The Influence Of Operation Parameter On Small-signal Stability In Power System

With the development of power industry, year-by-year increment of load demand and pursuit of profit, the transmission line power increases continuously and the operation condition of power system is gradually stressed. The problem of small-signal stability, that is, low frequency oscillation becomes serious, which is one of the most important factors to limit power transmission and impair network security.PSS (Power System Stabilizer) and FACTS devices were applied to improve the low frequency oscillation. But from system operation point of view, however, damping controllers may not always be effective enough to solve the small-signal security problem for three reasons. First, implementation of a damping controller usually requires lengthy design, manufacture, installation, and commission procedures. Therefore, it cannot meet the short-term solution for problems in operation studies. Second, a power transaction limited by the small-signal stability problem often occurs only for a short period of time. It may not be an efficient way to mitigate such a problem by adding new controllers. Third, even with appropriate damping controllers in place, there are always situations in which specific operation conditions fall beyond what the controllers are designed for and, therefore, additional remedial measures are necessary to accommodate the operation conditions.In practical power system, the small-signal stability closely relates to power system operation parameter. The changes of operation parameters influence the distribution of power flow and the state matrix of linearized system, thus, the oscillation mode. In small-signal stability analysis, the sensitivity of eigenvalue with respect to operation parameter may show the trend of damping ratio changing with operation state and provide measures for the improvement of damping. Therefore, studying the influence of operation parameter on small-signal stability in power system is an important and meaningful job.Based on the background above, this thesis focuses on the problem of operation parameter influencing on small-signal stability, furthermore, the active power optimal dispatch strategy and so on. The main contributions of the thesis are as follows:1. Based on Small-Signal Stability Analysis Package, SSAP (the software applied in China Southern Power Grid), developed by Shanghai Jiao Tong University, the sensitivity of damping ratio with respect to operation parameter, such as nodal injection power and voltage magnitude of PV-bus, is derived, according to the characteristic of Universal Component Connection Modeling method, UCM. The calculation results are verified by typical IEEE test systems and compared with disturbance simulation. It shows that the sensitivity results derived are identical with that obtained from disturbing network within a suitable error range. The sensitivity shows correctly the trend and extend of operation parameter influencing on small-signal stability.2. The problem of small-signal stability in power system considering load uncertainty is investigated. Load is expressed as an interval number. According to the optimization theory, a necessary condition of reaching the optimum is derived. The thesis presents an iterative search method taking the damping ratio sensitivity with respect to load for search direction. Then, guiding by the sensitivity, the extreme damping ratios of weak oscillation mode are solved under typical load levels, and the damping ratio distribution in the whole load range can be further determined. The validity of the method is proved by test systems. Comparison results illustrate that the suggested method is correct and superior to the Monte Carlo Simulation method by its calculation speed. According to the law of damping ratio changing with the load, more overall stability information can be provided.3. The thesis establishes a nonlinear optimization model to solve the upper and lower limits of damping ratio for oscillation mode under interval uncertain operation condition. The influence of power flow distribution on small-signal stability is studied in the range of generation and load interval. A Successive Linear Programming method (SLP) is proposed, by which the interval damping ratio and corresponding operation conditions at its interval limits are obtained. Calculation results can be used to evaluate the influence of interval uncertainty on a certain mode and provide information for improvement. The effectiveness of the proposed methodology is validated by two test systems.4. The damping ratio optimization model under interval uncertainty can be transformed into a model for solving the generation output scheme to improve the weak damping, used in large scale network after being expanded and simplified. And a generation rescheduling strategy is obtained to improve poor damping of oscillation mode. The methodology is tested in Guangdong Grid analysis.5. The thesis investigates the problem of optimal active power dispatch with small-signal stability constraints. A new optimal active power dispatch model with small-signal stability constraints is built. A SLP method is applied to solve the new model. The rescheduling scheme is superior to the conventional one from the perspective of considering stability and economy at the same time.