Harmonic Power Sharing Research Articles

A Decentralized Nonlinear Harmonic Power Sharing Scheme Considering Harmonic Residual Capacity and Working Conditions of Fundamental Load

A Decentralized Nonlinear Harmonic Power Sharing Scheme Considering Harmonic Residual Capacity and Working Conditions of Fundamental Load

Distributed Dynamic Event-Triggered Control for Accurate Active and Harmonic Power Sharing in Modular On-Line UPS Systems

Due to the mismatched line resistance in the modular online uninterruptible power supply (UPS) systems, accurate active power and harmonic power sharing is an arduous work. To tackle this issue, this article proposes a distributed dynamic event-triggered control for the power sharing to accurately compensate the effect of mismatch among the line resistances. Benefiting from the proposed control strategy, the UPS modules adaptively regulate the virtual resistance at the fundamental and harmonic frequencies, and hence, the accurate active power and harmonic power sharing are achieved. In addition, the suggested technique allows the UPS modules to exchange information at only the event-triggered times, which greatly reduces the number of communication among the neighboring units. Furthermore, the system's stability is analyzed using a Lyapunov function. Finally, experimental case studies are provided to evaluate the performance of the proposed control strategy, showing its effectiveness in achieving the accurate power sharing, and plug-and-play capability.

A Decentralized Impedance Reshaping Strategy for Balanced, Unbalanced and Harmonic Power Sharing in Islanded Resistive Microgrids

In an islanded microgrid, the wire impedance mismatches among distributed generation (DG) units lead to inaccurate power sharing results, which influences the system’s normal operation. The output impedances of DG units can be reshaped to deal with this issue. In this paper, an impedance reshaping strategy for the resistive microgrid is proposed based on the combination of droop control and adaptive virtual impedance regulation (AVIR). Firstly, DG units operate in droop mode and the steady-state reactive power from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Q-f$</tex-math></inline-formula> droop is restored as the reference. Then the droop loops are removed and an AVIR based on the error between instantaneous reactive power and its reference is implemented. Since reactive power sharing with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Q-f$</tex-math></inline-formula> is relatively accurate, the impedance mismatches could be eliminated by the virtual impedances introduced with the proposed strategy, all the accurate active, reactive, unbalanced and harmonic power sharing could be realized simultaneously. In this strategy, communication among DG units or between DG unit and central control is not required, neither the wire impedances nor bus voltage or load current is measured, which reveals the high system reliability. Finally, the simulation and experimental results are presented to verify the effectiveness of the proposed control method.

Distributed Event-Triggered Control for Reactive, Unbalanced, and Harmonic Power Sharing in Islanded AC Microgrids

For several reasons, particularly due to the mismatch in the feeder impedance, accurate power sharing in islanded microgrids is a challenging task. To get around this problem, a distributed event-triggered power sharing control strategy is proposed in this article. The suggested technique adaptively regulates the virtual impedances at both fundamental positive/negative sequence and harmonic frequencies and, therefore, accurately share the reactive, unbalanced, and harmonics powers among distributed generation units. The proposed method requires no information of feeder impedance and involves exchanging information among units at only event-triggered times, which reduces the communication burden without affecting the system performance. The stability and interevent interval are analyzed in this article. Finally, experimental results are presented to validate the effectiveness of the proposed scheme.

Virtual Impedance Control Scheme to Compensate for Voltage Harmonics With Accurate Harmonic Power Sharing in Islanded Microgrids

In case of voltage harmonic compensation at the point of common coupling (PCC) in islanded microgrids, it is important to share the harmonic load power correctly among the distributed generators (DGs) to use the DGs' capacity effectively. In previous DG control schemes, it is impossible to satisfy the requirements of PCC voltage quality and accurate harmonic power sharing simultaneously with only a virtual impedance controller (VIC). In this article, we propose an enhanced DG VIC to provide accurate harmonic power sharing along with voltage harmonic compensation in islanded microgrids. The proposed VIC is developed based on simple integral controllers with two controllable parts and no information about the feeder impedances or load currents. The control performance is theoretically analyzed using a small-signal state-space model to evaluate the system dynamics and stability. The feasibility and effectiveness of the method are validated by simulated and experimental results.

An Adaptive Virtual Impedance Control Scheme Based on Small-AC-Signal Injection for Unbalanced and Harmonic Power Sharing in Islanded Microgrids

To address the unbalanced and harmonic power sharing issue among parallel inverters caused by feeder impedance mismatch in islanded microgrids, an adaptive virtual impedance control method is proposed based on the injection of an extra small ac signal (SACS) in the output voltage of each inverter. Similar to the principle of active power–frequency droop, the frequency of the injected signal droops with the output unbalanced and harmonic power, while the active power produced by the injected SACS is detected to adjust the virtual impedance at the fundamental negative sequence and selected harmonic frequencies, which will tune the distribution of unbalanced and harmonic power in the system. When the injected SACSs of each inverter synchronize with each other and reach a common frequency in steady state, the virtual impedance of each inverter will be matched to each other for evenly sharing the unbalanced and harmonic power. This proposed method requires neither communication links among parallel inverters nor feeder impedance information. Furthermore, the control parameter design method based on modeling and stability analysis of the proposed control structure is discussed in detail. Finally, simulation and experimental results are provided to validate the effectiveness of the proposed scheme.

Harmonic Power Sharing With Voltage Distortion Compensation of Droop Controlled Islanded Microgrids

Harmonics are found to have deleterious effects on power system equipment including transformers, capacitor banks, rotating machines, etc. However, the distributed generation interfacing converters (DICs) are able to improve power quality and system efficiency in case of adding a flexible control strategy. This paper describes the voltage distortion generated by nonlinear loads and proposes a new harmonic droop control to reduce the voltage harmonic distortion at the point of common coupling (PCC) and to share the harmonic power between parallel islanded inverters controlled by classical frequency droop method. The main objective it to effectively compensate the harmonic currents drawn by each DIC to supply the nonlinear loads in parallel with voltage harmonic damping at PCC. The operation principle of the proposed harmonic droop is explained in details. Modeling and stability analysis are developed in order to define the control parameters suitable for proper operation. Simulation and experimental results are presented to show the competence of the proposed algorithm in achieving harmonic power sharing and in improving the voltage harmonic distortion at the PCC.

Notice of Removal: Consensus-Based Distributed Control for Accurate Reactive, Harmonic, and Imbalance Power Sharing in Microgrids

This paper investigates the issue of accurate reactive, harmonic, and imbalance power sharing in a microgrid. Harmonic and imbalance droop controllers are developed to proportionally share the harmonic power and the imbalance power among distributed generation (DG) units and improve the voltage quality at the point of common coupling (PCC). Further, a distributed consensus protocol is developed to adaptively regulate the virtual impedance at fundamental frequency and selected harmonic frequencies. Additionally, a dynamic consensus based method is adopted to restore the voltage to their average voltage. With the proposed methods, the microgrid system reliability and flexibility can be enhanced and the knowledge of the line impedance is not required. And the reactive, harmonic, and imbalance power can be proportionally shared among the DG units. Moreover, the quality of the voltage at PCC can be greatly improved. Simulation and experimental results are presented to demonstrate the proposed method.

A New Virtual Harmonic Impedance Scheme for Harmonic Power Sharing in an Islanded Microgrid

This paper proposes a control strategy for harmonic power sharing in an islanded microgrid, without affecting the voltage quality at the load bus. The harmonic power distribution in an islanded microgrid depends on the line impedance. The proposed control strategy employs negative virtual harmonic impedance to compensate for the effect of line impedance on harmonic power delivery. The influence of mismatched line impedances on nonlinear load distribution is minimized by virtually varying the effective line impedance at harmonic frequencies. The fundamental active and reactive power sharing is implemented using the droop technique and the proposed method operates independently, without affecting the linear load sharing. Furthermore, a new droop method, called $ {Z_{h}} - H $ droop, is also proposed to estimate the virtual harmonic impedance value adaptively. The proposed concept of negative virtual harmonic impedance and $ {Z_{h}} - H $ droop is validated using simulation and digital signal processor-based experimental studies.

An Enhanced Islanding Microgrid Reactive Power, Imbalance Power, and Harmonic Power Sharing Scheme

To address inaccurate power sharing problems in autonomous islanding microgrids, an enhanced droop control method through online virtual impedance adjustment is proposed. First, a term associated with DG reactive power, imbalance power, or harmonic power is added to the conventional real power-frequency droop control. The transient real power variations caused by this term are captured to realize DG series virtual impedance tuning. With the regulation of DG virtual impedance at fundamental positive sequence, fundamental negative sequence, and harmonic frequencies, an accurate power sharing can be realized at the steady state. In order to activate the compensation scheme in multiple DG units in a synchronized manner, a low-bandwidth communication bus is adopted to send the compensation command from a microgrid central controller to DG unit local controllers, without involving any information from DG unit local controllers. The feasibility of the proposed method is verified by simulated and experimental results from a low-power three-phase microgrid prototype.