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• Methodology for assessing the operational flexibility of Virtual Power Plants. • Methodology to facilitate the integration of renewable energies into the power system. • Graphical representation of operational flexibility of distributed energy resources. • Assessment of the available flexibility to facilitate decision-making under uncertainty. • Participation of Virtual Power Plants in the ancillary services and power reserve offer. Distributed energy resources (DERs) are elements that actively participate in the supply of renewable energy and contribute to the decarbonization of the power system. However, they lack two factors necessary to take advantage of their operational flexibility: observability and controllability. In this sense, Virtual Power Plants (VPPs) are a feasible alternative to provide the necessary requirements for the optimal management of a set of distributed units. Therefore, knowledge of the technical and energy characteristics of each unit that makes up the VPP is a necessary condition for the effective integration of DERs into the power system. This paper proposes a methodology to graphically represent, quantify and exploit the aggregate operational flexibility of a set of units. The proposed methodology is based on five metrics related to active and reactive power, which serve as a tool to facilitate the VPP Operator's decision-making under uncertainty. Consequently, achieving the coordinated operation of several distributed units makes it possible to achieve common objectives. For instance, frequency and voltage regulation, compliance with a planned power curve, or dealing with the variability of renewable energies. The proposal is applied to a theoretical case study and through real operational tests between a hydroelectric unit and a photovoltaic plant. Finally, it is shown that the results obtained are a useful tool in real-time. [ABSTRACT FROM AUTHOR]

Providing a cost-efficient and sustainable energy is one of the critical features in modern societies. In response to this demand, this paper proposes a comprehensive framework for optimal day-ahead operational planning of smart distribution systems considering both normal and emergency conditions. The proposed procedure for normal mode minimizes the operation costs and provides sustainability using the seamlessness index. By adjusting this index, the system can be adapted to achieve the desired self-sufficiency level along a specified planning horizon thanks to exploiting the sufficient local generations. The operational planning of emergency mode is integrated into the proposed framework to provide the optimal schemes which can handle the possible abnormal conditions using the available local generations and guarantee the desired resiliency level. In emergency mode, the proposed self-healing strategy will sectionalize the isolated area of the distribution system into island partitions to provide reliable power supply to the critical loads continuously. A set of key operational metrics including power loss, load priority, and system-related constraints are integrated into the proposed island partitioning procedure which promotes the functionality of the method. The proposed framework is implemented on a modified PG&E 69-bus distribution system and is investigated through different case studies. The results of case studies demonstrate significant improvements and benefits which are obtained by applying the proposed framework. [ABSTRACT FROM AUTHOR]

This paper presents a new droop-based control strategy for hybrid microgrids (HMG) with improved power sharing. When ac microgrids (AC-MG) and dc microgrids (DC-MG) are present in a distribution grid, there is an opportunity to interconnect them via an interlinking converter (IC) and form a HMG. Power sharing and adequate voltage/frequency control are the main functions of AC-MG and DC-MG control systems in standalone mode. When it comes to a HMG, active power sharing throughout the whole system must also be properly achieved by controlling the IC active power throughput. Furthermore, the possibility of participation of IC in AC-MG reactive power adds some complexity to a HMG control system. In this paper, a new decentralized control strategy is presented for a HMG which relies on regulating the voltage magnitude of a common bus in each microgrid. In this regard, new droop characteristics for sources across both microgrids as well as IC are proposed. The proposed droop characteristics result in better active/reactive power sharing across both microgrids and at the same time results in better voltage regulation. The derivation of new droop characteristics is thoroughly discussed in this paper. Simulation results are used to show the improvement achieved. [ABSTRACT FROM AUTHOR]

Electric distribution networks are experiencing fast changes mainly due to the integration of distributed energy resources. These changes originate the active distribution networks, where several distributed energy resources are integrated, usually using power converters. In these active networks, the conventional control, operation, and protection schemes used in power systems are unsuitable, and as a consequence, new approaches must be proposed and tested. This paper analyses the distance relay performance in active distribution networks; as the main contribution, a communication-less compensation strategy is proposed to deal with the effects of fault resistance and the distributed energy resources' infeed and remote currents. The obtained results demonstrate the excellent performance of the proposed compensation strategy aimed at determining the true value of the estimated positive sequence impedance. This compensation can be easily integrated into the software of conventional distance relays to overcome fault resistance, infeed, and remote current effects. • A detailed error analysis for conventional distance relays is presented. • A strategy to compensate conventional distance relays is proposed. • A critical case for active networks considering converters is validated. • Results validate the proposed distance relay compensation. [ABSTRACT FROM AUTHOR]

Given the significant concerns regarding carbon emissions from fossil fuels, global warming and energy crisis, renewable distributed energy resources (DERs) are going to be integrated in smart grids, which will make the energy supply more reliable and decrease the costs and transmission losses. Unfortunately, one of the key technical challenges in power system planning, control and operation with DERs is the voltage regulation at the distribution level. This problem stimulates the deployment of smart sensors and actuators in smart grids so that the voltage can be stabilized. The observation from the microgrid incorporating DERs is transmitted to the control center via wireless communication systems. In other words, the proposed communication infrastructure provides an opportunity to address the voltage regulation challenge by offering the two-way communication links for microgrid state information collection, estimation and stabilization. Based on the communication infrastructure, we propose a least square based Kalman filter algorithm for state estimation and an optimal feedback control framework for stabilizing the microgrid states. Specifically, we propose to optimize the performance index by using semidefinite programming techniques in the context of smart grid applications. At the end, the efficacy of the developed approaches is demonstrated using a microgrid incorporating multiple DERs. [ABSTRACT FROM AUTHOR]

• A framework for DER aggregator to participate in day-ahead market considering security check is proposed. • A single-leader-multi-follower Stackelberg game model is established. • A linearized power flow model of DS with substation transformer tap is proposed. • An A-RBRD algorithm is proposed to solve the Stackelberg game model. • Performance of the model and algorithm are verified using a constructed and a realistic T&D system. With increasing distributed energy resources (DERs) integration, the strategic behavior of a DER aggregator in electricity markets will significantly affect the secure operation of the distribution system. In this paper, the interactions among the DER aggregator, energy and reserve markets, and distribution system are investigated through a single-leader-multi-follower Stackelberg game model with the DER aggregator as the leader and the independent system operator and distribution system operator as the followers. To guarantee the operation security of the distribution system, security check problems under three different scenarios are involved in the follower level, which is linearized using a mixed-integer linearized power flow model. Then, using the strong duality theorem, the proposed model is converted into a bi-level mixed-integer linear (BMILP) programming model with only mixed-integer linear follower-level problems. Next, an accelerated relaxation-based bi-level reformulation and decomposition algorithm is proposed to solve the BMILP problem. Finally, case studies are carried out on a constructed integrated transmission and distribution (T&D) system and a practical integrated T&D system to verify the effectiveness of the proposed model and algorithm. The simulation results indicate that the available downward reserve of the DER aggregator will decrease with the security limitation of the distribution system. [ABSTRACT FROM AUTHOR]

Highlights: [•] A MAS based management and control strategy for distribution grid is researched. [•] A three-level hierarchical decentralized coordinated hybrid control is designed. [•] The energy management strategies are drafted based on CPN. [•] The discrete coordinated control strategies are drafted for real-time operating mode reconfiguration. [•] The simulation studies testify the effectiveness of the proposed control. [ABSTRACT FROM AUTHOR]

• Four discrete/continuous-time online optimal power flow algorithms are designed. • The proposed algorithms are suitable for active distribution network optimization. • The proposed algorithms are fully evaluated and compared via detailed case studies. • The proposed discrete-time prediction-correction algorithm is the most recommended. This paper discusses and analyzes the latest online algorithm applications for time-varying optimal power flow problems in active distribution networks with numerous power-electronics-interfaced distributed energy resources, which bring significant time-varying characteristics to the optimal operation of active distribution networks. Conventional offline optimization algorithms are time-consuming and unable to track time-varying optimal solutions in highly variable environments. In contrast, online algorithms are characterized by fast calculation speeds and the ability to continuously track optimal solutions, thus having great potential in solving optimization problems that vary rapidly over time. In this paper, we design online time-varying optimal power flow algorithms based on the latest online algorithms, including three discrete-time algorithms (i.e., correction-only, prediction-correction, and correction-extra correction algorithms) and one continuous-time algorithm (i.e., prediction-correction interior-point method). According to our survey of the current applications of online algorithms, this study represents the first time that discrete-time prediction-correction and correction-extra correction algorithms have been applied in solving time-varying optimal power flow problems in active distribution networks. Detailed case studies, including correctness, validity, and sensitivity analysis, are carried out on a modified IEEE 33-node system. The numerical results reveal the advantages, disadvantages, and applicability of the proposed algorithms. Moreover, the characteristics of the discrete-time and continuous-time algorithms are compared intuitively. Finally, based on the numerical results, a guide for how to select an appropriate online time-varying optimal power flow algorithm for active distribution network optimization is provided, wherein the discrete-time prediction-correction optimal power flow algorithm is the most frequently recommended choice. [ABSTRACT FROM AUTHOR]

• A congestion management model considering the time sequence of P2P energy trading based on the CDA mechanism is proposed. • A two-tier market coordination mechanism of the P2P energy market and ancillary service market is proposed. • A cost and benefit analysis of different market participants is conducted. • Economic constraints are specially considered in the congestion management model. • The simulation results of two network scales verify the effectiveness, economy, and scalability of the proposed method. In the deregulated energy market environment, small-scale peer-to-peer (P2P) energy trading can increase the distributed photovoltaic power generation consumption and promote local energy balance. However, it also increases the possibility of security constraints violations during the operation of the utility grid. To solve physical network congestion caused by distributed P2P energy trading, we propose a method that considers the time sequence of P2P energy trading based on a continuous double auction (CDA) mechanism. In addition, to make full use of the users' flexibility to solve the network congestion, a two-tier market (P2P energy market and ancillary service market) coordination mechanism is proposed. The congestion management model includes two parts. One is congestion responsibility determination and congestion cost allocation in the P2P energy market, and the other is flexible services purchasing in the ancillary service market for minimizing congestion costs. The cost and benefit constraints are specially added in the congestion management model to reduce the economic loss of the grid and users. The proposed method is tested on 11-bus and IEEE 33-bus radial distribution systems to illustrate the effectiveness and scalability of this method. The simulation results show that the proposed mechanism and model effectively perform congestion management considering distributed P2P energy trading, and the results are profitable to users participating in both markets. [ABSTRACT FROM AUTHOR]

Abstract: Recently, there has been great interest in the integration of large numbers of small generation and storage resources at the distribution level. This will require new control strategies for efficient system performance. One issue that has not been addressed sufficiently is the coordinated dispatch of large numbers of these units. In this paper, an optimal distribution power flow strategy is proposed and implemented. The algorithm decomposes the overall system problem into two components: economic dispatch for energy and ancillary services based on market prices at the system level; and loss minimization at the distribution level. A combined quadratic programming and sectioning algorithm is used to find the solution. Simulation results show the effectiveness of the approach. [Copyright &y& Elsevier]

• A distributed online algorithm is proposed for solving optimal power flow (OPF). • It can drive the operation of distribution systems to OPF solutions continuously. • It does not need the operation details of each distribution area. • It applies to the distribution systems with arbitrary topologies for each area. • It reduces the calculation time sharply compared with the centralized methods. In this paper, a scalable distributed online algorithm is proposed for solving optimal power flow (OPF) in real time. The operation of distribution systems can be continuously driven towards given OPF targets only depending on local and some aggregated information of each distribution area. Particularly, the proposed algorithm is applicable to the distribution system where each area operates in arbitrary topologies. The algorithm uses the primal-dual gradient method to update DER set-points recurrently based on the sensitivities of the OPF targets; moreover, we decompose the sensitivities into several separate items and give the method to calculate these separate items in a distributed way. The decomposition and distributed calculation methods are all deduced under the general power flow model that completely specifies the physics of electrical networks with arbitrary topologies. The effectiveness of the algorithm is verified using a modified 502-node distribution system. Compared with the online algorithms requiring to process some operation details in a centralized way, the proposed algorithm can achieve almost the same convergence properties with reducing the calculation time sharply. [ABSTRACT FROM AUTHOR]

• Introducing a new framework for a TVPP in different energy markets. • Examining the abilities of local heat markets in improving a TVPP's performance. • Proposing the model in a multi-carrier energy environment. • Extracting the optimization model as a bi-level problem. Distributed energy resources (DERs) play an important role in the future vision of distribution energy networks. This paper with the aim of promoting DERs role in energy markets proposes a new framework for the optimal bidding strategy of a Technical Virtual Power Plant (TVPP) in different markets. In the proposed framework, it is assumed that the TVPP energy power is distributed throughout a multi-carrier energy network (MCEN) which consists of district heat networks (DHNs) and a distributed electricity network (DEN). In the MCEN, CHP units are the linkage between DHNs and DEN. With the goal of maximizing the profit of TVPP in different local and wholesale energy markets, a bi-level optimization model is formularized to set scheduling of DERs. As the upper level, the TVPP sets the schedule of its DERs considering technical constraints of DHNs and DENs. In the lower level, however, the TVPP runs an optimization problem to estimate the energy price in wholesale energy markets considering the forecasted bids of other rivals. The proposed model is implemented on a test system and the results confirms that the local market creates an opportunity for CHP owner, TVPP and the heat consumers to make more profit as well as achieving better technical performance for distribution energy systems. [ABSTRACT FROM AUTHOR]

• A distributed online OPF algorithm for distribution systems is proposed. • It formulates the second-order OPF approximations in a distributed way. • It only needs local measurements and boundary information for each area. • It can continuously drive the distribution operation towards given OPF targets. • The algorithm can cope with various setups for the OPF models. This paper proposes a distributed online OPF algorithm for distribution systems with high integration of electronic-interfaced distributed energy resources (DERs). The algorithm can coordinate the update of DER set-points on fast time scales only depending on local measurements and boundary information for each distribution area. Comparing with the existing distributed online algorithms that require some special setups (e.g., without considering the constraints on currents), the proposed algorithm can cope with various setups for the objective functions and constraints. The algorithm is based on solving the Taylor approximations of OPF problems by a distributed solution. In this solution, we design a strategy to formulate the second-order Taylor information in a distributed way by decomposing the sensitivities of the OPF targets among different areas; moreover, we design a method to simplify the constraints on DER outputs to accelerate the solution. The validity of the algorithm is verified by case studies, which show that it can continuously drive DER outputs towards the time-varying OPF targets composed of power losses and operation costs of DERs while satisfying the constraints on voltages and currents. [ABSTRACT FROM AUTHOR]

• A two-stage resilient operational framework of ADNs is proposed that compromises between operation cost and operational risk due to equipment outage and renewable resources uncertainties. • The energy storage systems are used as reserve resources in contingency mode modeled by the second stage. • A minimum level of state of charge can be evaluated as the required reserve capacity of the distribution systems. Resect severe economic losses caused by distribution system equipment outage have highlighted the importance of improving the system resiliency and reliability. In active distribution networks (ADNs), the distributed energy resources (DERs) managed by dynamic isolated microgrids in contingency mode provide an alternative approach to enhance the system resiliency and continue supplying critical loads after equipment outage. How to incorporate this ADNs capability into a short-term DERs scheduling is a challenging issue. In response to this challenge, in this paper, a two stage risk based decision making framework for operation of ADNs is proposed to coordinate 24-h DERs' scheduling and outage management scheme, in a way to be immune against contingency by creating optimal dynamic multi-micrigrid, micro-turbine and energy storage island operating and load shed plan. The first stage is the normal operation condition that operation cost should be minimized considering of the uncertainties of renewable resources, the electricity price and customers loads. The second stage is the operation in contingency condition. In the second stage, the main objective is to keep the shed load at minimum level. Numerical results and sensitivity analysis from modified 33-bus IEEE network are further discussed to demonstrate the efficiency of the solution approach. [ABSTRACT FROM AUTHOR]

• To analysis a complicated line configuration of DC microgrids, a voltage sensitivity analysis is derived based on a power flow analysis. • Based on voltage sensitivity analysis, an off-line design methodology of droop control for multiple bi-directional distributed energy resources is proposed to improve the droop control performance. • The proposed design method is applied to a 5-bus meshed line network with bi-directional distributed energy resources and loads, three loads. • PSCAD simulations and experimental results demonstrated the improved power sharing and voltage regulation performance. The analysis and design of droop control applied to a bi-directional distributed energy resources in a DC microgrid are presented. The effects of line resistance on the power sharing and voltage regulation performance are analysed. To interpret a complicated line configuration, a voltage sensitivity analysis is derived based on a power flow analysis. Based on this analysis, the droop control design methodology is proposed to improve the power sharing accuracy and voltage regulation performance. Stability analysis is performed to analyse the influence of the control parameters and line resistance on the stability of the controller. The design method is applied to the 5-bus meshed line network model with three bi-directional distributed energy resources, three loads, and two non-dispatchable distributed energy resources. The improved power sharing accuracy and voltage regulation performance are verified using PSCAD simulations and the experimental system. [ABSTRACT FROM AUTHOR]

• Initial fault response of the IBDERs is higher than inverter's fault current limit. • Voltage drop, grid impedance, control parameters influence on IBDERs fault response. • Highly accurate IBDERs' fault responses models including the initial peak current. • Easy to implement models for existing fault calculation solvers. • Suitable for calculations in large-scale systems with high proliferation of IBDERs. • Suitable for system planning in systems with high proliferation of IBDERs. Considering the development trends of renewable energy technologies, it is easy to understand why the future direction of the development for the power system will require a significant change in an approach to system planning and modeling. This is particularly the case with a high amount of inverter based distributed energy resources that can introduce non-linearity and higher fault current contribution, especially during the transients. In that regard, it is important to develop a new and a more precise model of the inverter based distributed energy resources, focusing on the initial fault response of the inverter. The most important factors, influencing the initial inverter response to the grid faults are investigated within the paper. Based on the experimental verification of these factors on the advanced laboratory prototype for testing the grid integrated renewable energy sources, this paper proposes a novel and accurate model of the inverter based distributed energy resources under grid faults. [ABSTRACT FROM AUTHOR]