The widespread application of ultra-high voltage direct current (UHVDC) technology is driving the transition of power systems towards a mode of regional grid interconnection. As synchronous generators in power systems are gradually replaced by power electronic equipment

such as new energy sources and large-scale HVDC systems

the power systems face increased frequency security risks. Fully utilizing the frequency support capabilities of various frequency regulation resources has become an effective approach. Within a region

flexible coordination between conventional units and new energy units should be considered; between regions

utilizing HVDC systems can achieve flexible frequency support. To this end

this paper establishes a frequency response model for an inter-regional HVDC interconnected system. This model considers the impact of HVDC

new energy units

and conventional units on frequency dynamics

and describes the relationship between power disturbances and frequency deviations in different regions. Based on this

a coordinated optimization model for multiple resources considering frequency stability is proposed. To ensure the tractability of frequency security constraints

this paper proposes an iterative algorithm of simplification-approximation-correction

which strictly ensures frequency security while embedding the optimization model. Case study results demonstrate the effectiveness of the proposed method and highlight the advantages of coordinating multiple resources in an HVDC interconnected system.