A 252 k V GIS partial discharge experimental platform was built.The inner grading electrode inside the insulating spacer

designed as an extractor-type loop UHF sensor

was combined with the internal and external UHF sensors to investigate their frequency responses and partial discharge（PD） detection characteristics.Four typical insulation defects

such as metal protrusion on high-voltage conductor

floating potential

metal shim left on the

fringe of spacer

and free particles

were selected for the investigation

and the PD development features under different applied voltage amplitudes were discussed.It is shown that the working frequency ranges of the internal sensor

external sensor

and extractor type loop UHF sensor are

<

1.5 GHz

500 MHz

1

.5 GHz and

<

1 GHz

respectively

and the extractor type loop UHF sensor gains the highest detection sensitivity at about 400 MHz.The detection results of different defects PD show that the metal protrusion PD on high-voltage conductor can be divided into the negative corona dominated stage

the coexistence stage of positive and negative corona

the positive corona dominated stage

and the "rabbit ears"shape stage according to the PRPD and PD severity level

and the space charge around the protrusion is considered the dominant role during the stage transition.The floating potential defect PD transits gradually from the protrusion discharge stage to typical floating discharge with symmetrical discharge amplitude

density and phase.The PD of the metal shim left on fringe of spacer transits from the quasi floating discharge into critical flashover state.The free particles PD can be divided into "jump"

"dance"

and "shuffling"stages and the PD is centralized around the zero-crossing point of applied voltage.For the defects of metal protrusion on high voltage conductor

floating potential

and free particles

the internal sensor achieves the highest PD detection sensitivity but the external sensor achieves the lowest one.While for the metal shim left on the fringe of spacer

the extractor type loop UHF sensor achieves the highest PD detection sensitivity.