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Highlights • An analytical framework for quantifying the flexibility of ADNs was proposed. • Flexibility of network transmission was analysed with the limits of physical network. • The benefits of controllable resources for flexibility improvement were explored. Abstract With the integration of high shares of distributed generators (DGs), it is increasingly difficult to cope with the various uncertainties and puts forward higher requirements for the operational flexibility in active distribution networks (ADNs). Controllability of ADNs has been significantly improved by the application of energy storage and power electronic devices. However, due to the difficulties in effective coordination of various controllable resources, the controllability cannot be fully translated into the system flexibility. In this paper, an analytical framework for quantifying the flexibility of ADNs is proposed, including the quantification of node flexibility, the matching of system flexibility, and the flexibility of network transmission. The proposed framework provides a novel perspective of flexibility to reinterpret operation issues of distribution networks. As the power imbalance and voltage deviation are even more severe caused by the high shares of DG integration, the indexes of flexibility from magnitude, frequency and intensity dimensions are deteriorated in ADNs operation. Through the spatial and temporal regulation of power flow, various controllable devices, such as soft open points (SOPs) and energy storage system (ESS), can effectively mitigate the power imbalance and voltage deviation to improve the indexes of system flexibility. Thus, under the unified analytical framework, the potential benefits of system controllability are fully utilized to provide effective countermeasures for the flexible operation of ADNs. Finally, case studies are performed on the modified IEEE 33-node system to quantify the operational flexibility of ADNs and verify the flexibility enhancement brought by controllable resources. [ABSTRACT FROM AUTHOR]

Highlights • A centralized parameter tuning model of local control curves was proposed for DGs. • The potential benefits of DG inverters were explored to realize voltage control. • A mixed-integer second-order cone programming (MISOCP) model was formulated. Abstract The increasing penetration of distributed generators (DGs) exacerbates the risk of voltage violations in active distribution networks (ADNs). The var capacity provided by DG inverters is a potential solution for voltage regulation. The conventional centralized control strategies limited by computation and communication burdens are difficult to meet the requirement of rapid voltage control. However, the local control strategies based on real-time measurements have a fast response to the frequent fluctuations of DG outputs. The performance of these local controllers depends on the tuning of the control parameters. This paper proposes a centralized-based method to determine the local voltage control strategies for DGs. A centralized parameter tuning model of control curves is built, in which Q - V and P curt - V curves of DG inverters are mathematically formulated based on piecewise linearization. The original mixed-integer nonlinear programming (MINLP) model is converted into an effectively solved mixed-integer second-order cone programming (MISOCP) model using convex relaxation. The potential benefits of DG inverters are explored to regulate both reactive and curtailed active power, based on local voltage measurements. Case studies on a modified PG&E 69-node distribution system are carried out to verify the effectiveness of the proposed method. Results show that power losses of ADNs are significantly reduced and voltage profiles are also improved. [ABSTRACT FROM AUTHOR]

There is an increasing awareness of resilience for active distribution networks (ADNs) as extreme fault conditions pose threats to the reliable operation of power supply. Islanding operation of ADNs with distributed generators (DGs) and energy storage system (ESS) can significantly serve the critical electricity demands and improve the power supply reliability. Considering the characteristics of DG, ESS and load, a time-series islanding partition model of ADNs is established based on soft open point (SOP). The proposed model exploits the benefits of operational flexibility provided by SOP to improve the level of load recovery. It further takes into account the coordination of the control strategies among regulation devices as well as the failure duration in island partition, while satisfying a variety of operational constraints. Specifically, a mixed-integer nonlinear programming (MINLP) is employed to solve the proposed model effectively. Finally, case studies on IEEE 33-node and IEEE 123-node test feeders verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

With the integration of high shares of distributed generators (DGs), both volatile DGs and demand-side resources make it increasingly difficult to cope with the various uncertainties and put forward a higher requirement for the operational flexibility in active distribution networks (ADNs). The highly integrated distribution-level power electronic devices represented by soft open point (SOP) significantly benefit the operation of ADNs. Due to the accurate and rapid power flow control provided by SOP, various flexible resources can be coordinated in the spatial and temporal aspects, which effectively increases the DG penetration of ADNs. In this paper, the multi-dimensional characteristics of distribution system flexibility are analysed and the maximum DG hosting capacity is adopted as the index to evaluate the flexibility brought by SOP. The flexibility evaluation model is further proposed to quantify the benefits of SOP-based flexible resources, such as conventional SOP, multi-terminal SOP and SOP with energy storage. Then, the original non-convex nonlinear model is converted into an effectively solved second-order cone programming (SOCP) model using convex relaxation. To tighten the deviation of convex relaxation to the predefined accuracy, a difference-of-convex programming (DCP)-based approach is developed to solve the proposed model. Finally, case studies are performed on the modified IEEE 33-node system to verify the effectiveness and efficiency of the proposed method. [ABSTRACT FROM AUTHOR]

The integration of distributed generators (DGs) exacerbates the feeder power flow fluctuation and load unbalanced condition in active distribution networks (ADNs). The unbalanced feeder load causes inefficient use of network assets and network congestion during system operation. The flexible interconnection based on the multi-terminal soft open point (SOP) significantly benefits the operation of ADNs. The multi-terminal SOP, which is a controllable power electronic device installed to replace the normally open point, provides accurate active and reactive power flow control to enable the flexible connection of feeders. An enhanced SOCP-based method for feeder load balancing using the multi-terminal SOP is proposed in this paper. By regulating the operation of the multi-terminal SOP, the proposed method can mitigate the unbalanced condition of feeder load and simultaneously reduce the power losses of ADNs. Then, the original non-convex model is converted into a second-order cone programming (SOCP) model using convex relaxation. To tighten the SOCP relaxation and improve the computation efficiency, an enhanced SOCP-based approach is developed to solve the proposed model. Finally, case studies are performed on the modified IEEE 33-node system to verify the effectiveness and efficiency of the proposed method. [ABSTRACT FROM AUTHOR]

Highlights • A combined decentralized and local voltage control strategy of SOPs was proposed. • Decentralized regulation was used for the power transmission of SOPs among areas. • The local reactive power curve of SOPs was optimally tuned in each area. • The potential benefits of SOPs were fully explored to realize fast voltage control. Abstract With the integration of high shares of distributed generators (DGs), it is increasingly difficult to cope with the voltage violations and puts forward a higher requirement for the operational flexibility in active distribution networks (ADNs). Soft open point (SOP) can realize accurate power flow control and continuous voltage regulation, which is usually operated by the centralized control strategy. However, the heavy burden of communication and complex global optimization process hinder its application on fast voltage control. This paper proposes a combined decentralized and local voltage control strategy of SOPs to rapidly cope with the frequent voltage fluctuations. Based on the dynamic network partition results, the decentralized optimization is applied to regulate active power transmission of SOPs among the connected areas. The Q - V control curve is optimally tuned to locally regulate the reactive power outputs of SOPs by using the intra-area information. Compared to the widely-used centralized strategy, the proposed combined control strategy of SOPs can effectively reduce the communication and computational burden with acceptable voltage control performance. Finally, the effectiveness of the proposed control strategy of SOPs is validated on the modified PG&E 69-node distribution system and modified IEEE 123-node distribution system. [ABSTRACT FROM AUTHOR]

Supply restoration from outages is essential for improving the self-healing ability of active distribution networks (ADNs). Soft open point (SOP) refers to a novel power electronic device installed to replace the traditional tie switch. Under fault situation, the coordination of multiple SOPs can provide voltage support and effectively expand the range of supply restoration. In this paper, a supply restoration method based on the coordination of multiple SOPs is proposed to improve the self-healing ability of ADNs. First, the coordinated service restoration strategy of multiple SOPs is proposed. Then, a self-healing oriented supply restoration method is proposed to restore loads from outages based on multiple SOPs. The contingency set is also established for practical application. Finally, the effectiveness of the proposed supply restoration method is validated on the IEEE 33-node distribution system. It shows that the proposed method can exploit the potential benefits of multiple SOPs and effectively improve the load recovery level of ADNs. • The impacts of position and capacity of SOPs on supply restoration were analyzed. • The supply restoration method of ADNs was proposed based on multiple SOPs. • Contingency set establishment was used in the practical application. • The potential benefits of SOPs were explored to restore loads from outages. [ABSTRACT FROM AUTHOR]