This paper presents a proof of concept and equivalent circuit analysis of a cascade arrangement of tunable HV testing transformers intended for field use. The transformers use a partial core with air completing the flux path and are tuned to resonate with insulation capacitance. This minimises the power drawn from the supply and the size and weight of the transformer. A prototype set of transformers were built to validate the model. Each transformer was modelled as a set of coupled inductors to determine the input impedance frequency response. Good agreement is shown between the modelled and measured input impedance. The inclusion of core loss resistance was shown to significantly increase the accuracy of the cascade model.