Electrical Tree Growth in LDPE: Fine Channel Development During Negative DC Ramp Down

This article identifies the threat to high-voltage direct current (HVDC) polyethylene insulation posed by the growth of fine electrical trees on the removal of high dc voltages. Such fine trees have been identified previously growing under ac voltages, especially in glassy epoxy resins. However, fine tree growth is distinguished as not being associated with measurable partial discharges (PDs) and so presents a particular threat to high-voltage (HV) system integrity. In the case reported, fine trees are seen to grow in low-density polyethylene (LDPE) from smaller traditional dark trees, on the removal of a negative dc voltage. The growth is seen even in the case of a relatively slow ramp down over many seconds. The growth of a fine tree leaves the system potentially vulnerable to rapid aging when re-energizing. For cable systems, the threat is that such a tree will grow and is essentially invisible to asset managers using the traditional method of PD measurement.