Accumulation of space charge within the dielectric material who is used in high-voltage direct –current (HVDC), is estimated from measured space charge distribution as a function of temperature using the pulsed electro acoustic (PEA) technique. A sandwich structure constituted by two nonidentical dielectric films Fluorinated Ethylene Propylene (FEP) and Low Density Polyethylene (LDPE) was subjected to an electric stress level of +14.3 kV/mm at three temperatures: 20 °C, 40 °C, and 60 °C. Space charge and electric field distributions were investigated for combinations of LDPE/FEP flat specimens. The time dependence of the space charge distribution was subsequently recorded at different temperatures under field (polarization) and short circuit conditions (depolarization). Experimental results demonstrated that temperature plays a significant role in space charge dynamics at the dielectric interface. The increase in temperature may result in an enhancement of charge injection from the electrodes, and increase the activation energy of charge, which in turn enhances charge mobility and conductivity. A great quantity of interfacial charge is introduced at high temperatures. The increase of the temperature increases charge injection, charge mobility, electrical conductivity, and filling of the trap, and the formation of the shallow trap.
Research Article
Thermal Influence on Space Charge Accumulation in Multilayer Dielectric Insulation for HVDC Systems
https://doi.org/10.21203/rs.3.rs-5214287/v1
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Space charge
dielectric interface
pulsed electro-acoustic technique
temperature influence.
THERMOPLASTIC materials are increasingly used in Electrical Engineering as high-voltage insulation. They are highly insulating media and exhibit excellent physical and chemical properties, with the possibility to be processed to comply with complex systems. Polymer can be used as a simple material or as combination of materials. It is well known that in high-voltage direct –current (HVDC), a combination of insulating materials is often used in cable accessories (i.e. joints and termination), which are considered to be the weakest part of a cable system, because of the presence of a dielectric interface between the cable insulation and that of the accessory [1, 2].
When an electric field is applied across a multi dielectric, the material becomes polarized because atomic and molecular charges are displaced from their equilibrium position. There are several Different polarization mecanisms can occur to create the space charges: electronic polarization, ionic polarization, Orientation polarization, ionic impurities from the manufacturing process, for derived from an electron injection phenomenon or holes, or resulting from dissociation of initially neutral species, surface discharge ,or, finally Interface polarization or Maxwell -Wagner-Sillars.
However, these space charges strongly affect degradation phenomena. Therefore, there is a growing interest in understanding space charge phenomena in insulating materials, not only in the bulk of a dielectric or at the physical interfaces between two identical materials [3–15], but also at the interface between two different media [17]. However, most of such work was conducted at room temperature, while space charge dynamics in insulation under DC electric stress at different temperatures has only received limited attention [18–25]. This study concerns polymers commonly used in the high-voltage cable manufacturing industry. Looking at research of the dielectric properties of polymers that cannot be carried out jointly on all existing polymers, field research was restricted to polymers commonly used in the high-voltage cable manufacturing industry. This is important from the point of view of implementation work on common materials, allowing first to reduce search costs purchase and ease of obtaining, and secondly, to dispose of a sum of most significant information about these polymers.
In our studies, we focus our attention on the interface between the two dielectric in which the contribution of migrating charge carriers dominates the interfacial polarization and when a temperature gradient is present across the insulation. To that purpose, space charge measurements were performed at different temperature conditions, and at a negative polarity of the test voltage.
Specimen characteristics
Space charge measurements were performed on two different types of flat specimens, Low Density Polyethylene (LDPE) and Fluorinated Ethylene Propylene (FEP).
Work was focused on LDPE and ethylene propylene copolymer Fluorinated FEP, based on the fact that they are widely used in the high-voltage cable manufacturing industry.
The LDPE and FEP were supplied by Good fellow SARL (www.goodfellow.com). Samples were purchased in the form of plates of dimensions 300 x 300 mm, thickness between 0.1–0. 25 mm. They are delivered in a plastic protection.
Before each experiment concerning the measurement of space charge, the sample is cleaned with ethanol and then dried and provided as such for the experimental tests.
Combinations of flat specimens are obtained by placing two different films onto each other, unless stated otherwise. No lubricants or specific chemical treatments were applied at the interface between the two plates. The nature of the interface was physical, i,e. the contact was provided by vertical compressive force of FEP flat on the LDPE. Figure 1.
The thickness of LDPE was typically ≈ 250 µm for a simple layer, and the thickness of FEP was 100 µm. The total insulation thickness was 0.35 mm. In Table 1, some electrical properties of the materials are shown.
Table 1: Electrical properties of the materials at T=25° C and E= 14.3 kV/mm
Experimental set-up
In this study, we used a conventional system of a pulsed electro-acoustic (PEA) set-up Fig. 2, where the sample is sandwiched between a thick aluminum electrode in contact with the piezoelectric sensor and semi-conduction (SC) electrode (i.e. a carbon black-load polyethylene film) to which is applied the voltage pulse of the PEA and the DC voltage for polarize the sandwich under test. Measurements were performed at different temperatures from T = 20° C to the 60°C limit for the PEA system in a temperature-controlled oven. In all tests, the measurements were started only after the temperature distribution was attained.
Again, three average temperature values were chosen: low, medium and high, to study the influence of climate change on the behaviour of spaces charges in the dielecteric materials.
Measurement protocol
The influence of temperature on charge build-up has been investigated on sandwich structures constituted by the association of two nonidentical peels.
In this part of the experiments, the sandwich was poled for 1 h at a field of + 14.3 kV/mm, and at different temperatures: 20°C, 40°C and 60°C. Charge profiles were recorded versus time in polarisation (volt on) and in depolarisation (volt off, the electrodes of the PEA system being grounded). The oven is controlled by a temperature controller. All samples were held at the set temperature for 1 h before any space charge measurements were taken. This is necessary to ensure that the samples have reached thermal equilibrium, as temperature affects the electrode of the PEA system.
Measurement at T = 20° C
Figure 3a shows that no significant charge accumulation can be detected in the layer FEP during the polarization time. There is only a trace of negative charge in the layer LDPE next to the cathode electrode.
The presence of charge within the insulation leads to a significant distortion of the electric field distribution: Fig. 3b.
The charge formation in the bulk can be best illustrated when the applied voltage is removed (see discharging: Fig. 3c): a small amount of negative charge can be observed in the region adjacent to the electrode Ex cathode. In the layer FEP next to the electrode Ex anode, there are very low densities of hetero-charge detected in the bulk, within the limit of the threshold of detection of the equipment.
Measurement at T = 40° C
When the test temperature was increased to 40° C the charge dynamic is more obvious than in the sample of the test at T = 20°C. As we can see, the temperature has a significant effect on the space charge behaviour.
The presence of positive and negative charges in the sample (Fig. 4a) advocates for the injection product by both electrodes. As expected, the amount of the injected charge increases with the duration of the applied voltage. At the end of the applied voltage, there is a clear indication of positive charge accumulation in the bulk FEP followed by its maximum adjacent to the anode. In the layer LDPE next to the cathode, there is a large amount of negative charge accumulated in the bulk, with its maximum close to the dielectric interface FEP/LDPE. It is clear that the amount of negative charge trapped in the layer LDPE is higher then the amount of positive charge trapped in the layer FEP. We note that the decay of negative charge in the layer LDPE is faster than the decay of positive charge in the layer FEP (Fig. 4c).
Measurement at T = 60° C: The experimental temperature was then increased to 60°C; the charge distributions and electric field are illustrated in Fig. 5. Similar to the situation at 40° C, there is a small amount of positive charge injection at the Al/ FEP interface, which is somehow surprising (as the Al anode is strongly injecting: Fig. 5a).
This is clear indication of the positive charges that accumulate in the vicinity of the anode, while the negative charges appear in the layer LDPE adjacent to the cathode. After 5 minutes of DC stressing, negative charges begin to accumulate at the FEP /LDPE interface near to the cathode.
Along with charge injection, the electrode peak moves slightly towards the centre of the sample, especially for the anode. Note that the displacement of the both mirror charges is due to the change of pressure during the tests, thus the thermal dilatation of material (Fig. 5a).
The remaining charge measured is shown in Fig. 3c after the applied voltage is removed. It is clear that a significant amount of negative charge at the dielectric interface decreases faster after about 1 h. The increasing of the charges accumulated at the dielectric interface may be interpreted as indicating the formation of shallow traps, as a result of the thermal dilatation. On the other hand, the higher temperature may cause most of these charges to gain more energy to liberate the traps.
As a result, charge decreases with time, negative charge decays very rapidly in the layer LDPE, while positive charge decays very slowly in the layer FEP.
In all tests the sample is composed of two different materials, and there is a discontinuity in the distribution of conductivity and permittivity at the interface between the two materials. In practice, the variation of conductivity at the dielectric interface favours locations for space charge accumulation.
to interpret the origin , the sign, and the value of the interfacial charge, we take into consideration several factors such as: the conductivity of two dielectric materials LDPE and FEP, the thickness of two sample, the nature of the electrode materials which come into contact with the dielectric, the value of the applied field, on one of these two dielectric materials, the results show that when the temperature increases, the negative interfacial charge increases, which in accordance with the work of F. Rogti [23-25]. The negative charge increases at the interface; this can be means by several assumptions:
· The LDPE conductivity is greater than the FEP conductivity.
· The thickness of the LDPE is less than the thickness of FEP.
· These charges meet the interface FEP / LDPE, and found it as a barrier, which blocked these charges.
· the electrode SC is injected electrons.
· The electric field in the LDPE layer is greater than in the FEP layer in the case of tests 40°C and 60°C.
· In addition the nature of the FEP, this influences the injection of Al anode electrode.
We deduct the height of barrier AL / FEP t is greater than the height of barrier LDP / SC.
The quantity of charge injected to the dielectric interface, depends on the metal work function.
Which means that the negative charge, request a work function, lower, to extract from the cathode electrode to the SC LDPE dielectric.
By comparing the amount of the positive charge which is lower than the amount of negative charge.
This meant that, the injection of positive charge, from the electrode Al anode to FEP, request higher output work.
Injection of charges depends on the internal and external factors:
Internal factors: the barrier height is depends a metal Fermi level, which is depend metal output of work.
External: the applied field and the operating temperature which is indirectly bound by the conductivity and by a result of the mobility of charges.
It is clear that the higher the temperature increases, the greater the quantity of charge accumulated at the FEP /LDPE dielectric interface. The latter result is contrary to some works that indicate that when the conductivity of two layers is different, a great quantity of interfacial charge is detected at lower temperatures. However, our results are in agreement with recent results elsewhere, such as those of Bodega [16].
The existence of positive and negative charge distributed in the two dielectric layers shows clearly that the injection occurs at both electrodes.
The charges injected from the electrodes migrate towards the opposite electrode. However, this is unlikely in the present case, as the time for the injected charge to travel across the sample could be just a few seconds (this is confirmed by a large amount of space charge being accumulated in the bulk of the sample at the beginning of the applied voltage).
Indeed, when the electrons move under the effect of the electric field, they are blocked on the walls of the LDPE. This blocking creates an accumulation of negative charge at the interface. This charge cannot cross the FEP interface easily, because of the discontinuity of conductivity (the conductivity of the FEP is much lower than that of the LDPE).
By comparing the results in all tests, it is evident that the amount of negative charge increased as the testing temperature was increased from 20°C to 60°C. This confirms that charge injection is enhanced by high temperatures. It is probably under the effect of thermal agitation that the electrons are extracted more easily from the cathode, because the injection barrier is reduced. Moreover, under the effect of temperature, the chains slip the one on the other; these movements facilitate the displacement of charge in the bulk.
In order to estimate the dynamics of the interfacial charges according to time in the cases of depolarization for various values of temperature, a quantification of space charge at the FEP/LDPE dielectric interface has been carried out, by integrating the value of the measured charge
The decay of space charge is related to the kind of trap and the energy gained by the charge carriers that is kept; in other words, the time of the decay depends on the transit distance, the size of the pores in particular, and the energy gained by these charges.
In all the tests, the negative charge decreases with time, and its decay is faster when the temperature is higher.
In the case of the test at 20 °C, a small amount of negative charge is detected at the dielectric interface, which decays very slowly.
In the case of the test at 60 °C, the decay of negative charge at the FEP/LDPE dielectric interface is much faster than in the tests at 40 °C and 20 °C.
It is believed that the increase in temperature increases the detrapping of charge carriers and their transport in the dielectric, and consequently reduces the decay time of these charges Figure 6.
On the one hand, the increase in temperature from T = 20 °C to T = 60 °C produces thermal dilatation, inducing changes (i.e. increases) in the sample thickness, changing the position of the polymeric chain (but otherwise keeping the physical distortion of the LDPE and FEP dielectrics), and widening the size of the pore; then the positive charge can surmount this pore more easily, and consequently the trap becomes shallow. On the other hand, the decrease in temperature to T = 20 °C cools the dielectric. This base temperature can decrease the thickness of the sample, reducing the motion of the polymeric chain.
The experiment has been carried out to understand the effect of temperature on the generation, distribution, injection and mobility of charge at the dielectric interface LDPE/FEP.
The results allow us to draw the following conclusions:
-When two materials have different conductivities, an accumulation of charge at the dielectric interface LDPE / FEP is required. Generally, a great quantity of interfacial charge is introduced at high temperatures. The increase in the temperature increases the conductivity of the FEP and LDPE dielectrics, and consequently the mobility of the charge carriers increases.
The injection of the interface anode / FEP is much less important than the cathode / LDPE injection. The negative charge in the LDPE is quickly discharged, while the positive charge in the FEP is not discharged.
At high temperature the electric behavior of FEP is strongly influenced by its conductive properties.
Both the conductivites of LDPE and FEP samples strongly depend on applied field and temperature.
The accumulation of charge at the interface increases the field slightly in the material of higher conductivity, but greatly increases the field in that of lower conductivity.
Under the thermal agitation effect, electrons are extracted from metal more easily. This argument is confirmed by the amounts of positive and negative charges.
Under the effect of temperature, the chains slide over each other, this facilitates displacement charges in the volume of the dielectric material. High temperatures facilitate the movement of charge carriers. The interfacial charge at the dielectric/dielectric interface increases with increasing temperature. At low temperatures, the movements of the charges are reduced. This argument is confirmed by the interfacial charge, which does not discharge quickly.
The high temperatures have a significant effect on the formation and the filling of the shallow traps at the dielectric interfaces. A significant quantity of interfacial charge is introduced at high temperatures. At higher temperatures, there is faster charge accumulation at the interface. The increase of temperature decreases the conduction barrier at the electrodes-dielectric interface, which leads to an increase in charge injection from the electrodes, and strong activation for the injection. High temperature plays two significant roles: when the material is subjected to a high voltage, the temperature favours the trapping of space charge in the dielectric material. When the material powers down, the temperature evacuates the traps, and space charges are freed from the dielectric material.
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The authors declare no competing interests.