Frequency and Voltage Control of a Grid of Microgrids

The rapid proliferation of Distributed Energy Resources (DERs) in recent years has resulted in significant technical challenges for power system operators and planners, mainly due to the particular characteristics of some of these systems that are interfaced with converters that alter the dynamic behavior of typically power systems. To accommodate the increasing penetration of DERs in power systems, microgrids have been formed to facilitate their integration. The operation of these microgrids could be further enhanced by interconnecting them to satisfy the overall system demand, and improve their stability if suitable control schemes are implemented. The control of microgrids has been extensively studied; however, coordinated operation, dynamics, and control of a grid that includes interconnected microgrids have not been sufficiently addressed in the literature, and thus this is the focus of this thesis. In the first stage of the thesis, a new microgrid interface based on Virtual Synchronous Generators (VSGs) is proposed to control the power exchange of interconnected ac and dc microgrids, and provide frequency support, voltage regulation, and virtual inertia for individual microgrids and the host grid as required, to improve both frequency and voltage dynamics for the overall system. Thus, a hierarchical distributed control technique is proposed, where the primary control of interfacing VSGs provides adaptive inertia for the ac systems, while a secondary distributed control of the system regulates the frequency and the voltages of the host grid and the interconnected microgrids, based on a consensus technique with limited information about the overall system. The proposed controller shares the total system load among the grid and microgrids, while minimizing the overall frequency and voltage deviations in all interconnected systems. The proposed interface and the controller are implemented, tested, and validated in detailed simulations for a grid-of-microgrids system. In the second stage of the thesis, an adaptive active power droop controller and voltage setpoint control in isolated microgrids for optimal frequency response and stability after disturbances is first proposed and presented, and then applied to the coordinated control of interconnected microgrids. The control scheme involves an optimal and model predictive control approach, which continuously adjusts the active power droop gains and the voltage setpoints of Distributed Energy Resources (DERs) to maintain the frequency of the system within acceptable limits, and enhance the primary frequency response of the system, while taking into account the active power sensitivity of the microgrid loads to the system's operating voltage. The proposed approach is also implemented, tested, validated, and compared via detailed simulations in a microgrid benchmark system and the developed grid-of-microgrids test system. The results demonstrate that the proposed VSG controlled interfaces limit severe frequency deviations during disturbances, and allow proper power sharing among the microgrids without causing significant power transients for the ac/dc systems, compared to existing techniques. Furthermore, the proposed secondary distributed and centralized frequency and voltage controllers maintain the power balance of the interconnected systems and regulate the microgrids' frequencies and dc voltages to nominal values, compared to conventional frequency controllers; however, the distributed control approach shows better overall frequency and dc-voltage dynamics and regulation than the centralized control approach

Secondary Frequency and Voltage Control of Islanded Microgrids via Distributed Averaging

In this work we present new distributed controllers for secondary frequency and voltage control in islanded microgrids. Inspired by techniques from cooperative control, the proposed controllers use localized information and nearest-neighbor communication to collectively perform secondary control actions. The frequency controller rapidly regulates the microgrid frequency to its nominal value while maintaining active power sharing among the distributed generators. Tuning of the voltage controller provides a simple and intuitive trade-off between the conflicting goals of voltage regulation and reactive power sharing. Our designs require no knowledge of the microgrid topology, impedances or loads. The distributed architecture allows for flexibility and redundancy, and eliminates the need for a central microgrid controller. We provide a voltage stability analysis and present extensive experimental results validating our designs, verifying robust performance under communication failure and during plug-and-play operation.Comment: Accepted for publication in IEEE Transactions on Industrial Electronic

Analysis of the effect of clock drifts on frequency regulation and power sharing in inverter-based islanded microgrids

© 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Local hardware clocks in physically distributed computation devices hardly ever agree because clocks drift apart and the drift can be different for each device. This paper analyses the effect that local clock drifts have in the parallel operation of voltage source inverters (VSIs) in islanded microgrids (MG). The state-of-the-art control policies for frequency regulation and active power sharing in VSIs-based MGs are reviewed and selected prototype policies are then re-formulated in terms of clock drifts. Next, steady-state properties for these policies are analyzed. For each of the policies, analytical expressions are developed to provide an exact quantification of the impact that drifts have on frequency and active power equilibrium points. In addition, a closed-loop model that accommodates all the policies is derived, and the stability of the equilibrium points is characterized in terms of the clock drifts. Finally, the implementation of the analyzed policies in a laboratory MG provides experimental results that confirm the theoretical analysis.Peer ReviewedPostprint (author's final draft

Ultimate boundedness of droop controlled Microgrids with secondary loops

In this paper we study theoretical properties of inverter-based microgrids controlled via primary and secondary loops. Stability of these microgrids has been the subject of a number of recent studies. Conventional approaches based on standard hierarchical control rely on time-scale separation between primary and secondary control loops to show local stability of equilibria. In this paper we show that (i) frequency regulation can be ensured without assuming time-scale separation and, (ii) ultimate boundedness of the trajectories starting inside a region of the state space can be guaranteed under a condition on the inverters power injection errors. The trajectory ultimate bound can be computed by simple iterations of a nonlinear mapping and provides a certificate of the overall performance of the controlled microgrid.Comment: 8 pages, 1 figur