There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

This paper presents a review of control strategies, stability analysis, and stabilization techniques for dc microgrids (MGs). Overall control is systematically classified into local and coordinated control levels according to respective functionalities in each level. As opposed to local control, which relies only on local measurements, some line of communication between units needs to be made available in order to achieve the coordinated control. Depending on the communication method, three basic coordinated control strategies can be distinguished, i.e., decentralized, centralized, and distributed control. Decentralized control can be regarded as an extension of the local control since it is also based exclusively on local measurements. In contrast, centralized and distributed control strategies rely on digital communication technologies. A number of approaches using these three coordinated control strategies to achieve various control objectives are reviewed in this paper. Moreover, properties of dc MG dynamics and stability are discussed. This paper illustrates that tightly regulated point-of-load converters tend to reduce the stability margins of the system since they introduce negative impedances, which can potentially oscillate with lightly damped power supply input filters. It is also demonstrated that how the stability of the whole system is defined by the relationship of the source and load impedances, referred to as the minor loop gain. Several prominent specifications for the minor loop gain are reviewed. Finally, a number of active stabilization techniques are presented.

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DC Microgrids—Part I: A Review of Control Strategies and Stabilization Techniques

DC microgrids (MGs) have been gaining a continually increasing interest over the past couple of years both in academia and industry. The advantages of dc distribution when compared to its ac counterpart are well known. The most important ones include higher reliability and efficiency, simpler control and natural interface with renewable energy sources, and electronic loads and energy storage systems. With rapid emergence of these components in modern power systems, the importance of dc in today's society is gradually being brought to a whole new level. A broad class of traditional dc distribution applications, such as traction, telecom, vehicular, and distributed power systems can be classified under dc MG framework and ongoing development, and expansion of the field is largely influenced by concepts used over there. This paper aims first to shed light on the practical design aspects of dc MG technology concerning typical power hardware topologies and their suitability for different emerging smart grid applications. Then, an overview of the state of the art in dc MG protection and grounding is provided. Owing to the fact that there is no zero-current crossing, an arc that appears upon breaking dc current cannot be extinguished naturally, making the protection of dc MGs a challenging problem. In relation with this, a comprehensive overview of protection schemes, which discusses both design of practical protective devices and their integration into overall protection systems, is provided. Closely coupled with protection, conflicting grounding objectives, e.g., minimization of stray current and common-mode voltage, are explained and several practical solutions are presented. Also, standardization efforts for dc systems are addressed. Finally, concluding remarks and important future research directions are pointed out.

/pdf/dc-microgrids-part-ii-a-review-of-power-architectures-69j8nn2rpd.pdf

DC Microgrids—Part II: A Review of Power Architectures, Applications, and Standardization Issues

Historically, centrally computed algorithms have been the primary means of power system optimization and control. With increasing penetrations of distributed energy resources requiring optimization and control of power systems with many controllable devices, distributed algorithms have been the subject of significant research interest. This paper surveys the literature of distributed algorithms with applications to optimization and control of power systems. In particular, this paper reviews distributed algorithms for offline solution of optimal power flow (OPF) problems as well as online algorithms for real-time solution of OPF, optimal frequency control, optimal voltage control, and optimal wide-area control problems.

https://ieeexplore.ieee.org/ielaam/5165411/8075182/7990560-aam.pdf

A Survey of Distributed Optimization and Control Algorithms for Electric Power Systems

In recent researches on inverter-based distributed generators, disadvantages of traditional grid-connected current control, such as no grid-forming ability and lack of inertia, have been pointed out. As a result, novel control methods like droop control and virtual synchronous generator (VSG) have been proposed. In both methods, droop characteristics are used to control active and reactive power, and the only difference between them is that VSG has virtual inertia with the emulation of swing equation, whereas droop control has no inertia. In this paper, dynamic characteristics of both control methods are studied, in both stand-alone mode and synchronous-generator-connected mode, to understand the differences caused by swing equation. Small-signal models are built to compare transient responses of frequency during a small loading transition, and state-space models are built to analyze oscillation of output active power. Effects of delays in both controls are also studied, and an inertial droop control method is proposed based on the comparison. The results are verified by simulations and experiments. It is suggested that VSG control and proposed inertial droop control inherits the advantages of droop control, and in addition, provides inertia support for the system.

Comparison of Dynamic Characteristics Between Virtual Synchronous Generator and Droop Control in Inverter-Based Distributed Generators

This paper proposes a consensus-based secondary frequency control scheme for islanded microgrids. The scheme only deals with droop equation and secondary control variables, and coincides with the hierarchical control concept. In this way, the effects of primary control level are integrated into the active power variation as a disturbance signal. Finite-gain L 2 -stability of the system, from the disturbances to the system states, is then investigated. Minimum requirements for convergence of the system to the desired operating point are also extracted theoretically. The scheme is then augmented with an online Kron reduction strategy, which prevents the spanning trees in the communication network from cutting, and provides the distributed generation units with smooth plug-and-play functionality, when they get disconnected physically. The effectiveness of the proposed scheme for different case studies is validated in MATLAB/Simscape Electrical™ software environment.

Kron Reduction and L 2 -Stability for Plug-and-Play Frequency Control of Microgrids

Voltage regulation and current sharing are known as two main goals of DC microgrids. Distributed secondary control based on dynamic averaging consensus algorithm has been recently introduced in the literature as a viable solution. This approach, not only provides a good voltage regulation, but also guarantees load sharing will be done according to the nominal capacity of agents among the microgrids units. However, This solution is based on time-triggered communication that led to high communication burden by unnecessary data exchanging. In this paper, to prevent needless data-exchanging, the abovementioned distributed secondary controller is equipped with an event-triggered communication strategy. This need-based communication strategy reduces the communication load, considerably. The system stability analysis under the event-triggered strategy is evaluated using the Lyapunov's stability approach. The proposed strategy is applied to a DC microgrid and its performance is validated under communication disturbances and different working conditions using Simulink/ MATLAB environment.

Need-Based Communication in Fully-Distributed Secondary Control of DC Microgrids

In this paper a fuzzy logic-based distributed secondary control is introduced for islanded microgrids. Inspired by techniques from cooperative control, the presented controllers use local information and neighbor communication to perform secondary control actions. Consensus protocols are used for regulating voltage and sharing the active and reactive power. To tune the communication weights used in the consensus-based distributed framework, a smart fuzzy logic-based method is presented. The presented method estimates the appropriate amount of communication weights according to the fuzzy states, and relation between inputs and outputs considering stability criteria. A microgrid with three sources is used to verify the effectiveness of the proposed contribution. MATLAB/SIMULINK® is used for modelling and simulation of the system.

A fuzzy inference model for distributed secondary control of islanded microgrids

Comprehensive small-signal modeling and Prony analysis-based validation of synchronous interconnected microgrids

In this paper, a complete small signal model of two networked microgrids (NMGs) is provided, which can be used in the stability analysis, stability margin derivation and controller design. This model is validated by comparing it with the simulated NMGs using the Prony analysis. In this method, the modes, calculated by eigenvalue analysis and contributing to a particular state variable, are identified using the participation matrix and compared with the natural frequencies of its waveform. The state variable waveform is locally measured from the simulated NMGs in the SimPowerSystems environment of MATLAB and its natural frequencies are calculated using Prony analysis. It is concluded that Prony analysis estimates the natural frequencies well and there is a good similarity between them and the calculated modes, which shows the modeling validation.

Qobad Shafiee

Papers

Kron Reduction and L 2 -Stability for Plug-and-Play Frequency Control of Microgrids

Need-Based Communication in Fully-Distributed Secondary Control of DC Microgrids

A fuzzy inference model for distributed secondary control of islanded microgrids

Comprehensive small-signal modeling and Prony analysis-based validation of synchronous interconnected microgrids

Model Validation of Power Electronics-based Networked Micro-grids by Prony Analysis