CONCERNING THE SPACE CHARGE ACCUMULATION IN DC POWER CABLE JOINTS INSULATION
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After an analysis of the structure and properties of the DC joints insulation components, the generation mechanisms and the measurements methods of the space charge in DC multilayer joints are presented. Then, the equations used to calculate the charge accumulated at the homogeneous interfaces of the joints and the electric field are given. To calculate the space charge density and the electric field, experiments on flat samples (XLPE and reductionEPDM) are performed and their dielectric properties are measured. Finally, it is shown that the charge accumulation leads to local enhancements of the electric field and to degradation and lifetime reduction of joints.Keywords:
Charge density
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The world’s first commercial High Voltage Direct Current (HVDC) transmission link was built in 1954 between the Swedish mainland and the island of Gotland. At that time, it was proved that HVDC transmission is technically feasible. Since then, HVDC cable systems have been used worldwide in electrical energy transportation. Most HVDC installations in use around the world today, use paper-insulated, oil-filled type cables. Extruded dielectric cables with cross-linked polyethylene (XLPE) has long been the preferred solution in HVAC applications due to a combination of low material and processing costs, reliability and appropriate mechanical and electrical properties. However, polymeric HVDC cables suffer greatly from space charge accumulation during dc voltage application and from ‘low’ depletion rate of accumulated space charge when the external field is removed. As a result, considerable modifications of the electric field distribution with respect to the geometric Laplacian field occur, especially in case of voltage polarity inversion. This may cause insulation degradation and premature breakdown. Manufacturers are trying to tackle the problems related to space charge phenomena by introducing additives to the insulation or semicon layers. The development of new polymeric materials with improved performance under dc electrical stress requires a thorough investigation of the properties governing charge injection, transport and trapping. Particularly mobility and trap depth distribution are very useful to describe and compare the behavior of different materials from the view point of charge dynamics and field modification. In this thesis, different polymeric mini-cables are examined under DC stresses with regard to their space charge dynamics. Two different types of XLPE insulation and four types of semi-conductive layers compose eight different combinations of mini-cables. The specimens are subjected to space charge measurements and conduction current measurements in order for their electric field thresholds to be determined. The threshold for space charge trapping is an important parameter for the design of insulation systems subjected to dc electrical fields. If the applied electric field exceeds the threshold, charge injected from the electrodes can accumulate in traps located at the interface with electrodes and in the insulation bulk. Furthermore, depolarization characteristics obtained at high electric fields and temperatures, are used in order to further investigate the performance of the mini-cables with respect to their apparent trap-controlled mobilities and trap depths, including the space charge distribution along the trap levels. The main goal of this thesis is to evaluate how the composition of insulation and semi-conductive layers affects the space charge dynamics in polymeric mini-cables.
High-voltage direct current
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Space charge accumulation under the DC field is one of the most challenging issues in the further development of HVDC extruded cable systems. The electric field distribution over the insulation thickness is strongly affected by space charge, which further control the long‐term reliability and life expectancy of the cable system. Although extensive efforts have been made to investigate the space charge behaviour in dielectrics, the charge generation and transport mechanisms are still not yet well understood. The space charge and field distribution in cable system cannot be calculated accurately by the conductivity model, which serves as an important reference for DC cable design. Thus this dissertation devotes to study space charge by numerical modelling and experimental investigation of space charge behaviour in cable insulation. A numerical modelling approach based on bipolar charge transport theory has been developed to simulate the space charge dynamics in polymeric insulation of coaxial geometry. Based on previous experimental observations, a threshold electric field (10 kV/mm for polyethylene‐based material) at which the charge injection takes place is introduced. The build‐up of space charge in the medium size cable insulation in the presence of temperature gradient has been modelled and the field inversion phenomenon has been verified under the applied voltage of 90 kV. Compared with the traditional conductivity model, the new modelling method performs better as it well describes the charge generation and transport mechanisms. The space charge and field distribution in cable system under the voltage polarity reversal have been calculated, and the significant field enhancement near the conductor suggests that particular attentions need to be paid on the DC cable design and the operation methods of polarity reversal. Considering the practical operation of HVDC cable systems, the thermal transient effects on the space charge behaviour in cable insulation have been investigated. It is found that the field inversion can only take place with a higher load current which represents a higher general temperature and a larger temperature gradient, and this phenomenon can still be maintained even with the temperature decreasing. It is suggested that different loading conditions need to be considered in designing DC polymeric cables. To study the hetero charge accumulation in cross‐linked polyethylene, the formation and transport of the ionic charges have been considered and fed into the bipolar charge transport model. By introducing the impurity molecules serving as the ion‐pairs with a dissociation coefficient, the modified model can be employed to investigate the hetero charge formation in XLPE material. Simulations have been performed in XLPE flat specimens, and it is found that if the ionic charges distribute predominantly and offset the electronic charges, the net charge distribution will exhibit as a hetero charge accumulation. A pulsed electro‐acoustic system for measuring space charge accumulation in cable insulation has been designed and built, and considerable hetero charges have been experimentally observed in the XLPE insulated cables. The modelling approach considering both the charge injection and ionic dissociation is employed to investigate the space charge behaviour in XLPE cables, and both the space charge and field distribution are consistent with the experimental observations in the original cables. It is found that the ionic charge behaviour interplays tightly with the electronic charge behaviour, and it is also suggested that the impurity concentration gradient needs to be considered in the degassed cables.
Polarity reversal
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In this study, the characteristics of the space charge at the interface inside the cable joints known as the weak part of the HVDC cable were experimentally identified. For the experiment, a double-layer sample was made of two types of polymer and after applying a DC voltage, the space charge at the interface was measured and analyzed. The polarity of the space charge formed at the interface was consistent with the Maxwell-Wagner model. The time dependence of space charge accumulation and disappearance will be further studied through additional experiments and analysis in the future.
Polarity (international relations)
Polarity reversal
Double layer (biology)
Interface (matter)
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Accumulation of space charge is well identified to be a significant element affecting the performance of high voltage cable insulation system. In the area of polymeric insulation, space charge have been widely investigated under various electric stresses. The aim of this paper is to investigate the charge accumulation and bipolar charge transport within cross-linked polyethylene (XLPE) material through numerical simulation. The simulationestablished in this paper was designed based from previous work proposed by R M Hill and J M Alison. The simulation characterizes the injection, trapping, transport and recombination of electrical charges. The numerical model is driven by three fundamentalcalculations that described the characteristics of space charge which consist of Poisson’s, Transport and Continuity equations. In terms of its width, the XLPE sample used for the simulation is uniformly segregated in order to determine the electric field at each division. The electric field in the simulation is determined by using the direct discretization of Poisson’s equation. In the simulation, mobile electrons/holes and trapped electrons/holes are considered, while by applying Schottky injection algorithm, the charge is injected into the simulation with holes and electrons are at the anode and cathode respectively. Then different values of applied voltage are considered and preliminary results have shown that the variation of electric field would influence the dynamics and accumulation of space charge within the material.
Poisson's equation
Cross-linked polyethylene
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The accumulation of space charge is a major concern in HVDC transmission systems. Although a considerable number of models have been proposed to calculate space charge in thin film and cable samples, there are still many questions that need to be answered, such as effect of defects on conduction and space charge accumulation. In the present paper a simulation model is presented for the calculation of space charge accumulation in the two dimensional (2D) flat plane geometry insulator when it contains a defect, under polarization, polarity reversal and depolarization cases. The results showed that the electric field was distributed uniformly and no space charge accumulated when the material conductivity was electric field dependent and no defects were present inside the sample. However, when conducting defects were introduced into the insulation a significant amount of space charge accumulated around the defects and this amount increased with time until a steady state was obtained. The space charge that accumulated under both positive and negative voltage polarities were found to decrease the electric field enhancement around defects but increase it on rapid polarity reversal. When the applied voltage was removed the space charge decayed at a much slower rate compared to the rate of accumulation under voltage due to the decrease in the field dependent conductivity of the insulation around the defect.
Polarity reversal
High Voltage
Charge carrier
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The space charge dynamic behavior and accumulation are important contributions to the formation of electrical trees, as recognized by cross-linked polyethylene (XLPE) cable insulation under alternating current electric stress. In this article, in order to investigate the reasons for the charge accumulation phenomenon revealed by experiments under ac stress, a bipolar space charge transport model with symmetrical and asymmetrical parameters is built under ac stress. The charge accumulation mechanism is analyzed based on the variation of space charge density with different voltage application time and voltage phase. The effects of each major physical parameter are simulated and analyzed, respectively. The results show that there is no obvious space charge accumulation with the transport model under symmetrical physical parameters. However, asymmetrical parameters in the transport model result in different motions between positive and negative charges, which greatly enhances the charge accumulation. Each main parameter has an influence on charge accumulation in different perspectives and extents. The applicability of the simulation model with asymmetrical parameters is evaluated by comparing experiment and simulation results. This study contributes to the understanding of space charge generation mechanism under ac voltage and serves as a basis for improving the stability of XLPE electric power cables.
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Space charge in polyethylene-based insulation materials and its effect on the local electric field under a dc environment have been extensively examined over the last few decades while the behavior of space charge under ac stress has received less attention. Space charge phenomenon under ac electric fields becomes an important issue with increased operating field strength in many applications, such as next generation high voltage cables. In this paper, a bipolar charge transport model has been developed to simulate space charge in polymers under ac electric fields. Obtained simulation results show that there is a small quantity of phase-dependent bipolar charge accumulation in the vicinity of the electrodes that does not move into the bulk under ac stress. This causes a slight distortion of the local field in the bulk. However, at lower frequencies less than 1 Hz, there is increased charge accumulation and penetration. Comparison with available experimental data suggests that the model is capable of describing the underlying physics of charge behavior when a dielectric material is subjected to ac electric fields. Due to the weak charge movement in the bulk, the conduction current density is small and hence the displacement component dominates the total current density and this increases linearly with ac frequency.
Charge conservation
Displacement current
Electric charge
Charge density
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Space charge behavior in cross-linked polyethylene (XLPE), ethylene-propylene rubber (EPR) and multi-layered XLPE/EPR subjected to electric field and temperature has been investigated using the pulsed electro acoustic (PEA) technique. A dual-layer structure constituted of two flat sheets, where one is XLPE and the other is EPR was used to study the charging properties of the electrode/dielectric and dielectric/dielectric interfaces. The time dependence of the space charge distribution was recorded at different temperatures and fields and short circuit condition. The results show that the increase of the electric field leads to an increase of the space charge density values, and the increase of temperature leads to an increase of the space charge density in the entire sample, excepting the area near the ground electrode.
Ethylene propylene rubber
Charge density
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The role of space charge under DC electric field on aging of cable insulation has been proven. In providing the space charge electric field and temperature are two effective factors. This paper investigates the influence of temperature on making space charge in a special group of materials used in high temperature (HT) application. These insulations usually are under normal electric field, but the effect of temperature on their space charge can be vital. For this, a list of commercial HT materials were selected and their space charge results under normal DC electric field and three different thermal conditions from room temperature (RT) to 200°C were compared. To analyze the results some electrical indexes like amplitude of space charge and maximum electric field enhancement (MEFEP) are utilized. Based on the results, Fluorinated ethylene propylene (FEP) has the lowest space charge and MEFE due to the HT condition among the studied materials.
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The dynamic behavior of the space charge in the crosslinked polyethylene insulation is one of the important reasons for the initiation of the aging of electrical branches in ac cables. In this paper, a bipolar charge model is established to numerically simulate the dynamic behavior of space charge in each AC cycle and the accumulation of space charge under long-term stress. Space charge under AC stress can be divided into two types: periodic charge and aperiodic charge. Space charge density and electric field are influenced by each other. It is found that the periodic charge at the electrode interface changes with the polarity and intensity of the electric field, and the accumulation rate is relatively slow. The aperiodic charge has a multi-period accumulation effect, which causes a certain degree of distortion of the electric field near the two electrodes. Compared with periodic charge, aperiodic charge has a more significant effect on cable insulation performance.
Aperiodic graph
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