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Space charge accumulation is an important issue for HVDC cable design. The space charge accumulation are determined by the charge transport property which can be monitored by the space charge and conduction current measurement. In this paper, based on PEA system and three-electrode system, the modified combined measurement system was developed to measure the space charge and current in LDPE samples simultaneously. The results show that the current gradually decreased with the accumulation of homocharges; the generation and movement of space charge packet resulted in the current peaks; the reduction of homocharge accumulation and the increase of heterocharge tends to increase the current.

EPDM, as a high-quality insulation material, is widely used in cable joints, but it bears a large compression force in the working environment, which may affect its insulation performance. We study the effect of pressure on charge transport in EPDM by calculating the change in the key parameter of carrier mobility in EPDM at different pressures. In this paper, by designing a unipolar carrier blocking experiment and combining improved PEA measurement equipment, the mobility of electrons in EPDM at six pressures of 0.2 MPa-1.2 MPa (interval 0.2 MPa) is calculated. In addition, the mobility data is analyzed in-depth through the hopping conduction model. It was found that the mobility in EPDM at different pressures did not change significantly, and the electronic transition distance and energy required for hopping after fitting the hopping conduction model also did not change much under different pressures. It is indicated that under normal working conditions, pressures will not affect the electrical performance of EPDM.

Underground cable and submarine cable are now widely used, and the locations of those cables sometimes are hard to be found. In this paper, a method of finding the location of the underground cable or submarine cable based on fluxgate magnetometer was proposed. 500 A was applied to the 35 kV AC cable circle via a high current generator. The magnetic field around the cable was collected when the magnetic fluxgate sensor passed underneath the cable. It is found that peaks and trough occur in the magnetic field lines, and they can be used to distinguish the cable. The position of the peaks and trough can be used to precisely locate the cable. It is also found that the sampling rate greatly influence the result. Only when the sampling rate is a divisor of the current frequency, good results can be obtained.

Electrical trees are artifacts resulting from aging of polymeric insulation in high electrical fields. Whilst there is some debate concerning the mechanism by which they grow, there is no doubt that their growth can lead to the ultimate failure of the host insulation. Studying electrical trees is mainly confined to measurement of associated partial discharges and observing the physical growth of the tree structure optically. This paper reviews developments in observations of the growth of trees in the laboratory. In particular, consideration is given to the benefits of generating three-dimensional replicas of real trees from X-ray computed tomography (XCT) and serial block face scanning electron microscopy (SBFSEM), and how these can facilitate better understanding of tree development mechanisms. It is concluded that both two- and three-dimensional imaging are required, and these need correlating with partial discharge measurements to develop models of tree growth and effective asset management tools.

The impact of air gaps between a metallic needle tip and the polymer on electrical tree growth has been studied in low density polyethylene (LDPE). Gaps of $115\ \boldsymbol{\mu} \mathbf{m},\ 212\ \boldsymbol{\mu} \mathbf{m}$ and $1037 \boldsymbol{\mu} \mathbf{m}$ have been experimentally investigated. Corona discharges of different types were measured within those air gaps. Accelerated treeing to breakdown was found in the sample with a $115\ \boldsymbol{\mu} \mathbf{m}$ air gap. In this case the streamer coronas were slightly higher in magnitude than the maximum partial discharge (PD) during the tree growth in the no-gap sample. In contrast, lower corona discharge magnitudes were associated with larger air gaps, and electrical trees gradually stopped propagating to the counter electrodes and developed into pine-branch tree structures. This work highlights the differences between electrical trees that effectively grew from a metallic electrode in direct contact with the polymer and a point of corona source.

Electrical tree growth through polymeric insulation is a key degradation process in HV cables. Partial discharges (PDs) are considered to be the energy conversion path for the tree growth. Several methods have been employed to study PDs in electrical tree channels, including the phase resolved PD (PRPD) pattern and pulse sequence analysis (PSA). Our recent work proposed a new method of estimating the PD inception and extinction voltages in non-conductive tree channel based on the PD signals. Here a typical non-conductive tree in polyethylene is considered. Both PRPD and PSA have been employed. It is found that after the PD inception, the PD inception and extinction voltages decrease with the tree growth, and become stable before breakdown. It is also found that the decrease of PD inception and extinction voltages causes an increase in the average number of PDs per cycle, and the shrinking of the dV-dV PSA plot. In turn, the average number of PDs per cycle, and the dV-dV plot can be used to monitor the change of PD inception and extinction voltage during the tree growth.

The movement of space charge packets has been reported in polyethylene and epoxy resin under high electric fields by several groups, and has been characterized by the speed of propagation. A number of explanations of the formation and transport of slow space charge packets has been proposed. However, the mechanisms creating fast space charge packets are not clear. Dissado et al have suggested that the formation of fast charge packets is due to discontinuous electromechanical compression and charge injection at the electrode-insulation interface, and the transport of charge packets is related to corresponding relaxation processes. In that model, charges travel in packets because of group polarization. This paper gives an alternative model considering the reduction of charge carrier activation energy due to charge density triggered polymer chain movement and subsequent chain relaxation times. It is shown that the generation and transport of fast charge pulses are readily simulated by a bipolar charge transport model with three additional parameters: reduced activation energy, a charge density threshold, and a polymer chain relaxation time.

Electrical trees are degraded paths in polymeric insulation and are the mechanism of electrical failure of high voltage insulation systems. Previous studies have confirmed the application of X-ray Computed Tomography (XCT) imaging of electrical trees using phase contrast enhancement. This work evaluates the feasibility of X-ray imaging of electrical trees to measure growth at several stages of growth in the same treeing sample. The impact of x-ray dose on the epoxy resin studied is determined experimentally. An example of multi-stage tree imaging using laboratory micro-XCT and reconstructed renderings is provided. Based on the three-dimensional tree models, characteristics such as the degraded volume and surface area can be quantified. Furthermore, a comparison between two stages is presented to show the growth of the electrical tree in a fixed period of time. Insight into the treeing characteristics is also discussed.

In order to restrict the field distortion in the insulation of high voltage direct current (HVDC) cable under temperature gradient, this paper proposes a nano-filler, the addition of which can restrict the space charge accumulation of low density polyethylene (LDPE) under temperature gradient. To reveal the mechanism of the nannocomposite, the trap depth distribution of the pure LDPE and LDPE nannocomposites under different temperatures were tested based on the Pulsed Electro-Acoustic (PEA) method. The parameters of trap depth distribution were achieved by PEA method under different temperatures(20°C,40°C,60°C and 80°C), and the trap depth distribution under different temperatures were all calibrated to that of 20°C. The results indicated that the trap depths of LDPE decrease with increasing temperature, whereas the trap depths of LDPE nanocomposites changes inobviously with temperature variation. It is considered that, the result contributes to the restriction of space charge accumulation under temperature gradient.

Simultaneous imaging of electrical tree growth and partial discharge measurement have allowed 5 stages of tree growth to be identified. Partial discharge (PD) characteristics have been collected from tree growth in needle-plane epoxy resin samples under 15 kV peak AC voltage, and have been shown to behave differently at different stages of tree growth. Initial slow growth is accompanied by large discharges (about 10 pC). Thereafter, growth of fine channels is associated with smaller PDs (about 0.3 pC). It was observed that the tree propagation is further driven by even smaller PDs below the equipment sensitivity. However, when the tree approaches the counter electrode, the PD magnitude increases again and a more substantial tree grows with a low propagation rate. Reverse tree channels then occur from the planar electrode towards the needle, together with higher magnitude PDs characterized by thick and dark branches before breakdown.

The aging parameter n can reflect the aging property of material to some degree and determine the test voltage of the type test and prequalification test of HVDC cables. However, the aging parameter has rarely been studied systematically, especially under temperature gradient. In this paper, step-stress test was carried out to estimate the ageing parameter n of two materials, XLPE and its nanocomposites under temperature gradient. A new method based on the cumulative damage has been used to obtain the ageing parameters, which can fully take the consideration of the voltage procedure and avoid the errors brought by the approximation. Aging parameters of XLPE and its nanocomposite under temperature gradient ΔT =0, 20, 40°C are 13, 15.4, 17.7 and 8.6, 9.4, 10, respectively. The aging parameter n reduces when nano-fillers are introduced into XLPE. As XLPE nonocomposites can depress space charge availably, the larger aging parameter n of XLPE may be explained in terms of space charge effect and thus more severe field distortion at higher DC voltage. It is also suggested that the electrostatic force induced by space charge can contribute to material aging.

Space charge will accumulate within polyethylene when stressed under DC high voltage, and a temperature gradient will form throughout the insulation when a HVDC cable is in service. Previous research has revealed that temperature gradient can enhance hetero charge accumulation at the low temperature end, but the high temperature only helps a small quantity of homo charge accumulate, and the mechanism was explained for hetero charge formation due to ionization of impurities within the insulation and blocked extraction of injected carriers. However there is no direct evidence to distinguish whether the hetero charge is injected carriers or the ions. This paper intends to investigate and explain the influence of temperature gradient onto hetero charge formation. It was concluded that temperature gradient across the sample thickness could affect the space charge through its influence to the mobility of carriers, and the temperature difference between two electrodes could also influence the entire space charge distribution through its influence to the charge injection and extraction. The space charge distribution in cross-linked polyethylene (XLPE) samples with different thickness under different temperature differences and temperature gradient was investigated. The results show that both the temperature difference between the electrodes and the temperature gradient across the thickness can enhance the hetero charge accumulation.

have not yet been clarified. The experimental results for PE under a temperature gradient show clear evidence of heterocharge next to the electrodes. This feature has been simulated previously using a bipolar injection model that includes blocked extraction. A direct comparison with PEA experiments however is difficult because the finite spatial resolution of the measurement causes the bulk space charge to overlap with that of the electrodes leaving the experimental values as a mixture of the two quantities that are generated separately in the simulation. Here we allow for the finite spatial resolution of the experimental technique by broadening the simulated charge density values using a Gaussian function. This allows us to compare the simulation and the measurements more effectively. The space charge distribution in PE is simulated by including a temperature dependent charge injection and charge transportation. Under these conditions it is found that the simulation results give a detailed match to the experimental results. They also show that both the temperature dependent charge injection and temperature dependent charge transportation are of importance in determining the space charge distribution in PE under a temperature gradient.

Polyethylene (PE) and oil impregnated paper are widely used as insulation materials in high voltage power equipment however, at high voltages the space charge inside the insulating material can distort the electric field distribution and thus greatly influence aging and degradation. Moreover, there is usually a temperature gradient across the insulation when the equipment is in service, and this can also influence the space charge accumulation. In order to study the effect of temperature gradient systematically a PEA system suitable for measurement under a temperature gradient has been developed and appropriate corrections for the measurement error introduced by the temperature gradient implemented. Separate determinations of the space charge distributions of PE and oil-impregnated paper at different temperatures and voltages have been made. The results for both materials showed some similar characteristics. Obvious heterocharge accumulates at the low temperature side and the higher the temperature difference between the two electrodes or the higher the applied voltage, the more the heterocharge accumulates. As a result the electric field is greatly distorted and the maximum field always appears at the low temperature side. The effects of temperature gradient on the space charge distribution are discussed in detail, and the differences between the two materials are identified. The conductivity of both materials has also been investigated, and the space charge induced by the conductivity gradient produced by a temperature gradient in both materials has also been evaluated. The results show that in oil-impregnated paper the space charge induced by a conductivity gradient plays a great part in space charge formation, while in LDPE the conductivity gradient contributes much less to the space charge. These differences are discussed.

In recent years, HVDC cables have been greatly improved, 500kV XLPE DC cable has been developed, and many XLPE DC cables of lower applied voltage have been used in HVDC systems. However, there still is not a unified standard for testing high voltage DC extruded cable systems, only a recommendation. Furthermore the recommendation only gives a lower limit of ageing parameter (n=10 in the inverse power law tL·Vn= constant), which directly determines the voltage used in the test. A more precise value of ageing parameter is needed in order to improve the testing method. The most common method of estimating the ageing parameter uses constant-stress tests at different voltages (fields) and is based on the inverse power model, but this method takes a lot of time and cannot give usable data when the applied voltage is low. An alternative method is to use step-stress tests based on the cumulative life model. This approach can give usable data quickly, but parameter evaluation is much harder than for the constant-stress tests and the approximations necessary for the data solution unavoidably brings errors. In this paper, a new method of estimating the ageing parameter of XLPE is proposed. Step-stress tests are also used but a new parameter evaluation method is presented, which avoids complicated approximations in the data solution. This new method can greatly reduce the errors and easily give the ageing parameter. A series of step-stress tests on 0.3 mm thick XLPE samples have been carried out to illustrate the method. The values of the ageing parameter, n, obtained from different methods are compared, and it is found that the new method gives a greater consistency between different test protocols, indicating a more accurate estimation of its value. However, a comparison between the new method and the constant stress accelerated life tests is still needed to confirm the accuracy of the new method.

The effect of the introduction of nano-fillers into polyethylene on space charge accumulation is usually studied in electric fields reaching 100kV/mm using thin 100 μm samples, which is much thinner than the insulation in actual HVDC cables. Our previous work showed that space charge distributions in LDPE and its nanocomposites have different thickness dependences at isothermal temperature (20°C and 60°C). This was explained using the bipolar model and trapping potential model. However, in actual cables the temperature is not isothermal, but has a gradient, so we have investigated the space charge distribution in unfilled XLPE and its SiO2 nanocomposites under different temperature gradients at 50kV/mm. It is found that the unfilled XLPE space charge distributions definitely have thickness dependence under a temperature gradient. The XLPE/SiO2 nanocomposites also show strong thickness dependence: the thin samples accumulate large heterocharge densities, but the thicker the sample is, the less the heterocharge accumulates and even under a temperature gradient the 0.5mm nanocomposites still suppress most of the heterocharge.

The introduction of nano-fillers into polyethylene can suppress space charge accumulation. The mechanism of this phenomenon hasn't been understood clearly, and has been explained in two different ways, i.e. an interface behavior that reduces charge injection and a bulk property that affects charge migration. In order to investigate which factor determines the space charge behavior in LDPE nano-composites, samples of triple-layer structure are measured. The experimental results show that the nano-fillers can reduce charge injection. However, the reduced charge injection cannot entirely remove the space charge. The bulk property also plays an important role in space charge suppression. Furthermore, a bipolar blocking phenomenon is found in our triple layer sample at high temperature. This can be explained by a shift in the apparent Fermi (charge demarcation) level near the electrode due to the trapping of injected charge, which increases the interface barrier.