Theoretical Studies On Economic Dispatch Of Power System To Address Uncertainties

The environment’s worsening day by day threatens human survival. A clean energy transformation is imperative, and the power system must develop in the green, low-carbon, environmental and energy-saving direction of intelligence. Under the new situation, power generation and consumption becomes diversified. On the basis of uncertain electrical load itself, the connection of renewable energy sources like wind and solar further increases uncertainty. Meanwhile, the form of active controllable variable and passive uncontrollable variable changes intrinsically, which presents the opposite features. Therefore, the balance between power generation and load changes into a state of pluralism and correlation. Economic dispatch and control is faced with serious and urgent contradiction.Economic dispatch of power system is to obtain an optimal result in dealing with the above contradiction with finite ability of regulation. The key is making decisions about the base point of adjustable, controllable amounts and the reserve of controllable amounts. The former is the schedule of expectation based on prediction, the latter is the scope of reserve deviated from expectation, and the coexistence must be organically combined together to ensure that the dispatch is executed as planned along with control. Obviously, the position of base point, the controllable scope and the variation of scope constitute three basic elements for economic dispatch of power system, and the three are closely related and indivisible.This thesis has investigated the uncertain factors in economic dispatch of power system under the before-mentioned background. The basic theory and methods of economic dispatch of power system aiming at dealing with uncertainties is studied and explored by considering the settlement of the contradiction between the active and the passive and economic law. The main work and innovation of this thesis are shown as follows:(1) The active variable and passive variable correlate and couple, meanwhile uncertain disturbances and uncertain fluctuations interlace and coexist, to address this complex contradiction, the concepts and metric definitions of uncertainties in economic dispatch of power system is proposed based on kinematics and geometry description. First, the definitions of dot, line, surface, space by analogy to economic dispatch, active variable and passive variable are given based on these notions. Second, the characteristics of uncertainties in the real power balance between the active variable and the passive variable are vividly described and quantitatively analyzed. Third, the decision problems about addressing fluctuations and intermittencies in the passive variable and disturbances in the active variable in economic dispatch are summarized as decision problems about the linkage between dot and interval, dot, line and interval, dot, surface and interval. The scope and path of economic dispatch are clarified, and the effect has a definite object. This study also lays a foundation for the following work.(2) An interval economic dispatch model of power system considering unit reserve responsiveness is proposed to deal with the uncertain characteristic of passive variable’s interval fluctuation. The operating characteristics are analyzed for three kinds of units, which are the down-regulation unit, the two-direction-regulation and the up-regulation, classified by running position, regulation direction and reserve responsiveness. Based on passive variable demand interval distribution, the concept of effective reserve responsiveness is proposed, and such a new constraint as interval satisfaction degree is clarified. An economic dispatching method is proposed which considers the constraint of passive demand interval uncertainty. Solve the model with Lagrangian multiplier method to deduce the symmetrical characteristic between the incremental cost rates of the first and third class; the equal difference existing in the three classes’variations of power, incremental cost rates and cost difference are analyzed and demonstrated, as well as the basic problems including interval reserve responsiveness limits, economic characters and physical essence discussed. A 10-unit system sample is analyzed to testify the effectiveness of this theory, which realizes optimal selection of reserve interval scope while minimizing unit reserve cost and indicates the reference value when it comes to uncertainty issues.(3) An economic dispatch model considering multiple uncertain intervals is established for different distributions and characteristics of multiple passive variables. In this paper the multiple fluctuant uncertainty sets of passive variables are divided into the pivotal uncertainty sets and the flexible uncertainty sets. Then an economic dispatch model considering multiple uncertain intervals of passive variables is proposed with the definition and description of load interval reserve constraint and renewable energy generation confidence reserve constraint. The model is solved by classifying units, and the response theory and feature of units’reserve is studied under different constraint impacts by analyzing two constraints. At last, the correctness and effectiveness are verified by simulations on 10-unit system. This research reflects a compromise decision-making thought, which develops the adaption of the interval economic dispatching method.(4) Unpredictable variations in the renewable energy generation give rise to power generation output unbalance, which could not be neglected in the economic dispatch issue. Aiming at this problem, by mean of analyzing two different kinds of characteristics for fluctuant uncertainty and intermittent uncertainty in uncertain variable changes, the concept of passive variable power output changing speed and relevant new constraint expression were constituted according to seeking the rigorous scene scope for the variations of load and renewable energy power generation based on the interval satisfactory degree constraint, and then the interval economic dispatching model of power system considering the intermittency was proposed. This research allowed power system economic dispatch to take into account both the demands for quantity and extent of uncertainties effectively in the decision for addressing and incorporating the renewable energy power generation, meanwhile the robustness of the scheduling solutions was efficiently improved.(5) Uncertainties in both decision variable and decided variable make economic dispatch of power system have the characteristic of diversification and result in difficulty in scheduling reserve capacity. In order to address the issue, this paper proposed an economic dispatching method to make the scheduled reserve more efficient after analyzing the characteristics and the effects of reserve deeply. Based on the concepts of active variable and passive variable and taking expected energy not supplied (EENS) as the reserve’s utility, this method scheduled active variable’s reserve and passive variable’s reserve respectively according to the basis of their uncertainty characteristics. The thermal generating was taken as active variable, load and the renewable generating as passive variable, analytical expressions of the reserve utility and the marginal reserve utility were deduced to constitute a new constraint which equals active variable’s marginal reserve utility and passive variable’s marginal reserve utility according to the law of diminishing marginal utility. The method is a new progress in economic dispatching modeling of power system. Also, an economic dispatching model is proposed under the constraint that active variable’s marginal reserve utility and passive variable’s marginal reserve utility are equal, and the solving method is given. This research realizes the best-matching between active variable’s reserve and passive variable’s reserve, and makes economic dispatch acquire advantageous progress.(6) To address the physical characteristic of generating units’ prohibited operating zones, an effective iterative algorithm is proposed to solve the economic dispatch problem when some of the on-line units have prohibited operating zones. For a unit with prohibited operating zones, the effective operating region will be broken into several isolated sub-regions between the upper limit and the lower limit of the output power, which constitutes a non-convex optimization problem. By deeply discussing and analyzing the impact of the units with prohibited zones to the other units in the system, the impact of the correlations among the units with prohibited zones and the state of the optimal solution for the economic dispatch issue, the problem-solving process is revised for three times, and still can be calculated via the conventional Lagrangian relaxation method. This research explores the economic law and the physical nature in the economic dispatch problem containing units with prohibited operating zones in solving the problem efficiently, and has certain value for engineering application.