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XLPE cables are widely used in urban distribution systems because of their excellent mechanical, heat and electrical properties. However, water tree, formed due to the existence of water and intense electric field, is one of the primary causes of aging breakdown. There used to be several methods, such as insulation impedance method and DC leakage current method for the detection of bridged water tree, which are non-effective for the diagnosis of non-bridged one. Residual Charge Method is known for its effectiveness in the diagnosis of non-bridged one. In this paper, pulse voltage is applied to the DC voltage charged XLPE cable after grounding. It is called 'Pulsed-type Residual Charge Method' which is found to be more effective for the diagnosis of non-bridged water tree. The effects of DC voltage, grounding time and pulse voltage on the amount of residual charge are studied. Also, the releasing mechanism of residual charge by pulse is discussed. Accumulated results show that the improved method has a high resolution for the diagnosis of water tree degradation degree.

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