Oil-paper capacitance plates is an important part of the main insulation of inverted oil-immersed current transformer（CT）and on line detection of insulation status is an important means in assuring safe and reliable operation of CT. Since the internal end screen of most inverted CT is welded and connected with the base permanently and the pipe on the secondary side has shunt effect

installation of sensor on the grounding end screen for PD pulse collection is neither convenient nor accurate. On this basis

in this paper

the primary winding method for measuring PD pulse signal at HV primary side of inverted CT is proposed. The fundamental principles of the primary winding method based on the structure characteristic of the inverted CT and the prorogation process of the PD pulses signal is given

the on line detection system including wide band current sensor

signal processor

fiber transmission system

operating and hooking device and PD detection and analysis program is developed. The sensitivity of this system is 1 V/A and the sampling frequency is 50 MHz

which can be used to test current signal of PD pulse generated in oil-paper insulation defects. A test platform with PD model of imperfect earth of end shield is built in the test laboratory to test the developed on line detection system. It is shown by the result that the primary winding method has good practicability and anti-interference ability. The PD pulse discharge signal of the oil-immersed CT can be extracted effectively

which lays a foundation for the further application of primary winding method in online detection of PD of oil-immersed CT.