Parallel Operation Of Inverters Research Articles

Structural Resemblance Between Droop Controllers and Phase-Locked Loops

It is well known that droop control is fundamental to the operation of power systems and now the parallel operation of inverters, while phase-locked loops (PLLs) are widely adopted in modern electrical engineering. In this paper, it is shown at first that droop control and PLLs structurally resemble each other. This bridges the gap between the two communities working on droop control and PLLs. As a result, droop controllers and PLLs can be improved and further developed via adopting the advancements in the other field. This finding is then applied to operate the conventional droop controller for inverters with inductive output impedance to achieve the function of PLLs, without having a dedicated synchronization unit. Extensive experimental results are provided to validate the theoretical analysis.

Universal Droop Control of Inverters With Different Types of Output Impedance

Droop control is a well-known strategy for the parallel operation of inverters. However, the droop control strategy changes its form for inverters with different types of output impedance, and so far, it is impossible to operate inverters with inductive and capacitive output impedances in parallel. In this paper, it is shown that there exists a universal droop control principle for inverters with output impedance having a phase angle between $- ({\pi }/{2})$ rad and $({\pi }/{2})$ rad. It takes the form of the droop control for inverters with resistive output impedance ( $R$ -inverters). Hence, the robust droop controller recently proposed in the literature for $R$ -inverters actually provides one way to implement such a universal droop controller that can be applied to all practical inverters without the need of knowing the impedance angle. The small-signal stability of an inverter equipped with the universal droop controller is analyzed, and it is shown to be stable when the phase angle of the output impedance changes from $- ({\pi }/{2})$ rad to $({\pi }/{2})$ rad. Both real-time simulation results and experimental results from a test rig consisting of an $R$ -inverter, an $L$ -inverter, and a $C$ -inverter operated in parallel are presented to validate the proposed strategy.

Parallel Operation of Modular Single-Phase Transformerless Grid-Tied PV Inverters With Common DC Bus and AC Bus

In order to enhance the efficiency and reliability of dc-module-type photovoltaic (PV) generation systems, the single-phase transformerless full-bridge grid-tied inverters with a common dc bus and ac bus are utilized as the dc–ac stage. The high-frequency circulating current issue of parallel-operated single-phase grid-tied inverters with different topologies and modulation strategies are investigated. The conditions of high-frequency circulating currents elimination without additional hardware costs are revealed and presented. The analysis indicates that there are no high-frequency circulating currents between parallel-operated inverters with bipolar pulsewidth modulation (PWM). The parallel-operated inverters with unipolar double frequency PWM have high-frequency circulating currents, but the effects of the high-frequency circuiting currents on the fundamental components of split-filter inductor currents can be mitigated. The parallel-operated inverters with a unipolar PWM also have high-frequency circulating currents, but their effects on the fundamental components of split-filter inductor currents cannot be reduced. The single-phase transformerless full-bridge inverters with low-leakage currents, such as H5 and highly efficient and reliable inverter concept, meet the elimination conditions of the high-frequency circulating currents. Hence, these inverters are free of high-frequency circulating currents. A prototype of dc module-type grid-tied PV generation system is built up, and experimental tests with different topologies and PWM methods verify the analysis.

Synchronization of Identical Oscillators Coupled Through a Symmetric Network With Dynamics: A Constructive Approach With Applications to Parallel Operation of Inverters

We consider the problem of synchronizing a group of $N$ identical oscillators, coupled by a symmetric network that is modelled by a multiple-input/multiple-output dynamical system. We provide results that can be used to establish asymptotic synchronization of a given system and also to construct identical feedback oscillators for which synchronization is guaranteed. These results are based on a new notion of passivity with respect to manifolds defined in the input and output spaces of a dynamical system. The problem under consideration is motivated by the design of control algorithms for the parallel operation of power inverters. Simulation results have shown that synchronization could be achieved in less than two AC cycles, depending on the electrical interconnection network.

Survey of Droop Control Technique for Parallel Inverter Operation

Increasing demand for large-scale power delivery with high reliability and quality necessitates the use of multiple distributed energy resources to be connected in parallel; for example, parallel inverters operation. Proper operation of parallel inverters requires sophisticated control techniques and many have been proposed and continue to evolve. This paper depicts the droop control techniques for the parallel inverter systems. The recent improvements and variants of this control technique are presented

Bounded droop controller for parallel operation of inverters

In this paper, the stability of parallel-operated inverters in the sense of boundedness is investigated. At first, the non-linear model of parallelled inverters with a generic linear or non-linear load is obtained by using the generalised dissipative Hamiltonian structure and then the robust droop controller, recently proposed in the literature for parallel operation of inverters, is implemented in a way to produce a bounded control output. The proposed controller is called the bounded droop controller (BDC). It introduces a zero-gain property and can guarantee the boundedness of the closed-loop system solution. Therefore, for the first time, the closed-loop stability in the sense of boundedness is guaranteed for parallelled inverters feeding generic non-linear/linear loads. The controller structure is further improved to increase its robustness with respect to initial conditions, numerical errors or external disturbances while maintaining the stability property. Moreover, the controller is tuned to avoid any possible limit cycles in the voltage dynamics. Real-time simulation results for two single-phase inverters operated in parallel loaded with a non-linear load are presented to verify the effectiveness of the proposed BDC.

Design of Quadratic Optimal Regulator System for State Space Model of Single phase Inverter both in Standalone and Gridtie Modes

Since in 3-phase systems, the reliability of 3-phase Inverter is not good, under such situations Paralleled Inverter Systems are used. Parallel Inverter operation has a major role in uninterruptible power system (UPS) applications. Most of the standalone inverter systems use a LC filter and proportional-integral (PI) controller in their control loops. When connecting the paralleled inverters to utility grids, the capacitor becomes redundant and thus either a pure inductor or an LCL filter can be used as inverter output stage. Compared with the L filter, the LCL filter is more attractive because it cannot only provide higher harmonics attenuation with same inductance value, but also allow inverter to operate both in standalone and grid-tie modes, which makes it a universal inverter for distributed generation applications. Output of Inverter should always be checked. In this paper State Space Model of Single phase Inverter is analysis in two modes which are: a) Grid-tie Mode b) Standalone Mode.A Quadratic Optimal Regulator System is designed and its unit step response is obtained to validate whether the whole system (inverter system along with designed QOR) is a stable system or not. This is done using MATLAB Program.

A control strategy for microgrid inverters based on adaptive three-order sliding mode and optimized droop controls

Robust control and seamless formation are the two crucial problems that affect smart microgrids. This paper proposes a new solution for microgrid inverters in terms of circuit topology and control structure. The combined three-phase four-wire inverter, which is composed of three single-phase full-bridge circuits, is adopted in this study. The control structure is based on the inner adaptive three-order sliding-mode closed-loop, the immediate virtual output-impedance loop, and the outer power control loop. Three significant contributions are obtained: (1) the microgrid inverters effectively reject both voltage and load disturbances with the adaptive sliding-mode controllers regardless of whether the inverters are operating in the grid-connected mode, islanding mode, or transition from the grid-connected mode to the islanding mode; (2) the virtual output impedance loop is applied to make a resistive equivalent output impedance of the inverters and to meet the requirements of the inverter parallel operation in the islanding mode; (3) the proposed droop method reduces the line inductive impedance effects and improves the power sharing accuracy by optimizing the droop coefficients. The theoretical analysis and test results validate the effectiveness of the proposed control scheme.

Virtual Flux Droop Method—A New Control Strategy of Inverters in Microgrids

The parallel operation of inverters in microgrids is mainly based on the droop method. The conventional voltage droop method consists of adjusting the output voltage frequency and amplitude to achieve autonomous power sharing without control wire interconnections. Nevertheless, the conventional voltage droop method shows several drawbacks, such as complicated inner multiloop feedback control, and most importantly, frequency and voltage deviations. This paper proposes a new control strategy in microgrid applications by drooping the virtual flux instead of the inverter output voltage. First, the relationship between the inverter virtual flux and the active and reactive powers is mathematically obtained. This is used to develop a new flux droop method. In addition, a small-signal model is developed in order to design the main control parameters and study the system dynamics and stability. Furthermore, a direct flux control algorithm is employed to regulate the virtual flux according to the droop controller, which avoids the use of proportional-integral controllers and pulse-width modulation modulators. Both the simulation and experimental results show that the proposed flux droop strategy can achieve active and reactive power sharing with much lower frequency deviation than the conventional voltage droop method, thus highlighting the potential use in microgrid applications.

A System-Level Control Strategy of Photovoltaic Grid-Tied Generation Systems for European Efficiency Enhancement

A European efficiency enhancement strategy for the grid-tied inverters with common dc bus and ac bus in parallel operation is proposed in this paper. Based on the inherent relationship between the dc bus voltage and the grid-feeding power, two dc bus voltage regulators with different output limits are employed in each grid-tied inverter. The operation of the dc bus voltage regulators depends on the grid-feeding power level. Therefore, the grid-tied inverter has priority up to the maximum efficiency operation point, and then, to operate at full load mode with the increase of the grid-feeding power. As a result, the European efficiency of the grid-tied system is improved without any communication link and extra hardware cost. The operation principle of the grid-tied inverter system is analyzed in detail with the implementation of the proposed control, and the design guidelines for key parameters are given and discussed. A prototype with three grid-tied inverters in parallel operation is built and experimental tests have been conducted on it in terms of steady-state performances and dynamic performances. The experimental results show that the operation modes of each grid-tied inverter depend on its sensing gain of the dc bus voltage. The European efficiency of the grid-tied inverter system with the conventional control and with the proposed control are 94.74% and 95.17%, respectively, which means the European efficiency has been enhanced by 0.43%. These results verify the feasibility and effectiveness of the proposed control strategy.

Robust Droop Controller for Accurate Proportional Load Sharing Among Inverters Operated in Parallel

In this paper, the inherent limitations of the conventional droop control scheme are revealed. It has been proven that parallel-operated inverters should have the same per-unit impedance in order for them to share the load accurately in proportion to their power ratings when the conventional droop control scheme is adopted. The droop controllers should also generate the same voltage set-point for the inverters. Both conditions are difficult to meet in practice, which results in errors in proportional load sharing. An improved droop controller is then proposed to achieve accurate proportional load sharing without meeting these two requirements and to reduce the load voltage drop due to the load effect and the droop effect. The load voltage can be maintained within the desired range around the rated value. The strategy is robust against numerical errors, disturbances, noises, feeder impedance, parameter drifts and component mismatches. The only sharing error, which is quantified in this paper, comes from the error in measuring the load voltage. When there are errors in the voltage measured, a fundamental tradeoff between the voltage drop and the sharing accuracy appears. It has also been explained that, in order to avoid errors in power sharing, the global settings of the rated voltage and frequency should be accurate. Experimental results are provided to verify the analysis and design.

Novel Digital Current Sharing Control Technique on Stand-Alone Photovoltaic Inverter

To share the loads and restrain circumfluence among parallel inverters in Stand- alone photovoltaic system (PV), it is described that a digital current sharing control technique for inverter parallel operation in detail in this paper. After the analysis of circumfluence of paralleled system on distributed-logical-control, characteristic of the power regulation, a novel digital power detection technique is deduced. With measuring and analyzing the instantaneous average current of all inverters and the output current of each inverter, both of the current components are used to control and regulate the amplitude and the phase of the output voltage. The proposed control technique on current sharing in the inverter paralleling system has been verified the good characteristic of restraining circumfluence by the results of the experiment.

Centralized Control for Parallel Operation of Distributed Generation Inverters in Microgrids

This paper presents a centralized control system that coordinates parallel operations of different distributed generation (DG) inverters within a microgrid. The control design for the DG inverters employs a new Model Predictive Control algorithm that allows faster computational time for large power systems by optimizing the steady-state and the transient control problems separately. An overall energy management system is also implemented for the microgrid to coordinate load sharing among different DG units during both grid-connected and islanded operations. The design concept of the proposed control system is evaluated through simulation studies under different test scenarios. The impact of the increased penetration of DG units on the distribution grid is also investigated using the proposed microgrid. The simulation results show that the operations of the DG units within the microgrid can be coordinated effectively under the proposed control system to ensure stable operation of the overall microgrid.

Voltage and Frequency Droop Control in an Autonomous Microgrid

In this paper, a method for the parallel operation of inverters in an autonomous microgrid system is adopted. This paper presents the resistive output impedance control scheme that allows multiple voltage source converters (VSCs) to operate in parallel in a VSC fed microgrid. The control loops are taking into account the special nature of a low-voltage microgrid, in which the line impedance is mainly resistive. In contrast with the conventional droop-control method, the proposed controller uses a virtual resistance without communication signals to achieve good power sharing, which is insensitive to line-impedance unbalances.

Multilayer Control for Inverters in Parallel Operation Without Intercommunications

In this paper, a multilayer control is proposed for inverters that are able to operate in parallel without intercommunications. The first control layer is an improved droop method that introduces power proportional terms into the conventional droop scheme, letting both active and reactive power to be shared among the inverters. The second layer is designed to compensate the voltage deviations caused by the aforementioned droop control, thus improving the load-voltage regulation of the system. The third layer is a quasi-synchronization control that roughly adjusts the angle of the inverter to be close to the common ac bus. This layer ensures that the phase difference of each inverter is inside a limited margin with the help of the phase signal sensed from the common ac bus. The principle of operation of the control scheme has been analyzed in detail. A small-signal model has been developed in order to study the system dynamics, which can be used for adjusting the main control parameters. A prototype consisting of a two 35-kVA-inverter system has been built and tested in order to verify the feasibility of the proposed approach.

Differential-Evolution-Based Optimization of the Dynamic Response for Parallel Operation of Inverters With No Controller Interconnection

In this paper, the use of differential evolution (DE), a global search technique inspired by evolutionary theory, to find the parameters that are required to achieve optimum dynamic response of parallel operation of inverters with no interconnection among the controllers is proposed. Basically, in order to reach such a goal, the system is modeled in a certain way that the slopes of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> - ω and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> curves are the parameters to be tuned. Such parameters, when properly tuned, result in system's eigenvalues located in positions that assure the system's stability and oscillation-free dynamic response with minimum settling time. This paper describes the modeling approach and provides an overview of the motivation for the optimization and a description of the DE technique. Simulation and experimental results are also presented, and they show the viability of the proposed method.

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.

Simulation and Analysis of a Compact Electronic Infrastructure for DC Micro-Grid: Necessity and Challenges

Complex circuitry of electronic infrastructure of compact micro-grids with multiple renewable energy sources feeding the loads using parallel operation of inverters acts as a deterrent in developing such systems. This paper deals with applicable techniques reducing the driving circuits in parallel power inverters used in micro-grid system (MGS), mainly focused on the distributed generation (DG) in islanded mode. The method introduced in this paper, gives a minimal and compressed circuitry that can be implemented very cost-effectively with simple components. DC micro-grids are proposed and researched for the good connection with DC output type sources such as photovoltaic (PV), fuel cell, and secondary battery. In this paper, the electronic infrastructure of micro-grid is expressed. Then discussed the reasons for its complexity and the possibility of reducing the elements of electronic circuits are investigated. The reason for this is in order to compact DC micro-grid system for electrification to places like villages. Digital Simulation in Matlab Simulink is used to show the effectiveness of this novel driver topology for parallel operating inverters (NDTPI).

Distributed Control for AC Motor Drive Inverters in Parallel Operation

A distributed control for pulsewidth modulation inverters in parallel operation to feed an asynchronous motor is proposed. The phase angle of the sinusoidal reference for each inverter is represented as a square signal and shared among the inverters and is pre-synchronized cycle-by-cycle with each other. And then the current sharing is regulated by introducing the circulating current into the mild adjustment of the sinusoidal references. All the sense and control for parallel operation are embedded in inverters distributed and a redundant power system is achieved. A prototype is built with experimental results given to verify the control proposed.

A Modular Grid-Connected Photovoltaic Generation System Based on DC Bus

In order to enhance the efficiency and reliability for distributed microgeneration, a modular grid-connected photovoltaic (PV) generation system is proposed. It consists of modular dc-dc converters and modular dc-ac inverters. The outputs of dc-dc converter and the inputs of dc-ac inverter share a dc bus. AC current coupling between the parallel-operated inverters, which is the key issue in this generation system, has been investigated. A current-decoupling method is proposed and implemented by regulating the currents of split-filter inductors, respectively. An optimal control strategy for the efficiency enhancement of PV generation system is proposed by utilizing the dispersion of control parameters. During power generation, only one modular dc-ac inverter is operating with nonfull load, and the other modular dc-ac inverters are operating with full load or at standby mode. A prototype of this modularized grid-connected PV generation system is implemented. The steady state and dynamic experimental results show that the fundamental components of two split-filter inductor currents in one inverter module are equivalent, which are decoupled completely, and only one inverter module operates with nonfull load among the parallel-operated inverter modules. The feasibility of the proposed system and the effectiveness of the control strategies have been verified by experimental results.