Review of Active and Reactive Power Sharing Strategies in Hierarchical Controlled Microgrids

Review of Active and Reactive Power Sharing Strategies in Hierarchical Controlled Microgrids

Review of Active and Reactive Power Sharing Strategies in Hierarchical Controlled Microgrids

Abstract:

Microgrids consist of multiple parallel-connected distributed generation (DG) units with coordinated control strategies, which are able to operate in both grid-connected and islanded modes. Microgrids are attracting considerable attention since they can alleviate the stress of main transmission systems, reduce feeder losses, and improve system power quality. When the islanded microgrids are concerned, it is important to maintain system stability and achieve load power sharing among the multiple parallel-connected DG units. However, the poor active and reactive power sharing problems due to the influence of impedance mismatch of the DG feeders and the different ratings of the DG units are inevitable when the conventional droop control scheme is adopted. Therefore, the adaptive/improved droop control, network-based control methods, and cost-based droop schemes are compared and summarized in this paper for active power sharing. Moreover, nonlinear and unbalanced loads could further affect the reactive power sharing when regulating the active power, and it is difficult to share the reactive power accurately only by using the enhanced virtual impedance method. Therefore, the hierarchical control strategies are utilized as supplements of the conventional droop controls and virtual impedance methods. The improved hierarchical control approaches such as the algorithms based on graph theory, multi-agent system, the gain scheduling method, and predictive control have been proposed to achieve proper reactive power sharing for islanded microgrids and eliminate the effect of the communication delays on hierarchical control. Finally, the future research trends on islanded microgrids are also discussed in this paper.
Published in: IEEE Transactions on Power Electronics ( Volume: 32, Issue: 3, March 2017 )
Date of Publication: 25 May 2016
INSPEC Accession Number: 16524185
Publisher: IEEE

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Reactive Power Flow Control for PV Inverters Voltage Support in LV Distribution Networks

Reactive Power Flow Control for PV Inverters Voltage Support in LV Distribution Networks

Reactive Power Flow Control for PV Inverters Voltage Support in LV Distribution Networks

Abstract:

This paper proposes a reactive power flow control pursuing the active integration of photovoltaic systems in LV distribution networks. An alternative power flow analysis is performed according to the specific characteristics of LV networks, such as high resistance/reactance ratio and radial topologies. The proposed solution gives high performances, in terms of rms-voltage regulation, by estimating the reactive power reference on each node considering the influence of the rest of the nodes in terms of active and reactive power demanded/generated by them. The local control of each photovoltaic system is based on the power converter control, interfacing these units with the grid and the loads respectively. The local control is designed on the basis of locally measured feedback variables. Photovoltaic units thus guarantee universal operation, being able to change between islanding-mode and grid-connected mode without disrupting critical loads connected to them, and allowing smooth transitions. Exhaustive results are also included and discussed in this paper.
Published in: IEEE Transactions on Smart Grid ( Volume: 8, Issue: 1, Jan. 2017 )
Date of Publication: 04 November 2016
INSPEC Accession Number: 16561204
Publisher: IEEE

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Distributed Optimal Control of Reactive Power and Voltage in Islanded Microgrids

Distributed Optimal Control of Reactive Power and Voltage in Islanded Microgrids

Distributed Optimal Control of Reactive Power and Voltage in Islanded Microgrids

Abstract:

This paper presents a distributed optimal control strategy for islanded microgrids, which allows performing reactive power sharing and voltage regulation without using a communication system. To perform the twofold objectives, a small signal model is first developed to reconstruct the system input-output relationship, which is evaluated through sensitivity analysis. A state estimator is then constructed to observe reactive power distribution and system voltages by local measurement. An optimal regulator is developed to perform both reactive power sharing and system voltage restoration. And the dynamic performance of the optimal controller is analyzed, from which the guideline for choosing controller parameters is formulated. The results obtained from sensitivity analysis, simulations, and experiments show that the proposed approach provides the expected reliability and flexibility for optimizing the reactive power sharing and system voltages restoration.
Published in: IEEE Transactions on Industry Applications ( Volume: 53, Issue: 1, Jan.-Feb. 2017 )
Date of Publication: 04 October 2016
INSPEC Accession Number: 16598461
Publisher: IEEE

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Approach to setting gateway reactive power control band for distribution networks with wind power

Approach to setting gateway reactive power control band for distribution networks with wind power

Approach to setting gateway reactive power control band for distribution networks with wind power

Abstract:

The integration of distributed wind farm leads to additional fluctuation of power flow in a high-voltage distribution network, which has become a major concern in automatic voltage control. This study proposes a slack optimal control method to determine the tolerance band of gateway reactive power (GRP) which takes into account voltage constraints, energy loss and excessive operations of control devices. Analysis of network characteristics shows that the GRP control band should be set according to both load levels and the outputs of distributed wind farm. Based on the analysis, slack optimal active loss difference strategy is proposed to set the slack optimal band of GRP. A slack optimal band matrix is defined and a partitioning intersection strategy is proposed to implement the differential setting of the slack optimal control band parameters of GRP. Simulation studies are conducted on a practical distribution network in China and the results show that the proposed method performs well in energy saving, control device regulating, voltage profile keeping, and reactive power balancing in a simple and practical way.
Published in: IET Generation, Transmission & Distribution ( Volume: 11, Issue: 3, 2 16 2017 )
Date of Publication: 09 February 2017
INSPEC Accession Number: 16638591
Publisher: IET

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Reactive Power Ancillary Service of Synchronous DGs in Coordination With Voltage Control Devices

Reactive Power Ancillary Service of Synchronous DGs in Coordination With Voltage Control Devices

Reactive Power Ancillary Service of Synchronous DGs in Coordination With Voltage Control Devices

Abstract:

This paper presents an optimal way to control steady-state voltage in distribution feeders using reactive power ancillary service that is provided by synchronous distributed generators (DGs). Based on load forecasts provided one day in advance, DG reactive power is dispatched on the hour in coordination with the switching operations of an on-load tap changer (OLTC) and shunt capacitors (SCs). This aims to reduce not only distribution line power losses but also the number of switching operations of the OLTC and the SCs, which affect the feeder voltage quality and switching device lifetime. For the reactive power dispatch, a mixed-integer nonlinear optimization problem is formulated using a multiobjective function and solved using a particle swarm optimization (PSO) algorithm. Modules using evolutionary and dynamic programming are incorporated into the PSO algorithm to be less susceptible to becoming trapped in local optima and have a better chance of reaching global optimum. Simulation case studies using small- and large-scale distribution networks were performed using MATLAB to demonstrate that the coordinated reactive power support of the DGs, OLTC, and SCs can achieve the objective effectively, resulting in improved voltage quality.
Published in: IEEE Transactions on Smart Grid ( Volume: 8, Issue: 2, March 2017 )
Date of Publication: 17 September 2015
INSPEC Accession Number: 16690045
Publisher: IEEE

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