Analysis And Control Of Low Frequency Oscillation In Regional Power System

Under the situation of interconnection power grids expanding and the operation condition increasingly close to their maximum, the risk of low frequency oscillation (LFO) is greatly increased. It impacts the modern interconnected power system deeply, even cause more serious malfunction than transient stability and became the primary factor affecting power system security and stability. Because of the characteristics of modern power system, like nonlinear, high dimension, multi-variable, strong binding, strong coupling and time-varying, there are still many problems in LFO analysis and control which are discussed and researched in this dissertation.The current investigation about power system LFO is summarized at first. High incidence of LFO incidents that off-line analysis cannot ensure safety and stabilization of interconnected power system and on-line analysis of LFO is urgently required. But the real-time modeling and huge calculation are the major difficulties. Aiming at the characteristics of regional power system, the method of LFO on-line analysis and modeling are studied in this dissertation, under the premise of retain the influence to the regional power system which caused by external grid. The full text consists of several parts:In the first part, the balance point eigenvalue analysis and the trajectory eigenvalue analysis are introduced and the relationship between oscillation modes on tie-line and eigenvalues of power system is studied too, which is the foundation of RMA method and mode-synthesis analysis method in the following parts. As it is known that the information of oscillation modes is involved in power oscillations on tie-line. If the relationship between the entire network balance point and the dominant modes on tie-line which obtained by trajectory eigenvalue analysis are determined, the LFO properties of regional power grid can be obtained on-line by analyzing the real time information of local regional power grid and tie-line power oscillations, instead of the whole network.Based on the content of the first part, a regional mode analysis (RMA) method is proposed in the second part, whose main idea is that, under the current system of power management, weakly damped regional oscillation modes are mainly concerned in regional power grid. Hence, through detailed topology and loadflow of regional grid, as well as real time operating condition of each tie-line connecting internal and external network which can be obtained from EMS and WAMS respectively, the RMA reduced-order model containing detailed model of regional network and real-time loadflow of tie-line can be constructed. Through this RMA model, oscillation modes related to regional power grid can be obtained without external power grid topology. The proposed model can also be used in LFO on-line analysis.The mode-synthesis analysis method is proposed in the third part. Means proposed in literature such as the eigenvalue trajectory method can only figure out the dominant modes on tie-line, without disturbance source. The method proposed in this part overcomes this limitation. Combing eigenvalue analysis with trajectory eigenvalue analysis, not only dominant modes on tie-line but also the associated units can be figured out which provides the essential information of installation sites of the PSS. Aiming at damping multi modes of oscillation, the broadband PSS is used based on phase compensation method. The broadband PSS can supply advance phase in wider band to suppress multi-mode power oscillations on tie-line.A new two-point-estimation (TPE) based power system LFO probability stability analysis method (TLPA) is proposed in the fourth part. The defined probability stability index and calculation method are given. With TLPA and uncertainties which may be already known, computation burden will be reduced for probability of occurring LFO. The same precision with less computation can be achieved using TLPA comparing to Monte Carlo method. So it is practical for large scale power system.The digital simulation results of the classical four-machine-two-region power system and a certain actual interconnected bulk power system demonstrates the practicability of the theory proposed in this dissertation.