Parallel Inverters Research Articles

Autonomous Control of Voltage and Frequency in Parallel Inverters of Microgrid

Integration of Distribution Generations (DGs) provides flexibility in power transmission. DGs reduce expenditure, expansion of transmission lines and transmission loss. DGs can work separately from the main grid with local loads and form a microgrid. In grid-connected mode, the voltage and frequency of the microgrid are regulated by the main grid. Voltage and frequency regulation in the islanding microgrid are crucial. This paper presents voltage and frequency control techniques for parallel inverters in microgrid. The proposed model works on close loop control of inverters and it can be used for plug-and-play operation of microgrid. The Microgrid model is developed in MATLAB/Simulink and the performance of the system is analyzed in grid-connected mode and isolation mode.

Research on current sharing control of parallel inverters used on electric motor emulator

AbstractParallel inverters have the advantages of low‐output harmonics and high‐parallel power, making them very suitable as the topology structure for an electric motor emulator. However, parallel inverters can also bring about circulating current issues, especially when using carrier phase shifted sinusoidal PWM (CPS‐SPWM) control mode, which significantly increases circulating current, leading to increased output harmonics and losses in the system. In order to suppress circulating currents, this paper provides a detailed analysis from both high‐frequency and low‐frequency perspectives in CPS‐SPWM control mode. A circulating current suppression method that combines hardware and software systems, adopting a cascaded coupled output network structure along with a corresponding error voltage space vector control method to achieve circulating current suppression, is proposed. By constructing a 6‐branch parallel output experimental platform, the feasibility and effectiveness of the proposed method have been verified, suppressing current circulating currents and improving current quality.

Fixed Switching Frequency Model Predictive Control for Parallel Inverters in Microgrids

A Fixed-Switching-Frequency Model Predictive Control (FSF-MPC) for Master-Slave inverters in microgrids is proposed in this paper. The Master is a three-phase, two-level inverter with an LC filter, while the Slave is a three-phase, two-level inverter with an LCL filter. The inverters are connected in parallel in a microgrid, composed of different loads. The voltage and current inner control loops of the Master-Slave FSF-MPC are presented for both inverters. Then, two modes of operation are proposed: grid-connected and islanded, and the primary control for both cases is developed. Finally, Hardware-in-the-Loop (HIL) results are presented for different operational conditions of the microgrid with load-related disturbances. The HIL results validate the good performance of the proposed Master-Slave FSF-MPC, including fast dynamic response, multi-objective control, and fixed switching frequency.

An Adaptive Droop Control for Power Sharing Between Three Phase Parallel Inverter in Autonomous Microgrid

The development of an adaptive droop controller is for power sharing between three-phase parallel inverters. Traditionally, droop controllers have been employed in conventional power systems to allocate the load among generators with varying capacities. Our approach involves the utilization of an adaptive control technique. This method is designed for a setup involving three-phase parallel inverters linked to an AC microgrid, to enhance the proportional distribution of active power despite changes in feeder network characteristics and loads. The proposed control mechanism automatically tunes the frequency droop parameter to address mismatch frequencies. The system includes two sets of three-phase inverters regulated by a combination of P-F droop control, an enhanced droop control loop, and PI controllers. The adaptive droop coefficient control is capable of addressing problems where power distribution inaccuracies arise due to different line loads as well as different inverter capacities.

A Systematic Literature Review on AC Microgrids

The objective of this work is to analyze and compare AC microgrid (ACMG) solutions to introduce the topic to new researchers. The methodology used to achieve this goal is a systematic literature review using five questions: (1) How have ACMGs evolved in five years? (2) What are the standards for ACMGs? (3) What are the different schemes for connecting MGs to the utility grid? (4) What are the different control schemes in ACMGs? (5) What is an appropriate way to compare results when working with ACMGs? The articles were published in Q1/Q2 journals as based on either the Scimago Journal Rank (SJR) and/or the Journal Citation Report (JCR) between 2018 and 2022 and were from three databases: (1) Web of Science (WoS), (2) Scopus, and (3) IEEE Xplore. Publications not describing pure ACMGs, review papers, publications not related to the questions, and papers describing work that did not meet a quality assessment were excluded, resulting in 34 articles being included in this review. Results show: (1) the energy sources and AC bus nature of microgrids over five years, (2) the identification and quantification of cited standards for microgrids, (3) the pros and cons of different schemes for connecting an AC microgrid to the main grid, (4) the control schemes, classified in a hierarchical control structure, and (5) the simulation tools and experimental benches used in microgrids. Most studies considered a generic energy source and a low-voltage three-phase AC bus, 16 standards were found, and the most cited standard was IEEE Standard 1547. The most common connection scheme to the utility grid was a direct connection, most of the works proposed a modification to a hierarchical control system scheme, and the most common simulation tool was MATLAB. The preferred experimental setup consisted of parallel inverters for testing a control scheme, a prototype when proposing a power electronic system, and a laboratory microgrid for testing fault detection methods.

A First-Order PLL-Synchronized Voltage-Supporting Control Method for Parallel Inverters With Near-Zero Line Impedance

A First-Order PLL-Synchronized Voltage-Supporting Control Method for Parallel Inverters With Near-Zero Line Impedance

New PI-synergetic hybrid control for suppressing circulating currents of an electrical drive system fed by two parallel inverters

New PI-synergetic hybrid control for suppressing circulating currents of an electrical drive system fed by two parallel inverters

Self-Voltage Balanced Switched-Capacitor Seven-Level Inverter for Asymmetrical Solar PV Source Applications

In this paper, a novel structure of self-voltage balanced switched-capacitor-based seven-level inverter (SVB-SCSLI) is proposed for asymmetrical input energy source applications. SVB-SCSLI develops a seven-level staircase waveform with a minimum number of semiconductor devices compared to existing multilevel inverters (MLIs). The proposed converter is more suitable for the solar PV system where unequal DC voltage sources are available. The SVB-SCSLI also resolves the main difficulty of connecting many inverters in parallel to achieve staircase voltage. Besides, it attains voltage balancing capability at DC-link by holding constant capacitor voltage at every cycle. The detailed loss and steady-state analysis of the SVB-SCSLI are examined. Furthermore, the proposed converter’s efficiency improves with better power quality than other topologies reported in the literature. Finally, a prototype is fabricated, and experimental results are presented to validate the feasibility of SVB-SCSLI.

Improved Hopf Oscillator-Based VOC Method for Fast Synchronization of Parallel Inverters in Standalone Microgrid

Improved Hopf Oscillator-Based VOC Method for Fast Synchronization of Parallel Inverters in Standalone Microgrid

Reactive Power Sharing in Parallel Inverters and Stability Improvement of Parallel Inverters in AC Microgrid Comprising Active Loads

Reactive Power Sharing in Parallel Inverters and Stability Improvement of Parallel Inverters in AC Microgrid Comprising Active Loads

Inductor integration for internal parallel inverters

SummaryIn addition to inheriting the merits of interleaved parallel inverters, the internal parallel structure (IPS) inverter has the advantages of reduced circulating current and lower switching loss. Currently, however, the suppression of circulating current and output current ripple individually results in an evident increase in the number and volume of inductors. In this work, an inductor integration scheme is developed to improve the IPS inverter. Two integrated inductors are developed for different application configurations. As a result, the circulating current and output current ripple are simultaneously suppressed by an integrated inductor, making each phase of the IPS inverter to be equipped with only one integrated inductor implemented by wounding two coils on one magnetic core. The feasibility of the inductor integration scheme for the IPS inverter is experimentally verified by a downscale prototype.

Improved Power Sharing Strategy for Parallel Connected Inverters in Standalone Micro-grid

The primary goal of integrating alternative energy systems such as solar and wind turbines into the power grid using power electronic devices is to meet the growing energy demands. Connecting inverters in parallel effectively enhance power capacity, reliability, and overall system efficiency. However, an uneven power distribution among the inverters is a significant limitation in these parallel connected inverters (PCI). This study focuses on a distributed generation (DG) unit comprising a solar photovoltaic system (SPV) and a battery energy storage system (BESS) connected to voltage source inverters (VSI) 1 and 2. The proposed approach aims to achieve uniform load/power distribution among the inverters with power management, maintaining a constant DC link voltage despite variations in solar irradiation and temperature. Additionally, the strategy targets the reduction of total harmonic distortion (THD) in the load current. The conventional droop control method is restricted in its ability to achieve accurate and uniform power distribution during load changes. An enhanced P-f and Q-E-based droop control technique (EDCT) and a fuzzy logic controller (FLC) have been proposed to address these challenges. Performance analysis using MATLAB/Simulink was conducted with two test cases, and a comparative assessment was carried out with a conventional controller, such as a Proportional Integral Controller (PIC) and Sliding Mode Controller (SMC), to showcase the effectiveness of the developed control technique. The proposed control method ensures equal sharing of active power of 2.5 and 4kW, 2.5 and 7.5kW and reactive powers of 2.5 and 4Vars, 2.5 and 7.5Vars among the inverters for the two cases even during the load change at t=0.5seconds. The settling time for DC link voltage and THD is effectively diminished to 0.03 and 0.25 seconds and 1.18% and 2.87% for the two test case studies of the proposed method, which are much lower than those of existing methods available in the literature.

Modeling and Attenuation of Common Mode Resonance Current for Improved LCL-Type Parallel Inverters in PV Plants

Modeling and Attenuation of Common Mode Resonance Current for Improved LCL-Type Parallel Inverters in PV Plants

Fuzzy-Based Adaptive VOC Methods for Parallel Inverters

Fuzzy-Based Adaptive VOC Methods for Parallel Inverters

DC-Link Dynamics Examination of the Parallel-Connected Three-Phase Inverters in Islanded and Grid-Connected Modes

This paper presents a comprehensive analysis of an AC microgrid setup, consisting of parallel three-phase inverters. It introduces a novel controller that integrates the tasks of controlling the DC-link voltage and implementing a current limiting approach. The design is informed by the characteristics of lead compensators and its main objectives are twofold: i) to regulate the DC-link voltage in order to minimize unwanted power exchange among the parallel inverters, and ii) to ensure the current-limiting capabilities of all inverters, regardless of the microgrid's operational state. This includes both standalone and grid-connected modes, voltage sags in the grid side, as well as transitional intervals between the two. In contrast to existing DC-link voltage control methods that do not incorporate a current limiting strategy, the proposed solution effectively restrains the dq axis current values in all operating conditions. This is crucial for preserving system stability, avoiding abrupt changes in DC-link voltage and current during sudden grid modifications and transitions. To validate the efficacy of the proposed controller, a series of simulations were carried out using Matlab/Simulink to evaluate its performance and compare it with an established method. The results explicitly demonstrate that the proposed controller successfully stabilizes the system, minimizes fluctuations in DC-link voltage, and restricts the dq axis current for each inverter without the potential of protection relay tripping.

Multi-objective based Hybrid Artificial Intelligence Controlled Parallel Inverter in Islanded and Grid Connected Operations

Background: The Integration of non-conventional energy systems (NCES), like solar, wind, etc., into the grid with power electronic devices is adapted to meet the demand. Parallel connection of inverters (PCI) is an efficient method to boost power handling capacity, reliability, and system efficiency. However, the main drawback is the unequal power sharing among the inverters while using the conventional droop control technique (CDC). In addition, the circulating currents (CC) flow between these PCI, leading to common mode voltage (CMV), current waveform distortion, and reduction in the system's overall performance. Objectives: This work consists of a photovoltaic system (PVS) and battery energy storage system (BES) as the distributed generation (DG) unit to voltage source inverter (VSI) 1 and 2. The multi- objectives of the suggested work are (a) the equal power/load sharing among two inverters, (b) effective minimization of CC and the CMV, (c) maintaining constant DC-link (DCL) voltage during different solar irradiation and constant temperature, and (d) the reduction in total harmonic distortion (THD) of load current. Methods: A novel approach related to the enhanced droop control (EDC) method with an adaptive neuro-fuzzy hybrid controller (ANFHC) was suggested here to overcome the above issues. The performance analysis of the suggested technique was done in the Matlab/ Simulink platform with islanded and grid-connected modes for different loads. Results: A comparative analysis with the available methods like the proportional integral controller (PIC) and sliding mode controller (SMC) was carried out to exhibit the viability of the developed control technique. Conclusion: This study focused on the operation and control of a PCI with ANFHC in islanded and grid-connected modes to address issues such as uniform power sharing and reduction of CC and CMV.

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.

A control method of parallel inverter for smart islanding of a local power system

Various types of distributed generators based on renewable energy become available today. The trends can make some small-scale power networks stand alone with an individual style and benefits, which is referred to as a concept of microgrid. To achieve its stable operation, a parallel inverter which is installed at the coupling point of the microgrid and utility can provide good performance. This paper deals with a control system of the inverter to keep high power quality by supporting the operation of islanding microgrid. The proposed system is tested under some situations including the condition of distributed generator with simulation studies.

Resonance coupling analysis of multiple differently parameterized grid-connected inverters in new energy generation

This study investigates the coupling problem between different types of resonances in the system of multiple differently parameterized inverters. The coupling between the inverters and the grid, as well as between differently parameterized inverters, can lead to different types of resonances, which have different resonance characteristics. Due to the absence of resonance coupling relationship analysis, it is difficult to form a unified and complete understanding of the system resonances that are formed by the superposition of different types of resonances. In this paper, the system model is established, and the expressionsof the interactive current circulating between the parallel inverters and input-grid current injecting the power grid can be obtained. Stability analysis of the interactive current and input-grid current reveal characteristics of different types of resonances, and thus the resonance coupling relationship is obtained. It helps to form a complete understanding of system resonance characteristics, and then obtain all system resonance cases. The effects of system parameters on the resonance characteristics and harmonic interaction are analyzed. Finally, the correctness of the theoretical analysis is validated through simulations and experiments.

Fault‐resilient control of parallel PV inverters using multi‐agent twin‐delayed deep deterministic policy gradient approach

SummaryGrid‐tied photovoltaic system (GTPS) are widely favored due to their inherent benefits. The interface of parallel power electronic converters in these systems is rapidly increasing. The inverter in GTPS has a very important role in power conversion, transfer, and control. However, their challenging working conditions contribute to susceptibility to power switching device failures. Traditional fault diagnosis and tolerant approaches exhibit limitations in control efficiency, model dependency, and slow recovery from fault. This study proposes a multi‐agent twin delayed deep deterministic policy gradient (MATD3PG) configuration for intelligent parallel inverter control, fault diagnosis, and fault‐tolerant operation in GTPS structure. By leveraging the multi‐agent reinforcement learning (RL) framework, an optimal control of the parallel inverter can be achieved, encompassing fault‐tolerant operation using MATLAB Simulink/PLECS. The major advantage the given technique offers is that it carries out optimum fault‐tolerant operation without causing the system derating. Experimental findings demonstrate that the proposed fault‐tolerant model based on RL traditional methods is able to ensure continuous supply to the connected loads even during fault events. Further, the transition time from fault occurrence to recovery is found to be 6 ms, which is quite less compared with the fault‐tolerant techniques presented in literature. Through real‐time fault diagnosis‐based results, the proposed approach ensures precise tracking of reference currents, quicker response times, uninterrupted supply, and smooth transition to the post‐fault operation mode.