Effect of Semiconductive Layer on Space Charge Accumulation in XLPE

Long time DC pressure on high voltage cables will lead to the accumulation of space charge in XLPE cables, thus endangering cable insulation. In order to study the effect of the thickness of semiconducting layer on the space charge in XLPE, the space charge in 10kV and 220kV XLPE sample with different thickness of semiconducting layer was measured and compared based on PEA method. Firstly, the samples were pressurized to the specified voltage, then kept this voltage for 30 minutes, then depressurized to 0, and lastly maintained for 90 minutes. The variation of space charge distribution during the pressurized stage was analyzed with the space charge density as the characteristic parameter. The results show that the space charge near the anode and cathode is accumulated by the semi-conductive coating during the period of maintaining pressure; the thicker the semi-conductive layer is, the more obvious the accumulation of space charge is; the longer the time of maintaining pressure, the more space charge accumulates.


Introduction
XLPE cables are widely used in power systems of various voltage levels because of their excellent electrical performance, simple structure, convenient laying and high reliability of power supply. However, the insulation structure of the cable will be damaged due to the bad technology in the process of cable processing and laying or the external damage during operation, and partial discharge will occur when the cable is subjected to voltage [1][2][3]. Long-term partial discharge will lead to insulation aging and deterioration until breakdown, resulting in serious accidents. Partial discharge detection of cables can effectively detect insulation defects of cables and eliminate hidden troubles in time. It is of great significance to ensure the safe operation of cable lines and even the whole power system.
In partial discharge detection, oscillation wave is widely used in cables with 35 kV and below voltage level because of its excellent performance, but it is seldom used in cables with 110 kV and above. One of the reasons is that people worry about the accumulation of space charge caused by DC pressurization process, which endangers cable insulation. In order to study the effect of semiconducting layer thickness on space charge in XLPE, it is necessary to prepare samples and measure space charge density in XLPE by electroacoustic pulse method.
Electroacoustic pulse (PEA) method is an effective method for measuring space charge of solid insulating materials. In recent years, scholars at home and abroad have used this method to study the space charge characteristics of XLPE insulating materials. Based on PEA method, there are a wide range of studies, mainly involving: 1)the influence of field strength, frequency, temperature, cross-linking by-products and other factors on space charge accumulation of XLPE materials; 2)the relationship between space charge distribution and insulation aging. Based on the key issues in the above research fields, many scholars at home and abroad have carried out a lot of experiments and conducted indepth research. Liu Jun and others in China have studied the space charge distribution characteristics of cables at different core temperatures [4][5], Zhong Qiongxia and others have studied the relationship between space charge characteristics and cross-linking byproducts under DC high voltage [6], Chen Zhengzheng and others have studied the relationship between space charge characteristics and electrical aging degree [7][8]. The phase characteristics of space charge distribution under AC inhomogeneous electric field were studied by Muratak et al. abroad [9]. Roy S L et al. studied the effect of polarizable impurities on space charge electric field distribution in cable polymers [10]. Tanaka T studied the space charge properties of XLPE/SiO2 nanocomposites [11]. YingLi et al. studied water content. Space charge distribution of branch XLPE cable under AC electric field [12]. Therefore, 10 kV and 220 kV XLPE samples with different thickness of semiconductive coatings were prepared in this paper. The change of space charge accumulation in XLPE during 30 minutes of maintaining pressure was studied by rapidly increasing voltage to 30 kV/mm, maintaining pressure for 30 minutes, rapidly lowering pressure to 0 and maintaining 90 minutes. The effects of different thickness of semi-conductive layer on space charge accumulation of 10 kV and 220 kV XLPE samples were investigated.

Sample preparation
The selected samples are 10 kV and 220 kV XLPE with sizes of 60 mm x 60 mm. he semiconductive layer on XLPE is prepared by spraying two and three layers of semi-conductive paint. The XLPE samples before and after spraying are shown in Fig. 1. XLPE specimens sprayed with semi-conductive paint were observed and measured under a microscope. The thickness of two voltage grade specimens was obtained as shown in Table 1. Figure 2 shows the measurement process of the sample.

Test System
The space charge measurement system based on electro-acoustic pulse (PEA) method is shown in Fig. 3. The DC voltage output is controlled by using LabVIEW software program, so that the linear boost of XLPE sample can be achieved. The structure of the test device is shown in Fig. 4

Testing Principle
The basic principle of space charge measurement by electroacoustic pulse method is to apply a high voltage pulse with very narrow pulse width on the sample. Under the action of this pulse, the space charge in the sample will generate corresponding pressure wave pulse. The pressure profile of the pressure wave pulse is related to the volume density distribution of space charge in the sample and is received by piezoelectric sensor. By measuring the pressure wave pulse, the distribution of space charge in the corresponding sample can be obtained.

Testing method
The purpose of this experiment is to study the effect of the thickness of semiconducting layer on space charge accumulation in XLPE. For this reason, the following test procedures were carried out for 10 and 220 kV samples with different thickness.
The XLPE samples were rapidly pressurized to 30 kV/mm by spraying semi-conductive paint on uncoated slices and 2 or 3 layers. After waiting for 10 seconds, the field strength was maintained for 30 minutes at this time point, and then the pressure was rapidly lowered to 0 kV and continued for 90 minutes. Then, new samples were selected to repeat the above process.
The curves of space charge density varying with sample thickness at 10s, 300s, 1200s and 1800s were selected for analysis and comparison.

Space Charge Distribution of 10kV XLPE under Different Semiconductive Layer Thicknesses
The test results are shown in Fig. 5-7. The peak value on the left side is the anode interface, the peak value on the right side is the cathode interface, the vertical axis is the space charge density, and the transverse axis is the sample thickness.
From the results, it can be concluded that during the process of high voltage direct current compression, there is a positive charge accumulation near the positive peak (positive electrode), and the space charge inside the sample is almost zero. Compared with uncoated slices, the negative charge accumulation phenomenon of coated slices is more obvious near the negative peak. Near the positive and negative electrodes, the polarity of charge accumulation is consistent with that of the positive and negative electrodes.

Space Charge Distribution of 220kV XLPE under Different Semiconductive Layer Thicknesses
The test results are shown in Fig. 8-10, and the parameters in the figure are the same.
From the test results, it can be concluded that the 220 kV sample is similar to the 10 kV sample in that it accumulates negative charges near the negative peak, and the sample with coating accumulates more and more in a wider range; all the heteropolar charges near the positive pole of the 220 kV sample are accumulated, and the charges near the positive pole of the sample with coating are more obvious; The positive charge accumulated and increased with the increase of coating thickness. The thicker the coating, the more the peak charge accumulated in 30 minutes.
In summary, for 10 and 220 kV samples, the charge accumulation at the positive and negative peaks (anode and cathode interface) of semi-conductive coatings is significantly higher than that without coatings, and the thicker the semi-conductive layer is, the more the charge accumulation is. However, for the accumulation of positive charges near the interface of 10kV anode and negative charges near 220kV anode, further research and demonstration are needed.

Conclusions
Through the research of the thesis, we have obtained the following main conclusions. During the maintaining pressure stage, the semi-conductive coating can accumulate space charge near the anode and cathode. The thicker the semiconducting layer is, the more obvious the accumulation of space charge is. The longer the pressure is maintained, the more space charge accumulates.