parallel inverters and at the same time maintain microgrid voltage level by injecting the needed reactive power that is determined automatically from the droop-DCVC controllers. Parallel inverter units with loads for a microgrid in the islanding mode are simulated using Matlab/Simulink and Opal-RT real-time simulation system. The hardware experiment is also conducted for two parallel inverters. Results show the effectiveness and excellent performance of parallel inverters in the microgrid by combining DCVC and droop methods.

A Unified Virtual Power Decoupling Method for Droop-Controlled Parallel Inverters in Microgrids

The coupling between the active and the reactive power of the droop-controlled inverter is critical in the low-voltage microgrid. The conventional virtual power-based control strategy can decouple the active and the reactive power of the inverter, whereas the active power of the load is probably not equally shared among the parallel inverters in the microgrid. To tackle this issue, this paper proposes an improved virtual power-based control method for the droop-controlled parallel inverters with a unified rotation angle in the power transformation. Meanwhile, the optimal value of the unified rotation angle is rigorously derived through investigating the relative stability of the system by innovatively adopting Routh Stability Criterion. Moreover, the proposed method is enhanced by introducing low-pass filters in the coupling path for further mitigating the coupling between the active and the reactive power of the inverter. Additionally, the stability analysis and parameter design for the proposed method are comprehensively presented. Finally, the effectiveness of the proposed method is validated by the simulation and experimental results.

A control method for parallel inverters in Microgrid based on sliding mode and droop controls

Robust control and flexible operation are the major objectives of islanded microgrid. A microgrid can have different configurations with linear loads and nonlinear loads. The harmonic current caused by nonlinear loads make Proportional Integral (PI) or Proportional Integral Derivative (PID) voltage controller far beyond excellent performance in case of microgrid operating in islanded mode. Additionally, the robustness of the PID closed loop system can not be guaranteed. The voltage control by using Proportional resonant (PR) controller are recommended. Although PR controller has the ability to sinusoidal signals. However, PR controller has a slight deviation of the frequency of selected harmonic component. This paper presents a Sliding Mode Control (SMC) for voltage control of parallel inverters operating in islanded microgrid. This controller can enhance the robustness of control system and reduced-state tracking error. The stability of the closed-system is verified by means of Lyapunov stability criterion. The control structure is based on the inner sliding mode closed-loop and the outer droop control loop. The main aim of this paper is to design inner controllers to enhance the dynamics of the microgrid. The results obtained from the simulation of Matlab.