Parallel-connected Inverter System Research Articles

Study of a Synchronization System for Distributed Inverters Conceived for FPGA Devices

In a multiple parallel-connected inverters system, limiting the circulating current phenomenon is mandatory since it may influence efficiency and reliability. In this paper, a new control method aimed at this purpose and conceived to be implemented on a Field Programmable Gate Array (FPGA) device is presented. Each of the inverters, connected in parallel, is conceived to be equipped with an FPGA that controls the Pulse-Width Modulation (PWM) waveform without intercommunication with the others. The hardware implemented is the same for every inverter; therefore, the addition of a new module does not require redesign, enhancing system modularity. The system has been simulated in a Simulink environment. To study its behavior and to improve the control method, simulations with two parallel-connected inverters have been firstly conducted, then additional simulations have been performed with increasing complexity to demonstrate the quality of the algorithm. The results prove the ability of the method proposed to limit the circulating currents to negligible values.

Transient Optimization of Parallel Connected Inverters in Islanded AC Microgrids

Along with the rapid growth in green energy utilization, microgrids are becoming an interesting part of the future power systems. This paper presents a transient trajectory optimization of parallel connected inverter system in an islanded microgrid. Optimal state and control transient trajectories are generated which drive each individual inverter from given initial conditions to their respective desired steady state equilibrium. This trajectory optimization is performed in decentralized manner and they are the open loop, local optimal control signals of the inverters. The Pontryagin’s minimum principle is employed to obtain the optimal state and control signal trajectories. Then, a stability analysis of the system with the local optimal control signals is done by a Lyapunov approach. Dynamic model of the system and the optimum trajectories are obtained in the d-q reference frame. Two example microgrid systems, one with three inverters and the other one with five inverters, were used to demonstrate the effectiveness and performance of the proposed approach which can be easily extended to any type of microgrid such as AC, DC, hybrid microgrids in both islanded or grid connected mode.

Droop control technique for equal power sharing in islanded microgrid

This paper presents a droop control technique for equal power sharing in islanded microgrid. In this study, the proposed controller is based on the frequency droop method, is applied to a robust droop controller in parallel connected inverters. The previous robust droop controller deals with voltage droop method. A modification has been formed against this controller by adding a fuzzy logic controller with the frequency droop method. The only sharing error which is concentrated in this paper is the error in sharing the rated frequency among the inverters. By adapting fuzzy in the robust droop, it tries to eliminate the frequency error, hence that the frequency reference of the inverters keeps maintain at 50Hz. A derivation of generalized models of a single-phase parallel-connected inverter system is shown. The simulation results show that the proposed controller with FLC is able to improve the stability of frequency reference and the performance of power sharing between the inverters under the inductive line impedance.

Decentralized Power Sharing Control for Parallel-Connected Inverters in Islanded Single-Phase Micro-Grids

In this paper, a Lyapunov function-based decentralized control scheme is proposed for a single-phase parallel-connected inverters system within islanded micro-grid. With this novel method, a proper voltage regulation (e.g., amplitude and waveform quality) and excellent power and current sharing performance can be well ensured even in the presence of unknown line impedance and unbalanced LC filter parameters. In addition, high reliability and robustness can be guaranteed since no inter-communication among inverters as well as no centralized controller is demanded, which also delivers advantages of scalability and redundancy. The stability of the closed-loop system is ensured by standard Lyapunov method. The designation of the decentralized controller is simple to implement but rather effective. Detailed simulation and hardware-in-the-loop experimental results are presented to validate the feasibility of the proposed scheme under different operating conditions.

Reduction in Current THD of Grid Parallel Inverters Using Randomized PR Control

In grid connected system, unity power factor current injection into the grid is vital. This can be achieved by choosing the right inverter topology, passive filter components, current controllers, and PWM switching scheme. This paper compares the output current harmonics profile between when using the conventional proportional resonant (PR) current controller and when using the modified PR current controller. By applying the latter technique, via experimental validation using TMS320F2812, the THD of the injected grid current in a parallel connected inverters system is improved.

Theoretical and Experimental Investigation of Networked Control for Parallel Operation of Inverters

This paper presents an inverter parallel system with a corresponding networked control strategy for parallel operations. In this system, all the modules have the same circuit configuration and two control loops: the inner control loop is used for output voltage regulation, and the outer loop is used for accurate power-sharing regulation formulated by a weighted power function with networked data. The method with low-bandwidth communication is employed to achieve the superior load-sharing accuracy compared to droop-only scheme. In addition, good robustness can be obtained in the case of communication failure. A state-space model has been developed to describe the dynamics of the active and reactive power of the networked parallel-connected inverter system. We also present the methodologies of predicting stability for system with time delay and data drop-out due to the use of network-in-the-loop of this networked parallel inverter system. Experimental results are also shown to validate the effectiveness and advantages of the proposed networked parallel inverter system.

Improved Instantaneous Average Current-Sharing Control Scheme for Parallel-Connected Inverter Considering Line Impedance Impact in Microgrid Networks

A new control scheme for parallel-connected inverters taking into account the effect of line impedance is presented. The system presented here consists of two single-phase inverters connected in parallel. The control technique is based on instantaneous average current-sharing control that requires interconnections among inverters for information sharing. A generalized model of a single-phase parallel-connected inverter system is derived. The model incorporates the detail of the control loops that use a proportional-resonant controller, but not the switching action. The voltage- and current-controller design and parameters selection process are discussed. Adaptive gain scheduling is introduced to the controller to improve the current and power sharing for a condition, where the line impedance is different among the inverters. The simulation results show that the adaptive gain-scheduling approaches introduced improve the performance of conventional controller in terms of current and power sharing between inverters under difference line impedance condition. The experiments validate the proposed system performance.

Model Predictive Control of Parallel-Connected Inverters for Uninterruptible Power Supplies

Uninterruptible power supplies (UPSs) have been used in many installations for critical loads that cannot afford power failure or surge during operation. It is often difficult to upgrade the UPS system as the load grows over time. Due to lower cost and maintenance, as well as ease of increasing system capacity, the parallel operation of modularized small-power UPS has attracted much attention in recent years. In this paper, a new scheme for parallel operation of inverters is introduced. A multiple-input-multiple-output state-space model is developed to describe the parallel-connected inverters system, and a model-predictive-control scheme suitable for paralleled inverters control is proposed. In this algorithm, the control objectives of voltage tracking and current sharing are formulated using a weighted cost function. The effectiveness and the hot-swap capability of the proposed parallel-connected inverters system have been verified with experimental results.

Wireless-Control Strategy for Parallel Operation of Distributed-Generation Inverters

In this paper, a method for the parallel operation of inverters in an ac-distributed system is proposed. This paper explores the control of active and reactive power flow through the analysis of the output impedance of the inverters and its impact on the power sharing. As a result, adaptive virtual output impedance is proposed in order to achieve a proper reactive power sharing, regardless of the line-impedance unbalances. A soft-start operation is also included, avoiding the initial current peak, which results in a seamless hot-swap operation. Active power sharing is achieved by adjusting the frequency in load transient situations only, owing to which the proposed method obtains a constant steady-state frequency and amplitude. As opposed to the conventional droop method, the transient response can be modified by acting on the main control parameters. Linear and nonlinear loads can be properly shared due to the addition of a current harmonic loop in the control strategy. Experimental results are presented from a two-6-kVA parallel-connected inverter system, showing the feasibility of the proposed approach