UNIT COMMITMENT PROBLEM UNDER REGULATED AND DEREGULATED POWER SYSTEMS

Abstract

This thesis presents the scenario of unit commitment problem (UCP) for both regulated and deregulated power systems to ensure economically efficient operation of power system utilities. To achieve an accurate unit commitment (UC) schedule for either utilities or companies with more number of generating units and unpredicted market behavior becomes a challenge for the researchers in the recent times. There are a number of factors that affect the economic decisions of power generators. These include operating and maintenance costs, output control, start-up costs and emission caps etc. In addition to these, appropriate dispatch of generators also based upon the physical characteristics and limitations of the plant. These can include ramp-up rates, ramp-down rates and minimum and maximum run times. Conventionally, systematic evaluation of large number of possible decisions is performed in UCP with multi stage scheduling and thereby minimizing the overall cost of generation. The exact solution of the UCP can be obtained through complete enumeration using conventional optimization technique such as Dynamic programming (DP) and its complexity increases further with additional units and constraints. The objective of UCP is to schedule the generating units economically (i.e. switching on / off) so as to meet the requirements of forecasted load and spinning reserve. The optimum schedule is obtained by minimizing the overall cost of the power generation while satisfying a set of operational constraints (OC). To get an UC schedule for a time period ofgenerally 24 hours each stage consists of (2 N° -1 ) combination ofstates and therefore, the maximum number of possible combinations of scheduled paths is (2 N° -1) , where Tis the hourly stages and Ng is the number of generating units. The maximum number of possible combinations of scheduled paths increases with the increase in the number of generating units and scheduled hours. Hence, to find an optimum UCP solution in a reasonable time is an important task because it will produce significant annual financial savings for both utilities and companies. At present, most of the power utilities through out the world have been either deregulated or moving towards deregulation. The deregulation of electric power systems has results in market based competition by creating an open market environment under the new entity so called independent system operator (ISO), to balance supply and demand inreal time and maintain system reliability and security. ^ Before deregulation, it is the generation part of utility responsible to minimize the production cost, and after deregulation, the generation company optimizes the operation of generating units to get maximum profit. So, the existing conventional UCP algorithms and models for the regulated markets need modification to meet the recent deregulated scenario. Generation companies (GENCOs) in deregulated environment are mainly interested in maximizing their own profits because they are independent and do function under unbiased regulator body. The total available generation of GENCOs is also expected to be much higher than the system demand so that many generation companies (GENCOs) can participate and compete in the deregulated power market. Independently each GENCO normally plan their unit schedule before 48/24 hour depending on market requirement for both generation and reserve to maximize the total profit against the predicted energy prices. When all the GENCOs are connected to the network simultaneously may fail to guarantee the profit. So a new attempt has also been made to observe the GENCO's behavior and change in profit when connected to the secure network. XXIV * The main objectives ofthe proposed research work are; 1) Development of unit commitment models for conventional regulated power utilities with operational, environmental and security constraints. 2) Development of profit based unit commitment models for deregulated power market with operational and environmental constraints. 3) Development of security constrained unit commitment model for deregulated power market. A profit based UCP has been proposed and analyzed for both power and reserve market. Profit based UCP schedules the generators of a single GENCO based on the forecasted information, such as demand and reserve energy (Mwhr) prices and their magnitudes, with an objective to maximize profit. Initially the proposed UCP formulation is subjected to generation, reserve and unit constraints only. In this formulation, the power generation and reserve generation together decides the profit for GENCO's. The exact scheduling plan of power and reserve depends on the way the reserve payments are made. In this research work two types of reserve markets are presented. In the first approach, reserve power is paid only when the reserve is actually used. In the second approach, GENCO receives the reserve price per unit of reserve power for every time period that the reserve is allocated and in this approach of reserve payment, the reserve price is much lower than the power price, otherwise higher. In the profit based UCP (PBUCP) model with operational constraints, a single GENCO with NG units (4, 5, 6, 14 and 18) is considered. The status of the generating units in the GENCO are determined for all the possible combination of states that satisfy the demand and spinning reserve requirements for each stage, then they are XXV allowed to perform economic power dispatch (EPD) and economic reserve power dispatch (ERPD) for the power market. For each stage, all the feasible states that satisfy the OC are stored. The state that gives maximum profit, satisfying the operational constraints, and meets both the forecasted generation and reserve for each stage are selected. Based on the above approach, the UC schedule that maximizes the profit has been obtained. Generally, power plants are operated on the criteria of least fuel cost strategies without considering the pollutants produced. Researchers proposed a price penalty factor (PPF) for solving the combined economic emission dispatch (CEED) problem which blends the emission costs with the normal fuel costs. Considering various environmental aspects and to meet the requirements of present mode of operation, there is a need to include emission constraints (EC) to get an optimum UC schedule. In this research work, a profit based UCP model with environmental constraints suitable to competitive environment has been formulated. A multi-objective function is solved considering both Economic load dispatch (ELD) and Economic emission dispatch (EED) simultaneously to obtain optimal fuel cost and optimal emission cost for the GENCOs. The CEED bi-objective problem is converted to a single objective function by adding a PPF and further extended to an efficient modified price penalty factor (MPPF). The unit combination which provides maximum profit and satisfying the necessary constraints in each stage is selected. The resultant (fuel and emission) profit based UC schedule will satisfy operational constraints with maximum profit. Conventional UCP models suitable to regulated environment with operational and emission constraints are also developed and exercised for different test case systems. For the secure operation of power network, it is essential to obtain UCP solutions with power flow constraints (PFC). A UC model with operational and power XXVI flow constraints has been developed to obtain a practical and economical UC schedule with minimum operating cost using hybrid Lagrangian multiplier (for economic dispatch) and Newton Raphson power flow algorithms. States those satisfy demand and spinning reserve requirements are allowed to go through power flow (PF) analysis. If converged, economic dispatch (ED) solutions are obtained including transmission losses in thenetwork without violating MVA limits. The slack bus power obtained from the PF solution is compared with the slack bus power obtained from ED, if the slack bus power in both the cases are very close to each other then the algorithm converges. Otherwise repeated PF is being carried out till convergence for the satisfactory unit combinations for the entire time horizon (24 hours). System security is one of the most important aspects of power system operation and should not be compromised. An algorithm for the security constrained UCP (SUCP) is developed to withstand contingencies on a daily and hourly basis. SUCP takes care of the voltage violation, power flow violations and (N-l) contingencies. The contingency analysis is not performed on the lines which are completely dedicated for generating and supplying the loads. In this research work, (N-l) contingency has been performed by removing one line from the system at a time andPFanalysis is carried out for the unit combination which satisfies the load demand and spinning reserve. This continues until all the lines are removed once for each possible state. The state which converges for PF for every line removal is selected. Both PF and ED solutions are obtained for those unit combinations satisfied the above criteria. A secure economic UC schedule so determined holds good even when any (N-l) contingencies occurs during the operation. A security and emission constrained UCP (SEUCP) model is developed for regulated power system to ensure a practical and economical UC solution. SEUCP model has also been developed considering practical peak load variations in the system. XXVll Finally security constrained unit commitment problem has been developed in the line with regulated power system suitable to deregulated power market for the ISO todecide 48/24 hour ahead schedule considering GENCOs operating behavior. ^ The performance of the proposed models are validated using IEEE 14 bus systems, IEEE 30 bus systems, IEEE 57 bus systems, IEEE 118 bus standard test systems and Indian utility 75 bus test systems in different stages of the work. The test systems have been modified to suit the deregulated environment whereas they remain > exact for regulated models. The solutions obtained for both regulated and deregulated models are quite encouraging and may be easily extended to practical utilities. The proposed models can be extended by including ramping cost so that the generator may be allowed to generate slightly beyond its allowable ramping limits under competitive power supply markets and emergency conditions of power industry. While solving the profit based UCP the ~* uncertainties in forecasted parameters such as demand / reserve, the probability that reserve is used and generated in real time should be taken into account to obtain high quality and more precise solution. The present work can be extended to still larger systems with more number of GENCO's. The non conventional techniques can also be applied to solve the profit based UCP with all the practical and business constraints in the future.