Invetigation Of Space Charge In HVDC Cable With Thick Insulation Layer

High voltage direct current(HVDC)cable is one of the key equipment in DC transmission.However,space charge is easy to accumulate in the insulation under dc stress,therefore,direct detection of space charge in actual full-size cables is necessary to ensure reliable operation of cables.In this paper,the HVDC full-size cable with thick insulation layer was investigated and a full-size space charge measurement system was developed based on pulse electro-acoustic(PEA)method.Experiments were done with the measurement system.In order to meet the testing demands for actual cables with thick insulation layer,a space charge measurement system suitable for full-size cable with thick insulation was designed.The electrode system was adopted with a linear contact structure.At the same time,a high-voltage pulse generator was developed with the maximum amplitude of 10 kV and the minimum pulse width of 200 ns,Also the pulse width could be adjusted according to the requirement.To ensure that the electrical pulse amplitude applied to the space charge measurement area was high enough to,stimulate space charge disturbance in thick insulation layer,an external capacitance coupling technology for pulse coupling was experimentally studied,as well as a direct cable coupling technology.In addition,to know the temperature distribution inside the cable insulation under an actual load,a simulation heating system was designed via the induction heating technology with current transformers,of which the temperature could be real-time monitored synchronously with a model cable cut from the cable for space charge measurement.Through space charge dynamic measurement of cable with 10 mm thickness,both pulse coupling technologies were verified to achieve dynamic space charge monitoring in thick insulated AC cable.It is found that there is a large number of hetero-polarity space charge producing from those cross-linked by-products,and accumulated near the interface between the semiconductor layer and the insulation.And it is deduced that traditional AC cable materials and manufacturing processes are not suitable for HVDC cable manufacture directly.