| Line commutated converter based high voltage direct current(LCC-HVDC)technology has been widely applied in the world with the advantages of long-distance and bulk-power transmission.Modular multilevel converter(MMC)has become the mainstream topology of voltage sourced converter based HVDC(VSC-HVDC)due to its advantages of modularity,superior harmonic performance and low loss,which has developed rapidly especially for power grid interconnection,large-scale renewable energy integration and so on.The growing application of MMC-HVDC technology results in that the MMC converter is more likely to be located in close mutual proximity with the inverter station of LCC-HVDC links,leading to the hybrid multi-infeed HVDC(H-MIDC)system.The electrical-control interaction between the LCC-HVDC and MMC-HVDC links makes the H-MIDC system show different stable operation region and oscillation characteristics from the traditional single-infeed HVDC system,introducing new challenges to the safe and stable operation of the power system.As a typical case of the H-MIDC system,the hybrid dual-infeed HVDC system can effectively reflect the interaction and oscillation mode between different types of converters in the H-MIDC system.Therefore,this thesis focuses on the electrical-control interaction and oscillation stability of the hybrid dual-infeed HVDC system,and the main contents are discribed as follows:1)The multi-input multi-output(MIMO)transfer function models of LCC-HVDC subsystem and MMC-HVDC subsystem are established respectively,and the corresponding equivalent single-input single-output(SISO)feedback control models of each subsystem are derived.By adopting the relative gain array(RGA)number,the impacts of AC system strength,control parameters on the interaction between control loops in LCC-HVDC/MMC-HVDC are quantitatively evaluated,and the instability mechanism of LCC-HVDC/MMC-HVDC under weak AC system conditions is revealed from the perspective of control loop interaction.Moreover,according to the different influence characteristics of the time delay in the control link on the stability of the LCC-HVDC/MMC-HVDC subsystem,the influences of the interaction between time delay and other electrical/control links on system stability are analyzed.2)The MIMO transfer function model of hybrid dual-infeed HVDC system is developed,and its equivalent SISO feedback control model is derived.The impacts of AC system strength and controller bandwidth on the interaction between different control loops and stability margin of hybrid dual-infeed HVDC system are quantitatively evaluated.The results show that,the decrease of the AC system strength of LCC-HVDC/MMC-HVDC subsystem,and the increase of their phase locked loop(PLL)bandwidth,will strengthen the coupling effect between the constant DC voltage control loop and the constant active power control loop in the hybrid dual-infeed system.Consequently,the interaction between the control loop of LCC-HVDC subsystem and that of MMC-HVDC subsystem is strengthened,and the control performance of the subsystem control loop is weakened,as well as the stability margin of the whole hybrid dual-infeed HVDC system.Furthermore,a closer electrical proximity between LCC-HVDC and MMC-HVDC subsystems will lead to the following two effects,i.e.,the increase of the distance between their geographical locations and the decrease of the equivalent short circuit ratio(ESCR)of the hybrid dual-infeed system,and the stabilizing effect due to the weakening of interaction between control loops brought by the former is weaker than the destabilizing effect due to the enhancement of interaction between control loops brought by the latter,thereby reducing the system stability.3)A decentralized cooperative oscillation suppression method for the hybrid dual-infeed HVDC systems is proposed.The same type of derivative feedback control is distributed in the LCC-HVDC subsystem and the MMC-HVDC subsystem,which can simultaneously work and cooperatively suppress the oscillation when the oscillation is occurred.The results show that,the derivative feedback control can effectively suppress the oscillation instability in LCC-HVDC subsystem and MMC-HVDC subsystem under weak AC system conditions.The decentralized cooperative oscillation suppression method based on derivative feedback control can effectively improve the stability of hybrid dual-infeed HVDC system connected to the weak AC system.Especially,when both the derivative feedback controls in LCC-HVDC and MMC-HVDC subsystems are in operation,they can exert a cooperative supporting effect on oscillation suppression,improving the stability of hybrid dual-infeed HVDC system to a greater extent.Additionally,the hybrid dual-infeed HVDC system,with the decentralized cooperative oscillation suppression method based on the derivative feedback control,presents the satisfactory robust stability when subjected to the disturbances of frequency/phase in the AC system and parameter uncertainty.4)The state space model and the high-frequency simplified impedance model that applied to the study of high-frequency oscillation for hybrid dual-infeed HVDC system are established.The system high-frequency oscillation characteristics with or without electrical proximity are analyzed,and the high-frequency oscillation mechanism is revealed.The results show that,for the hybrid dual-infeed HVDC system where LCC-HVDC and MMC-HVDC subsystems are fed into the same AC bus(i.e.,without the electrical proximity),the presence of LCC AC filter makes it free from the risk of high-frequency oscillation.For the hybrid dual-infeed HVDC system with electrical proximity,the AC filter of LCC subsystem can still suppress the high-frequency oscillation when the LCC and MMC subsystems are in close proximity to each other,and its suppression effect decreases with the increase of the electrical proximity.Besides,a method to determine the critical electrical proximity for hybrid dual-infeed HVDC system free from the risk of high-frequency oscillation is proposed,and the effectiveness is verified by theoretical and simulation analysis. |
