Analysis And Control Of Power System Frequency Closed-Loop

On March 28,2016,an ultra-low-frequency oscillation,which appeared frequently at the isolated system with a hydropower unit in the 1970s,occurred in the Yunnan asynchronous interconnection test.It raises the power system operators' and researchers' concern to the power system frequency closed-loop problem.When studying this power system frequency closed-loop problem,a simple single machine model or an equivalent model is often used.But this model lacks strictly mathematical proof and cannot reflect the effect of the excitation system and PSS.How to simplify the power system complete model to the study model of this problem,what is its mathematics meaning and physics essence,and how to mitigate this problem,is the goal of this thesis.For the first time,this thesis strictly proves the research model of power system frequency closed-loop problem from mathematical theory.Since the most effective way to solve this problem is to design the speed regulation system in the frequency closed loop,this thesis also proposes an independent coordinated design method of the governor to suppress the power system frequency closed-loop instability.At the same time,power system stabilizers can be used to suppress low-frequency oscillations(power-angle oscillations)in power systems,so whether they can be used to suppress ultra-low-frequency oscillations will be answered in this thesis.And a PSS design method to suppress frequency closed-loop instability is proposedThe main work and innovations of this thesis are as follows:1)The mechanism and key factors of ultra-low-frequency oscillation are studied.From the formula derivation of the single-machine single load system,it is proved that the ultra-low-frequency oscillation and the low-frequency oscillation are different problems.The former is a frequency closed-loop problem and the latter is a power angle closed-loop problem.The key factors of frequency closed-loop stability were found,which is the foundation of this research At the same time,some arguments about the power system ultra-low-frequency oscillation were discussed,including whether the damping torque method can be directly used to analyze the effect of the speed governor system on the power system frequency closed-loop2)Starting from the power system multi-machine rotor dynamic model,the linear transfor-mation is used to obtain the state equation including the common mode and the rotor relative motion mode.A model reduction method based on the invariant manifold theory is used to decouple the common mode and the relative rotor motion mode.The common mode equation obtained by decoupling is the power system frequency closed-loop model.This is the first time that the research model of the power system frequency closed-loop problem has been strictly proved.3)Based on the Nyquist stability criterion and the mathematical significance of the Nyquist diagram,this thesis deduces the sufficient conditions to ensure the stability of the unit feedback closed-loop system of parallel transfer functions.Using this corollary,a speed governor parame-ter independent coordination optimization method is proposed to prevent frequency closed-loop instability of the multi-machine system4)Based on Modal Induced Torque Coefficients(MITC)theory,the full model of the multi-machine power system is reduced to the power system multi-machine rotor dynamic model.Using this model reduction result and the model reduction method decoupling the common mode and the relative rotor motion mode,a PSS design to suppress the frequency closed-loop instability of the multi-machine system is proposedThis thesis demonstrates the essence of the ultra-low-frequency oscillation(frequency closed-loop stability)problem on three aspects-mathematical theory,power system modeling,and physical significance.And this thesis proposes a speed governor independent design and the power system stabilizer(PSS)design to prevent the frequency closed-loop instability of the power system multi-machine system.This thesis forms a set of solutions for the power system closed-loop problem from the perspective of modeling,analysis,and control.