Phenomena on cumulative breakdown process of organic glass under microsecond pulses-comparison and basic understanding

Tree ageing is a common failure pattern of polymers in high voltage devices. The cumulative breakdown process of PMMA under microsecond (μs) pulses was observed with an on-line microscope, a microsecond pulse generator, a needle-plane electrode and organic glass (PMMA) samples. Aside from the common phenomena, some abnormal phenomena were observed, which include: (1) sub-trees grow from the tip of firstly-initiated tree rather than the needle; (2) dark region emerges in the vicinity of the needle and successive bubbles are ejected; (3) `growth-ring-shape' mechanical crack increases in front of the needle pulse by pulse. These phenomena were compared with those under a nanosecond (ns) pulse and those under an AC voltage. Similarities and differences between these phenomena were generalized. By comparison, it is concluded that the local strong field around the tips of the trees and huge energy released in the cumulative breakdown process are the main dynamics for the tree to grow under μs pulses. The local strong field corresponds to the electron breakdown mechanism and is responsible for the sub-tree initiation phenomenon. The huge energy corresponds to the thermal and the mechanical breakdown mechanisms, and is responsible for the gas release phenomenon and the `growth-ring' phenomenon.