Considering the availability and derivation of Roscoe's equation for absolute viscosity calcula- tion,the method to adopt λ_0 value,so-called λ_f,was proposed by experimentally deter- mining relational curve of ...Considering the availability and derivation of Roscoe's equation for absolute viscosity calcula- tion,the method to adopt λ_0 value,so-called λ_f,was proposed by experimentally deter- mining relational curve of the measured height,H,of metal melt vs logarithmic oscillation decrement,λ,extrapolating H into zero,if such corresponding λ value is taken.Experimental results show that the proposed method not only satisfies the availability of original equation but also eliminate the effects of:“end and slippage”,so as to improve the accuracy for liquid Cu and Sn examinations.展开更多
The power grid with large proportion of hydropower may engage the ultra-low frequency oscillation (ULFO),which threatening the operation of system.To the NY Power Grid system,whose dimension is over thousands,this pap...The power grid with large proportion of hydropower may engage the ultra-low frequency oscillation (ULFO),which threatening the operation of system.To the NY Power Grid system,whose dimension is over thousands,this paper reveals the cause of ULFO with the phase portrait method,i.e.,lies in the saturation of the different limits of the governor system of the generators,in other words,the occurrence of limit cycle is due to the non-smooth bifurcation.And furthermore,it proposes two methods to suppress ULFO with the philosophy of destroying the condition of non-smooth bifurcation.In detail,first,the phenomenon of ULFO in NY Power Grid,the damping characteristics and the limits in the governor system of the hydropower units are presented.Second,based on the non-smooth bifurcation theory,the mathematical basis of the oscillation in the non-smooth system caused by the saturation of limits is introduced.And then,the non-oscillation and oscillation dynamic of the system corresponding to without/with limits in the governor system are analyzed.Furthermore,it discusses the influence of the saturation of single limit on the characteristics of ULFO with the phase portraits.Moreover,the phase portraits of the ULFO under different faults is analyzed,and it shows that the limiting saturation occurs with a large enough disturbance.Thus,the ULFO is resulting from the negative damping of generation and the saturation of multiple limits under a large enough disturbance.In addition,according to the above mechanism,two strategies to suppress the ULFO,i.e.,the governor parameters tuning and the fault strength relief are designed.The simulation shows the effectiveness of proposed strategies.展开更多
文摘Considering the availability and derivation of Roscoe's equation for absolute viscosity calcula- tion,the method to adopt λ_0 value,so-called λ_f,was proposed by experimentally deter- mining relational curve of the measured height,H,of metal melt vs logarithmic oscillation decrement,λ,extrapolating H into zero,if such corresponding λ value is taken.Experimental results show that the proposed method not only satisfies the availability of original equation but also eliminate the effects of:“end and slippage”,so as to improve the accuracy for liquid Cu and Sn examinations.
基金National Natural Science Foundation of China (Nos. 51477050, 50707035) and 111 plan (No. B08013).
文摘The power grid with large proportion of hydropower may engage the ultra-low frequency oscillation (ULFO),which threatening the operation of system.To the NY Power Grid system,whose dimension is over thousands,this paper reveals the cause of ULFO with the phase portrait method,i.e.,lies in the saturation of the different limits of the governor system of the generators,in other words,the occurrence of limit cycle is due to the non-smooth bifurcation.And furthermore,it proposes two methods to suppress ULFO with the philosophy of destroying the condition of non-smooth bifurcation.In detail,first,the phenomenon of ULFO in NY Power Grid,the damping characteristics and the limits in the governor system of the hydropower units are presented.Second,based on the non-smooth bifurcation theory,the mathematical basis of the oscillation in the non-smooth system caused by the saturation of limits is introduced.And then,the non-oscillation and oscillation dynamic of the system corresponding to without/with limits in the governor system are analyzed.Furthermore,it discusses the influence of the saturation of single limit on the characteristics of ULFO with the phase portraits.Moreover,the phase portraits of the ULFO under different faults is analyzed,and it shows that the limiting saturation occurs with a large enough disturbance.Thus,the ULFO is resulting from the negative damping of generation and the saturation of multiple limits under a large enough disturbance.In addition,according to the above mechanism,two strategies to suppress the ULFO,i.e.,the governor parameters tuning and the fault strength relief are designed.The simulation shows the effectiveness of proposed strategies.
