Research On Suppression Approaches Of Ultra-low Frequency And Low Frequency Oscillations In Power Systems

With the wide application of ultra-high voltage AC/DC transmission technologies in power grids,large-scale power transmission across regions,the penetration of clean energy is gradually increasing.These facts are making the small-signal stability problem in power systems,such as ultra-low frequency oscillation and low-frequency oscillation,become increasingly serious,which threaten the safe and stable operation of the power gird.Ultra-low frequency oscillations often occur in power systems with high hydro-power penetration.Engineering experience shows that this problem is closely related to the primary frequency regulation,but the mechanism is still lacking in-depth understanding and analysis.To study the ultra-low frequency oscillation problem,a simple unified frequency model is often used to replace the complex complete model,but the applicability of the model in the ultra-low frequency oscillation frequency band still lacks strict mathematical proof.The control design based on the hydro-generator governor can effectively suppress the ultra-low frequency oscillation.However,the current method generally only considers the stability,and the performance conflict between the stability and the tracking performance has not been widely reported.The control design of the hydro-generator governor should consider both stability and tracking performance,and should have certain robustness.Power system stabilizers can be used to suppress ultra-low frequency/low frequency oscillations in power systems,but today the ideal phase-frequency characteristic method widely used to set its phase is derived based on a single-machine infinite system.The effectiveness explanation of this method in multi-machine power systems is still lacking.Compared with the traditional power system stabilizer,the control structure of PSS4B power system stabilizer has more frequency band branches and parameters,which can achieve a better oscillation damping performance.Due to the complicated structure and numerous parameters,PSS4B parameter tuning is still a difficulty remains to be solved.The main contributions of this dissertation are as follows.1.The mechanism and key factors of ultra-low frequency oscillation are studied.The complete system model can be approximated to the single-frequency system model in the ultra-low frequency oscillation frequency band is proved using the multi-machine power system Phillips-Heffron model,the linear matrix theory and grid characteristics.The analytical expression of the ultra-low frequency oscillation mode eigenvalue real part under weak damping is derived from the single-frequency system model,based on which explain the reason why using the damped torque method to analyze the ultra-low frequency oscillation can get correct conclusions,and the phase conditons the power system stabilizer should be met to suppress ultra-low frequency oscillations.The influence of synchronous generator on Nyquist vector stability margin is studied based on the single-frequency system model.A frequency-sweeping based method for ultra-low frequency oscillation stability analysis is proposed.The analysis results indicate that the ultra-low frequency oscillation characteristics are dominated by the dynamics of the governor and prime mover of each unit in the system,the high hydro-power penetration is the root cause,and the hydro-power unit governor parameters has a great influence on the damping of the ultra-low frequency oscillation.2.The performance conflict of the governor control design in hydro-power dominant power system are studied.A design method of hydro-power unit governor control considering performance conflict is proposed.It is proved that adjusting the governor parameters in a power system containing a hydro-power unit will inevitably have the opposite effect on the ultra-low frequency oscillation stability and tracking performance,that is,performance conflict.Such performance conflict cannot be ignored in power systems with high hydro-power penetration.An optimization model with the best tracking performance as the optimization target,and the ultra-low frequency oscillation stability requirement as constraints,is proposed.By solving the optimization model,the hydro-power unit governor parameters that have a negative effect on stability can be optimized,thereby suppressing ultra-low frequency oscillations.3.The structural singular value theory is introduced into the design of ultra-low frequency oscillation control.A robust design method for hydro-power unit governor parameters considering multi-objective is proposed.The uncertainties caused by the change of power system operation condition is treated as perturbations in the single-frequency model,and the method of reconstructing the single-frequency model into the extended M-△ model is derived.On this basis,a parameter design method for hydro-power unit governor is proposed to find the structural singular value of the extended M-△ model less than 1.The method guarantees both the stability and the tracking performance requirements are met under uncertainties.4.The control design method of PSS4B power system stabilizer for suppressing ultra-low frequency oscillation and low frequency oscillation is studied.The ideal phase-frequency characteristic method can be used to tuning PSS phase is demonstrated using the multi-machine power system Phillips-Heffron model,the matrix theory and grid characteristics.Based on the multi-input and multi-output system Nyquist stability criterion,a control design method for simultaneously setting PSS4B parameters of multiple units is proposed.The critical gain constraint is considered in the method,the magnitude-frequency and phase-frequency characteristics of PSS4B are both adjusted,and multiple weakly damped oscillation mode are simultaneously damped.