Flexibility Of Distributed Energy Resources Research Articles

Common TSO-DSO market framework with no upfront priority to utilize DER flexibility

Grid integration of distributed energy resources (DERs) is vital to provide flexibility services, required to ensure efficient system operations at both transmission and distribution levels. To prevent negative interactions between transmission system operators (TSO) and distribution system operators (DSOs) while procuring flexibility services from DERs, it is imperative to prioritize them. However, for system's cost-effectiveness allocating DER flexibility to the operator with the highest need is preferable rather than giving upfront priority. To address this, a novel coordinated decision-making approach for a common TSO-DSO market is proposed. This approach considers global balancing for the TSO and distribution congestion management for the DSO as objectives, using them as deciding factor to allocate DER flexibility to the system operator with the greatest need to fulfil its objective. Results show proposed method enhances DSO's flexibility transfer to TSO and maintains reactive power at T-D interface. Gap between TSO's DER flexibility needs and availability, influenced by congestion and line losses, is critical for prioritization. Specifically, network capacity and line losses constitute 18.79 % and 3.42 % of offered DER flexibility, respectively. Reactive power deployment improves Q/P ratio, reducing standard deviation from 0.19 to 0.018. Framework reduces conflicts, provides clear flexibility allocation rules without upfront priority.

Stacking Revenues From Flexible DERs in Multi-Scale Markets Using Tri-Level Optimization

Rapid proliferation of flexible Distributed Energy Resources (DERs) as a result of Net Zero Emissions objectives entails a profound shift in the paradigm of local and national energy systems. Currently, DERs' simultaneous participation in multiple markets is generally restricted, which undermines their profitability. With the aim of increasing the number of business cases for them, a tri-level optimization problem that seeks the maximisation of revenues from DERs is proposed. The optimization problem considers simultaneous participation of different flexible DERs, such as, Electric Vehicles (EVs), Battery Energy Storage Systems (BESSs) and Heating, Ventilation and Air Conditioning (HVACs), in national and local markets. Markets are cleared sequentially, and the model is recast into a tractable single-level problem using its dual formulation and strong duality condition. Results from a case study based on the IEEE 14 bus transmission network, a realistic distribution network and SimBench dataset demonstrate the effectiveness of the proposed approach in increasing profits compared with a baseline scenario.

Optimal Cluster Scheduling of Active–Reactive Power for Distribution Network Considering Aggregated Flexibility of Heterogeneous Building-Integrated DERs

This paper proposes an active–reactive power collaborative scheduling model with cluster division for the flexible distributed energy resources (DERs) of smart-building systems to resolve the high complexity of the centralized optimal scheduling of massive dispersed DERs in the distribution network. Specifically, the optimization objective of each cluster is to minimize the operational cost, the power-loss cost, and the penalty cost for flexibility deficiency, and the second-order cone-based branch flow method is utilized to convert the power-flow equations into linearized cone constraints, reducing the nonlinearity and heavy computation burden of the scheduling model. Customized virtual battery models for building-integrated flexible DERs are developed to aggregate the power characteristics of flexible resources while quantifying their regulation capacities with time-shifting power and energy boundaries. Moreover, a cluster division algorithm considering the module degree index based on the electrical distance and the flexible balance contribution index is formulated for cluster division to achieve information exchange and energy interaction in the distribution network with a high proportion of building-integrated flexible DERs. Comparative studies have demonstrated the superior performance of the proposed methodology in economic merits and voltage regulation.

Computational Performance Study on the Alternating Direction Method of Multipliers Algorithm for a Demand Response Peak Shaving Application

This article quantifies and benchmarks the computational performance of the distributed alternating direction method of multipliers (ADMM) optimization algorithm applied to a slow-dynamics demand response peak shaving application. We propose a hierarchical demand response architecture, in which a commercial aggregator acts as the supplier and system-level flexibility service provider to a portfolio of residential prosumers. The prosumers are equipped with flexible distributed energy resources that we model based on convex operation constraints. The optimal day-ahead peak shaving control of the portfolio is realized in a distributed way through an entirely parallel implementation of the distributed ADMM optimization algorithm. In this scenario, we evaluate the ADMM algorithm's overall speed of convergence and conduct multiple large-scale numerical simulation experiments on a compute cluster to compare and assess the algorithmic performance of ADMM against a centralized reference benchmark optimization. Our simulation results show that the distributed ADMM algorithm is superior compared to a purely centralized optimization. The findings are of particular use to commercial demand response aggregators that show interest in the deployment and improvement of distributed coordination and optimal control concepts for future slow-dynamics demand response applications and services.

Advanced Flexibility Support through DSO-Coordinated Participation of DER Aggregators in the Balancing Market

Future power systems with a high share of intermittent renewable energy sources (RES) in the energy portfolio will have an increasing need for active power balancing. The integration of controllable and more flexible distributed energy resources (DERs) at the distribution-grid level represents a new solution and a sustainable alternative to conventional generation units for providing balancing services to the transmission system operator (TSO). Considering that the extensive participation of DERs in ancillary services may lead to the violation of limits in the distribution network, the distribution system operator (DSO) needs to have a more active role in this process. In this paper, a framework is presented that allows the DSO, as the central coordinator of the aggregators, to participate in the balancing market (BM) as a balancing service provider (BSP). The developed mathematical model is based on the mixed-integer second-order cone programming (MISOCP) approach and allows for determination of the limits of active power flexibility at the point of the TSO–DSO connection, formation of the dependence of the price/quantity curve, and achievement of the optimal dispatch of each DER after clearing the balancing market. The simulation results are presented and verified on modified IEEE distribution networks.

An equivalent model‐based asynchronous dispatch method for clusters of flexible distributed energy resources

AbstractWith the increasing number of flexible distributed energy resources, it is difficult for the distribution network to control various devices directly. Therefore, managing and dispatching the resources in a cluster way is required. This article proposes an equivalent model‐based asynchronous dispatch framework, which is composed of three layers, the distribution network, clusters, and distributed energy resources. With this framework, economic dispatch is carried out with the equivalent models of clusters, simplifying the dispatch problem of the distribution network. The numerical test results show that the proposed method can significantly reduce the calculation time and maintain high accuracy in the economic dispatch problem.

Stochastic Strategic Participation of Active Distribution Networks With High-Penetration DERs in Wholesale Electricity Markets

With the increasing penetration of distributed energy resources (DERs), traditional distribution networks as load-serving entities in wholesale electricity markets, now evolve towards active distribution networks (ADNs) which can proactively participate in wholesale markets by optimally controlling the DERs in their networks. A stochastic bilevel optimization model is proposed in this paper for the strategic participation of ADNs and DERs to provide energy and grid services in wholesale electricity markets. The bilevel optimization model can capture the interactions between the ADN and the wholesale energy and ancillary service markets, considering the uncertainties of DERs in the ADN. In the upper-level model, the ADN makes optimal decisions on energy and reserve bidding considering the availability, uncertainties, and flexibility of DERs. The joint energy and reserve market-clearing of the independent system operator (ISO) is modeled as the lower-level problem. Using strong duality theory and Karush-Kuhn Tucker (KKT) conditions, the proposed bilevel optimization problem is reformulated as mathematical programming with equilibrium constraints (MPEC) problem and further converted into a computationally-solvable mixed-integer second-order-cone programming (MISOCP) model. The simulation results demonstrate the effectiveness of the model and the interactions between an ADN and wholesale electricity markets.

An Effective Method to Estimate the Aggregated Flexibility at Distribution Level

The exploitation of flexibility services provided by distributed energy resources (DERs) located in the distribution grid becomes increasingly important for the secure and reliable operation of power systems. These resources are typically small-scale and located close to end-users, however their aggregated flexibility can offset the imbalance caused by the stochastic variations of the renewable power sources. This paper proposes a methodology to estimate with increased accuracy the available flexibility of DERs in an Active Distribution Grid (ADG) at the points of interconnection with the transmission system. An optimization-based approach is used to estimate the available maximum active and reactive power flexibility at each TSO-DSO substation for specific directions formed by constant active to reactive power ratios. In order to keep calculations manageable and calculate the available flexibility with high accuracy, the search directions are sampled from a distribution that is iteratively updated at each step. Three variations are proposed for the angle sampling, both in the angle domain and in the p-q domain. Each variation is tested on a modified 18-bus radial distribution system and its effectiveness and convergence is compared.

Customized Rebate Pricing Mechanism for Virtual Power Plants Using a Hierarchical Game and Reinforcement Learning Approach

In the transition to a two-sided electricity market, energy users are turning into prosumers who own the flexible distributed energy resources (DERs) and have the potential to provide services to the power system. Virtual power plants (VPPs) aggregate DERs to join the electricity market and respond to system signals. It is urgent to develop a new pricing mechanism for VPPs to allocate the payoff from the electricity market to prosumers. This paper proposes a customized rebate package pricing mechanism for a VPP retailer to reward prosumers for supporting the power system. The retailer’s pricing strategies are determined based on a Stackelberg game, considering the heterogeneous prosumers’ dynamic selecting process based on an evolutionary game. The extended replicator dynamics is proposed to take the future payoff into account and guarantee the evolutionary equilibrium. Moreover, a new reinforcement learning algorithm based on the Cross learning model is developed to solve the evolutionary game with less computational effort. The simulation results verify the effectiveness of the proposed customized rebate package pricing mechanism, which can efficiently reward prosumers’ flexible resources in supporting the system while maximizing the retailer’s utility to achieve a win-win outcome.

Distributed energy resources flexibility as volumetric options on electricity

Distributed energy resources flexibility as volumetric options on electricity

Network-secure bidding strategy for aggregators under uncertainty

The widespread adoption of distributed energy resources (DER) is creating an opportunity for aggregators to transform DER flexibility into electricity market services. In a scenario of high DER integration, aggregators will need to coordinate the optimisation of DER with the distribution system operator (DSO) in order to avoid congestion and voltage incursions in the distribution networks. This coordination task is notably complex since both network and DER operation are impacted by multiple sources of uncertainty. To address these challenges, this paper proposes a new bidding strategy for aggregators of prosumers to make robust network-secure bidding decisions in day-ahead energy and reserve markets. The bidding strategy computes robust network-secure bids without jeopardising the data privacy of aggregators and the DSO. The data privacy is preserved by using the alternating direction method of multipliers (ADMM) to decompose a stochastic network-secure bidding problem into bidding and network subproblems and solve them separately and in parallel. The uncertainty of the prosumers is incorporated in the bidding problem through scenarios of load, renewable generation, and DER preferences. Our experiments show that the proposed bidding strategy computes robust bids against distribution network problems, outperforming deterministic and stochastic state-of-the-art bidding strategies in terms of cost and network observability.

A Network-Aware Distributed Energy Resource Aggregation Framework for Flexible, Cost-Optimal, and Resilient Operation

To efficiently use the ubiquitous behind-the-meter distributed energy resources (DERs) in distribution systems for providing grid services, this paper presents a hierarchical control framework for DER optimal aggregation and control. We first develop a convex optimization model to evaluate the DER flexibility, and then use a convex model-predictive-control based approach to dispatch those DERs. The hierarchical control framework consists of a utility controller, community aggregators and multiple home energy management systems. The flexibility of the DERs is evaluated by each controller in the hierarchy such that the resultant flexibility is feasible given its operational domain. Based on the determined flexibility, the hierarchical controllers then compute optimal setpoints for the DERs to help the distribution system regulate node voltages and provide other distribution grid services. Numerical simulations performed on a model of a real distribution feeder in Colorado, using actual DER data in a residential community demonstrate that the proposed approach can effectively alleviate voltage issues and support resilient operation.

10 kV SiC MOSFET Based Medium Voltage Power Conditioning System for Asynchronous Microgrids

Distributed energy resources (DERs) and microgrids have seen tremendous growth and research activities in recent years. Flexible DERs and asynchronous microgrids (ASMG) can have many system-level benefits over fixed DERs and conventional microgrids. The key enabler for flexible DERs and ASMG is a power converter based power conditioning system (PCS) as the interface between DERs/microgrids and the medium voltage (MV) distribution grid. High voltage (HV, >3.3 kV) silicon carbide (SiC) based MV converter is now a promising solution for the PCS. This article presents development and testing of a 10 kV SiC MOSFET based MV PCS for 13.8 kV ASMG. MV PCS converter design addressing high dv/dt issue generated by fast switching of the 10 kV SiC MOSFET is presented. The developed PCS is successfully tested at 25 kV dc 13.8 kV ac voltages and 100 kVA power. Grid support functions are also demonstrated with the developed PCS prototype and hardware tests beds, validating HV SiC converter benefits for ASMG.

Aggregate Temporally Coupled Power Flexibility of DERs Considering Distribution System Security Constraints

The proliferation of distributed energy resources (DERs) makes it possible for the transmission system operator to exploit the DERs' power flexibility. Modeling the aggregate power flexibility of different types of DERs in the distribution system continues to be a vexing scientific problem. This paper rigorously derives the exact mathematical expression of aggregate temporally coupled power flexibility of DERs considering linear distribution system security constraints. A flexibility optimization method and a backtracking elimination method are proposed to calculate the parameters in the aggregate flexibility model. Based on the exact model, an approximate model is proposed to reduce the complexity, which can be used for practical applications. We further developed a transmission system economic dispatch model considering the power flexibility of distribution systems. Finally, numerical tests are carried out on IEEE test systems to verify the correctness of the derived model and the effectiveness of the proposed methods.

MV-LV network-secure bidding optimisation of an aggregator of prosumers in real-time energy and reserve markets

The large-scale adoption of commercial and residential distributed energy resources (DER) is transforming passive consumers into active prosumers. This new paradigm opens a door for aggregators to transform DER flexibility into electricity market services. However, it also brings new challenges for the distribution system operator (DSO) since aggregator actions may cause voltage and congestion problems in medium voltage (MV) and low voltage (LV) distribution networks. This paper addresses these challenges by proposing a new bidding optimisation strategy for an aggregator of prosumers to make network-secure bidding decisions in real-time energy and reserve markets. The bidding strategy uses the alternating direction method of multipliers on a rolling horizon framework to negotiate MV-LV network-secure bids between the aggregator and DSO, without jeopardizing the data privacy of either agent. The experiments run on a real-world case study show that the proposed bidding strategy outperforms state-of-the-art strategies, by computing MV-LV network-secure bids in fast execution times. Furthermore, the experiments also show that most of the DER value can reach the market with the proposed bidding strategy, even in heavily network-constrained scenarios of DER (up to 87% of the potential value in the most extreme DER scenario).

Aggregate Operation Model for Numerous Small-Capacity Distributed Energy Resources Considering Uncertainty

Numerous small-capacity distributed energy resources (DERs) pose technical challenges and increase the management complexity for power system operators. This article proposes an aggregate operation model to efficiently manage a large number of DERs with heterogeneous parameters. The aggregate operation model includes the approximate feasible region and the equivalent operational cost function. First, considering the coupling of power, energy, and regulation service, the operational flexibility of multiple categories of DERs is modeled by a general polytope. Second, by scaling and translating a basic homothetic polytope, a maximum inner aggregation approach is formulated to exploit the potential flexibility of DERs and guarantee solution feasibility. The uncertainty of DERs is modeled by the distributionally robust chance-constrained program (DRCCP), and the equivalent cost function of the DER aggregator is rigidly derived. Last, this study theoretically explains that the aggregate feasible region can be enlarged by introducing the DER clustering process, and a spectral clustering algorithm based on the multi-scale similarity metric method is developed to classify DERs into different aggregators. Numerical simulation results indicate the great potential of the proposed method to enhance system operational flexibility with high computational efficiency.

The Role of Regulators in Promoting the Procurement of Flexibility Services within the Electricity Distribution System: A Survey of Seven Leading Countries

This paper identifies and explores regulatory issues that may have an impact on the use of flexibility services by distribution utilities to solve grid constraints. This can be done by flexible distributed energy resources which can be instructed, for instance, to reduce export generating capacity or increasing consumption. We want to identify how regulation can better support the development of the future distribution utility in its role as neutral market facilitator, enabling more competition in local flexibility markets and optimal use of resources. A set of questionnaires were designed to capture the insights around important aspects of the regulation of flexibility markets (utilities’ network incentives, network tariff structure, market design for flexibility markets, etc.). These were sent to distribution utilities, energy regulators, energy marketplaces, energy associations and relevant experts from seven jurisdictions. The responses suggest a collective interest in the procurement of flexibility services by distribution utilities from distributed energy resources. New regulations, the adaptation of current rules and recent consultations reflect this. However, the amount of progress with and preferences for key regulatory changes differ across jurisdictions.

Virtual power plant implementation scheme in Shenzhen city

AbstractTo meet the challenge of the “three highs and one limit” of the Shenzhen power grid operation, the virtual power plant (VPP) is proposed to effectively utilize the flexible distributed energy resources (DERs) to optimize the operation scheme of the distribution network. Firstly, based on the investigation of the available and controllable DERs of the Shenzhen power grid, typical application scenarios representing the characteristics of the Shenzhen power grid are formulated. Secondly, the framework and roadmap of the typical regional VPP in Shenzhen are proposed. Thirdly, a coordinated optimal scheduling strategy for the VPP is proposed to solve the voltage issues caused by overload or other reasons in Shenzhen's distribution network. Finally, a case study is carried out on the distribution networks using that of the Shenzhen Potevio New Energy company, Shenzhen Bay Science and Technology Eco‐Park are connected. The results show that the voltage level of the distribution network can be improved by the proposed optimal control strategy of the VPP, and verify the effectiveness of the proposed VPP scheme and the benefits of the VPP for the Shenzhen power grid operation.

Network-Secure and Price-Elastic Aggregator Bidding in Energy and Reserve Markets

The increasing uptake of distributed energy resources (DER) is displacing the dispatchable generators responsible for providing frequency response in our power system. To compensate for the withdrawal of such generators, the market participation of DER fleets through aggregators has been suggested in the literature. Unfortunately, these works are either limited to price-taking aggregators or neglect the distribution network. The first limitation leads to inelastic bids which do not reflect DER flexibility, while the second results in bids that do not lie within the network boundaries. To mitigate these challenges, we propose a price-elastic aggregator bidding approach together with a network conforming layer. Our approach enables aggregators to offer a range of network-secure bid bands for different prices. To preserve the independent role of each stakeholder and share the computational burden, we decompose the problem into aggregator and network subproblems, solved sequentially. Moreover, we confer aggregators the possibility to provide reactive power support to enable the network to accept greater throughput. Finally, since our approach ensures network feasibility prior to bids reaching the market, it minimizes the disruption to existing wholesale market structures. Our results on a 141-bus network shows 19% higher benefits for aggregators, compared to the inelastic bidding approach.

Hierarchical Distribution System Adaptive Restoration With Diverse Distributed Energy Resources

Distributed energy resources (DERs) can be utilized as alternative power sources for fast service recovery in cases of distribution system outages. However, the diverse characteristics and stochastic nature of DERs impose challenges on the optimal operation of diverse resources during the restoration phase. In this article, we introduce a hierarchical structure to coordinate distribution system entities and propose an adaptive restoration strategy to sustain a reliable power supply to outage loads in an unbalanced distribution system utilizing DERs. An efficient system reconfiguration model is developed to guarantee the radiality of restoration islands. The flexibility of DERs is evaluated by a novel method that focuses on the cumulative energy consumption, the superiority of which is also proven. Model predictive control (MPC) is adopted to enable the adjustment of system topology and DER operation strategies based on up-to-date data. The performance of the proposed restoration model is validated on a modified IEEE 123-bus test system. Moreover, the effectiveness of the proposed novel flexibility assessment and the MPC-based adaptive restoration strategy are verified through comparative studies.