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

Microgrids are now emerging from lab benches and pilot demonstration sites into commercial markets, driven by technological improvements, falling costs, a proven track record, and growing recognition of their benefits. They are being used to improve reliability and resilience of electrical grids, to manage the addition of distributed clean energy resources like wind and solar photovoltaic (PV) generation to reduce fossil fuel emissions, and to provide electricity in areas not served by centralized electrical infrastructure. This review article (1) explains what a microgrid is, and (2) provides a multi-disciplinary portrait of today's microgrid drivers, real-world applications, challenges, and future prospects.

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Microgrids: A review of technologies, key drivers, and outstanding issues

The significant benefits associated with microgrids have led to vast efforts to expand their penetration in electric power systems. Although their deployment is rapidly growing, there are still many challenges to efficiently design, control, and operate microgrids when connected to the grid, and also when in islanded mode, where extensive research activities are underway to tackle these issues. It is necessary to have an across-the-board view of the microgrid integration in power systems. This paper presents a review of issues concerning microgrids and provides an account of research in areas related to microgrids, including distributed generation, microgrid value propositions, applications of power electronics, economic issues, microgrid operation and control, microgrid clusters, and protection and communications issues.

State of the Art in Research on Microgrids: A Review

Natural disasters can cause large blackouts. Research into natural disaster impacts on electric power systems is emerging to understand the causes of the blackouts, explore ways to prepare and harden the grid, and increase the resilience of the power grid under such events. At the same time, new technologies such as smart grid, micro grid, and wide area monitoring applications could increase situational awareness as well as enable faster restoration of the system. This paper aims to consolidate and review the progress of the research field towards methods and tools of forecasting natural disaster related power system disturbances, hardening and pre-storm operations, and restoration models. Challenges and future research opportunities are also presented in the paper.

Research on Resilience of Power Systems Under Natural Disasters—A Review

Microgrids with distributed generation (DG) provide a resilient solution in the case of major faults in a distribution system due to natural disasters. This paper proposes a novel distribution system operational approach by forming multiple microgrids energized by DG from the radial distribution system in real-time operations to restore critical loads from the power outage. Specifically, a mixed-integer linear program is formulated to maximize the critical loads to be picked up while satisfying the self-adequacy and operation constraints for the microgrids formation problem by controlling the ON/OFF status of the remotely controlled switch devices and DG. A distributed multiagent coordination scheme is designed via local communications for the global information discovery as inputs of the optimization, which is suitable for autonomous communication requirements after the disastrous event. The formed microgrids can be further utilized for power quality control and can be connected to a larger microgrid before the restoration of the main grids is complete. Numerical results based on modified IEEE distribution test systems validate the effectiveness of our proposed scheme.

Resilient Distribution System by Microgrids Formation After Natural Disasters

In contingency analysis of large-scale power systems, the high-order contingencies involving a relatively large number of component failures are traditionally assumed to have a very low occurrence probability. However, this may underestimate the probability of some ${N}$ - ${k}$ contingencies and thus pose risk to the reliable system operations considering that those contingencies would cause severe consequences. In this paper, we investigate the problem of hidden ${N}$ - ${k}$ contingencies and cast the problem of evaluating the hidden probability of an ${N}$ - ${k}$ contingency as a mixed integer linear programming problem. The simulation results on the IEEE 14-bus and 118-bus systems verify the effectiveness of the proposed model.

A Mixed Integer Programming Model for Evaluating the Hidden Probabilities of $N$ - ${k}$ Line Contingencies in Smart Grids

The increasing wind penetration challenges power system operations. In this letter, we use historical wind data to reveal a security risk within a dispatch interval, called the intrainterval security risk, from the perspective of excessive burden placed on the regulation due to the nonlinear wind variations during the interval. This can compromise the operational security under high wind penetration scenarios. The issue deserves attention as the higher wind penetration is expected in the near future. To the best of our knowledge, this is the first time such risk is revealed.

An Intrainterval Security Risk Regarding Regulation Burden Due to Wind Variation in High-Wind-Penetrated Power Systems

A system disturbance can cause severe exceedances of the power system operating limits in which the transmission network overloads, if not being timely mitigated, can initiate cascading failures. So, an optimal and efficient postevent overload mitigation (PEOM) tool is necessary. However, research in the postevent mitigation is relatively limited as compared to that for the normal state and pre-event actions. Some existing dispatch methods consider the postcontingency scenario, but they do not fully address the postevent security and may lead to security issues such as power imbalance or violations of network security criterion. To address these issues and to improve the mitigation efficiency, this paper proposes a PEOM tool. PEOM implements a time-optimal load shedding for managing the power imbalance, ensures the time-dependent network security during the mitigation time period, and enhances the efficiency by prioritizing the mitigations. The simulations on the IEEE 118-bus and 2383-bus systems verify the effectiveness and efficiency of PEOM.

Optimal Transmission Overloads Mitigation Following Disturbances in Power Systems

Contingencies occurring in multi-terminal high voltage direct current (MT-HVDC) grids can result in DC voltage/flow violations and also affect the frequency stability of the connected multiple asynchronous grids. To recognize and control such notable operating risks, a novel flexible risk-limiting optimal power flow (FROPF) for the MT-HVDC grid with vast wind generation is proposed in this paper. Within the two-stage FROPF structure, the pre-contingency operation of grid-side voltage-source converters (GVSCs) is optimized to minimize MT-HVDC grid power losses. Immediately following an outage occurring in the MT-HVDC grid, various fast-acting corrective actions of GVSCs are utilized to hedge against the overall risk exposure, including the wind power curtailment risk of the MT-HVDC grid and the rate-of-change-of-frequency (RoCoF) violation risk imposed on associated asynchronous grids. Reformulation techniques are introduced to ease the computational complexity of the optimization model. Case studies of two MT-HVDC grids demonstrate the effectiveness of the proposed FROPF.

Toward Flexible Risk-Limiting Operation of Multi-Terminal HVDC Grids With Vast Wind Generation

Security-constrained economic dispatch is executed based on consecutive dispatch intervals in real time. The short-term variations of wind power can cause the system to enter an unsecured operating region inside a dispatch interval, which is referred to as the intra-interval security (IIS) problem in this paper. IIS may not be addressed by existing dispatch methods as the traditional dispatch is to ensure the security at the interval end. This can lead to IIS risks in an interval when a high-wind-penetrated system encounters large intra-interval variations (IIVs) of wind power. To address this issue, an IIS preventive dispatch approach is proposed in this study. This approach models the wind power IIVs and develops a preventive dispatch to ensure the IIS over the interval in terms of line overloads while taking into account the tradeoff between security and economics. The proposed approach is implemented in a linear-programming model whose global optimum can be efficiently obtained by adding Benders-like cuts. Case study on the IEEE 118-bus system proves that a high level of nonlinear IIVs can cause the IIS issue (inside of an interval) even with accurate wind forecasts at both interval ends. The simulations show that some lines can have severe overloads inside the dispatch interval in spite of having flows below their capacity limits at both interval ends, and demonstrate that the proposed approach can mitigate such intra-interval overloads even under the worst case IIV scenario.

Liang Che

Papers

A Mixed Integer Programming Model for Evaluating the Hidden Probabilities of $N$ - ${k}$ Line Contingencies in Smart Grids

An Intrainterval Security Risk Regarding Regulation Burden Due to Wind Variation in High-Wind-Penetrated Power Systems

Optimal Transmission Overloads Mitigation Following Disturbances in Power Systems

Toward Flexible Risk-Limiting Operation of Multi-Terminal HVDC Grids With Vast Wind Generation

Intra-Interval Security Based Dispatch for Power Systems With High Wind Penetration