Research On Security And Stability Control Technology Of Multi-Infeed Direct Current System

Constructing the “new-type power system” with new energy as the main body is a key measure to achieve the strategic goal of “Carbon Peaking and Carbon Neutrality”.However,the new energy resources and load centers in China are in the reverse distribution pattern,and it is urgent to build the new energy cross-regional transmission channels.Due to the advantage of long-distance and large-capacity power transmission,the high voltage direct current(HVDC)transmission technology becomes an important means of developing and utilizing new energy.In China,there are more than 30 HVDC projects in operation at present,including more than 10 HVDC projects connected to East China Power Grid and Guangdong Power Grid,forming the multi-infeed direct current(MIDC)system.Due to the close electrical distance and strong coupling between multiple converter stations within the MIDC system,the AC system fault may cause oscillation,commutation failure and HVDC blocking,which may threaten the stable operation of the power grid.Therefore,it is necessary to carry out the research of the MIDC security and stability control technology.This article is funded by the National Key Research and Development Program of China “Commutation Failure Prevention Technology of Multi-Infeed Direct Current System”,the Major Science and Technology Project of Hunan Province“Research and Demonstration on Key Technology of Large-Scale Energy Storage System Supporting the Safe and Efficient Operation of Power Grid” and other projects.The researchs are completed relying on the State Key Laboratory of Electric Power Hige-Efficient and High-Quality Conversion,and National Electric Power Conversion and Control Engineering Technology Research Center.Aiming at the problems of small-signal instability,commutation failure,and the power disturbance caused by the HVDC blocking during the fault evolution process,this article focuses on the small-signal stability analysis and damping control technology,the commutation failure mechanism analysis and prevention technology,the large-signal stability analysis and emergency power support technology of MIDC system.The MIDC security and stability control technology has been reseached considering multiple interaction factors,such as small-signal interaction,power grid state variable interaction,and power electronic equipment interaction,which can provide theoretical basis and technical support for ensuring the safe and stable operation of the actual projects.The main research contents and results include:(1)Aiming at the small-signal stability of the MIDC system,the small-signal interaction mechanism and additional damping control strategy of the MIDC system are studied.First of all,the influence of circuit parameter and control parameter on the damping characteristics of the MIDC system is analyzed based on the small-signal interaction path.And it shows that the small-signal interaction between the line commutated converter-based high voltage direct current(LCC-HVDC)and the voltage source converter-based high voltage direct current(VSC-HVDC)provides negative damping,which has an adverse influence on the stability margin of the MIDC system.Then,with the proposed additional damping controller in the VSC terminal voltage loop,the reactive power/amplitude coupling damping coefficient increases at the oscillation frequency,which results in the increase of the total damping coefficient,and ensuring that the MIDC system maintains small-signal stability on the premise of strong reactive power regulation capability.Finally,the correctness of the small-signal interaction mechanism of the MIDC system and the effectiveness of the additional damping control strategy are verified by the hardware-in-the-loop experiments.(2)Aiming at the successive commutation failure of the MIDC system,the mechanism and prevention strategy of successive commutation failure considering reactive power and harmonic interaction are studied.First of all,for the impact of reactive power interaction,the increase of reactive power exchanged on tie line leads to the decrease of AC voltage amplitude of healthy line during the successive commutation failure development stage,which promotes the occurrence of successive commutation failure.For the influence of harmonic interaction,the voltage distortion components in AC voltage of healthy line caused by harmonic interaction play a negative role on commutation process,which increases the probability of successive commutation failure.Then,a successive commutation failure prevention strategy of MIDC system considering reactive power and harmonic interaction is proposed.For the improvement of the faulty line control strategy,by disabling commutation failure prevention controller and modifying the parameter in constant extinction angle controller based on reactive power impact index,the reactive power interaction characteristic is improved.For the improvement of the healthy line control strategy,by triggering it earlier based on voltage distortion index,the startup speed of trigger control is accelerated.Finally,the effectiveness of the proposed successive commutation failure prevention strategy is verified under different fault types,and the generality of the proposed method is further verified in the Henan multi-infeed UHVDC system model.(3)Aiming at the subsequent commutation failure of the MIDC system,the mechanism and prevention strategy of subsequent commutation failure considering the interaction of grid side energy storage are studied.First of all,the subsequent commutation failure mechanism of the MIDC system is analyzed considering the influence of grid side energy storage during the fault duration stage and fault clearance stage.Then,a subsequent commutation failure prevention strategy of MIDC considering the interaction of grid side energy storage is proposed.For the improvement of the MIDC control strategy,in order to decrease the impact of the current error controller output reduction,the lower limit of the constant extinction angle controller is dynamically modified based on the AC voltage amplitude.And the influence of the DC current oscillation on the trigger angle order value fluctuation is weakened by the band-stop filter.For the improvement of the grid side energy storage control strategy,to ensure the sufficient commutation voltage demand area,the grid side energy storage continuously outputs the reactive power during the AC voltage and DC power recovery stage.Finally,compared with conventional reactive power control strategy and other methods,the effectiveness of the proposed subsequent commutation failure prevention strategy is verified under various fault types.(4)Aiming at the power disturbance problem caused by the blocking of MIDC system,the large-signal stability of VSC-HVDC system is analyzed,and the emergency power support strategy of multiple HVDCs power coordination is proposed.Firstly,under grid voltage and power disturbance,the large-signal stability criterion of VSC-HVDC is deduced based on the mixed potential function,and the influence of circuit parameter and control parameter on the large-signal stability of VSC-HVDC is analyzed.Then,on the basis of ensuring the large-signal stability of non-blocking DC lines,the emergency power support strategy of VSC-HVDC and LCC-HVDC power coordination is proposed under the condition of faulty line blocking.The proposed emergency power support strategy prioritizes the active power support of VSC-HVDC.Otherwise,the non-blocking VSC-HVDC and LCC-HVDC coordinate the active power and reactive power.Finally,the correctness of VSC-HVDC large-signal stability analysis and the effectiveness of the proposed emergency power support strategy are verified by simulation.