Bilevel Programming Theory In Power System Studies

The transmission transfer ability has great influence on power system reliability and safety, represented by the interconnected transmission networks and power interchange between areas. This is even more severe with the increase of uncertainty in power market and instantaneous variation in electricity exchange process. It tends to have over-loaded branches and over-voltages. This brings forward the problem of how to evaluate the transmission transfer ability accurately and efficiently. Available Transfer Capability (ATC), based on the electricity contract under power market, is a physical quantity to evaluate the transfer ability of transmission networks. Many studies have been done on the methodology of ATC evaluation, however, how to consider the uncertainty appropriately still remains unsolved, such as the problem of calculating the Transmission Reliability Margin (TRM). This paper focuses on the problems mentioned above and brings forward original ideas and methodology. The majior contributions are summarized as follows:1. Enhanced the concept of min-max transfer capacity, which is the conservative transfer ability against the optimistic transfer ability that ATC evaluates.2. Constructed a TRM model of bi-level optimization based on leader-follower decision-making model, and an analytic method of computing TRM is proposed. A bi-section search algorithm simplifies and speeds up the calculation.3. Proposed the min-max transfer capability model considering steady voltage stability. The BiGA algorithm used in this model is shown to be robust.With the increase of electric loads in power system and the growing unbalance between power demand and supply, the amount of power that should be transferred over the interconnected transmission network becomes greater. Therefor, power grid tends to be over-loaded. To enhance the structure and equipments of the transmission network is the ultimate way to solve this problem. However, it requires the large amount of investment. Another effective method is, under the condition of maintaining power system safety and stability, to utilize distributed generators as more as possible based on the existed network. This helps to ease the tensity of power supply in regional systems, make fully use of resources and lower the amount of power transferred between areas. Distributed generators are usually connected via distribution network, which makes revolutionary change of the radial structure of distributed network. A series of problems, such as voltage regulation, reactive power balance and relay protection, will have effect on power system operation. Voltage/var control equipment is a major device in distribution network. This work studies the influence of the interconnection of distributed generator on these devices and the allowable penetration level considering these influences. The major achievements are summarized as follows:1. The distributed generators unit commitment has a certain degree of randomness, as influenced by natural causes or man-made impact. With considerably large capacities of distributed generators, the shutting-down or starting up of distributed generators may cause the voltage/var control device working abnormally. This paper discusses the influence and proposes a model for evaluating the maximum allowable penetration level.2. Analyses the condition of simple oscillation and VQC oscillation regulation, discusses the influences on the oscillation with the interconnection of distributed generators, indicates that the interconnection of distributed generators may cause the regulation of voltage/var control device to oscillate, and presents a model of allowable penetration level calculation considering tap changer oscillations.3. Investigates the difference between the allowable penetration level of single-bus and multi-bus injection. Proposes the concept of the min-max allowable penetration level, along with its mathematic model and algorithm.