Security-constrained Unit Commitment Problem Research Articles

Hierarchical and Robust Scheduling Approach for VSC-MTDC Meshed AC/DC Grid With High Share of Wind Power

This paper proposes a novel hierarchical and robust scheduling paradigm for the emerging voltage source converter (VSC) based multiterminal high voltage direct current (VSC-MTDC) meshed AC/DC hybrid system with high share of wind power. Considering the multilevel structure of the VSC-MTDC meshed AC/DC system, the scheduling problem is decomposed into three interactive levels according to the hierarchical analytical target cascading (ATC) technique. The low level is a two-stage adaptive robust security-constrained unit commitment problem for the AC system, which is solved by the column-and-constraint generation (C&CG) algorithm; the middle level is a day-ahead power transmission optimization problem for the VSC stations, where a day-ahead operation model for the VSC station is presented to fully use its flexible and controllable power adjustment capability to promote wind power accommodation; and the high level is a multi-period optimal power flow problem for the DC grid. An integrated ATC and C&CG algorithm is proposed to enable the hierarchical and robust scheduling. The parallel solution is used to accelerate the hierarchical scheduling. Simulations show that this formulation is effective in dealing with wind power uncertainty, promoting wind power accommodation, and improving the calculation efficiency.

Optimal scheduling of reconfigurable grids considering dynamic line rating constraint

Power system automation is an effective tool from both economical and technical aspects to improve the optimal operation of power generators. In this regard, Security Constrained Unit Commitment (SCUC) incorporating Dynamic Thermal Line Rating (DTLR) of overhead transmission lines can boost the system security effectively. Using Transmission Switching (TS) tool in SCUC problems leads to cost reduction. Still, one of the main challenges arisen in TS problems is the excessive number of switching in lines, which decreases the lifespan of power switches. In this paper, an Improved Linear AC Optimal Power Flow (ILACOPF) is proposed by using TS and considering Heat Balance Equation (HBE) as a security constraint. Merging dynamic thermal line rating (considering the weather conditions) in SCUC with TS, besides decreasing the number of switching and increasing the lifespan of power switches, causes a remarkable reduction in operating costs. In this power flow, system losses are modeled by linear formulations. Moreover, a linear approximation of the heat losses due to power flow through lines is proposed. To solve the proposed model, Benders' decomposition approach is applied. The performance of the proposed framework has been evaluated on 6-bus and 118-bus IEEE test systems.

A Cost Effective Solution to Security Constrained Unit Commitment and Dispatch Problem using Hybrid Search Algorithm

The Security constraints unit commitment (SCUC) is a powerful scheduling technique used in power markets for daily planning. SCUC is a framework that combines two common algorithms in the electricity industry: Unit Commitment (UC) and Economic Dispatch (ED), while adding a new dimension – Security. Load demand is changing continuously due to variation of load of industrial, residential and commercial sectors. Thus, it is very important to decide which generating unit should be kept on and how much power should be dispatched so that time varying load demand can be meet on Hourly basis and there should be no scope of occurrence of loss of load hour (LOLH). In the proposed research, the recently developed hybrid meta-heuristics search algorithm i.e. GWO-RES has been applied to solve the security constrained Unit Commitment and dispatch problem of Electric power system. The efficiency of the proposed hybrid algorithm has been tested for standard IEEE-14 Bus, 30-Bus and 56-bus system and it has been experimentally found that GWO-RES performs much better than hybrid GWO-PS and GWO-RS algorithm.

Stochastically coordinated transmission and distribution system operation with large-scale wind farms

With the high penetration of renewable energies in modern power systems, deterministic coordination algorithms are facing two major problems: one is degradation in accuracy if fewer scenarios are utilized for uncertainty evaluation while second is the high computational time if a high number of scenarios are considered for better accuracy. In both cases, the efficiency of the algorithm is degraded. To solve these problems in coupled transmission system and distribution systems (TSDS), probabilistic coordination algorithms are adopted to solve with less effort. In this paper, a TSDS probabilistic coordination model is proposed to solve the coordinated security-constrained unit commitment problem. A mean and standard deviation matching based probabilistic analytical target cascading algorithm has been utilized for evaluation of TSDS coordination problem. Instead of solving each scenario as a separate problem, the proposed algorithm considers a single coordination problem with probabilistic characteristics as shared variables and hence, achieves fast convergence. Different case studies are performed to prove the efficacy of the proposed algorithm. Results verify that the proposed algorithm reduces computational time and resources for large-scale systems.

Security-Constrained Unit Commitment Problem With Transmission Switching Reliability and Dynamic Thermal Line Rating

In security-constrained unit commitment (SCUC) problems, one approach to decrease operation costs is using a transmission switching (TS) tool. In SCUC problems with TS, one of the main challenges is that there is no limitation for the number of switching of circuit breakers (CB) in the system. In this article, the reliability of CB is merged into the SCUC problem with the TS and is considered as a limiting factor for switching. With a more reliable CB, the overall reliability of the system will be increased. So, it can be concluded that the reliability of a CB affects the amount of load shedding. Reliability of a CB is a nonlinear equation based on the number of switching in a period. An approach is presented to linearize the switch reliability equation. In this article, the power flow model uses an improved linear ac optimal power flow and a dynamic thermal line rating (DTLR) model, which considers the weather conditions. Other than CB reliability, DTLR in SCUC problems affects the number of switching and, as a result, operation costs will be significantly decreased. The proposed model is empowered by Bender's decomposition and is tested on 6-bus and 118-bus IEEE test systems.

An Improved Constrained Order Optimization Algorithm for Uncertain SCUC Problem Solving

Studying the faster and more efficient method of solving the uncertain security-constrained unit commitment (SCUC) problem is an urgent need for the development of power systems under the background of large-scale wind power access and power dispatching. This study proposes an improved constrained order optimization (COO) algorithm to solve the uncertain SCUC problem. First, the data-driven discrete variable identification strategy is incorporated into the COO rough model, and then, the invalid security constraints identification strategy is incorporated into the COO accurate model. Finally, the improved COO algorithm combines the discrete variable identification with the invalid security constraint identification to make the uncertain SCUC decision. The results of the IEEE 118-bus test system showed that, compared with the traditional COO algorithm, the improved COO algorithm proposed has higher accuracy and better efficiency.

Optimization of the integration among traditional fossil fuels, clean energies, renewable sources, and energy storages: An MILP model for the coupled electric power, hydraulic, and natural gas systems

This paper presents a Mixed Integer Linear Programming (MILP) formulation to optimize the Security Constrained Unit Commitment problem, integrating three areas: electrical, hydraulic, and natural gas. The model considers thermal generation from natural gas units and other fossil fuel units, hydraulic generation, pumped storage units, hydraulic head effects for the generation and pumping modes, power flow transmission constraints, natural gas supply and transportation, gas flows in single pipelines and pipelines with compressors, and line-pack for gas pipelines. The original model is hard to solve due to nonconvexities and nonlinearities of several constraints, but with the application of correct linearization techniques, accurate MILP models can be efficiently obtained and solved. The proposed MILP model differs from most of other approaches available in the literature in that it applies to several types of power generation, and includes a large number of real characteristics of the integrated system.A modified IEEE 31-bus with 2 pumped stations and 7-node natural gas system is presented to illustrate the effectiveness of the proposed model; test cases are solved by commercial package Gurobi. Test results indicate that the proposed approach provides feasible solutions with convenient CPU times. The obtained results are discussed and analyzed through simple graphs.

A Mixed-Integer Convex Programming Algorithm for Security-Constrained Unit Commitment of Power System with 110-kV Network and Pumped-Storage Hydro Units

The secure operation of 110-kV networks should be considered in the optimal generation dispatch of regional power grids in large central cities. However, since 110-kV lines do not satisfy the premise of R << X in the direct current power flow (DCPF) model, the DCPF, which is mostly applied in the security-constrained unit commitment (SCUC) problem of high-voltage power grids, is no longer suitable for describing the active power flow of regional power grids in large central cities. Hence, the quadratic active power flow (QAPF) model considering the resistance of lines is proposed to describe the network security constraints, and an SCUC model for power system with 110-kV network and pumped-storage hydro (PSH) units is established. The analytical expressions of the spinning reserve (SR) capacity of PSH units are given considering different operational modes, and the SR capacity of PSH units is included in the constraint of the SR capacity requirement of the system. The QAPF is a set of quadratic equality constraints, making the SCUC model a mixed-integer nonlinear non-convex programming (MINNP) model. To reduce the computational complexity of solving the model when applied in actual large-scale regional networks, the QAPF model is relaxed by its convex hull, and the SCUC model is transformed into a mixed-integer convex programming (MICP) model, which can be solved to obtain the global optimal solution efficiently and reliably by the mature commercial solver GUROBI (24.3.3, GAMS Development Corporation, Guangzhou, China). Test results on the IEEE-9 bus system, the PEGASE 89 bus system and the Shenzhen city power grid including the 110-kV network demonstrate that the relaxed QAPF model has good calculation accuracy and efficiency, and it is suitable for solving the SCUC problem in large-scale regional networks.

Chance-constrained CAES and DRP scheduling to maximize wind power harvesting in congested transmission systems considering operational flexibility

As enlarging share of renewables brings up a promising future for clean power generation, nonetheless, it imposes new challenges into the secure operation of power systems such occurrence of distasteful congestion, discriminatory locational marginal pricing (LMP) and also increasing uncertainty and inflexibility. To address these issues, a novel chance constrained two-stage programming is developed, where in the first stage social welfare of system is maximised while in the second stage a stochastic security constrained unit commitment problem is executed along with compressed air energy storage (CAES) and demand response program (DRP) to minimize both operation costs and wind curtailment. Both DRP and CAES are cooperatively applied to maximize wind proliferation and social welfare, alleviate the congestion of network, smooth LMP at different nodes, and improve technical characterizations of system. The problem is formulated as an exact mixed integer non-linear programming (MINLP) considering operational flexibility by means of power capacity for up/down power regulation and then is solved using primal-dual interior point solver. Finally, a case study based on modified IEEE 30-bus transmission system with three zones is performed and the results are duly expressed and analysed to corroborate the pertinence of the proposed model.

Using the Matpower Optimal Scheduling Tool to Test Power System Operation Methodologies Under Uncertainty

Short-term power system operational planning problems that consider multi-stage uncertainties pose significant challenges, not only in the design of tractable optimization frameworks for implementing them, but also in the testing and benchmarking of such frameworks. This paper presents an implementation using the open-source M atpower Optimal Scheduling Tool (MOST) to study and compare a stochastic day-ahead, security-constrained unit commitment problem with a more traditional deterministic approach. The comparison is based on a testing methodology for day-ahead plans designed to produce expected performance estimates with minimal biases from modeling assumptions. Emphasis is given in the proposed stochastic approach to explicit modeling of the operational characteristics of the technologies available, their spatial and temporal coupling, and the regulatory constraints that assure reliability and adequacy. The problem formulations and testing methodology are described and simulation results from MOST are presented, with discussion of implications for future market design. All of the code and data to replicate the simulations is provided online.

Impact of solar energy on the integrated operation of electricity-gas grids

Photovoltaic energy is one of the clean and efficient energies which has been developing quickly in the last years. As the penetration of solar plants is increasing in the electricity network, new problems have arisen in network operation. This paper models a high penetration factor of solar energy in the electricity network and investigates the impact of solar energy growth on both the generation schedule of different power plants and in the natural gas transmission network. Fuel management of gas power plants is modeled through simulation of the natural gas transmission network. To this end, an increase in the penetration of solar energy in the electricity network inevitably leads to a sudden increase in the output of gas fired units and a linear and integrated model with the electricity and the natural gas transmission networks has been presented to analyze both of them at the same time to better depict the impact of a high penetration of the solar energy in natural gas transmission grids. In this method, natural gas transmission network and Security Constrained Unit Commitment (SCUC) are presented in a single level program. Gas network constraints are linearized and added to the SCUC problem. The stress imposed on the gas network due to a sudden increase in the load of the electricity network is investigated. Conclusions are duly drawn.

Security-Constrained Unit Commitment With Natural Gas Pipeline Transient Constraints

The interdependencies of power systems and natural gas networks have increased due to the additional installations of more environmental-friendly and fast-ramping natural gas power plants. The natural gas transmission network constraints and the use of natural gas for other types of loads can affect the delivery of natural gas to generation units. These interdependencies will affect the power system security and economics in day-ahead and real-time operations. Hence, it is imperative to analyze the impact of natural gas network constraints on the security-constrained unit commitment (SCUC) problem. In particular, it is important to include natural gas and electricity network transients in the integrated system security because the impacts of any disturbances propagate at two distinctly different speeds in natural gas and electricity networks. Thus, analyzing the transient behavior of the natural gas network on the security of natural gas power plants would be essential as these plants are considered to be very flexible in electricity networks. This paper presents a method for solving the SCUC problem considering the transient behavior of the natural gas transmission network. The applicability of the presented method and the accuracy of the proposed solution are demonstrated for the IEEE 118-bus power system, which is linked with the natural gas transmission system and the results are discussed in this paper.

Transmission Constraint Filtering in Large-Scale Security-Constrained Unit Commitment

When solving the security-constrained unit commitment problem, one of the most complicating factors is handling the large number of transmission constraints, corresponding to both base case and N − 1 line contingency scenarios. Although it is well known that only a few of these constraints need to be enforced, identifying this critical subset of constraints efficiently remains a challenge. In this letter, we propose a novel and simple iterative contingency-screening procedure that is able to eliminate 99.4% of the constraints selected by existing iterative methods, allowing for the solution of much larger-scale problems. We report computational results in realistic instances with up to 6468 buses and 9000 transmission lines. The method was also independently implemented and evaluated at MISO, where it performed faster than alternative methods.

Implementation of User Cuts and Linear Sensitivity Factors to Improve the Computational Performance of the Security-Constrained Unit Commitment Problem

Power system operators must schedule the available generation resources required to achieve an economical, reliable, and secure energy production in power systems. This is usually achieved by solving a security-constrained unit commitment (SCUC) problem. Through a SCUC the System Operator determines which generation units must be on and off-line over a time horizon of typically 24 h. The SCUC is a challenging problem that features high computational cost due to the amount and nature of the variables involved. This paper presents an alternative formulation to the SCUC problem aimed at reducing its computational cost using sensitivity factors and user cuts. Power Transfer Distribution Factors (PTDF) and Line Outage Distribution Factors (LODF) sensitivity factors allow a fast computation of power flows (in normal operative conditions and under contingencies), while the implementation of user cuts reduces computational burden by considering only biding N-1 security constraints. Several tests were performed with the IEEE RTS-96 power system showing the applicability and effectiveness of the proposed modelling approach. It was found that the use of Linear Sensitivity Factors (LSF) together with user cuts as proposed in this paper, reduces the computation time of the SCUC problem up to 97% when compared with its classical formulation. Furthermore, the proposed modelling allows a straightforward identification of the most critical lines in terms of the overloads they produce in other elements after an outage, and the number of times they are overloaded by a fault. Such information is valuable to system planners when deciding future network expansion projects.

Simultaneous operation of wind and pumped storage hydropower plants in a linearized security-constrained unit commitment model for high wind energy penetration

Abstract The operation of pumped storage hydropower plants (PSHPs) in the security-constrained unit commitment (SCUC) problem with the consideration of high wind energy penetration has become a practical issue for power systems. In this paper, first, a comparison is made between the operation of fixed-speed and adjustable speed PSHPs in the proposed linearized SCUC model based on the calculation of added values in the market. Then, for the participation of wind power plants (WPPs) and considering their uncertain production, a scenario-based method using real data and the Weibull distribution function is proposed. Also, considering WPPs may have a surplus energy in comparison to the network load in the condition of high wind energy penetration, which will be lost without use, in the modeling of WPPs, a mathematical variable as the curtailed wind energy is considered. With the simultaneous security-based coordination of adjustable speed PSHP units and WPPs in the unit commitment problem, the use of WPPs has become more effective. Also, the WPPs can participate in the market with higher capacity than the separate operation. The various numerical studies with increasing WPPs penetration factor are presented to demonstrate the effectiveness of the adjustable speed PSHP units for the IEEE 24-bus test system.

Two-Stage Robust Security Constrained Unit Commitment Considering the Spatiotemporal Correlation of Uncertainty Prediction Error

Robust optimization (RO) is an important tool to solve the security-constrained unit commitment (SCUC) problem for a power system with large-scale wind power. The main disadvantage of RO is that it is overly conservative, and the conservativeness of RO can be attributed to the uncertainty set used in the formulation. This paper proposes a two-stage robust SCUC model considering the spatiotemporal correlation of the uncertainty prediction error. First, based on the historical data, a polyhedral uncertainty set that can describe the spatial-temporal correlation of uncertainties is established, and the analytical relationship between confidence probability and the uncertain set is given. Second, a two-stage robust SCUC model with the objective of minimizing the operating cost under the forecasting scenarios is proposed based on the polyhedral set. Third, the Benders decomposition method is used to solve the proposed model according to its characteristics. The simulation results on the modified IEEE-30 and IEEE-118 bus system demonstrate that the proposed method can reduce the conservativeness of RO and guarantee the security and economy of the unit commitment.

Extensive [formula omitted] recycling in power systems via Power-to-Gas and network storage

The recycling of carbon dioxide (CO2) into synthetic fuels via Power-to-Gas (PtG) could represent an important instrument for achieving the complete decarbonization of the energy sector. To address such issue, this paper calculates the investments in PtG units, grid reinforcements and renewable installations that allow the almost complete recycling of the CO2 emissions of a countrywide electric power system. Furthermore, this work evaluates the feasibility of gas and electric operations in the new system configuration. The analysis is enabled by coupled gas and electric network modelling. The necessary PtG station installations and overhead line reinforcements are identified via scenario-based cost optimization. Hourly operations of electric power plants are scheduled as a sequence of day-head security-constrained unit commitment problems. A transient gas flow model assesses the capability of the gas network to act as short- and long-term storage of synthetic gas. The developed framework is applied to the electric and gas transmission networks of Great Britain, whose investments and operations are investigated for increasing renewable capacity levels based on the 2030 Gone Green case. Results show that almost complete CO2 recycling is achieved if the installed renewable capacity is approximately three times as large as the 2030 Gone Green estimates. The investments comprise 114 GW of PtG capacity and the construction of 23 electric parallel lines. Remarkably, gas network operations do not represent a limit to the storage of large amount of synthetic methane. Moreover, PtG stations are preferentially installed at locations with large RES capacity and foster large renewable penetration; only small curtailments occur even for large renewable capacity levels. These results support decision makers by quantifying the techno-economic implications of the presented extensive CO2 recycling strategy.

Geographic information system and weather based dynamic line rating for generation scheduling

Transmission line ratings influence the economic and security aspects of power system operation, generation and planning. The use of geographical information systems in power system generation and planning activities is gaining importance. This paper proposes a geographical information system and weather based dynamic line rating (GISWDLR) and investigates its effectiveness in a security constrained unit commitment problem (SCUCP). Here, the dynamic line ratings (DLRs) of conductors at different locations along the transmission line are calculated using the geographic parameters and weather parameters. Thereafter, the minimum value of DLRs at all indentified locations of the transmission line is taken as DLR of the transmission line. Here, binary real coded particle swarm optimization (BRPSO) is employed to solve the SCUCP. The proposed method is validated using the benchmark IEEE 30 bus system. Here, results obtained using the proposed GISWDLR, static line rating (SLR) and conventional UCP are compared. Also, the effectiveness of BRPSO is validated by comparing it with the conventional enhanced priority list based method.

Security constrained unit commitment scheduling: A new MILP formulation for solving transmission constraints

This paper presents a new Mixed Integer Linear Programming model (MILP) to account for the Security-Constrained Unit Commitment Problem (SCUC). Transmission Constraints are introduced through bus balances, line power bound flows, and bus voltage angle differences. Line status is also considered. Binary variables regarding line status (active or inactive) are introduced for this purpose.These variables allow discrete decisions on the connectivity of buses, reducing the angle coupling between buses, with several advantages.Three examples are solved. The results indicate that this method can obtain feasible solutions with CPU times of 2.5 s (for a 6-bus system) and 500 s (for the IEEE 118-bus system), and they reached cost savings up to 4.9% of the total generating cost for one day of programming horizon, in comparison with classical models.Relations of the network are illustrated graphically, and an analysis of the results is presented through new evaluation indexes.

Distributed Dispatch Approach for Bulk AC/DC Hybrid Systems With High Wind Power Penetration

For bulk AC/DC hybrid transmission systems with high wind power penetration, this paper presents a distributed dispatch approach to the security-constrained unit commitment (SCUC) problem. To fully use the flexible adjustment capability of the HVDC tie-line to promote inter-regional wind power accommodation, an operation model for the HVDC tie-line is presented. Then, a distributed SCUC approach based on the analytical target cascading technique is proposed, where the day-ahead SCUC problem is decomposed into an upper-level master problem and parallel subproblems of lower level regional dispatch. The master problem is in charge of determining the day-ahead transmission plan for the HVDC tie-line, and the lower-level dispatch centers independently solve their SCUC problems in parallel in accordance with the hierarchical and partitioned power scheduling mode. Simulations show that considering the flexible and controllable nature of the HVDC tie-line, the proposed distributed dispatch approach for bulk AC/DC hybrid systems has the advantages of promoting inter-regional wind power accommodation and improving the economics of the overall system operation while relieving the peak regulation pressure of the receiving-side system.