| Polymeric materials are widely applied in many fields such as electric power engineering,energy engineering and aerospace engineering due to their superb properties.The polymer cable,as a crucial part of the high voltage direct current power transmission system,endures high electric field strength during work time.Polymers are vulnerable to space charge accumulation in high direct current electric field,which will distort the local electric field,enhance the thermal movement of carriers,intensify the energy storage and release process,age the insulation and eventually impose critical threats on lifetime.Polymers are also used in radiant environment situations such as spacecrafts.However,the radiation of high energy particles,collision of plasma,and irradiation of ultraviolet lights make them easy to accumulate charges on the surface and in the bulk of the polymers,thus causing electrostatic discharge.Space charge accumulation phenomenon mainly results from the existence of shallow and deep traps and space charge behavior is strongly related to the distribution of traps in polymers.It is very beneficial to perform simultaneous measurement of space charge and trap energy density,and it will help to comprehensively understand the mechanism of carrier transport in detail.However,the lack of proper equipment hinders the development of simultaneous measurement of space charge and trap energy density.This paper focuses on the crucial issue of the relation between space charge behavior and trap energy density distribution,and elaborates a measurement system that can simultaneously measure space charge and trap energy density of the same specimen.The space charge,conduction current and trap energy density distribution of cross-linked polyethylene(XLPE)and nano-MgO/XLPE composite were measured by this system.The relation between space charge and conduction current,and the relation between space charge and trap energy density distribution were carefully investigated to analyze the carrier transport process.The most difficult part of the development of simultaneous measurement system lies in how to coordinate the experiment setup contradiction of space charge measurement and current measurement.For solving this problem,firstly,we performed the simulation and theoretical analysis of the interference of voltage pulse(which is indispensable in space charge measurement)in current measurement.It is found that the voltage pulse results in a current pulse that can affect the current measurement or even destroy the current measuring equipment.Therefore,a “four electrodes” setup,including high voltage electrode,space charge measurement electrode,current measurement electrode and grounded protection electrode,is adopted to isolate the space charge measurement area and current measurement area.In addition,a time-division measuring strategy is adopted to alternately perform space charge measurement and current measurement.The liquid nitrogen and a heating coil are set to control the temperature of the specimen during measurement.Besides,a nanosecond voltage pulse generator with high repetition rate is developed based on the Blumlein transmission line.The pulse generator is realized by a fast MOSFET that can make the generator operate in 3 MHz repetition rate.This pulse generator realizes the fast measurement of space charge.According to the classic thermally stimulated current analysis theory of single energy level,it is found that the thermally stimulated current equations caused by different charge sources are similar.A modified thermally stimulated current analysis method is proposed on the basis of an iterative algorithm,solving non-negative linear least square problem.This method does not require preknowledge of the number and location of the thermally stimulated current peak,and can automatically analyze the whole thermally stimulated current curve.It has good repeatability and can avoid human error.This analysis method can be used to calculate the trap energy density distribution in polymers.Through the simultaneous measurement of space charge and conduction current in XLPE and nano-MgO/XLPE composite at the temperature range of 20 to 60 °C,it is found that the electric field threshold of space charge injection of nano-MgO/XLPE is higher than that of XLPE,which means that the addition of nanoparticles can inhibit the space charge accumulation to some extent.As for the conduction current characteristics,it is found that the turning electric field in J-E curve of nano-MgO/XLPE is higher than that of XLPE.Moreover,according to the analysis of J-V scaling curves,it can be induced that deep traps exist in the bulk of both materials and the spatial trap distribution in nano-MgO/XLPE is more even than that in XLPE.Besides,according to the experiment results of the simultaneous measurement of space charge and current,it is found that the negative differential mobility is the main reason of space charge packet formation.The simultaneous measurements of space charge and thermally stimulated current of XLPE and nano-MgO/XLPE are performed in the temperature range of-60 to 100 °C.The space charge measurement results show that many positive charges are accumulated near the anode in polarization stage and gradually migrate to the bulk of XLPE.During cooling stage,the movement of space charge slows down.During thermal stimulation stage,the amount of space charge decreases dramatically.Whereas,not so much space charge accumulated in nano-MgO/XLPE during the polarization stage and cooling stage.The trap energy density distribution shows that there are many deep traps in 1.15 eV energy level in XLPE,whereas the traps in nano-MgO/XLPE are distributed more evenly in energy levels.The combined analysis results show that most of the space charges in XLPE are distributed in deep traps,whereas less space charge is accumulated in nano-MgO/XLPE due to the excess trap energy levels induced by the addition of nanoparticles. 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