Moisture ingression accelerates the ageing and degradation of the oil-paper insulation. Rapid and accurate moisture assessment of power apparatus in the field is thus essential. In this paper

an innovative design of a microstrip petal-like complementary split ring resonator (MPCSRR) is proposed for the moisture assessment of the oil-paper insulation in the GHz range. The MPCSRR has advantages in concentrating the electromagnetic field

significantly increasing the number of resonance peaks

and achieving a sensitivity of approximately 3–4 times higher than the traditional microstrip ring resonators (MRRs). A mathematical model for analysing the dielectric response is established

combining the theoretical analysis and equivalent circuit model. The effects of microstrip width and substrate thickness on impedance matching

as well as the incidence depth and test sensitivity of the resonator

are explored using simulation. The experimental results demonstrate that the MPCSRR achieves rapid moisture detection within tens of seconds and accurately characterises the wetness of the oil-paper insulation. A moisture assessment model based on Lasso regression achieves an accuracy of 4%

confirming the reliability and validity of the proposed method. The results also verify that the sensitivity of the MPCSRR is also higher than that of the traditional MRR

showing its potential for precise and efficient moisture detection in oil-paper insulation.