Inverters In Microgrid Research Articles

Expeditious Identification of IGBT Switch Fault in Bidirectional Microgrid Inverter Linked to Distributed Energy Resources

Expeditious Identification of IGBT Switch Fault in Bidirectional Microgrid Inverter Linked to Distributed Energy Resources

Distributed cooperative grid synchronization strategy for multiple parallel grid-supporting inverters in AC microgrid

In an AC microgrid (MG), the grid-supporting inverters (GSIs) are crucial components, which can regulate the frequency and voltage and enhance power supply reliability. To provide a reliable utilization of distributed generation (DG) and ensure uninterrupted operation of critical loads, a seamless handover between islanded and grid-connected modes is required for an AC MG with multiple parallel operating GSIs. Traditionally, the phase angle and frequency are usually detected by the phase-locked loop (PLL) units. Furthermore, the centralized control structure is widely used to obtain the global information of the system for grid synchronization, which will reduce the system reliability and flexibility. In this paper, a distributed cooperative grid synchronization strategy is proposed for multiple parallel GSIs. A seamless handover can be performed under the proposed control strategy that only needs to transmit the synchronization information to the leader DG without using any PLL units, which is more simple, effective, and flexible. The results of the simulation and StarSim Hardware-in-the-loop experiment are presented to confirm the effectiveness of the proposed control strategy.

The Improved Control Method of Parallel Microgrid Inverters

The parallel of inverters is inevitable in the operation of distributed generation with a Microgrid. However, due to the difference in line impedance between each parallel inverter and the public AC bus in the microgrid, the m available control method is insufficient to overcome the disadvantages such as unbalancing distribution of power, large circulating current, and poor dynamic characteristics, etc., especially when the line impedance difference is large. To conquer the above problems, the paper proposed an improved droop control strategy, in which a differential module is adapted to the droop control to improve the dynamic response. Further, a modified adaptive virtual resistance is proposed, so that the system can maintain high response speed, high stability, and robustness. These can effectively benefit power sharing and circulation suppression of parallel inverters while ensuring excellent dynamic characteristics of the system. Finally, the effectiveness of the improved control method is verified by the simulation results.

VSG Control for Cascaded Three-Phase Bridge Based Battery Inverter

With the increasing number of new energy sources connected to the grid, the unbalanced output of three-phase grid-connected inverters and the lack of no inertia and damping characteristics in the traditional microgrid control system will seriously affect the stability of voltage, frequency, and power angle for microgrids. This paper proposes a novel cascaded three-phase bridge inverter topology for the battery system used for the electric vehicle. Compared with traditional cascaded H-bridge inverters, the proposed multilevel inverter can achieve self-adaptive balance for three phases. The mathematical model of a cascaded three-phase bridge inverter is established in this paper. Based on the voltage and current equations of a multilevel inverter, a new modulation strategy named carrier phase-shifted-distributed pulse width modulation (CPSD-PWM) was developed, which is more suitable for cascaded three-phase bridge inverters. The harmonic analytic equations of carrier phase-shifted pulse width modulation (CPS-PWM) and CPSD-PWM are constructed by the double Fourier analysis method. Compared with the traditional PWM modulation strategy, the CPSD-PWM can reduce the output harmonics and improve the balance of the three-phase output, which can realize the three-phase adaptive balance in the cascaded three-phase bridge inverter. This paper develops a cascaded three-phase bridge multilevel power converter system based on the virtual synchronous generator (VSG) control strategy. The voltage and frequency of inverter output can be accurately controlled in both island mode and grid-connected mode through active power-frequency regulation and reactive power–voltage regulation, and the stability of primary frequency regulation for the multilevel microgrid inverter can be improved by collaborative optimization of virtual inertia and virtual damping. The CPSD-PWM modulation strategy and VSG control strategy are verified by the simulation results and experimental data for the cascaded three-phase bridge inverter.

A Novel Narendra‐based adaptive anti‐disturbance control strategy for the microgrid inverters: A novel MG VSI adaptive anti‐disturbance control strategy based on Narendra theory

AbstractThe control performance of the voltage source inverter will be reduced when it operates under complex microgrid (MG) conditions. Traditionally, problems such as high controller order and complex design may occur when robust control and secondary regulation are adopted for the inverters. Meanwhile, when the conventional adaptive control is adopted, the adaptive gain may be too high, resulting in increased sensitivity of the system to external disturbances. To address the above problems, a novel Narendra‐based adaptive anti‐disturbance control strategy for microgrid voltage source inverter is proposed in this paper. Firstly, a Narendra adaptive‐based controller is constructed, which enables the controller to adaptively adjust the system parameters to change the inverter output voltage quality. Secondly, the extended state observer (ESO) is introduced to observe and suppress the total disturbance of the system, which can make the system less sensitive and ensure the stability of the system operation. Thirdly, according to the Lyapunov stability theory, the Lyapunov function is chosen to prove the global asymptotic stability of the system, and an adaptive law is designed. Finally, the comparative experiment shows that the proposed control strategy based on Narendra adaptive anti‐disturbance can maintain excellent voltage control performance in complex working conditions, and then ensure that the inverter's output voltage always meets power quality standard in complex working conditions.

Hybrid Switching Control-Based DC to AC Inverter for Microgrid Using Renewable Energy Sources

Hybrid switching control-based direct current (DC) to alternating current (AC) inverter is a method of controlling the flow of power from DC sources such as solar panels or wind turbines to AC loads in a microgrid. In a microgrid, renewable energy sources are often used to supplement traditional sources of energy and improve overall energy efficiency. To simulate micro-grid systems, an effective three-legged IGBT inverter has been developed using Matlab R2016a/Simulink. The inverter switches are controlled by a pulse controller, which creates the switching gate pulses. This work models the output voltage and current waveforms of the three-phase 180° voltage source inverter that supplies nonlinear loads. To lower the total harmonic distortion (THD) of the current and voltage outputs of the three-phase voltage source converter (VSC), a filter circuit and a two-level pulse width modulation (PWM) generator have been presented. This pulse-width modulation generator compares the modulating signal and a high-frequency triangular carrier wave. Sinusoidal pulse width modulation (SPWM) is used for PWM control. Moreover, the hybrid switching control-based DC-to-AC inverter utilizes a combination of pulse width modulation (PWM) and hysteresis current control techniques to maintain a constant AC output voltage and frequency. PWM control is used to regulate the voltage, while hysteresis current control is used to regulate the current. Additionally, an inductor-capacitor-inductor (LCL) filter is used to improve the stability of the power converter by providing a damping effect on the system. Overall, the LCL filter is a versatile and effective tool for improving the performance of power inverters, especially in applications where high efficiency and low harmonic distortion are important. IUBAT Review—A Multidisciplinary Academic Journal, 6 (1): 25-46

Modified control structures for sharing of asymmetrical powers amongst inverter based distributed energy resources

Renewables interfaced through inverters in microgrids, may be controlled using centralized, decentralized or distributed approaches. The decentralized droop control involves mimicking the performance of a synchronous generator of the power system, so as to share load power proportionately amongst parallel connected sources. When the conventional droop control is used, only the fundamental real powers can be shared in proportion to the source ratings, which does not ensure satisfactory performances in the presence of unbalanced/ asymmetrical loads. Several schemes have been worked upon to achieve proportional load power-sharing with power quality aspects: in the presence of harmonic and unbalanced loads. This paper proposes five modifications for the popular virtual conductance vs. negative sequence power (G vs. Q-) approach, aimed at improving the sharing of the unbalanced currents amongst the sources to improve the power quality and microgrid system reliability. The eigenvalue analysis for determining the parameter range for assuring stability and comparisons with fixed virtual conductance values are discussed. The proposed modifications are discussed and demonstrated through simulations on a two inverter-based distributed generator (DG) system, a modified 13 bus system, and using experimental results on a lab-scale microgrid.

Power control in microgrid using improved virtual impedance method

AbstractIn this article, a method is proposed to increase the accuracy of reactive power‐sharing for parallel‐connected inverters in Microgrid, this method is done by automatically adjusting the values of the virtual impedance to adjusting the output voltage of the inverters. The virtual impedances are automatically adjusted to compensate for the difference in the output voltage of the inverters due to the influence of the line impedances. The output voltage of the inverters will be adaptively adjusted according to the change of the load, this method greatly improves the accuracy in the reactive power sharing The correct power sharing for the inverters will ensure the stability of voltage and frequency in the Microgrid. The control method is simple and does not need to know the line impedance parameter. The feasibility and effectiveness of the proposed strategy are proven by simulation and experimental results.

Droop control strategy for microgrid inverters: A deep reinforcement learning enhanced approach

To better tap into the potential of distributed renewable energy generation, microgrid system has become an emerging technology. As the bridge of microgrids, the inverters can flexibly convert distributed DC power input into AC power output. It is verified that the traditional droop control strategy for microgrid inverters has inherent defects of uneven reactive power distribution. To this end, this paper proposes a droop control strategy as a multi-objective optimization problem while considering the deviations of bus voltage and reactive power distributions of microgrids. Then, the optimization problem is further formulated as a Markov decision process and solved by a deep reinforcement learning (DRL) algorithm called deep deterministic policy gradient to obtain a dynamic optimal droop coefficient control strategy. Simulation results demonstrated that our DRL-based strategy eliminates the uneven reactive power distribution without voltage drop.

Improving voltage imbalance in inverter‐based islanded microgrids during line‐to‐line short circuits

AbstractLine‐to‐line short circuits are the transient disturbances that cause the highest currents as well as the largest voltage imbalances in inverter‐based microgrids. This paper presents a control scheme for grid‐forming inverters in islanded microgrids that limits the current to a safe value during these types of short circuits while providing a lower voltage imbalance compared to state‐of‐the‐art current limiting techniques. This control scheme is based on reducing both the amplitude of the reference voltage with the maximum amplitude of the reference current and the instantaneous reference voltage with only the inductive component of the virtual impedance. The combination of these two control actions provides low voltage imbalance, as the theoretical and experimental studies reveal. This voltage imbalance improvement is the main contribution of this paper and is maintained when the microgrid supplies linear and non‐linear loads. In the theoretical study, the paper includes control design guidelines to satisfy static and dynamic specifications. The theoretical predictions are validated by means of experimental results measured in a laboratory microgrid. A practical performance comparison of current limiting techniques based on experimental tests is also reported.

Improved Repetitive Control for an LCL-Type Grid-Tied Inverter with Frequency Adaptive Capability in Microgrids

Repetitive control (RC), which can track any periodic signal with a known integer period with zero steady-state error, is widely used for current control of grid-tied inverters in microgrids. However, the inherent one fundamental period time delay, leads to poor dynamic performance. Furthermore, the performance of conventional RC (CRC) will degrade when operating at a high variation grid frequency. Therefore, this paper proposes a frequency adaptive improved RC (FA-IRC) for grid-tied inverters. The improved RC (IRC) consists of a repetitive controller with a modified internal model filter, plus a proportional controller. In comparison to the CRC, the IRC has a good dynamic response, because it provides a higher gain and a wider bandwidth at the resonant frequency. Moreover, to achieve the frequency adaptability of the IRC, a fractional delay, based on a finite impulse response (FIR) filter, is built into the IRC system, to ensure that the resonant frequency of the IRC is approximately equal to the actual grid frequency and harmonic frequency. Stability analysis and characteristic analysis of the FA-IRC system are reported in this paper. Simulations are conducted, to demonstrate the validity of the proposed method.

Frequency Restoration of Grid-Forming Inverters in Pulse Load and Plug-in Events

This article presents a restoration technique using grid-forming inverters in an islanded microgrid during pulse load and plug-in events. In a microgrid powered by droop-controlled inverters, frequency is the variable that is accessible to all inverters for adjusting their power contributions. The problem is that the microgrid frequency deviates from its nominal value after a load change. The presented method enables the inverters to restore the frequency to the nominal value and regulate their bus voltage amplitudes. The frequency and voltage restorations are performed without communication while achieving the desired power-sharing between grid-forming inverters. The restoration is activated after detecting any active or reactive power changes. In this article, the dynamic model of an inverter equipped with enabled restoration paths is developed to verify the stability of the inverter controller under various conditions. The frequency and voltage restorations are examined during pulse load and plug-in events. This study is performed using a laboratory-scale islanded microgrid powered by two 208 V, 60 Hz, 5 kVA inverters.

Active and Reactive Power Control of the Voltage Source Inverter in an AC Microgrid

This paper presents the mathematical model and control of a voltage source inverter (VSI) connected to an alternating current (AC) microgrid. The VSI considered in this paper is six switches three-phase Pulse Width Modulated (PWM) inverter, whose output active and reactive power is controlled in the dq reference frame. The control strategy presented here is state feedback control with disturbance cancellation. This disturbance signal is either provided by a voltage sensor or estimated using a presented extended high gain observer (EHGO). The control strategy without EHGO requires a current sensor and a voltage sensor, and the control strategy with EHGO requires only a current sensor. The EHGO is saving the requirement of a voltage sensor. The stability analysis of the presented control strategy is showing that the error is ultimately bounded in the presence of disturbance, formed due to Pulse Width Modulated (PWM) inverters. The microgrid is simulated using the SimPowerSystems Toolbox of MATLAB/Simulink. The simulation results are also showing the effectiveness of the proposed control strategy, that the output active and reactive power control is achieved with ultimately bounded errors. The comparison of the proposed control with the PI-based control scheme is also presented, and it is shown that better reference tracking with the desired settling time of “0.04 s” is achieved with the proposed control.

MPC-based three-phase unbalanced power coordination control method for microgrid clusters

The microgrid cluster group-level bus voltage is high, the capacity is large, and the interconnection bus is connected to various microgrid subsystems. It is challenging for the microsource inverters in the subsystems to directly participate in the imbalance regulation of the group-level bus voltage, so it is currently difficult to solve the three-phase imbalance of the group-level bus voltage. In this paper, a particular structure of a series microgrid is applied to the cluster system, and a three-phase unbalanced control strategy based on model predictive control for the cluster system is proposed. First, the grid-connected current prediction control model of the series microgrid inverter using an LCL filter is established, a medium-voltage high-capacity three-level neutral point clamped (NPC) power electronic converter is introduced into the system and used as the central inverter of the DC microgrid, and the model prediction control method is applied to this central inverter. Second, considering possible load asymmetry operating conditions of the group bus, the p-q-o method is used to calculate the load imbalance current component that must be compensated, which is used as a part of the output current of the series microgrid or DC microgrid, based on which the three-phase unbalanced power coordinated control method between two heterogeneous microgrids is studied. The simulation results show that the series microgrid and DC microgrid exhibit good three-phase unbalance suppression under load unbalanced conditions, and the coordinated control between them can effectively improve the three-phase unbalance suppression capability of the system, which confirms that the proposed control strategy can achieve high-quality voltage output of the microgrid cluster system in multiple scenarios.

Improved power computation method for droop‐controlled single‐phase VSIs in standalone microgrid considering non‐linear loads

AbstractComputation of active and reactive powers is a crucial step in droop‐controlled single‐phase voltage source inverters (VSIs) in standalone microgrid since the performance and stability of the power‐sharing strategy are strongly influenced by its speed and accuracy, especially in the case of non‐linear loads. Here, an improved performance of power‐sharing among single‐phase droop‐controlled VSIs in an islanded microgrid, considering DC component and nonlinear loads is presented. To achieve this goal, an enhanced power‐sharing control scheme including a Multiple Enhanced Second‐Order Generalized Integrator Frequency‐Locked Loop (MESOGI‐FLL) for power calculation is proposed. As a result, the proposed power computation technique provides high rejection capability of DC component and current harmonics, hence, perfect estimation of the fundamental component of the inverter output current and its 90◦ phase‐shifted component. This strategy makes the power calculation method‐based control scheme immune to disturbance effects of the DC component and the high current harmonics. Detailed analysis, mathematical modelling of MESOGI, as well as a comparison with recent methods, are also provided. Simulation and experimental tests were carried out and the obtained results have shown the effectiveness and robustness of the proposed power‐sharing controller even under nonlinear load operating conditions.

Dynamic virtual resistance-based droop control for seamless transition operation of multi-parallel microgrid inverters

Dynamic virtual resistance-based droop control for seamless transition operation of multi-parallel microgrid inverters

Multi-Rate Sampling Control Design and Stability Analysis for Frequency and Voltage Regulation in Islanded Microgrids

The wide use of inverters in microgrids motivates the development of digital controllers with sampling systems. This paper proposes a multi-rate sampling frequency and voltage regulation method for inverters-based distributed generators (DGs). The fast-sampling signals are adopted to achieve the primary control, which is composed of the VSG-based frequency control and the voltage droop control with a low-pass filter (LPF). Based on the unified primary control model, the low-sampling distributed model predictive control (DMPC) is proposed as secondary frequency and voltage control. Considering the microgrid network, the inverter-based DGs supply power for both the local load and loads on other buses. The proposed DMPC method can overcome the disproportionate impedances of interconnecting lines among buses. Then the frequency and voltage of islanded microgrid can be restored to rated values with accurate power sharing. The DMPC-based secondary control and the primary control have different sampling rates. The stability and dynamic performance of the proposed multi-rate sampling control system are investigated for the first time. Finally, both simulation and experiment tests are carried out to validate the effectiveness of the proposed control strategy.

Power Synchronization Compensation Strategy Based on Second-order Compensation Links for Voltage-controlled Inverters in Microgrids

Power Synchronization Compensation Strategy Based on Second-order Compensation Links for Voltage-controlled Inverters in Microgrids

Sliding mode control strategy for microgrid inverter systems

Sliding mode control strategy for microgrid inverter systems

Design of Adaptive Fuzzy Sliding-Mode Control for High-Performance Islanded Inverter in Micro-Grid

In this paper, an adaptive fuzzy sliding-mode control (AFSMC) system is investigated for an islanded inverter to achieve a high-performance power supply. A sliding mode control (SMC) law is designed initially to obtain both the voltage tracking error and the current tracking error of the inverter involved, to realize both the output-voltage regulation and the current protection with global stability. Moreover, to deal with uncertainties in the practical inverter system without the chattering phenomenon, an adaptive fuzzy system embedded with a self-adjustive translation width is developed to replace the switch term of the SMC. In addition, the adaptation laws, derived from the Lyapunov stability theorem, adjust the AFSMC parameters online to guarantee optimal and robust performance. Furthermore, the superior control performance of the proposed AFSMC is verified by a numerical simulation in MATLAB, producing experimental results on the prototype in comparison with the conventional SMC.