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For monitoring the insulation status of medium- and high-voltage power cables, on-line partial discharge (PD) diagnosis tests such as detection, location and identification are used to find the defects or faults and to assess the aging degree of the cable insulation, especially for XLPE cable systems. However, owing to signal propagation attenuation and noise pollution, the PD signals are weak, which impose challenges for PD detection. Fine detection methods require a sensor with high sensitivity and wide bandwidth to detect the PD pulses. Rogowski air coil sensor (RACS) technique has been used for on-line PD signal detection in recent years. It requires reasonable methods to raise the upper frequency under the condition of high sensitivity. Traditional RACS has narrow bandwidth because of the limitation of the discrete resonance frequencies. A novel physical design for on-line RACS is proposed. It is aimed at applying transmission line theory and adjustable balance impedance to restrain or eliminate the first resonance notch frequency. Based on this design, the bandwidth of RACS increases effectively, whereas the sensitivity remains essentially unchanged. A real RACS on the basis of coaxial cable windings is designed and developed. In comparison with the traditional capacitance sensor and RACS, experimental results verified that the RACS based on the proposed novel design technique can not only increase bandwidth but also improve the sensitivity of PD detection.