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The residual charge method is expected to be a powerful tool to detect the water tree degradation of polymer insulated power cables. In this method, a dc voltage is first applied to electrify the degraded region. A part of the charge is deeply trapped and stays stably, even after the dc voltage is removed and the system is short-circuited. An ac voltage is subsequently applied in order to release the trapped charge. The dc component current, or its integration, is considered to have a correlation with the degree of degradation. This paper discusses the detail of the mechanism by which the dc component signal appears. The specimen was carefully prepared as to contain water trees with a uniform length. Space charge measurement was performed by pulsed electroacoustic method. The decay process of charge under ac voltage was interpreted into the energy depth at which the charge had been trapped, assuming that the charge was de-trapped by the process of Poole-Frenkel type. It was suggested that the charges trapped between 0.8-1.2 eV can be detected as the dc component current. The shallower would be released by thermal energy at ambient temperature while the system was short circuited. The deeper would not be released by realistic ac voltages. The result showed that the transition of dc component current in the residual charge method represents the behavior of space charges having been trapped at different energy levels. It is believed that the results give useful information to apply the method to power cables.