Research On Dynamic Reactive Power Control Strategy Of Multi-infeed Hvdc System

High voltage direct current transmission technology,as an important method for long-distance,large-capacity power transmission and regional grid interconnection,has been widely used in China.The central and eastern region of China is power load center,where the multi-infeed HVDC system has gradually been formed,as a result of rapid economic development.The HVDC converter stations,where semi-controlled thyristor are generally used as converter element,have opposite reactive power characteristic compared with the conventional generator.Once severe fault occurs in the receiving end,commutation failures may happen in multiple converter stations if insufficient reactive power is provided.Moreover,there is a risk that subsequent commutation failures may occur during the fault recovery process.As a dynamic reactive power compensation device,the synchronous condenser,which has been installed in some HVDC converter stations,can send a large amount of reactive power to support the voltage recovery when the system voltage drops.This paper focuses on the research of reactive power control with the participation of synchronous condenser and the main researches are as follows:Firstly,Aiming at the influence of AC system parameters on the degree of AC-DC coupling and DC interaction,the multi-infeed HVDC transmission system is taken as the research object,and the general expressions of the multi-infeed short circuit ratio and multi-infeed interaction factors on AC system parameters are derived.Taking the three-infeed system as an example,the relationship between the AC system parameters and the multi-infeed short circuit ratio and the multi-infeed interaction factor is studied.And trend of the multi-infeed short circuit ratio and the multi-infeed interaction factor is pointed out when the AC system equivalent impedance or interconnection impedance between two HVDC systems changes.Secondly,aiming at the influence of new generation large capacity synchronous condenser on commutation failure resilience in multi-infeed HVDC system,an evaluation system containing commutation failure criterion,commutation failure immunity index,multi-infeed short circuit ratio and multi-infeed interaction factor is established.In the case of different multi-infeed short circuit ratios,the influence of synchronous condenser on commutation failure immunity index,local commutation failure immunity index and concurrent commutation failure immunity index is quantitatively studied.The results show that when synchronous condenser is installed in some inverter station the commutation failure resilience of this inverter station is improved and the influence from other inverter stations is reduced.Finally,an HVDC additional control that generates a trigger pulse advance changing with the field current of synchronous condenser is proposed.Aiming at the problem of high reactive power demand in the preliminary period of fault recovery,a control strategy combining the HVDC additional control and an unequal-rate recovery current command is adopted.The simulation results show that the HVDC additional control using field current as input signal can effectively reduce the power fluctuation during the fault recovery process,and can cooperate with the unequal rate recovery current command to effectively reduce the reactive power during the fault recovery process.Thus,each HVDC system can return to a stable state in an orderly manner according to the strength of the system,reducing the risk of subsequent commutation failures during fault recovery.