| HVDC transmission technology based on modular multileve l converter is a new type of transmission technology developed in recent years.It is widely us ed in the transmission of long distance,large capacity and high vol tage overhead transmission lines,and can realize the interconnection of multiple AC systems across regions.This is the key technology to realize the optimal allocation of energy resources in China in the future,and also the development trend of power gr id in the future.MMC-HVDC plays a very important role in the field of flexible DC transmissio n in China.However,due to its special topology,circulating current will be generated between three-phase bridge arms during normal operation of MMC.The circulating current will not only affect the stability of the converter,but also increase the loss of the system.When the grid voltage is unbalanced,the nature of phase to phase circulating current will change.The circulating current contains positive sequenc e component,negative sequence component and zero sequence component of double frequency.These will directly affect the stable operation of converter station.Therefore,this thesis mainly studies the circulating current control strategy of MMC-HVDC system.At present,MMC-HVDC technology has been widely used all over the world,but the circulating current component generated by the internal operation of MMC will seriously affect the stability of the system.For MMC-HVDC system,the mathematical model of circulating current between phases of MMC is established.From the mathematical expression of the instantaneous power of the upper and lower arms of MMC,it is concluded that the circulating current is composed of DC component and a series of even harmonic components,among which the second and fourth harmonic components are the main ones.In this thesis,a MMC circulating current suppression method based on notch filter and improved proportional resonance(IPR)controller is proposed.The notch filter is us ed to extract the second and fourth harmonic components in the circulating current.After the IPR controller suppresses the harmonic components,the voltage compensation is obtained to compensate the modulation wave signal.Finally,the simulation is carried out on PSCAD,and compared with the traditional PI control method,it is proved that the proposed method can effectively suppress the inter phase circulating current,and the suppression effect is better than that of the PI control method.When the grid voltage is unbalanced,the nature of the circulating current between the three phases of MMC changes.Besides the negative sequence component of the second harmonic generation,the circulating current also contains positive sequence and zero sequence components.On this basis,this thesis proposes a circulating current control strategy based on t he combination of second order generalized integrator(SOGI)and IPR controller,which can effectively suppress the sequence components in the circulating current regardless of whether the MMC is in the state of grid voltage balance or imbalance.Finally,the MMC-HVDC transmission system is built by PSCAD simulation platform,and the simulation is carried out under both steady-state and single-phase short-circuit to ground fault conditions.The results show that the control method can effectively restrain t he circulating current between phases and improve the stability of the system. |
