Voltage Security Assessment And Coordinated Control Of Multi-infeed HVDC In Power System Recovery

With the rapid growth of power demand and requirements for the low costs,power systems are operating near security limit,leading to frequent blackouts.To reduce the loss of blackout,it is necessary to study the rapid and safe recovery of power systems.In recent years,the HVDC transmission systems develop rapidly,and multi-infeed HVDC systems have been formed in load intensive areas.So the security and control strategy of multi-infeed DC systems in recovery is necessary.The fast start-up and power control characteristics of HVDC is helpful to speed up the recovery process,but also introduces risks.We know the operation of HVDC is closely related to AC voltage.Improper regulation during recovery may cause voltage violations,loss of stability,HVDC commutation failure and other security issues.This paper studies on the voltage security issues in the start-up phase and power recovery phase of multi-infeed HVDC systems,and gives security constraints and optimized control strategies to ensure the security of multi-feed HVDC systems participating in recovery,which will speed up recovery process.The main contributions and innovations are described as following:1)This paper proposes a multi-infeed equivalent short circuit capacity index(MES)which takes into account the reactive power and voltage characteristics of HVDC systems.This index is used to accurately measure the power impact on AC voltage during HVDC systems start-up,and provides the start-up conditions.Fristly,the principle of traditional short-circuit capacity index is analyzed to analogically give the definition of MES.Then the reactive power and voltage characteristics expressions of three control methods for HVDC systems is deduced to modify node reactance matrix of power system.And the MES of converter bus is calculated using self-impedance.Combined with MES,the complete start-up conditions of HVDC are given for the multi-feed HVDC system participating in recovery.The simulation analysis of an actual example confirms the accuracy of MES and the practicability of constraints2)This paper proposes a minimum load margin index which can accurately measure the global voltage stability of AC/DC system's operating points.The optimal power recovery scheme is calculated to maximize DC power recovery under the premise of meeting voltage stability requirement.Firstly,a load margin calculation method considering operating characteristics of HVDC transmission system is deduced.The minimum load margin is gotten by using the normal vector of voltage stability region boundary to modify load growth direction,which can globally reflect the "distance" between system operating status and voltage instability.Geometrical characteristics of voltage stability region of AC/DC system is analyzed to obtain the characteristics of the optimal recovery point.Then the maximum recovery of DC power and optimal recovery scheme is calculated.The reliability of the voltage stability index and the effectiveness of the optimal recovery scheme are verified in an example.3)Aiming at the problem of commutation failure caused by too fast restoration of DC power,the constraint on the maximum change of DC current set value is deduced.The progressive coordinated control method of DC power recovery is given for multi-infeed HVDC system participating in recovery.Through electromagnetic transient simulation,the instantaneous electrical parameters changes during DC power adjustment is obtained.It is found that the sudden rise in DC current caused by DC power adjustment is the main cause of commutation failure by analyzing the influence of parameters on extinction angle of inverter.Then the anti-commutation failure constraint of recovery control is deduced.Taking the minimum integral of DC power deficit over time as the optimization goal and considering anti-commutation failure constraints,control time limits and power flow constraints,a progressive coordinated control method is obtained by optimizing the power recovery control of HVDC systems.The simulation proves that the control method is useful to ensure recovery safe and smooth.