Unit Commitment Problems with Operational Power Flow and Environmental Constraints
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Solutions to Practical Unit Commitment Problems with Operational, Power Flow and Environmental Constraints
I. Jacob Raglend and Narayana Prasad Padhy1
In this paper, an algorithm to solve environmental constrained unit commitment problem (UCP) with operational and power flow constraints has been developed to plan an economic and secure generation schedule. Both Economic load dispatch (ELD) and Economic emission dispatch (EED) have been applied to obtain optimal fuel cost and optimal emission of generating units for the entire time horizon. The unit commitment solution for the environmental constrained problem has been formulated as a multiobjective problem by considering both ELD and EED simultaneously. The common economic emission dispatch (CEED) bi-objective problem is converted to single objective function by adding a price penalty factor. A modified price penalty factor is proposed to solve this problem. The UCP solutions without operational and power flow constraints are not practical due to secure operation of the power system network. This proposed algorithm introduces an efficient unit commitment (UC) approach considering environmental constraints along with power flow constraints that obtains the minimum operating cost satisfying both unit and network constraints. In the proposed model repeated optimal power flow (OPF) for the satisfactory unit combinations under the given study period has been carried out to obtain UC solutions with both operational and power flow constraints. This proposed algorithm has been tested for environmental constrained UCP on IEEE 30 bus and Indian utility practical systems with and without power flow constraints scheduled for 24 hours. The solutions obtained are quite encouraging and useful in the economic emission environment. The algorithm and simulation are carried through Matlab environment.
Index Termsâ€Economic dispatch, Combined economic emission dispatch, Price penalty factor, Unit commitment, Dynamic programming, Netwon raphson, Lagrangian multiplier, optimal power flow.
LECTRIC power is generally be higher during the day time and early evening when industrial loads are high, lights are on and so forth lower during the late evening and early morning when most of the population is asleep. In power system operation and control, due to variation of load demand and non-storable nature of electrical energy, given the hourly load forecasting over a period of a day or a week ahead, the system operators should schedule the on/off status as well as the real power outputs of the generating units to meet the forecasted demand over the time horizon. The resultant Unit Commitment (UC) schedule should minimize the system production cost during the period while simultaneously satisfying the load demand, spinning reserve, ramp constraints and the operational constraints of the individual unit. Scheduling the on and off times of the generating units and minimizing the cost for the hourly generation schedule is the economics to save great deal of money by turning units off (decommiting) when they are not needed. By incorporating UC schedule, the electric utilities may save millions of Dollars per year in the production cost. The system security is still the most important aspect of power system operation and cannot be compromised. UCP is an important optimization task in the daily operation planning of modern power systems  - . A survey of literature on the UC methods reveals that various numerical optimization techniques have been employed to approach the UC problem. Traditional and conventional methodologies such as exhaustive enumeration, priority listing, dynamic programming, integer and linear programming, branch and bound method, lagrangian relaxation, interior point optimization etc. are able to solve UCP with success in varying degree  - .Gent and Lamont have started the early work on minimum emission dispatch . Optimal power dispatch problem considering practical constraints has been solved by Fletcher's quadratic programming method . Nanda, Hari and Kothari explore the feasibility of developing a classical technique based on co-ordination equations to solve Economic Emission load dispatch with line flow constraints . Hota et al. proposed a sequential quadratic programming technique to solve CEED problem by assigning weighting factors for generation and emission cost functions . Researchers proposed a price penalty factor for solving the CEED problem which blends the emission costs with the normal fuel costs . In this paper a modified price penalty factor is introduced to find the exact economic emission fuel cost with respect to the load demand. In this paper, the UCP is solved by considering both EED and ELD with operational power flow constraints. The UC schedule for the generating units considering only the unit constraints may not satisfy the power flow constraints and leads to insecure operation of the network. So to obtain the practical UC solutions the model must consider both the operational and power flow constraints. The unit commitment solution for a system can be obtained with repeated OPF algorithms. Repeated optimal power flow for the satisfactory unit combinations under given study period be carried out to obtain unit commitment solutions with unit and network constraints. This paper presents the UC schedule with CEED for IEEE 30 bus systems and Indian utility practical system with and without power flow (PF) constraints scheduled for 24 hours.
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