Research On Reactive Power Coordinated Control Strategies In Parallel Hybrid-HVDC System

With the widely application of LCC-HVDC and the development of VSC-HVDC,some hybrid HVDC systems composed of LCC-HVDC and VSC-HVDC are gradually formed in some areas of China.But the LCC-HVDC and the VSC-HVDC of existing hybrid HVDC systems are running independently without coordinated control approach,and the VSC-HVDC.are not fully utilized.Thus,it is important and meaningful to study the coordinated control approach between the LCC-HVDC and the VSC-HVDC in hybrid HVDC system.The thesis carried out a series of researches about the reactive power control of LCC-HVDC under low load and AC filter switching condition,commutation failure under fault conditions,then some coordinated control approaches are put forwarded.Firstly,reactive power characteristics of LCC-HVDC under low load condition are analyzed.And due to the restriction of the AC filter control approach,the reactive power of AC filters is more than that of consumption of LCC-HVDC causing the rise of AC voltage.To solve the above problem,LCC-HVDC is usually equipped with low load reactive power auxiliary control approach,but this approach makes the converters of LCC-HVDC work with larger electrical stress,and long time operation in this condition may lead to the reduction of the lifetime of the converter valve.For this reason,a low load reactive power coordinated control approach for parallel hybrid-HVDC system is proposed in this thesis to compensate for the unbalanced reactive power between LCC-HVDC and AC system utilizing VSC-HVDC.Meantime,the parameters of the proposed control approach are designed and the application of the proposed approach in a simulation system indicates its effectiveness.Secondly,due to the increasing DC power of LCC-HVDC,the capacity of single filter is also increasing.Thus,the switching of AC filter brings about greater disturbance to AC system.To solve this problem,LCC-HVDC projects are all equipped with reactive power auxiliary control approach of AC filter,however,this approach will cause the fluctuation of other parameters when reducing the disturbance of the AC voltage.For this reason,a reactive power coordinated control approach of AC filter based on VSC-HVDC is proposed in this thesis.Meantime,the parameters of the proposed control approach is designed and the application of the proposed approach in a simulation system indicates its effectiveness.Finally,to mitigate the commutation failure of LCC-HVDC,a transient reactive power coordinated control approach based on extinction angle of LCC-HVDC is proposed.And for symmetric fault and asymmetric fault cases,the transient reactive power coordinated control approach is simulated from the aspects of AC system intensity,commutation failure immunity index,commutation failure probability,fault recovery time and fault detection delay.The simulation results show that the proposed control approach can effectively mitigate the commutation failure and improve the fault recovery characteristics of LCC-HVDC in parallel hybrid-HVDC system.