In the high voltage direct current(HVDC)transmission system

there is a risk of harmonic instability when the DC system is connected to a weak AC system. For the single pole 12-pulse converter

a risk assessment method of harmonic instability in HVDC transmission system considering the effect of two-terminal converters is proposed based on the modulation theory of converter switching function

and under the condition of saturation of converter transformer core. The evaluation method takes into account the commutation angle of the commutation process and the mutual effect between the two-terminal AC system and DC system. When the equivalent harmonic impedance of the DC system is calculated

the commutation impedance of two-terminal converters is taken into account. A validation model is established

in which the existing criteria and the proposed criteria are used to make risk assessment under different network parameters and compared with the simulation results in the time domain. The comparison results verify the accuracy and superiority of the proposed evaluation method. The simulation results further demonstrate that the system harmonic impedance

commutation angles and core saturation will affect the harmonic instability of system.