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.

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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

Constant power loads (CPLs) impose instability issues in DC microgrids due to their negative impedance characteristics. This paper studies the problem of voltage control design of DC microgrids with CPLs. It is assumed that the power of CPLs is uncertain and belongs to a given interval leading to an infinite number of equilibrium points of the system. We develop a polytope model for DC microgrids with uncertain CPLs. Using this model, a robust two-degree-of-freedom feedback-feedforward voltage control framework is then proposed. The voltage controller is obtained by a solution of a set of linear matrix inequalities. The voltage control design strategy for each distributed generation (DG) unit is scalable and independent of the other DGs. The effectiveness of the proposed control approach is evaluated through simulation studies in MATLAB/SimPowerSystems toolbox.

Scalable Robust Voltage Control of DC Microgrids With Uncertain Constant Power Loads

In this article, a decentralized secondary control (SC) based on the active power estimation (APE) is presented. This achievement is realized by employing the unique feature of frequency as a global variable in autonomous ac microgrids (MGs). The APE is merely based on the droop coefficient of $P-{\omega }$ characteristics. The decentralized SC, utilizing a consensus protocol, restores the MG frequency to the nominal value while maintaining accurate power sharing of the droop mechanism. The consensus protocol is estimation-based and does not require communication infrastructure. In addition to the proposition of stability analysis method, experimental results with four distributed generation units (DGUs) also verify the effectiveness of the proposed SC structure.

Decentralized Frequency Control of AC Microgrids: An Estimation-Based Consensus Approach

This paper proposes a distributed control strategy for voltage and reactive power regulation in ac Microgrids First, the control module introduces a voltage regulator that maintains the average voltage of the system on the rated value, keeping all bus voltages within an acceptable range Dynamic consensus protocol is used to estimate the average voltage across the Microgrid This estimation is further utilized by the voltage regulator to elevate/lower the voltage-reactive power (Q-E) droop characteristic, compensating the drop caused by the droop mechanism The second module, the reactive power regulator, dynamically fine-tunes the Q-E coefficients to handle the proportional reactive power sharing Accordingly, locally supplied reactive power of any source is compared with neighbor sources and the local droop coefficient is adjusted to mitigate and, ultimately, eliminate the load mismatch The proposed controllers are fully distributed; ie, each source requires information exchange with only a few other sources, those in direct contact through the communication infrastructure A Microgrid test bench is used to verify the proposed control methodology, where different test scenarios such as load change, link failure, and inverter outage are carried out

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Team-oriented adaptive droop control for autonomous AC microgrids

During the past few years, microgrids (MGs) have been becoming more attractive as effective means to integrate different distributed energy resources (DERs). To coordinate active and reactive power sharing among DERs, conventional droop control method is widely used as a decentralized control scheme. However, sharing powers among the sources based on the units' rated capacities is not an optimal solution in terms of economy and efficiency. In this paper, a new adaptive droop-based control strategy is proposed for AC MGs to optimally share MG load between corresponding units. The mentioned control strategy is developed in two levels. The upper control level is a mixed-objective optimization algorithm that provides optimal set-points for power generations considering system's constraints and goals, while the lower control level is responsible for tracking the reference signals coming from the upper level. To demonstrate the effectiveness of the proposed control strategy under different operating scenarios, simulation results in a benchmark MG are also presented.

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Optimal adaptive droop control for effective load sharing in AC microgrids

In this paper a decentralized Model Predictive Controller (MPC) is proposed to load-frequency control in deregulated power systems under harsh conditions such as loss of power generation or loss of controllers in some control areas. To nullify these effects on frequency deviations, the MPC controller uses feed-forward control strategy in each area. The performance of the proposed controller is validated in different scenarios on a multi-area power system. Comparing simulation results with recent proposed robust LMI based PI control strategy, it can be seen that the decentralized MPC scheme effectively regulates frequency without using any emergency control strategy even in severe conditions.

Qobad Shafiee

Papers

Scalable Robust Voltage Control of DC Microgrids With Uncertain Constant Power Loads

Decentralized Frequency Control of AC Microgrids: An Estimation-Based Consensus Approach

Team-oriented adaptive droop control for autonomous AC microgrids

Optimal adaptive droop control for effective load sharing in AC microgrids

Decentralized Model Predictive load-frequency control for deregulated power systems in a tough situation