Frequent sampling for partial discharge（PD） oil chromatographic analysis is likely to cause negative pressure inside the oil-less power equipment

and accessing detection impedance will endanger the long-term reliability of the end screen grounding. In view of the above problems

we propose a joint on-line monitoring method for insulation defects of oil-less equipment based on pressure and ultra high frequency（UHF）. An oil immersed vertical TA partial discharge detection scheme based on pressure and UHF is designed

and the relationship between the pressure and the discharge magnitude under different degrees of bubble insulation defects is studied. The experimental results show that PD causes a significant increase in pressure inside equipment. There are following rules for pressure changes: at about 30 minutes of the initial discharge

the pressure remains the same. Thereafter

when the magnitude of discharge is constant

the pressure increases approximately linearly with time. When the discharge magnitudes are 500 pC

1 000 pC

and 2 000 pC

the oil pressure growth rates are 0.001 97 kPa/min

0.002 89 kPa/min

and 0.003 45 kPa/min

respectively. The external UHF sensor can detect electromagnetic waves leaking from non-metallic gaps

which effectively solve the problem of large error in oil pressure detection during the initial period of PD. Based on the above results

a joint diagnosis scheme based on pressure and UHF is proposed

which will provide a new idea for insulation detection of oil-less equipment.