Submarine DC cables are the key channels for power transmission from offshore wind farms. Their insulation performance is directly related to the reliable operation of grid-connected systems. Since traditional monitoring methods are difficult to be applied to high-voltage and long-distance submarine cables

this paper proposes an active injection-based online insulation monitoring method for submarine DC cables using transfer functions. First

the amplitude-frequency characteristics of the transfer function under different relative permittivity are analyzed. It is revealed that the transfer voltage ratio and transfer current ratio exhibit an opposite variation trend with insulation degradation

so using the difference between the two as the characteristic parameter can significantly improve the monitoring sensitivity. Furthermore

utilizing the high controllability of power electronic devices

an active injection control strategy based on the switching of redundant submodules is designed

which can generate characteristic signals in the target frequency band. In addition

to reduce the algorithm complexity

analytical calculation is converted into model matching

and an insulation condition monitoring method based on Manhattan distance is proposed. Finally

PSCAD simulation results show that the proposed method can effectively realize online insulation monitoring for ±250 kV/20 km submarine DC cables.