Main Insulation of High-Potential, High-Voltage Transformers in Inductive Cascade Generators

It is argued that the main insulation of HV transformers is the major roadblock to a wide acceptance of cascade generators (CGs) with inductive energy coupling in industry. Typical HV, high-potential, high-frequency (HF), high-power transformer designs are depicted, and detailed electrostatic analyses of their insulation structures are performed, both in 2-D and 3-D. It is shown that in the air gaps formed between the cores and potted insulation, and possible air voids in layer insulation, partial discharges (PDs) are imminent. Full-scale insulation models for U-core transformers were manufactured and subjected to long-term pulsed testing at levels substantially exceeding the targeted operational stresses. It is further argued that pulsed testing is appropriate for both pulsed and dc high-voltage power supply (HVPS). The main insulation failures were associated with PD in the layer insulation. Overall PD currents were measured in time domain, and energy invested in the insulation degradation was calculated on the base of these measurements. Inverse power law was used to estimate life at operational stresses, limited to electrical aging only. The predicted life is long enough to move the main insulation out of the risk zone in the generator design.

I. Introduction

The mainstay of industrial high-voltage power supplies (HVPSs) for dc and long-pulse operation for a range of tens to several 100 kV for many decades have been cascade generators (CGs) [1], [2]. CGs can be roughly divided into two categories, those with capacitive and inductive energy coupling between the cascades. CGs with capacitive coupling are often built by so-called “Cockcroft–Walton (CW)” schemes [3], although there are many popular schemes [4] deviating from the original CW design. (There is another variety of CGs with capacitive coupling, aka as Dynamitrons [5], used usually in the sub-MV to MV range.) We will call CGs with inductive coupling inductive cascade generators (IGs), or inductive adders. They will be the focus of this work.