Isolated AC Microgrids Research Articles

Performance analysis of modified sine cosine optimized multistage FOPD‐PI controller for load frequency control of an islanded microgrid system

AbstractThis research work proposes a modified sine cosine algorithm (mSCA) to optimize the parameters of a multistage fractional‐order PD‐PI (MSFOPD‐PI) controller for load frequency control of an isolated AC microgrid. Microgrid considered here is a cluster of renewable energy sources like photovoltaic (PV), wind turbine generators (WTG) which diverges from their optimal operating point due to change in its environment. Initially, the performance of proposed mSCA algorithms is compared with the original sine cosine algorithm (CSA) well as other techniques like Crow search algorithm (CSA), Artificial bee colony (ABC), Cuckoo Search (CS), Gravitational search algorithm (GSA), Dragonfly (DA) and Genetic algorithm (GA) techniques using benchmark test functions reported in the literature. In the next stage, proposed mSCA is applied to optimize MS FOPD‐PI controller parameters under different fluctuating scenarios of source and load. To demonstrate the supremacy of the proposed MS FOPD‐PI controller the performances are compared with conventional PID, PI and I controllers. A sensitivity analysis is made by varying the system parameters to justify the controller's robustness.

Robust Cooperative Control of Isolated AC Microgrids Subject to Unreliable Communications: A Low-Gain Feedback Approach

Inherent nonlinearities and uncertainties are inseparable parts of the real-life engineering systems, which make theoretical analyzes and control system designs more challenging. Usually, most of the physical parts in control systems such as controllers and actuators are subjected to saturation. On the other hand, uncertainties are ubiquitous in the physical systems, which demand some necessities in the design of the control system. Additionally, unreliability in communication networks is another practical constraint in multiagent systems, which is required to be considered. In this article, based on the mentioned issues, a distributed robust algorithm is proposed for the semiglobal stabilization of the voltage and frequency of isolated (islanded) ac microgrids considering parameters uncertainties, static nonlinearity of actuators, and unreliability in the communication network. In this regard, by employing an adaptive low-gain feedback protocol, the robust performance of the closed-loop system is guaranteed. Finally, to evaluate the proposed control strategy’s performance, digital time-domain simulations are carried out on a test microgrid system in MATLAB/Simulink environment, and the results are compared with several previously reported methods. Simulation results and comparison with previous works reveal the proposed method’s effectiveness and accuracy in regulating the microgrid voltage/frequency and providing accurate proportional active power sharing.

Control Scheme of Photovoltaic Inverter for Voltage Improvement in Isolated AC Microgrids

This paper presents a control scheme for a PV inverter in isolated three-phase AC microgrids. The proposed control scheme allows performing the voltage regulation by reactive power control of the PV inverters without communication systems. The PV inverter in the microgrid commits to supply the active power generated from the PV array first, and then the remaining capacity of the PV inverter represents the available reactive power that can contribute in the microgrid voltage regulation. The proposed control scheme allows the PV inverters to deliver or to absorb the reactive power depending on the measured voltage at the connection point of the PV inverter and the available reactive power of the PV inverter. The tested microgrid consists of two power inverters, one for interfacing energy storage batteries controlled by a droop control scheme in order to regulate the voltage and the frequency of the microgrid. The second inverter interfacing the PV array controlled by the proposed control scheme in order to support the microgrid by the active power generated from the PV array and the available reactive power. The results obtained from the simulations show that the proposed approach provides the expected voltage regulation in the tested microgrid under different operating conditions.

Single-Phase Consensus-Based Control for Regulating Voltage and Sharing Unbalanced Currents in 3-Wire Isolated AC Microgrids

Chilean National Commission for Scientific and Technological Research/Formation of Advanced Human Capital Programme (CONICYT/PFCHA)/Doctorado Nacional/2017-21171858 Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT) CONICYT FONDECYT 1170883 National Agency of Research and Development/Associative Research Program (ANID PIA)/Basal AFB180003 Solar Energy Research Center (SERC), Chile ANID/FONDAP/15110019 National Agency of Research and Development (ANID)/Basal FB0008 National Fund for Scientific and Technological Development (FONDEQUIP) EQM160122

PI-Consensus Based Distributed Control of AC Microgrids

The conventional voltage observer based distributed control can make a tradeoff between voltage restoration and reactive power sharing. However, it suffers from estimation error when there exist input disturbances, such as initial values and communication delays. To address this problem, this paper proposes a novel distributed secondary control based on PI consensus algorithm. A voltage controller and a frequency controller are designed separately to realize the restoration of voltage/frequency and also accurate reactive/active power allocation. Moreover, a detailed small-signal model with PI consensus algorithm is established to show the impact of the proposed control strategy on the system stability. Compared with the conventional distributed control, the proposed control scheme is simpler with higher control accuracy under input disturbances. Simulations are conducted to verify the effectiveness of the proposed control strategy in an isolated ac microgrid.

Distributed Pinning Droop Control in Isolated AC Microgrids

This paper presents a distributed pinning droop control mechanism for accurate active and reactive power sharing in isolated converter-fed microgrids (MG). Under this framework, both power converters and loads in the MG are treated as agents in a multiagent system (MAS). Since only a fraction of the agents in the network have access to critical information, the required communication bandwidth and control cost can be significantly reduced without degrading the dynamical performance of the power sharing compared to consensus-based droop control techniques. Theoretical studies of this pinning-based distributed droop control, including pinning localization, grid partition, and convergence criteria, will be explored in detail. In order to validate the feasibility and the correctness of the proposed distributed droop control method, an MAS platform is developed for a test-bed MG system in Taiwan. Simulation tests under normal conditions, loading conditions, and plug-in and plug-out scenarios are investigated in MATLAB/Simulink and the Java Agent Development Framework. Simulation results demonstrate that the proposed pinning-based distributed droop control can indeed improve autonomous operations of an isolated ac MG.

Control strategy of hybrid energy storage system in diesel generator based isolated AC micro-grids

This paper proposes an AC micro-grid structure, which was based on diesel engine, synchronous generator and hybrid energy storage (HES) subsystem, consisting of battery and ultra-capacitor. In system operation, the diesel generator works as the sole voltage source of the micro-grid under islanding mode and the HES cooperate to achieve the power balance of the system. The corresponding energy management strategy was proposed, where the net power of the micro-grid is separated into high and low frequency components. The high frequency part is suppressed by the ultra-capacitor unit, whereas the low frequency part is compensated by the diesel generator and the battery unit together. Through the designed energy management strategy, power balanced of the micro-grid system can be achieved. MATLAB/SIMULINK simulation results under different conditions have verified this proposed micro-grid structure and the proposed energy management strategy.

Consensus-Based Secondary Frequency and Voltage Droop Control of Virtual Synchronous Generators for Isolated AC Micro-Grids

As the penetration of renewable energy sources is increasing in the AC micro-grid, the stability of the closed-loop system has raised a major concern since conventional distributed interface converters (DICs) used in the AC micro-grid do not have a rotating mass, and hence low inertia. High penetration of DIC-based micro-grid may result in poor frequency and voltage response during large disturbance. In order to overcome this difficulty, the virtual synchronous generator (VSGs) was proposed recently in which the DIC mimics conventional synchronous generators (SGs) by designing proper parameters of the SG into each local droop control mechanism of the DIC. Meanwhile, due to the recent advances of distributed control, the concept of consensus-based control can be applied to study this droop control problem of VSGs. One important feature of this consensus-based control is that it can be implemented on each local DIC with communications among their neighboring DICs. In contrast to most existing secondary control schemes, no central controller is required. Under this framework, if DICs are redesigned as VSGs, the frequency and voltage of each DIC can be restored to their pre-specified values obtained from the steady-state analysis. In addition, the proper real and reactive power sharing still can be achieved according to the nominal rating of each DIC. The stability of the closed-loop system is ensured by the transient energy function under certain mild conditions. Numerical experiments of a 14-bus/6-DIC micro-grid system on real-time simulators are performed to validate the effectiveness of the proposed control mechanism.