| Fractional frequency transmission system(FFTS)combines the advantages of HVAC and HVDC transmission system,and thus is a very promising solution for large-scale offshore wind power transmission.Modular multilevel matrix converter(M~3C)which enjoys advantages of high power quality,easy scalability and direct high-voltage and high-power AC-AC conversion has attracted much attention in offshore FFTS.However,the input side and output side of M~3C are directly coupled with different frequencies,which brings challenges to its modeling and control.In view of the above problems,this article has mainly carried out the following research work:Firstly,the topology and working principle of M~3C is introduced in detail.The different working states of cells are analyzed and summarized.Then the average model of arm is established.For each subconverter,the input and output voltages/currents are decoupled through the differential-common-mode equivalent circuits.To facilitate capacitor voltage balance control,αβ0modeling is applied to the arm power of each subconverter.The mathematical expressions of each power term are calculated and the relationship between the fundamental component of the differential-mode current and the low-frequency component of the arm power is revealed,which provides theoretical foundation for capacitor voltage balance control.The mechanism of capacitor voltage fluctuation is studied,and the expression of ripple amplitude and fluctuation characteristics at each frequency are obtained.Secondly,a novel system control strategy of M~3C-FFTS is proposed for balanced input and output conditions,in which the positive sequence active component,negative sequence active and reactive components of the differential-mode current are utilized to realize grid side active power control and capacitor voltage balancing.According to the harmonic characteristics of capacitor voltage,notch filters are used in the outer loop of capacitor voltage to eliminate the unwanted AC components,and vector proportional integral controllers are used in the inner loop of current to achieve precise control of the differential-mode current.In order to obtain good voltage control performance,the classical voltage and current double loop control based on dq coordinate system is adopted in the fractional frequency side.For the internal cell capacitor voltage balance of the arm,the individual capacitor voltage balancing strategies based on carrier phase modulation and the nearest level modulation are given respectively.Then,the effects of capacitor voltage dynamics in modulation on fundamental frequency voltage coupling characteristics and arm currents are analyzed.The effectiveness of the proposed modeling and control strategy is verified by a 220 k V,400 MW M~3C system implemented in Matlab/Simulink.Finally,this paper proposes the control strategy of M~3C under unbalanced condition.First,the arm power under unbalanced grid condition is calculated,and the power flow law among arms is summarized.Next,the influence of low frequency circulating currents on the arm power is analyzed,and the open-loop arm power compensation strategy based on constructing circulating currents is proposed to redistribute the arm power.In order to realize the separation of positive and negative sequence voltage,the double sequence phase locked loop based on dual second-order generalized integrator is adopted and its working principle is analyzed in detail.In addition,this paper also proposes a closed-loop arm capacitor voltage balance control strategy through real-time regulating amplitudes of circulating currents.The current control schemes for two different capacitor voltage control strategies under unbalanced condition are given respectively.The validity of the control strategies is verified by the above simulation model.The static error free balance control of DC component of capacitor voltage under unbalanced condition is realized by superimposing arm power compensation strategy. |
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