Parallel Inverters Research Articles

Feature-Reduced Stability Analysis of Islanded Photovoltaic Microgrid Inverters

A smart grid environment is prone to data explosion while controlling a microgrid system. Islanded Microgrid’s stability analysis involves a large number of system state variables thus consuming more computational memory due to parallel connected inverter dynamics. Parallel inverters generate reference voltage and frequency using droop controllers, unlike grid-connected inverters where the primary grid provides the reference voltage and frequency. This paper develops feature-reduced stability analysis of the parallel inverters thus reducing the computational memory of its stability analysis. Principal component analysis being a feature extraction technique is applied to reduce the number of variables determining stability. MATLAB is used to develop the average model of a parallel inverter with an LCL filter and a three-phase AC load. Evaluation of the stability analysis using the state variable analysis with the virtual resistance method is simulated. Simulation validates stability analysis of the model with reduced state variables. An average model developed using MATLAB and PCA carried out using Python clearly indicated the validation of the dimensionality reduction in the stability analysis. The reduced number of variables is validated for a stable range of the parallel inverter droop controller. Both cases validated the dimensionality reduction in the stability analysis of parallel inverters.

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.

Secondary-Frequency and Voltage-Regulation Control of Multi-Parallel Inverter Microgrid System

As an important form of distributed renewable energy utilization and consumption, the multi-parallel inverter microgrid system works in both an isolated and grid-connected operation mode. Secondary-frequency and voltage-regulation control are very important in solving problems that appears in these systems, such as the distributed secondary-frequency regulation real-time scheme, voltage and reactive power balancing, and the secondary-frequency regulation control under the disturbances and unbalanced conditions of a microgrid system. This paper introduces key technologies related to these issues, such as the consensus algorithm and event-triggered technique, the dynamic and adaptive virtual impedance technique, and the robust and self-anti-disturbance control technique. Research and design methods such as small-signal state-space analysis, the Lyapunov function design method, the impedance analysis method, μ-synthesis design, and the LMI matrix design method are adopted to solve the issues in secondary-frequency regulation and voltage regulation. As the number of inverters increases, the structure of the microgrid becomes more and more complex. Suggestions and prospects for future research are provided to realize control with low-communication technology and a distributed scheme. Finally, for the case study, the droop-control model and primary frequency/voltage deviation of a multi-parallel inverter microgrid system is analyzed, and a state-space model of a multi-parallel inverter microgrid system with a droop-control loop is established. Then, the quantitative relationship between the primary frequency/voltage deviation and the active and reactive power output in the system is discussed. The methods and problems of centralized and decentralized secondary-frequency regulation methods, secondary-frequency regulation methods based on a consensus algorithm and an event-triggered mechanism, reactive power and voltage equalization, power distribution, and small-signal stability of the multiple parallel inverter microgrid system regarding the virtual impedance loop are analyzed.

Interleaved operation of parallel three-level inverter and suppression of circulating current

Circulating current suppression has been a key problem for non-isolated photovoltaic grid-connected parallel inverters, but traditional circulating current suppression method cannot meet the aims of reducing circulating current and output current harmonic at the same time. This paper analyzes the five-level voltage output capability based on the interleaved parallel three-level inverter through the interaction of the two parallel inverters, then proposes the interleaved operation idea based on Model Predictive Control Strategy. Futhermore, based on the basic vector of zero common-mode voltage in this system, this paper proposes a method of suppressing circulating current suitable for interleaved parallel three-level inverters, and derives a detailed modulation implementation process. Simulation shows that while suppressing the circulating current, the proposed method can greatly reduce the harmonic distortion of the parallel output current, which achieves the goals of suppressing the circulating current and reduce output current harmonics simultaneously in the non-isolated parallel photovoltaic inverter.

A Novel Droop Control Strategy of Reactive Power Sharing Based on Adaptive Virtual Impedance in Microgrids

Droop control strategy is widely used to control parallel inverters in a microgrid. However, due to the mismatched line impedance, the traditional droop control strategy is difficult to evenly share the reactive power and restrain the circulating current among multiple parallel inverters. In this article, an adaptive virtual impedance-based droop control strategy without communications is proposed to solve the reactive power sharing problem caused by the difference of line impedance. The reactive power output and voltage of each distribution generator are introduced into the adaptive virtual impedance controller to achieve a tradeoff between reactive power sharing accuracy and voltage drop. Small-signal stability analysis of the inverters equipped with the proposed controller is conducted to determine the optimal control parameters. Simulation and experimental results demonstrate the effectiveness of the proposed droop control strategy.

A novel space vector modulation strategy for legs parallel three-level inverters to reduce common-mode voltage

In order to increase the converter’s rated power, the parallel connection of legs between inverters has been an effective way. The common-mode voltage is one of the main problems of this inverter. Based on the special connection mode of legs parallel inverter, the output voltage can be regarded as the output of a five-level inverter. Therefore, this paper proposes a space vector modulation based on the virtual five-level to suppress common-mode voltage. The traditional SVPWM algorithm contains a lot of trigonometric function calculation, which slows down the response speed of the system. This paper proposes a novel coordinate system division method, which avoids trigonometric function calculation and reduces the amount of calculation. The effectiveness of this method is verified by Matlab / Simulink simulation.

Seamless Transition for Parallel Inverters With Novel Self-Adaptive Hybrid Controller and Presynchronization Unit

This article proposes a seamless transition strategy between grid-connected (GC) and stand-alone (SA) states for a microgrid (MG) consisting of parallel inverters. Most inverters in MG operate with novel self-adaptive hybrid controllers, and one inverter near the point of common coupling (PCC) acts as presynchronization (PS) unit. When islanding happens, the inverters based on self-adaptive hybrid control can inherently transfer from grid current control in the GC state to droop control in the SA state, critical islanding detection is needless. When the grid restores, the PS unit can regulate its reactive power to influence the output voltage phase of other inverters in MG and realize the PS of MG and grid. The main advantages are concluded into three points. First, seamless transition of parallel inverters is realized without reconfiguration of control structure, and power-sharing among parallel inverters is achieved without communication lines. Second, unlike droop control, the self-adaptive hybrid control can regulate the grid current of the inverter accurately when the grid fluctuates or is distorted. Third, the PS unit can save the complexity and cost of remote communication for PS. The effectiveness of the proposed seamless transition strategy is verified by simulation and experiment results.

A new virtual oscillator control for synchronization of single-phase parallel inverters in islanded microgrid

ABSTRACT This paper describes the concept of virtual oscillator control (VOC) for parallel-connected single-phase inverters (SPIs). The principal idea is to introduce a series of weakly coupled oscillators (Deadzone oscillator-based VOC and Van der Pol oscillator-based VOC) that can be used for the regulation of single-phase power inverters in an islanded microgrid (MG). Its dynamic equations are used to provide the frequency and amplitude references of the inverters. In these traditional methods, there is always the presence of a 3−order harmonic in the output voltage, which causes a significant amount of 3−order harmonic current in the system. The non-linear dynamical equations of the oscillator are analyzed and its non-linear current source (NCS) is made simpler in order to develop new VOC for SPIs that can effectively get rid of the 3−order harmonic component in the oscillator’s output voltage. Finally, an extensive comparison of Deadzone-based VOC (Dz-VOC), Van der pol-based VOC (VdP-VOC), and new VOC-based controllers with linear and nonlinear loads is presented. The new VOC-based controller minimizes the 3-order harmonic component in the output voltage and achieves a quicker response compared to the conventional VdP-VOC-based controller. Simulation results of the Dz-VOC, VdP-VOC, and proposed new VOC-based control methods with different loads (Resistive, Linear RLC, non-linear) were compared and analyzed in detail. The total harmonic distortion (THD) of the current in Dz-VOC, VdP-VOC, and new VOC are 1.98%, 1.11%, and 0.21%, respectively. The 3rd harmonic is dominant in both Dz and VdP VOCs, while in the new VOC the 3rd harmonic is very less and below 0.1%. The new VOC is also showing good current sharing and fast voltage synchronization (within 0.2 s). Hardware experimentation is also carried out to analyze the efficacy of the proposed new VOC controlled SPI in standalone MG. The results clearly depict that the new VOC control strategy is quite efficient in handling the output voltage harmonics and situations of different loadings in the standalone MG.

Optimization of Voltage Dynamic Performance at Inverter Output with Machine Learning and Intelligent Virtual Impedance

With the continuous improvement of industrial production and manufacturing level, all kinds of equipment are developing in the direction of refinement and precision, especially the equipment manufacturing industry such as automobile manufacturing, equipment, integrated circuits, chip manufacturing, and other semiconductor industries. An intelligent virtual resistance control strategy based on machine research is proposed for rational power distribution and cycle suppression when multiple inverters are operated in parallel in low-voltage microgrids. Based on this, the control strategy of temporary power reduction of inverters is improved to improve the parallel inverter power distribution accuracy and ensure the reliable dynamic response of the constrained current in the circuit between parallel inverters. Finally, a multi-inverter parallel system model is established to verify the correctness and effectiveness of the proposed control strategy. The voltage waveform under nonlinear load conditions is improved and high-frequency harmonics are suppressed, which has obvious advantages compared with the general virtual impedance design method.

Carrier Transition Techniques for Parallel Connected VSCs Using Cross-Coupled Inductors

Parallel inverters can be connected using cross-coupled inductors (CIs) to produce high-quality line voltages in three-phase ac systems. Since no fundamental flux is present in the inductor cores, their size can be made considerably smaller than alternative inductor configurations. Interleaved switching and phase-shifted carriers (PS-PWM) can be used to produce multilevel three-phase line voltages. A modified approach (MPS-PWM) improves the quality of the line voltages by changing the phase of the carriers, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">carrier swapping</i> , based upon the magnitude of the modulating signals. Since carrier swapping in MPS-PWM can cause the inductor circulating currents and flux patterns to jump and have dc offsets, two carrier transition techniques are presented to eliminate these jumps: <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">carrier manipulation</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pulse injection</i> . These techniques represent open-loop PWM control, dependent only on the carrier phase change being used, and can easily be applied to an arbitrary number of parallel-connected inverters. The circulating currents and inductor flux patterns follow predictable patterns, allowing the CIs to be designed more precisely. The relative merits of using MPS-PWM over PS-PWM are assessed using measurements of the load current total harmonic distortion and the line voltage harmonic volt–seconds. The feasibility of the parallel inverter system described is verified using simulations and results from an experimental laboratory prototype.

Parallel inverter control using different conventional control methods and an improved virtual oscillator control method in a standalone microgrid

Partly because of advances in power electronic converters, the share of renewable energy in power generation is steadily increasing. The main medium of interface for integrating renewable energy sources to the utility grid is the power electronic inverter. Virtual oscillator control (VOC) is a time-domain approach for controlling parallel inverters in a standalone microgrid (MG). The concept is to simulate nonlinear deadzone oscillator dynamics in a system of inverters to ensure a stable AC MG in the absence of communication. VOC is a time-domain and self-synchronizing controller that simply requires the measurement of filter current, whereas traditional droop control and the virtual synchronous machine (VSM) require low pass filters for active and reactive power calculations. In this work, a particle swarm optimization (PSO)-based VOC method (VOC-PSO) is proposed, in which the parameters of the VOC are designed using the PSO algorithm. The system performance using droop, VSM, VOC, and VOC-PSO controllers are investigated using MATLAB and Opal-RT real-time digital simulator platforms. The results show that the proposed VOC-PSO gives improved performance over other control strategies. The efficacy of the proposed VOC-PSO control method is also demonstrated by the experimental results.

Modified Droop Strategy for Wide Load Range Efficiency Improvement of Parallel Inverter Systems

Parallel inverters are used in many modern applications, and thus, improving the inverter system efficiency plays a key role in energy savings. The conventional droop strategy used for power sharing among inverters, however, leads to a low efficiency especially at light loads, as the low power demand is divided among inverters, forcing them to process a fraction of the low power at a low efficiency according to their efficiency curve. To avoid such operating conditions, a communicationless modified droop strategy is proposed in this article to select an optimal number of inverters to process fractions of the power demand that leads to a higher system efficiency considering the efficiency curve of the inverters. To achieve this objective at very light load situations, an online-inverter detection method is developed so that each inverter detects the online inverters and the unnecessary inverters turn <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> . The proposed method is employed in a system with three single-phase parallel inverters to evaluate the effectiveness of the method. It is observed that the proposed strategy can improve the system efficiency by up to 14% at light loads compared with the conventional droop. Additionally, the reliability of the system is enhanced by extending the lifetime of inverters with higher power ratings, which are considered as valuable assets of the system. Detailed derivations, simulations, and experimental results are presented to validate the proposed method.

Performance Enhancement of ICN-Based Single-Stage AC–DC Converters Using Reconfigurable Inverters

This article presents a reconfigurable inverter design for the recently proposed impedance control network (ICN) based single-stage ac–dc converters, targeting improvement in their efficiency and power density. This reconfigurable-inverter design effectively compresses the range of input voltages to the ICN converter by operating its two inverters in parallel at low line and in a stacked fashion at high line input. In addition to retaining the wide range zero voltage switching and near zero current switching operation characteristic of the ICN converter, the proposed reconfigurable inverter design benefits all the converter's transistors and magnetic components as it simultaneously reduces the peak voltage stress on its inverter transistors, rms currents through its rectifier transistors, the resonant inductance requirement, and the step-down burden on its transformer, thus improving the overall performance. A novel active voltage balancing scheme for the stacked mode of operation of the two inverters is also proposed. An enhanced version of this active voltage balancing strategy is also discussed, which ensures soft-switching of the inverter transistors by deliberately introducing slight inverter voltage imbalances of appropriate magnitude. A universal-input (90–265 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> ), 20-V output, 330-W, 300-kHz reconfigurable-inverter-based ICN ac–dc converter prototype is built and tested. This prototype achieves a power density of 44.5 W/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , and peak efficiencies of 94% at low line (120 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> ) and 93.8% at high line (230 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> ). Compared to the basic ICN converter prototype, this prototype achieves 17% higher power density and has 23% and 9% less loss at low and high line input, respectively.

A fuzzy sliding mode controller for power quality improvement of solar PV interleaved parallel inverters in a micro-grid

The paper attempts to develop a fuzzy based sliding mode controller (FSMC) for improving the quality of power in an interleaved inverter system. The framework involves boost inverters in parallel, operated from a solar PV source for offering the requirements of an ac load. It orients to evolve a rule based principle for engaging the gain of the FSMC in an effort to improve the quality of power delivered to the load through the inverter. The strategy augurs to derive the reference from the action of the controller and there generates the pulse width modulation (PWM) pulses for the switches in the inverter. The etiquettes incite to ensure equal sharing of current among the inverters in parallel and assuage to regulate the load voltage over the operating range. The MATLAB based simulation fosters brings out the merits of the controller over the fuzzy proportional integral (FPI) controller in terms of a lower THD and higher HSF for the output voltage. The prototype further serves to validate the simulated performance and illustrate its ability to suit real world systems.

Coupled Inductors for High-Frequency Drives With Parallel-Connected Inverter Legs

In this article, coupled inductors with cross-coupled windings are described and designed for voltage source converters with parallel-connected inverter legs in each phase. The nature of the cross-coupled windings produces a very low effective series output inductance, related to the intralimb leakage inductance rather than the higher interlimb leakage. The effective inductance between the parallel inverter output terminals can be made high to reduce circulating currents and is related to inductor magnetizing inductance. These two features make the use of parallel inverters attractive for generating multilevel high-frequency fundamental pulsewidth modulation output voltages with a very low fundamental voltage drop across the coupled inductors. Sample magnetic designs are presented for a three-phase system using three inverter legs connected in parallel in each phase. The effective output inductance of the cross-coupled winding arrangement is compared with that of the coupled windings on separate limbs without cross-coupling and is shown to be 97% smaller. The performance of the two inductor winding arrangements is demonstrated with simulation and experimental results of an 11 kW (300 Vdc, 208 Vac/30 A) laboratory prototype operating with fundamental frequencies in the kilohertz range up to 5 kHz.

Harmonic Resonance Characteristic of Large-Scale PV Plant: Modelling, Analysis, and Engineering Case

Harmonic amplification and interaction of the PV plant which contains multiple parallel inverters influence the renewable power generation and operation seriously. In this article, the resonance mechanism and characteristics of a real large-scale PV plant are explored based on its plant-level circuit model. The component and system models are established first. The distribution rules of wideband resonance frequency from the perspective of the point of common coupling (PCC) and the inverter are respectively determined. Two different resonance bands excited by LCL filter and underground cable are further identified. At last, the practical case of harmonic resonance in the studied PV plant is investigated. Both the measured data and the simulation results support the analytical conclusions. This article combines the theoretical study with the engineering practice, which provides beneficial guidance on the resonance prevention.

Dynamic Circulating Current Suppression Method for Multiple Hybrid Power Parallel Grid-Connected Inverters With Model Reference Adaptive System

Aiming at the problem of large harmonic distortion of grid-connected current in high-power grid-connected power generation system under weak sunlight, multiple hybrid power parallel inverters (MHPPIs) composed of some identical high-power inverters and one low-power inverter in parallel are proposed. According to the irradiance change of sunlight, the low-power inverter operates alone or cooperates with the switching process between the high-power inverters to ensure the low grid-connected current harmonics. Where, any inverters operate in four common working processes, such as soft start-up, steady-state operation, abrupt change of current, and soft stop. Due to the short running time of the dynamic process, most of the existing researches focus on the circulating current suppression under the steady-state condition. However, in engineering practice, the dynamic circulating current in the switching process between inverters is more likely to cause the over-current protection and other safe and stable operation problems of parallel inverters. Therefore, a dynamic circulating current suppression method with dynamic impedance matching for parallel inverters is proposed in this article. The working process of the MHPPIs is equivalent to that of two inverters. The dynamic working process is analyzed in detail, and the dynamic impedance mismatch is the main cause of the dynamic circulating current, so the dynamic virtual inductance matching is introduced to suppress it. Meanwhile, an online inductance identification method based on the model reference adaptive system is presented to compensate for the influence of inaccurate inductance parameter on the circulating current suppression and current control. An experimental prototype of equivalent MHPPIs is built and experimental results prove the validity of the proposed control method.

Mitigation of Circulating Current in Three Phase Quasi-Z-Source Parallel Inverters With PV and Battery Storage

This article presents an insightful analysis on parallel operation of three-phase quasi-Z-source inverters with photovoltaic (PV) and battery storage on input side. Coordinated operation of PV and battery during charging and discharging mode through self-tuned mechanism is also explained. The analysis also includes PV-battery source dynamics, which establishes an interlink between common-mode voltage (CMV), filter ground, and the load neutral currents. This has been utilized to propose a new controller, which reduces the circulating current in quasi-Z-source parallel inverters fed with PV-battery. The performance of proposed controller is verified through experimental validation under equal and unequal load-sharing conditions. Results establish the correctness of analysis and effectiveness of the proposed controller.

Research on Input-Parallel Single-Switch Wireless Power Transfer System With Constant-Current and Constant-Voltage Output

The full-bridge can further improve the output power through parallel inverters. The inverter output voltage of single-switch <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> -resonant circuit is a combination of trapezoidal wave and half-sine wave; there is no relevant research on parallel connection of single-switch <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> -resonant circuit. Therefore, this article presents a novel input-parallel single-switch <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> -resonant circuit; this topology adopts switchable secondary networks for constant-current (CC) and constant-voltage (CV) output, and prevents shoot-through of power switches. The decoupling between transmitters simplifies the analysis and calculation. The input-parallel structure avoids the unbalanced input voltage of each inverter and improves the stability of the wireless power transfer system. CC and CV output modes can be achieved by controlling one relay without adding dc–dc converters or changing the switching frequency. This article includes decoupling equivalent analysis of coils, topologies analysis of CC mode and CV mode, calculation of equivalent input ac voltage source, design of magnetic coupler, and circuit parameters. Finally, a 1-kW experimental prototype is built to verify the theoretical analysis; the maximum efficiency can reach up to 92.5%.

Solar Interfaced Series Inverter With Provision of Common DC Bus Grounding

In this work, a new series solar inverter configuration is proposed to share the power in terms of voltage, unlike parallel inverter configurations. In a single-stage parallel inverter, elevated dc potential and circulating current due to common mode voltage (CMV) would degrade the solar inverter’s life. The proposed topology eases the stress on the dc bus and protects the solar inverter from the issues associated with elevated dc potential (potential induced degradation effect, switch operating voltage stress, etc.). The inherent boosting capability of the proposed series inverter with two modular inverters is demonstrated through two switching algorithms. In the first switching algorithm, the switching combinations are devised to yield the maximum voltage across the load. The second switching algorithm demonstrates the method of eliminating CMV by choosing the appropriate switching combination of two inverters. The eliminated instantaneous CMV would give the provision of operating proposed topology with common dc bus along with the flexibility of dc bus grounding. For the proposed configuration with devised switching techniques, the closed-loop controller is designed by computing the plant’s equivalent characteristic impedance using a state-space model. The effectiveness of the proposed configuration, along with the closed-loop control, is validated through hardware.