| Developing history of power system stability study, background and significance in generator stability are introduced. It aims at the problems of generator model and reference frame, practical analysis of generator's static angle stability, practical turbo-generator power system stability and low-frequency oscillation suppression, has obtained the research results. The selected topics bear academic significance and practical applied values.At present, the generator model in power angle stability control based on the constant speed revolving x y reference frame generally, and takes x as the synchronized axis approximately. When system transit the new steady state after large disturbance, the rotor angle (relative to the x axis) will change unceasingly with time integral, even if the change of system frequency is not big, so that the control objective of rotor angular deviation is not definite. In addition, the power equation, used the rotor opposite angle (relative other rotors), loads replaced with the permanent impedance parts, based on linear network superimposition, therefore more complex and big error with the reality system. Considering the rotor inertia and ignoring network (including the stator windings) transient, the generator terminal voltage vector is synchronous with network, and much faster than the rotor angle in change after disturbing shown by the simulation. Therefore generator terminal voltage vector is selected as quasi-synchronous reference axis relative to rotor, and the generator model in two reference frames is established. Analysis shows the model is applicable to large and small disturbances, reservation of the characteristic of the power angle as the stability criterion, and adaptation to the practical control in power angle stability.With respect to stability analysis of the static power angle and evaluation of the stability margin, the study reveals that the traditional analysis methods of power angle stability depend on mandatory decoupling of system state values, or rely on system accurate model and parameters, which are easily invalidated with the state changes in the non-linear power system. Profits from the physical concept of static stability in kinetics space, the whole power system is mapped to the elastic mechanics space, but understands the relations between partial stability and the entire system stability intuitively in multi-generator power system. Based on this mapping, the static angle stability has analysed depend upon generators and the system decoupling. The practical stability criterion and indicator of stability margin are provided by real-time node status messages using wide-area measurement System.After implementation of a digital electro-hydraulic (DEH) control system, the effect of primary frequency regulation (PFR) is not ideal in some times. The main problems are existence of oscillations and insufficient PFR capability. Through detailed studies of actual power-frequency regulation control, it is discovered that he dynamic PFR process at the power output control module demonstrates feed-forward open-loop characteristics when subjected to abrupt disturbances, causes "reverse regulation" appearing in the initial period of the PFR, and initiates the process vibration. On the other hand, when the system is at steady state or leading regulate state by pre-set value, it mainly illustrates the closed-loop feedback characteristics, debase PFR static regulation capability, and debase the regional power system frequency stability. The simulation results prove the correctness of qualitative analysis and quantitative analysis.The source and cause of oscillation are studied as the main problem for the resonance-type low-frequency oscillation at present. Through the analysis, it's concluded that practical turbo-generators power-frequency control mode makes mechanical power damping decrease when network side oscillation power disturbance occurs. When disturbance frequency is close to resonance frequency, mechanical power resonance will be triggered. Simulation research based on the typical parameters model demonstrates the above analysis is correct, and oscillation of some machine initiated by small disturbance may be coupled and amplified between generating units if resonance frequencies of generating units are close to each other, amplitude limiter make unequal amplitude oscillation become equal amplitude and frequency changed low-frequency oscillation mode.By studying damping modes of generator stability control, it manifests that existing damping is a passive one, which has the same phase with rotation deviation, can not reduce the unbalanced torque. If disturbance energy not be consumed, oscillation will not be eliminated. Therefore, for the suppression of forced oscillations, an active damping mode aimed to reduce unbalanced torque is put forward, and the control effects of the two kinds of damping are compared. Academic analysis shows amplitudes both of the rotor speed and rotor angle are rather small by additional active damping control. Based on the two damping theories, governor power system stabilizers are devised. Simulation results show the controller based on active damping has better restraining effect to resonance-type low-frequency oscillation aroused by power network side disturbance. A new improved Prony algorithm was presented for identifying power system low frequency oscillation mode. The amplitudes were used as weights and trained in neural network, to avoid some problems, such as irreversibility of ill-conditioned matrix during practical calculation, or poor accuracy of amplitude and phase angle gained by matrix inversion. This improved algorithm overcame poor Anti-Jamming in Prony algorithm. The simulation result show that this improved algorithm can remove interference effectively, identify the dominant mode of low frequency oscillation reliably and accurately, and decrease calculation amount. Especially, it has great advantages when oscillation signal exists interference noise, or amount of sampling points is excessive. |
