Flexibility Of Power System Operation Research Articles

Optimal configuration of energy storage for alleviating transmission congestion in renewable energy enrichment region

High-penetration renewable energy development causes transmission congestion in power system operation. Such transmission congestion in short period can be alleviated by energy storage configuration, instead of investing and expanding new transmission lines. This paper presents an optimal configuration method of energy storage for alleviating transmission congestion in renewable energy enrichment region. In order to obtain the characteristics of renewable energy outputs, a scenario generation technique for renewable energy outputs considering spatio-temporal correlation is first proposed. A Monte Carlo-based approach for evaluating the transmission congestion is proposed for identifying the potential locations of energy storage installation. Finally, an optimal configuration method of energy storage based on stochastic programming is proposed for alleviating transmission congestion. Numerical experiments are carried out on a modified IEEE-RTS 24-bus system and a practical 129-bus system. Numerical results show that energy storage can improve the flexibility of power system operation and the utilization of renewable energy generation. Especially, in the practical 129-bus system, the proposed method reduces renewable energy curtailment by 625.26 MWh and 444.43 MWh under fall and winter typical scenarios, respectively. And, investing some energy storage devices is more economical when the duration of transmission congestion is relatively short.

Retrofit Planning and Flexible Operation of Coal-Fired Units Using Stochastic Dual Dynamic Integer Programming

The continuous growth of wind power requires greater power system operational flexibility owing to the variability and uncertainty of wind power generation. Retrofitting existing coal-fired units is a demonstrated cost-effective method for improving system flexibility. This study proposes a decision support tool that provides optimal investment decisions to enhance power system operational flexibility. A novel flexibility retrofit planning (FRP) model is proposed in which the embedded operational problem is a stochastic mixed-integer nonlinear programming (SMINLP) unit commitment problem. In the operational unit commitment model, novel mixed-integer nonlinear formulations are proposed to represent the coal-fired unit attribute (minimum power output, startup and shutdown times, and maximum ramp rate) modifications after retrofitting. A new solution method based on the Benders decomposition (BD) and stochastic dual dynamic integer programming (SDDIP) algorithms is proposed to solve the proposed FRP model. The modified IEEE 39-bus, 57-bus, and 118-bus test systems were used to verify the effectiveness of the proposed model and solution method. The results demonstrate that the proposed FRP model can balance the retrofitting and operational costs to minimize the total cost and accommodate more wind generation. The proposed solution method is computationally efficient for solving FRP models.

An optimal bidding and scheduling method for load service entities considering demand response uncertainty

With the rapid development of demand-side management technologies, load serving entities (LSEs) may offer demand response (DR) programs to improve the flexibility of power system operation. Reliable load aggregation is critical for LSEs to improve profits in electricity markets. Due to the uncertainty, the actual aggregated response of loads obtained by conventional aggregation methods can experience significant deviations from the bidding value, making it difficult for LSEs to develop an optimal bidding and scheduling strategy. In this paper, a bi-level scheduling model is proposed to maximize the net revenue of the LSE from optimal DR bidding and energy storage systems ESS scheduling by considering the impacts of the uncertainty of demand response. An online learning method is adopted to improve aggregation reliability. Additionally, the net profit for LSEs can be raised by strategically switching ESS between two modes, namely, energy arbitrage and deviation mitigation. With Karush–Kuhn–Tucker (KKT) optimality condition-based decoupling and piecewise linearization applied, this bi-level optimization model can be reformulated and converted into a mixed-integer linear programming (MILP) problem. The effectiveness and advantages of the proposed method are verified in a modified IEEE RTS-24 bus system.

Day‐ahead flexibility enhancement via joint optimization for new energy vehicle fleets and electric vehicle charging/hydrogen refuelling stations

Power systems are striving for the decarbonization goal for long-term sustainability. As a result, the renewable energy sources (RES) have been playing an increasingly important role in the generation portfolio. Power system flexibility maintaining the power balance under uncertain and variable RES generation has become a major concern during the energy transition. This paper proposes to apply new energy vehicles (NEV) including electric vehicles (EVs) and fuel cell vehicles (FCVs) as day-ahead flexibility resources to make revenue by providing comprehensive capacity/energy flexibility in the ancillary service market. First, the detailed joint optimized operational strategies of the NEV fleet and the EV charging/hydrogen refuelling stations are formulated. Second, the design and service clearing methodology of the ancillary service market for flexibility enhancement is introduced. A game-theoretic model is applied to determine the equilibrium price for both the demand and suppliers (namely NEV fleets) of operational flexibility. Third, case studies using IEEE benchmark systems are provided to validate the proposed methods. The results demonstrate that the joint optimization of NEV fleets, charging stations, and hydrogen refuelling stations can improve the power system operational flexibility, reduce the charging expense of NEV owners, and reduce the carbon emission by utilizing the green hydrogen.

A Comprehensive Overview of Dynamic Line Rating Combined with Other Flexibility Options from an Operational Point of View

The need for flexibility in power system operation gradually increases regarding more renewable energy integration, load growth, etc., and the system operators already invest in this manner to enhance the power system operation. Besides, the power system has thermally sensitive assets such as lines, transformers, etc. that are normally operated under highly conservative static ratings. There is a growing trend in this regard to use the actual capacity of such assets dynamically under varying operating conditions leading to a dynamic thermal rating concept which is referred as dynamic line rating (DLR) approach specifically for lines. This study provides a comprehensive overview of existing perspectives on DLR and combination with other flexibility options from an operational point of view. Apart from the existing review studies more focused on implementation category of DLR concept, the concentration on more operational stage from the power system operation point of view leads the difference of this study compared to the mentioned studies. A categorization of the DLR implementation for either being sole or combined usage as a flexibility option is further realized. Besides, a geographically categorized analysis on existing practical evidence on DLR concept and implementations is also presented in this study.

Using Energy Storage to Relief Transformer Overloading after Coal-Fire Generator Retirement

Energy storage system (ESS) is believed to be a significant source of power system operational flexibility given renewable generation with high penetration. This work reports a unique scenario of relieving the overloading of a 500kV transformer after the retirement of a coal-fire generator. ESS is proven to be an ideal solution in terms of technical feasibility and economic performance. A look-ahead operation strategy is to utilize ESS to mitigate the identified transformer overloading bottleneck. A case study using a synthetic version of Zhejiang power grid confirms the effectiveness of the proposed ESS based approach.

Economically Optimal Uncertainty Set Characterization for Power System Operational Flexibility

Assessing the operational flexibility of the grid with high penetration of renewable resources remains an issue of critical importance. Operational flexibility insufficiency may bring about two major problems: 1) there may exist no feasible solution for operation under uncertain conditions due to insufficient available flexibility capacity; 2) even if the solution feasibility criterion is ensured, dispatch limitations of the flexible resources may force the system operating point to deviate from the optimal economic point with increased redispatch. In this paper, an optimal uncertainty set at the unit commitment time scale is proposed and characterized as a reliable operational metric. A novel robust-based framework is developed to quantify the economically optimal uncertainty set such that both feasibility and optimality robustness are guaranteed. Furthermore, the flexible capabilities of storage units, nonquick-start, and quick-start generating units are formulated to illustrate the effectiveness of the proposed metrics. Two proposed algorithms based on column and constraint generation are adopted to solve the proposed formulations. The performance of the suggested formulations is compared with the scenarios, where only the feasibility robustness is taken into account. Numerical simulations on the modified IEEE 73-bus system will verify the effectiveness and efficiency of the suggested framework.

Co-optimization of transmission topology and generation dispatch incorporating demand response and renewable energy uncertainty in day-ahead electricity markets

The variability of renewables requires a higher degree of flexibility in power system operation. At present, there are a variety of solutions which are being utilized, particularly demand response and transmission switching. This paper presents a model for co-optimization of transmission topology and generation dispatch based on a two-stage stochastic optimization. Demand response and renewable energy uncertainty are integrated into the proposed model. The uncertainty pertaining to renewable energy sources is presented through a set of scenarios. The model is a mixed-integer linear programming (MILP) problem and can be applied for the day-ahead market clearing. The results implemented using a 5-bus system demonstrate the effectiveness of the proposed model.

Cascading Collapse of a Large-Scale Mixed Source Microgrid Caused by Fast-Acting Inverter-Based Distributed Energy Resources

Power electronic converter-interfaced distributed energy resources (DERs) are being increasingly deployed for achieving high energy efficiency, power quality, and flexibility of power system operation and controls. They facilitate access to a wide array of energy sources, including renewables, fuel cells, microturbines, variable speed engine-generator sets, etc. However, recent tests carried out at the Consortium for Electric Reliability Technology Solutions (CERTS) Microgrid have indicated that their deployment in the mixed source microgrid can cause a cascading collapse during extreme events. To investigate the problem, simulation models of two types of DERs are developed in PSCAD/EMTDC software and validated with the experimental test results. Furthermore, the validated models are used to study a cascading collapse problem in a large-scale mixed source microgrid on the benchmark IEEE 33-bus test system. In this paper, three alternative techniques are evaluated to prevent the cascading collapse in the large-scale microgrid caused by fast-acting inverter-based DERs.

Enhancing Power System Operational Flexibility With Flexible Ramping Products: A Review

With the increased variability and uncertainty of net load induced from high penetrations of renewable energy resources and more flexible interchange schedules, power systems are facing great operational challenges in maintaining balance. Among these, the scarcity of ramp capability is an important culprit of power balance violations and high scarcity prices. To address this issue, market-based flexible ramping products (FRPs) have been proposed in the industry to improve the availability of ramp capacity. This paper presents an in-depth review of the modeling and implementation of FRPs. The major motivation is that although FRPs are widely discussed in the literature, it is still unclear to many that how they can be incorporated into a co-optimization framework that includes energy and ancillary services. The concept and a definition of power system operational flexibility as well as the needs for FRPs are introduced. The industrial practices of implementing FRPs under different market structures are presented. Market operation issues and future research topics are also discussed. This paper can provide researchers and power engineers with further insights into the state of the art, technical barriers, and potential directions for FRPs.

Demand Response of Ancillary Service From Industrial Loads Coordinated With Energy Storage

As one of the featured initiatives in smart grids, demand response is enabling active participation of electricity consumers in the supply/demand balancing process, thereby enhancing the power system's operational flexibility in a cost-effective way. Industrial load plays an important role in demand response because of its intense power consumption, already existing advanced monitoring, and control infrastructure, and its strong economic incentive due to the high energy costs. As typical industrial loads, cement plants are able to quickly adjust their power consumption rate by switching on/off the crushers. However, in the cement plant as well as other industrial loads, switching on/off the loading units only achieves discrete power changes, which restricts the load from offering valuable ancillary services such as regulation and load following, as continuous power changes are required for these services. In this paper, we overcome this restriction of poor granularity by proposing methods that enable these loads to provide regulation or load following with the support of an onsite energy storage system.

Transmission-Constrained Unit Commitment Considering Combined Electricity and District Heating Networks

Wind power integration could be restricted by inflexible operation of combined heat and power (CHP) units due to the strong linkage between power generation and heating supply in winter. Utilization of the heat storage capacity of existing district heating network (DHN) is a cost-effective measure to enhance power system operational flexibility to accommodate large amounts of variable wind power. In this paper, transmission-constrained unit commitment (UC) with combined electricity and district heating networks (UC-CEHN) is formulated with a linear DHN model to coordinate short-term operation of electric power and district heating systems. The heat storage capacity of the DHN is modeled by capturing the quasi-dynamics of pipeline temperature. Both deterministic and robust models are developed to incorporate UC with the linear DHN model. Case studies are carried out for two test systems to show the potential benefits of the proposed method in terms of wind power integration and efficient operation.

Analyzing operational flexibility of electric power systems

Operational flexibility is an important property of electric power systems and plays a crucial role for the transition of today's power systems, many of them based on fossil fuels, towards power systems that can efficiently accommodate high shares of variable Renewable Energy Sources (RES). The availability of sufficient operational flexibility in a given power system is a necessary prerequisite for the effective grid integration of large shares of fluctuating power in-feed from variable RES, especially wind power and Photovoltaics (PV). This paper establishes the necessary framework for quantifying and visualizing the technically available operational flexibility of individual power system units and ensembles thereof. Necessary metrics for defining power system operational flexibility, namely the power ramp-rate, power and energy capability of generators, loads and storage devices, are presented. The flexibility properties of different power system unit types, e.g. load, generation and storage units that are non-controllable, curtailable or fully controllable are qualitatively analyzed and compared to each other. Quantitative results and flexibility visualizations are presented for intuitive power system examples.

Collaborative Demand Response Optimization of Electric Vehicles and Storage Space Heating for Residential Peak Shaving

Residential Demand Response (DR) is believed to be a feasible tool for increasing the power system operational flexibility and efficiency. In this paper, we develop a demand response methodology for residential peak load shaving. We present an optimal demand response model for scheduling the EV and storage space heating load in tandem. Realistic case studies based on Finnish household data is performed to showcase the effectiveness of the proposed methodology and the results are thoroughly compared with business as usual case. The simulation result suggests that proposed methodology can bring economic savings to the customers and reduce the peak power problem as well.

Impacts of Energy Storage on Short Term Operation Planning Under Centralized Spot Markets

Energy Storage Systems (ESS) are an alternative to provide flexibility in power system operation. ESS have been foreseen as crucial technologies to facilitate the transition to a low-carbon energy matrix that integrates higher level of renewable variable energy resources. In this paper, the integration of grid-scale ESS in short term operational planning under a centralized cost-based electricity market is analyzed through an adapted stochastic unit commitment formulation to provide energy arbitrage, primary or/and secondary reserve. Several computational simulations are performed in a realistic study case to discuss techno-economic effects of ESS integration in detail. Results obtained show that value of storage is increased when ESS jointly provide different services. Additionally, ESS enable greater participation of renewables into generation mix, while reducing impacts on flexible power plants. Influence of ESS in marginal price signals is also discussed. The undertaken discussion provides insight information for agents in liberalized markets.

Allocation and Circuit Parameter Design of Superconducting Fault Current Limiters in Loop Power System by a Genetic Algorithm

In near future, many Independent Power Producers (IPPs) will participate in power generations according to their own strategic contracts by the deregulation. Loop or mesh systems can be designed to balance the power flow and to regulate the voltage resulting to the flexibility of power system operation, improvement of reliability and economical efficiency. Nevertheless, they bring to the problem of increased fault levels which may raise beyond the withstand capability of existing circuit breakers in the power systems. Short-circuit current is strongly related to the cost of apparatus and the effective use of power transmissions. Therefore, the introduction of Superconducting Fault Current Limiters (SFCLs) becomes an effective way for suppressing such a large short-circuit current. In this paper, first the authors evaluate the behavior of the S/N transition-type SFCL by considering the sub-transient and transient effects of generators in order to obtain smaller SFCL circuit parameters, i.e. the resistance of the superconducting coil and the reactance of the current-limiting inductor. Then the authors propose a method by using a hierarchical genetic algorithm (HGA) combined with a micro-genetic algorithm (micro-GA) to search for the optimal locations and the smallest SFCL circuit parameters simultaneously. The flexibility in defining the required objective function by using the proposed method makes it possible to evaluate the requirement of SFCLs in large power systems. Analysis by computer simulation has been carried out in an example loop power system.