Accurate Reactive Power Sharing Research Articles

A Small-AC-Signal Injection Based Decentralized Secondary Voltage Control for Parallel Inverters With Accurate Reactive Power Sharing in Islanded Microgrids

A Small-AC-Signal Injection Based Decentralized Secondary Voltage Control for Parallel Inverters With Accurate Reactive Power Sharing in Islanded Microgrids

Accurate Reactive Power Sharing Strategy for Droop-Based Islanded AC Microgrids

In islanded ac microgrids, the mismatched impedances of the interfacing feeders between the inverters and the load bus cause poor reactive power sharing when the conventional frequency and voltage droop control technique is employed. Such operation endangers the whole microgrid reliability as it may lead to overloading certain inverters and, consequently, triggering protection relays and causing cascaded failure. Thus, this article proposes an accurate reactive power sharing strategy that considers the mismatched feeder impedances in islanded ac microgrids. It is based on the optimal tuning of the virtual complex impedance for each inverter. The proposed strategy has several advantages. First, it has a physical meaning as it establishes an explicit relationship between the mismatched values of the actual resistive–inductive feeders and the assigned values for the proposed virtual complex impedance to each inverter. Hence, further degradations in the microgrid voltages are prevented. Second, there is no need for prior knowledge about the actual feeder impedances in the design stage as they are estimated online from the available measurements. Finally, the proposed control is reliable and fault-tolerant as it copes with unexpected failures such as failure of (or sudden switching <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> ) some inverters and communication disruptions/delays. It ensures accurate power sharing even under the primary controllers after losing communication links with the secondary controller. The simulation and experimental verification results are presented to validate the performance of the proposed technique.

Distributed secondary controller to ensure proportional sharing of reactive power in AC microgrid

In an ac microgrid, reactive power sharing accuracy is affected due to unequal values of interconnecting cable impedances. To resolve this issue, secondary controllers are used to compensate the effect of cable impedances. Various types of secondary controllers are suggested in the literature which includes linear proportional plus integral (PI) controllers to minimize the difference in reactive power sharing created by conventional E−Q droop law. The reference value of the PI controller is the average value of reactive powers supplied by the sources. When the PI controller is used to minimize the difference between the actual and reference value of reactive power, the difference between the algebraic sum of reactive powers supplied by the sources and the reactive powers demanded by the loads has a nonzero value. Therefore, the performance of these controllers becomes poor in the case of ac microgrid having sources of unequal ratings. To resolve this issue, a proportional reactive power-sharing (PRPS) controller-based distributed secondary controller is proposed in this paper. The PRPS controller modifies the droop gain of the E−Q droop control loop of each source in such a way that each source supplies reactive power equal to its proportional value of reactive power. The proportional value of reactive power supplied by each source is the power when the equivalent output impedance of all sources as seen by the loads are identical. The key advantage offered by the proposed controller is the accurate reactive power-sharing in the case of ac microgrid having sources of unequal ratings. The proposed controller ensures zero value of the difference between the algebraic sum of reactive powers supplied by the sources and the reactive powers demanded by the loads. This ensures accurate sharing of reactive power among the sources. The validation of the proposed controller is carried out for islanded mode of operation of ac microgrid. Further, the proposed controller requires of low bandwidth communication for its implementation. The effect of the proposed controller on the stability of the system is demonstrated using reduced order small-signal model. The effect of communication delay on the performance of the system is analyzed with the help of roots locus plots. To validate the efficacy of the proposed controller, detailed simulation studies are carried out in Matlab/Simulink.

Successive-Approximation-Based Virtual Impedance Tuning Method for Accurate Reactive Power Sharing in Islanded Microgrids

Due to the mismatched line impedances in islanded microgrids, distributed generation units with conventional droop tend to have barely satisfactory reactive power sharing accuracy. To handle this problem, this article proposes a virtual impedance tuning method based on successive approximation to accurately compensate the mismatch among line impedances. First, in each distributed generation (DG) unit, an adaptive virtual impedance regulation based on the error between the actual output reactive power and its reference is established and combined with the conventional Q - V droop control. Then, these two control schemes operate alternately for several cycles, and in each cycle, the reactive power reference of the virtual impedance regulation is updated by the latest estimate from the Q-V droop control. Therefore, the virtual impedance of each DG unit will be tuned successively to an appropriate value to achieve accurate power sharing among DG units based on local information. Meanwhile, to ensure all the DG units switching between these two control schemes synchronously, a common triggering signal from the microgrid central controller is needed to start the internal time sequence of each DG unit. Furthermore, a comprehensive design procedure for key parameters is presented based on convergence analysis and small-signal modeling of the tuning process. Finally, the effectiveness of the proposed method is verified by simulation and experimental results. This article is accompanied by a video demonstrating the virtual impedance tuning process.

Consensus-Based Cooperative Droop Control for Accurate Reactive Power Sharing in Islanded AC Microgrid

The conventional droop control technology, which can achieve autonomous power sharing among the distributed generation units, suffers from inaccurate reactive power sharing due to its dependence on line impedances. In this article, a novel consensus-based cooperative droop control technique where the droop slope gain is adaptively adjusted is proposed to improve the reactive power-sharing accuracy with the help of a sparse communication network. Through detailed small-signal analysis, it is verified that the proposed droop technique presents excellent immunity against communication delay. Laboratory test results are presented and the performances of the proposed control technique under different operation scenarios are verified, including the load change, communication link failure, and plug-and-play operation.

A Modified $Q-\dot{V}$ Droop Control for Accurate Reactive Power Sharing in Distributed Generation Microgrid

The droop-control technology, which can achieve an autonomous power sharing among distributed resources interface converters, provides the advantages of enhanced system reliability and reduced complexity. However, the reactive power sharing accuracy of the conventional Q - V droop method can be degraded due to its dependence on the line impedances. The Q - V̇ droop-control method was proposed to reduce the influence of line impedance. The reactive power sharing accuracy is improved but still affected by the V̇ restoration operation. In this paper, a modified V̇ restoration mechanism is proposed. Its influence on the power sharing is theoretically explained and analyzed. Both simulation and experiment results are presented to verify the proposed method.

Notice of Removal: Distributed Adaptive Virtual Impedance Control for Accurate Reactive Power Sharing Based on Consensus Control in Microgrids

To achieve accurate reactive power sharing regardless of the effects of mismatched line impedance, this paper proposes a reactive power sharing method that employs both consensus control and adaptive virtual impedance control for islanded microgrids. The consensus control is used to find the reactive power mismatch among distributed generation (DG) units. The reactive power mismatch term is fed to a proportional integral controller to generate the virtual impedance correction. With fully distributed regulation of the DG virtual impedance, the load reactive power will be shared accurately among DGs and the circulating line currents among DGs are effectively suppressed. The control strategy does not require the knowledge of the line impedances. Also, the average voltage restoration based on a dynamic consensus control is proposed to recover the decreased output voltage of each DG due to the droop action and the added virtual impedance. Simulation results show the effectiveness of the proposed method in achieving load reactive power sharing and the voltage restoration.

Accurate Reactive Power Sharing in an Islanded Microgrid Using Adaptive Virtual Impedances

In this paper, a reactive power sharing strategy that employs communication and the virtual impedance concept is proposed to enhance the accuracy of reactive power sharing in an islanded microgrid. Communication is utilized to facilitate the tuning of adaptive virtual impedances in order to compensate for the mismatch in voltage drops across feeders. Once the virtual impedances are tuned for a given load operating point, the strategy will result in accurate reactive power sharing even if communication is disrupted. If the load changes while communication is unavailable, the sharing accuracy is reduced, but the proposed strategy will still outperform the conventional droop control method. In addition, the reactive power sharing accuracy based on the proposed strategy is immune to the time delay in the communication channel. The sensitivity of the tuned controller parameters to changes in the system operating point is also explored. The control strategy is straightforward to implement and does not require knowledge of the feeder impedances. The feasibility and effectiveness of the proposed strategy are validated using simulation and experimental results from a 2-kVA microgrid.