QuickFlex: a Fast Algorithm for Flexible Region Construction for the TSO-DSO Coordination

Most of the new technological changes in power systems are expected to take place in distribution grids. The enormous potential for distribution flexibility could meet the transmission system's needs, changing the paradigm of generator-centric energy and ancillary services provided to a demand-centric one, by placing more importance on smaller resources, such as flexible demands and electric vehicles. For unlocking such capabilities, it is essential to understand the aggregated flexibility that can be harvested from the large population of new technologies located in distribution grids. Distribution grids, therefore, could provide aggregated flexibility at the transmission level. To date, most computational methods for estimating the aggregated flexibility at the interface between distribution grids and transmission grids have the drawback of requiring significant computational time, which hinders their applicability. This paper presents a new algorithm, coined as QuickFlex} for constructing the flexibility domain of distribution grids. Contrary to previous methods, a priory flexibility domain accuracy can be selected. Our method requires few iterations for constructing the flexibility region. The number of iterations needed is mainly independent of the distribution grid's input size and flexible elements. Numerical experiments are performed in four grids ranging from 5 nodes to 123 nodes. It is shown that QuickFlex outperforms existing proposals in the literature in both speed and accuracy

Aggregating DERs VAR Capability Curve to Support the Grid in an Integrated T-D System

The multitudes of inverter-based distributed energy resources (DERs) can be envisioned as geographically distributed reactive power (var) devices (mini-SVCs) that can offer enhanced var flexibility to a future grid as an ancillary service. To facilitate this vision, a systematic methodology is proposed to construct an aggregated var capability curve of a distribution system with DERs at the substation level, analogous to a conventional bulk generator. Since such capability curve will be contingent to the operating conditions and network constraints, an optimal power flow (OPF) based approach is proposed that takes curtailment flexibility, unbalanced nature of system and coupling with grid side voltage into account along with changing operating conditions. Further, the influence of several other factors such as revised integration standard 1547 on the capability curve is thoroughly investigated on an IEEE 37 bus distribution test system. Finally, a T-D cosimulation is employed to demonstrate how DER aggregated flexibility can potentially enhance the decision domain for the transmission grid leading to improved performance

Volt/var control with high solar PV penetration in distribution systems and its impact on the transmision grid

With increasing distributed energy resources (DERs), both the distribution and transmission power systems are witnessing new challenges and opportunities. Voltage rise and voltage fluctuations are becoming major issues in the distribution network due to high solar PV penetration. Nevertheless, the volt/var control (VCC) capability of solar PV devices (inverter based DERs, in general) opens up various opportunities for both the distribution and transmission systems. In this work, we address the challenges in the distribution network and also explore the potential opportunities for the transmission system that can be provided by VCC capability of DERs in high PV penetration environment. The first part of our work addresses the voltage challenges faced by the distribution system due to solar PV penetration by utilizing VVC capabilities of PV smart inverters. In this part, we focus on the slope sensitive local droop VVC recommended by the recent integration standards IEEE1547, rule21 and addresses their major challenges i.e. selection of appropriate parameters under changing conditions, issue of control being vulnerable to instability (or voltage oscillations) and bad set-point tracking performance i.e. high steady state error (SSE). This is achieved by proposing a local real-time adaptive VVC which has two major features i.e. a) it is able to ensure both low SSE and control stability simultaneously without compromising any of the objectives, and; b) it dynamically adapts its parameters to ensure good performance in a wide range of external disturbances such as sudden cloud cover, cloud intermittency and substation voltage change. Moreover, the adaptive control does not depend on the feeder topology information. The proposed control is implementation friendly as it fits well within the integration standard framework and depends only on the local bus information. The performance is compared with the existing droop VVC methods under several scenarios on a large unbalanced 3-phase feeder (IEEE 123 bus test system) with detailed secondary side modeling. The second part of our work focus on investigating the impact of DER VVC on the bulk transmission grid. We present a hypothesis that the multitudes of inverter-based DERs can be envisioned as geographically distributed reactive power (var) devices (mini-SVCs) that can offer enhanced var flexibility to a future grid as an ancillary service. To facilitate this vision, a systematic methodology is proposed to construct an aggregated var capability curve of a distribution system with DERs at the substation level, analogous to a conventional bulk generator. Since such capability curve will be contingent to the operating conditions and network constraints, an optimal power flow (OPF) based approach is proposed that takes curtailment flexibility, unbalanced nature of system and coupling with grid side voltage into account along with changing operating conditions. Further, the influence of several other factors such as revised integration standard 1547 on the capability curve is thoroughly investigated on an IEEE 37 bus distribution test system. In the last part of the work, we investigate the DERs\u27 impact on the long-term voltage stability assessment on an integrated T-D system. Finally, a T-D cosimulation is employed to demonstrate how DER aggregated flexibility and var support can potentially enhance the transmission grid performance on an integrated T-D test system

On the Assessment of the Flexibility Region in Inter-DSO Local Markets.

Renewable Distributed Energy Resources (RDERs) are being rapidly deployed in energy systems to meet net zero emissions objectives. RDERs may cause operational issues in these systems, which exposes both Transmission System Operators (TSOs) and Distribution System Operators (DSOs) to congestions and imbalances. Recently, efforts were made towards the definition of Inter-DSO Local Flexibility Markets (LFMs) to mitigate those issues. However, DSOs do not have properly tools that analyse their flexibility when participating in these markets. This paper presents a linear approach for assessing their flexibility region and the associated costs. The novelty of the proposed approach lies in the flexibility assessment in Inter-DSO LFMs and in the linearization of the problem, which enables the analysis of non-linear DERs such as batteries. The feasibility of the proposed approach is demonstrated using a case study based two IEEE 34 bus systems which represent two different DSOs participating in an Inter-DSO LFM. The flexibility of each DSO is analysed in two scenarios, when the interconnection is active and when it is congested. Results reflect how RDERs can effectively procure flexibility for both situations and how the proposed approach captures their non-linear behaviour.This work was partially supported by Junta de Andalucia (Spain) Project Ref: P20\_01164, by Ministerio de Ciencia e Innovación through project TED2021-132339B-C42 and by the University of Málaga. Á. Paredes was also supported by the FPU grant (FPU19/03791) funded by the Spanish Ministry of Education. The authors thankfully acknowledge the computer resources provided by the SCBI center of the University of Málaga. Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Exploring Operational Flexibility of Active Distribution Networks with Low Observability

Power electronic interfaced devices progressively enable the increasing provision of flexible operational actions in distribution networks. The feasible flexibility these devices can effectively provide requires estimation and quantification so the network operators can plan operations close to real-time. Existing approaches estimating the distribution network flexibility require the full observability of the system, meaning topological and state knowledge. However, the assumption of full observability is unrealistic and represents a barrier to system operators' adaptation. This paper proposes a definition of the distribution network flexibility problem that considers the limited observability in real-time operation. A critical review and assessment of the most prominent approaches are done based on the proposed definition. This assessment showcases the limitations and benefits of existing approaches for estimating flexibility with low observability. A case study on the CIGRE MV distribution system highlights the drawbacks brought by low observability.Comment: This paper has been accepted to the IEEE Belgrade Powertech 2023. It has 6 pages, 4 figures, and 2 table