Analysis And Control Of Low Frequency Oscillations In Power Systems

Damage severity of low frequency oscillation has exceeded that of transient stability and low frequency oscillation has been the main concern which affects safe and stable operation in modern power system. According to figures, low frequency oscillation leads to a quarter of State Grid Corporation500kV line transmission capacity limitation and most of the lines are interties across the province, large power lines and load center lines.With expansion of interconnected power grids risk of low frequency oscillation emergence which easily causes large area blackout accident, therefore it is is particularly urgent to analyse weakly damped oscillation mode and put forward practical and feasible control strategise which increases the small signal stability and suppress low frequency oscillation of power grid.Small signal stability of a power grid(hereinafter referred to as A power grid) is analysed and strategies using permanent magnet synchronous machine (PMSG) improving system damping to suppress low frequency oscillation is studied in this paper.The basic principle of small signal analysis is described in this paper. The principle and method of small signal stability analysis is introduced, the basic theory of the complex frequency domain method, steps and related concepts is expounded and small signal stability analysis model of single machine infinite system and multimachine system is analysed.Small signal stability problems of A power grid is analysed. Negative damped oscillation mode and weakly damped oscillation mode of the A power grid is determined when PSS inputs and exits. The effect of PSS in small signal stability of A power grid is studied and results are tested by time domain simulation.An additional damping strategy for restraining inter-area low frequency oscillation utilizing rotational kinetic energy of permanent magnet synchronous generator(PMSG) is studied.Mechanism of PMSG additional damping strategy suppressing low frequency oscillation is explained.Secondly,control strategy of PMSG is established, additional damping strategy through changing rotor speed is studied and parameter installation method is given.4machine2area system including PMSG is simulated with constant wind speed and variable wind speed in PSCAD.Result shows that additional damping strategy is able to significantly enhance the system damping, restrain inter-area low frequency oscillation effectively and have an advantage over easy parameter installation and easy Engineering realization.