Coordinated Control And Harmonic Suppression For Low Frequency AC Access Of Multi Terminal Offshore Windfarms

Offshore wind power has the advantages of large capacity,stable output and long generation time.It is a new energy form with great development potential.At present,offshore wind power is still concentrated in coastal water areas.In view of the power generation potential of far-reaching sea areas,offshore wind power is also developing towards the trend of large capacity and long distance.Compared with the widely used HVDC and HVAC transmission modes,low frequency alternating current(LFAC)transmission improves the transmission capacity and distance of AC cable by reducing the transmission frequency,saves the offshore compensation station,and its key technical equipment,such as circuit breaker,is not as complex as DC equipment.Therefore LFAC is an ideal transmission mode for wind power transmission in deep sea area.At present,the research on LFAC mainly focuses on the single input single output system and the valve level control of the new converter,but the research on its application in multi terminal system and the problem of long-distance transmission is insufficient.In this paper,a coordinated control method is proposed to extend the LFAC converter station level control model suitable for far-reaching offshore wind power transmission to the transmission system with multiple offshore and onshore ports.Considering the influence of long-distance cable,the traditional control method is improved,and the harmonic influence caused by long cable parameters and the suppression measures are analyzed and studied.The main contents of this paper are as follows:(1).Based on a control mode of constant power at the sending end and constant voltage at the receiving end,a single input single output control model of converter station level based on back-to-back voltage source converter(BTB VSC)is built.The dual loop controller is selected as the basic control structure of VSC.The controller is constructed for the functions of three main VSCS,and for the typical parameters of deep sea wind power,The voltage and power control functions of the control model are verified by simulation.(2).Extend the original single input single output control model to apply on multi terminal system.From the point of view of port power control,the control mode of constant power at the sending end makes it easy to realize the expansion of multiple offshore ports,but when the number of transmission lines increases,the control mode of constant voltage at the receiving end can not realize the input power control of onshore ports.Aiming at this problem,this paper improves the control model of onshore converter station,introduces an extra loop droop control to change the given mode of voltage frequency on the basis of double loop controller,and deduces the droop control equation of PQ coupling considering the non-inductive characteristics of long AC cable,so as to construct the constant voltage droop coordinated control mode between onshore ports.The effect of coordinated control is simulated in Simulink.(3).For the harmonic instability problem in LFAC system during long-distance transmission,the impedance measurement method is used to analyze the harmonics of transmission line,and the harmonic suppression is carried out by adjusting the system parameters.Due to the existence of cable distribution parameters,the voltage on long-distance LFAC transmission lines is prone to waveform distortion due to resonance.Impedance analysis divides the whole LFAC system into two subsystems: source and load.By measuring the frequency response of port impedance,the harmonic frequency of the system is analyzed by the stability criterion of amplitude frequency characteristics.According to the amplitude frequency characteristic curve of the port impedance,the system parameters are adjusted to make the original resonant frequency meet the stability criterion,so as to realize the harmonic suppression at this frequency.