Penetration of power electronics in the grid has produced a new species of stresses, characterized by fast-rising pulsed waveforms with microsecond rise times repeating at several tens of kilohertz. Analyzing their impact on existing and future insulation systems requires pulse modulators, most often with pulse transformers (PTs), to perform aging and breakdown tests. PT design for klystron loads has been studied extensively albeit for either low repetition rates or short pulse durations. However, capacitive dielectric loads impose additional complex constraints on optimizing leakage ( ${L}_{\sigma }$ ) and parasitic capacitance ( ${C}_{d}$ ) in order to minimize rise time ( ${T}_{r}$ ) and overshoot ( $V_{\text {pk}}$ ). Ensuring consistent output pulse shape is crucial since breakdown is sensitive to voltage magnitude. This article discusses these challenges through the design procedure of a modulator prototype capable of producing bipolar pulses up to 14 kV with rise times $ < 2 ~\mu \text{s}$ at frequencies between 10 and 50 kHz. Major challenges, especially core selection, winding design, PT parasitic optimization, breakdown detection, and failure modes, are highlighted. A new PQR equation is derived to model modulators with capacitive loads. Finally, the output pulses are applied across oil-impregnated paper samples to generate statistics on insulation breakdown strength and lifetime at 10 and 50 kHz. Results illustrate a reduction in lifetime and breakdown strength at 50 kHz. This is possibly due to the nonhomogenous distribution of dielectric losses within the oil-paper leading to local hotspots and eventual thermal breakdown. Furthermore, a critical field ${F}_{c} = {21}$ kV/mm is found below which the slope of the lifeline decreases dramatically, thereby indicating a shift in the aging mechanism. Potential reasons for this phenomenon are also discussed. ... Read more

With the increasing penetration of power electronic interfaces in the power grid, insulation materials will begin to experience stresses at higher frequencies than the conventional 50 Hz AC. This article studies the lifetime curves of oil-impregnated paper (OIP) under pulsed stresses and compares them at 10 kHz and 50 kHz. A pulse modulator is constructed consisting of a rectified DC supply feeding an H-bridge pulse driver connected to a 4: 200 pulse transformer. The modulator is used to apply medium voltage pulse waveforms with rise-times of T r≈ 1.8 µs across single-layer OIP samples. The results clearly show that an increase in pulse frequency significantly accelerates insulation ageing. However, it is also observed that below a certain threshold of field strength, the slope of the lifeline decreases dramatically thereby indicating decelerated ageing. Possible reasons for this phenomenon are also discussed in this article. ... Read more