Research On Space Charge Evolution Mechanism In HVDC Cable Insulation And Accessory Composite Insulation Under Temperature Gradient

At present,the localization demand for HVDC extruded cables and accessories is very urgent.The problem that has limited the development of the insulation of the cables and accessories for a long time is the accumulation of space/interface charge,which is the source of insulation failure.The accumulation of space/interface charge will induce local electric field distortion,cause partial discharge,accelerate insulation aging and lead to insulation breakdown,which seriously threatens the long-term safe and reliable operation of cables.The fuzzy cognition of the evolution mechanism of space charge is the bottleneck restricting the development of its insulation.The research of space charge characteristics has been regarded as the key basis for the insulation design and evaluation of HVDC cables and accessories.In the actual operation of HVDC cables,the Joule heat generated by the load current flowing in the cable conductor will form a high temperature gradient(TG)inside the insulation layer and low temperature outside.However,most of the current researches mainly focus on constant temperature conditions and do not consider the effect of TG.In view of the research on the space/interface charge accumulation characteristics and transport mechanism of the composite insulation of HVDC cables and accessories under TG that needs to be improved,the unclear mechanism of the effect of TG on the space/interface charge behavior,and the lack of the research on the nano-modified accessories composite insulation interface charge regulation under TG,this thesis takes HVDC cable insulation and accessory composite insulation as the research objects,and uses the pulsed electro-acoustic(PEA)method to carry out the experimental measurement,and uses the carrier transport under electro-thermal multi-field coupling to carry out the numerical simulation,and uses theoretical analysis as means.It has carried out the research on the space charge distribution characteristics of HVDC cable insulation in steady-state and transient processes under electro-thermal multifield coupling,the space/interface charge distribution characteristics of HVDC cable insulation/accessory insulation bi-layer dielectric under electro-thermal multi-field coupling,the interface charge distribution characteristics of nanomodified bi-layer dielectrics under TG.Based on the PEA space charge measurement platform for the full-scale HVDC cable,this thesis realizes the controllable TG of cable insulation by controlling the load current flowing through the cable,measures the spatiotemporal distribution characteristics of space charge in HVDC cable under the TG,and researches the influence of TG on the space charge aecumulation.It is found that the increase of the TG strengthens the charge injection at the interface of the conductor shielding layer and the insulating layer,which causes the injected charge to spread more into the bulk insulation.At the same time,it also promotes the accumulation of more heterocharge near the interface between the insulating layer and the insulating shielding layer.Combined with the bipolar charge transport model and the ion charge transport model based on the coaxial structure,this thesis establishes a simulation model of space charge for HVDC cable insulation under the electric-thermal multi-field coupling,and simulates space charge and electric field distribution in the HVDC cable with thick insulation under the steady state and transient state(voltage polarity inversion).Thus,it obtains the evolution law of space charge and electric field spatiotemporal distribution during the conversion of thick insulation steady-state to transient-state to steady-state process of HVDC cable,and reveals the influence mechanism of polarity reversal period and TG on its space charge behavior and electric field distortion.The research has found that the maximum transient field strength in the HVDC extruded cable with thick insulation appeared at the initial stage of the complete voltage polarity reversal,which is located near the interface between the conductor shielding layer and the insulating layer;Higher TG and longer polarity reversal period both exacerbate the maximum transient field strength,resulting in more severe electric field distortion,when the load current is 1000 A and the polaity reversal period is 120 s,the maximum transient field strength distortion rate is respectively 1.21 times and 1.35 times of 30 s and 6 s;Higher TGs also lead to more severe distortion of the maximum steady-state field strength both before and after the voltage polarity reversal,when the polarity reversal period is 120 s,the maximum transient field intensity distortion rate is respectively 2.31 times and 1.81 times of the maximum steady-state field intensity distortion rate before and after polarity reversal at the load current of 1000 A,and respectively 2.16 times and 1.75 times of the maximum steady-state field intensity distortion rate before and after polarity reversal at the load current of 800 A.Based on the PEA pressure wave generation and propagation characteristics in bi-layer dielectrics and the influence of TG on the pressure wave characteristics,this thesis proposes a signal recovery method for the PEA space charge characterization in bi-layer dielectrics under TG.At the same time,according to the PEA measurement system with a controllable TG used for a filmtype dielectric,the spatiotemporal distribution characteristics of space/interface charge in the bi-layer dielectrics under the TG are obtained,and the influence of TG on the space/interface charge accumulation characteristics is also studied.The research has found that the interface charge increased first and then decreased with the increase of TG.Combined with bipolar charge transport model,ion charge transport model and surface states,a simulation model of space charge for bi-layer dielectrics under electro-thermal multi-field coupled is established.Compared with the PEA measured results of the space charge in bi-layer dielectrics under the same DC field strenght and TG,the validity of the simulation model is verified.In this way,the space charge accumulation and the electric field distribution characteristics in the bi-layer dielectrics under the TG are mastered,and the influence mechanism of the TG on the charge behavior and the electric field distortion is revealed.It is found that this model can well reproduce the interface charge behavior in bi-layer dielectrics under TG;Under the TG,the maximum field strength in the bi-layer dielectrics all appear in the bulk of higher-temperature dielectric close to the insulating interface,and the increase of TG leads to more serious electric field distortion.The maximum field intensity distortion rate under the TG(ΔT=35℃)are respectively 1.21 times under the TG(ΔT=15℃)and 2.53 times without TG(ΔT=0℃).In this thesis,the interface charge accumulation characteristics method of cross-linked polyethylene(XLPE)/carbon black(CB)-doped silicone rubber(SR)bi-layer dielectric interface under TG are researched,so as to obtain the effects of temperature and nano-doping concentration on the DC electrical conductance and dielectric properties of CB/SR nanocomposites,analyze the effects of nanodoping concentration and TG on the space/interface charge accumulation and reveal the regulation mechanism of the interface charge under TG.According to the research,it is found that the 1.5 wt%CB/SR nanocomposite at the temperature of 25℃-65℃ has the minimum steady-state conductance current value.Under no TG(ΔT=0℃)and TG(ΔT=35℃),when the doping concentration is 1.5 wt%,the insulating interface formed by the CB/SR nanocomposite and the XLPE material has the least space charge accumulation.However,the TG effect weakens the suppression effect of the interfacial charge accumulation under the doping concentration of 1.5 wt%.The interfacial charge regulation mechanism under TG can be explained according to the interface model of electric double layer and multi-core in nanocomposites.