As power-electronic (PE)-based systems become increasingly common in the electric power grid

the insulation systems used in medium- and high-voltage (HV) applications will be exposed to high-frequency (HF) electric fields. Therefore

the insulation materials must be characterised using HF waveforms. However

generating these waveforms presents a significant challenge due to the large reactive power associated with the dv $v$/dt $t$. This paper proposes a resonant test system with a ferrite-based transformer for HF insulation testing. The resonant circuit is formed by the transformer's leakage inductance and the insulation sample capacitance

with an adjustable frequency tuning capacitor. The system can be driven with an inverter or linear power amplifier. Increasing the test voltage level while maintaining the same test frequency presents several challenges: transformer core grounding

high resonant current and implications for bobbin and insulation design. This paper investigates these challenges and proposes an oil-insulated resonant transformer

capable of extending the test voltage to 23 kVpk for HF insulation tests at around 40 kHz. High-frequency breakdown tests are performed on enamelled copper wire in various insulation media using the prototype resonant test system

highlighting the importance of the dielectric's thermal performance.