Parallel Three-phase Inverters Research Articles

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.

Composite Sliding Mode Control of Phase Circulating Current for the Parallel Three-Phase Inverter Systems

The phase circulating current (PCC) of the parallel three-phase inverter systems dramatically affects the power quality and conversion efficiency of the power grid. In this paper, a composite suppression strategy is proposed to solve the PCC issue by using the sliding mode control (SMC) approach and improved virtual impedance droop control. Taking the commonly used 2-group parallel three-phase inverter as an example, an inter- and intra-classification model is established by analyzing the sources of PCC. In order to suppress the inter-PCC, the traditional virtual impedance droop control is given, following the improved substitute by combining SMC. And the variables of the bus voltage, Q-U loop, P-f loop, and the virtual-induced reactance are also introduced for the robust control of the impedance droop. On the other side, a SMC-based suppression approach is designed to solve the issue of the intra-PCC. Its idea is to introduce a regulation factor for the space vector pulse width modulation (SVPWM) so that the zero-sequence voltage can be eliminated and the influence of the intra-PCC can be relieved. Comparative simulations and experiments validate the effectiveness of the methods proposed in this paper.

Simulative Study to Reduce DC-Link Capacitor of Drive Train for Electric Vehicles

E-mobility is an emerging means of transportation, mainly due to the environmental impact of petroleum-based fuel vehicles and oil prices’ peak. However, electric vehicles face several challenges by the nature of technology. Consequently, electric vehicles have a limited travel range and are extremely heavy. In this research, an investigation is carried out on different measures to reduce the DC-link capacitor size in the drive train of an electric vehicle. The investigation is based on software simulations. The DC-link capacitor must be dimensioned with regards to relevant points of operation, which are defined by the rotation speed and torque of the motor as well as the available DC-link voltage. This also includes the field-oriented control (FOC). In order to optimally operate a three-phase inverter in the electric drive train, a suitable type and sizing of the capacitor was studied based on mathematical equations and simulations. Two measures were examined in this study: firstly, an auxiliary passive notch filter introduced in the electric drive train circuit is explored. Based on this measure, an advanced modulation scheme exploiting the control of individual currents within segmented windings of the PMSM is investigated in detail. It was seen that saw-tooth carrier modulation used in the parallel three-phase inverter is found to reduce DC-link capacitor size in the electric drive train circuit by 70%.

Sensorless Robust Flatness-Based Control With Nonlinear Observer for Non-Ideal Parallel DC–AC Inverters

Parallel operation of multiple AC/DC inverters is favorable in hybrid AC/DC microgrids to avert consecutive conversion stages and increase the system’s efficacy. Yet, several emerging technical challenges impede the expansion of such layout such as the circulating currents, sensitiveness to the input voltage disturbances, load variations and complexity of the control structures caused by using several required measurements. This paper proposes a one-loop sensorless controller which is based on the flatness technique for a non-isolated power supply consisting of n-parallel inverters. The proposed control scheme primarily relies on employing a nonlinear online observer to estimate the line inductor currents and the dc link voltage via information from the input voltage, output voltage, and load conditions to avoid using excessive sensors. In this way, the system reliability is improved by reducing burdens of the communication delays and/or the failures, signal noise, thus the system is featured by simple control. Besides, the system entire losses are modeled by equivalent voltage sources and one current source which implicitly represent all types of the losses by using an online nonlinear estimator for the control purposes. The proposed controller not only has high dynamic performance, wide-bandwidth, low voltage THD but also robust to the abrupt variations in the load and the input voltage. To validate the applicability of the proposed control method and the observer, both simulations and experimental investigations are performed for two paralleled three-phase inverters setup. The obtained results assure the effectiveness of the proposed control method in regulating the output voltage of the parallel DC/AC inverters with fewer number of the sensors against the fluctuations of the input dc voltage and the load perturbations.

Controlling the Direct Connected Parallel Three-Phase Voltage Source Inverters by Using Neural networks

This paper demonstrates that neural networks can be used effectively for the controlling the direct connection of parallel three-¬phase voltage source inverters..A unique feature of the parallel three-phase inverters is a zero--sequence circulating current. This work proposes a new zero-sequence control for parallel three- phase voltage source inverter using a neural networks to assist the Proportional-Integral controller against the variation on the two inverters to maintain the value of a zero-sequence current within an acceptable value. The controller of the zero-sequence current can be implemented within an individual inverter and it is independent of the other control loops of the inverter, therefore, it greatly facilitates design and expansion of parallel system.Simulation and results can be obtained by using mathematical software such as MATLAB version seven to show the performance of the controllers and the closed loop system.Two-layers feedforward neural networks FNN's containing a Levenberg-Marquardt training algorithm are used.

Research on Parallel Control Technology of Three-phase Inverter Based on Multiple Proportional Resonance Controller

The parallel operation of multiple inverters can increase the fault tolerance of the system and prevent the occurrence of major accidents. At the same time, it also has strong flexibility, which can reduce the volume and weight of the system and increase the capacity of the system, which is very attractive in the application of uninterruptible power supply. In the inverter parallel system, the circulation will be generated between the inverters due to the existence of the path. Circulation can cause current distortion, reduce system performance, and even damage equipment in severe cases. In order to effectively suppress the generation of circulation, this paper proposes a multiple proportional resonance control strategy for the parallel three-phase inverter system, that is, the voltage outer loop is controlled by PI+PR, and the current inner loop is controlled by PR. Two three-phase inverter parallel systems are built in MATLAB/Simulink simulation environment to compare and analyze the control effect of uncontrolled, traditional PI control and multiple PR control. The simulation results show that the proposed control method has a good suppression effect on the circulation generated in the inverter parallel system, even under the condition of unbalanced load.

Robust Model Predictive Control (MPC) for large-scale PV plant based on paralleled three-phase inverters

In this contribution a robust Model Predictive Control (MPC) is proposed to enhance the power quality of a large-scale PV plant connected to the grid through Paralleled Voltage Source Inverters (PVSIs) with common AC and DC buses. Paralleling inverters allow handling high-power export and offer advantages in terms of redundancy which ensure the system reliability. However, due to the physical differences and parameter disparities between the inverters, zero sequence circulating currents will flow through it, which will disturb the performance of the system. Hence, the control goal is to regulate the currents injected into the grid, suppress the zero-sequence circulating current (ZSCC). Consequently, this study proposes an MPC algorithm that is based on optimization approach which allows minimizing circulating currents. In order to show its effectiveness and performance of the proposed control, a comparison with linear PI controller is included. In addition, design control and tuning procedure are detailed. Simulation results show the performance of the proposed controller in ensuring power quality, and suppressing circulating currents. To verify the real-time feasibility of the proposed control scheme, Hardware-In-the-Loop (HIL) setup is carried out with means of Opal-RT and dSPACE rapid prototyping systems.

Power sharing strategy in islanded microgrids using improved droop control

This paper presents a power sharing control method for 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 with two or three inverters are connected in parallel. Simulation results demonstrate the accuracy of the proposed control method. Futhermore, an experimental setup was built in the laboratory. Results obtained from the experimental setup verify the effectiveness of the proposed method.

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.

Sliding Mode Control and Modified SVM for Suppressing Circulating Currents in Parallel-Connected Inverters

Parallel operation of multi-inverters is an effective way to increase the power capacity and the reliability of power electronic systems. However, in parallel inverter operation, one of the most crucial problems is the zero-sequence circulating current (ZSCC). The ZSCC flows between parallel inverters, which results in current distortion, harmonic loss, unbalanced load sharing, and a decline in the overall system performance. In this paper, a sliding mode ZSCC controller for paralleled three-phase inverters is proposed. The suggested controller takes advantage of zero-vector redundancies to adjust their dwell time and cancel any voltage difference that can generate ZSCC between the paralleled inverters. The appropriate adjustment quantity is determined using a sliding mode controller, and as a result, a better ZSCC cancelation can be achieved at different operation conditions. Experimental results of two parallel inverters show the feasibility of the proposed ZSCC elimination method.

Synchronization of the carrier wave of parallel three-phase inverters with virtual oscillator control

In this paper, a method to synchronize the carrier wave of parallel three-phase inverters without communication is proposed. This method is based on the digital-based virtual dead-zone oscillators coupled with each other through electrical connections between parallel inverters. These oscillators are synchronized based on the phenomenon of synchronization in networks of coupled oscillators. Thus, the carrier waves of different inverters are confirmed to be synchronized since they are generated by the oscillators. The model of the circulating current and the sufficient condition of global synchronization of oscillators are derived in this paper. The methodology for controller design is outlined based on the aforementioned analysis. Experimental results for a system with three inverters are given to illustrate the effectiveness of the proposed control.

Resonance Interaction of Multiparallel Grid-Connected Inverters With LCL Filter

This letter investigates the resonance characteristics and stability problem caused by the interactions of multiparallel LCL -filtered inverters. Compared to single grid-connected inverter, the multiinverter system presents a more challenging resonance issue, where the inverter interactions may excite multiple resonances at various frequencies. This letter proposes a modeling and analysis method based on the current separation scheme. It reveals that an interactive resonant current that circulates between the paralleled three-phase inverters may arise, depending on the current distribution and the system resonance characteristics. This is different from the common resonant current injected into the grid and has yet been discussed before. The analysis with the root loci in the discrete z -domain are carried out with two stability constraints identified, which correspond to the conditions of triggering the interactive resonant current and common resonant current, respectively. Simulations and experimental results verify the theoretical analysis.

New adaptive droop control with combined line impedance estimation method for parallel inverters

This paper presents a new load sharing control between paralleled three-phase inverters in an islanded-microgrid based on the line impedance estimation online by the use of the Kalman filter. We can solve the mismatch of power sharing when the line impedance changes due to the temperature and frequency, significant differences of line parameters and requirements of Plug-and-Play mode of inverters connected to the microgrid. Moreover, the paper also presents a new Droop control method working with the line impedance which is different from the Droop traditional algorithm when the line impedance is assumed pure resistance R or pure inductance X. In the paper, the line impedance estimation for parallel inverters uses the least squares method combined with Kalman filter. In addition, secondary control loops are designed to restore the voltage amplitude and frequency of the microgrid by using a combined nominal value SOGI-PLL with generalized integral block and phase lock loop to exactly monitor the voltage magnitude and frequency phase at common PCC. Control model has been simulated in Matlab/Simulink with three voltage source inverters connected in parallel for different ratios of the power sharing. The simulation results have shown the accuracy of the proposed control method. Therefore, the proposed adaptive droop control method based on line impedance estimation can be an alternative one for load sharing control in islanded microgrids.

A New Adaptive Control Strategy of active and reactive power sharing in Islanded Microgrids

This paper proposes a new control sharing method for parallel three-phase inverters in an islanded microgrid. The proposed technique uses adaptive PIDs combined with the communication among the parallel inverters to accurately share active power and reactive power among the inverters via adjusting the desired voltage if there is a distinct difference between line impedance and the load change in the microgrid. Moreover, the paper also presents the response ability of the inverters to maintain the error within the allowed limits as the transmission line is interrupted. The proposed technique has been verified in a microgrid with three parallel distributed generation-inverter units using Matlab/Simulink. In the simulation, as the droop control using the communication information among the inverters, the sharing errors for active power and reactive power are around 0.2% and 0.6%, respectively. As the connection between the microgrid and transmission line is interrupted, the sharing errors for active power and reactive power increase to 0.4% and 2%, respectively. The simulation results have indicated that the proposed technique is superior to the traditional droop control in terms of the accuracy and stability. Therefore, the new proposed technique can be a favor alternative model for active power and reactive power sharing control of parallel inverters in an islanded microgrid.

A New Way of Controlling Parallel-Connected Inverters by Using Synchronous-Reference-Frame Virtual Impedance Loop—Part I: Control Principle

A novel simple and effective autonomous current-sharing controller for parallel three-phase inverters is proposed in this paper. The proposed controller provides faster response and better accuracy in contrast to the conventional droop control, since this novel approach does not require any active or reactive power calculations. Instead, a synchronous-reference-frame (SRF) virtual impedance loop and an SRF-based phase-locked loop are used. Stationary analysis is provided in order to identify the inherent mechanism of the direct and quadrature output currents in relation to the voltage amplitude and frequency with different line impedances by means of the system transfer functions. Comparison experiments from two parallel inverters are presented to compare the control performance of the conventional droop control and the proposed control with different line impedances. In addition, experimental results from a setup with three parallel 2.2-kW inverters verify the effectiveness of the proposed control strategy in different scenarios.

Circulating zero-sequence current control of parallel three-phase inverters

A novel circulating zero-sequence current-control method for common DC-source parallel inverter systems is proposed and implemented in FPGA. The follower inverter can closely track the three-phase discontinuous PWM modulation waveforms of the master inverter and the circulating current caused by asynchronous modulation waveforms can be reduced. The controller is implemented in FPGA, and the requirement of high-speed control and measurement can be satisfied without any extra burden added to the processor of the drive system. The theoretical analysis of the experimental results verifies the performance of the proposed circulating-current-control method.