Polypropylene Laminated Paper Research Articles

Surface Flashover Characteristics of the Interface Between PPLP and Epoxy According to Tensile Stress Under Cryogenic Environments

Stop joint boxes (SJB) serve to maintain constant cooling efficiencies by separating the cooling sections on superconducting DC cable systems. SJBs are composed of polypropylene laminated paper (PPLP) and an epoxy spacer. PPLP is wound against a superconductor and epoxy spacer to improve surface flashover performance. The interface between the insulating paper and epoxy is vulnerable to surface flashover due to the extreme distortion of electric fields at the interface. In addition, PPLP is subject to constant tensile stress that affects the dielectric characteristics of insulating paper. The breakdown characteristics of insulating paper with diverse tensile stress in cryogenic environments has been previously reported. However, research with regard to the surface flashover characteristics of composite insulation structures under cryogenic environments depending on tensile stress has not yet been reported. Therefore, in this paper the DC surface flashover characteristics of the interface between PPLP and epoxy was investigated in liquid nitrogen in accordance with the tensile stress of PPLP. TM systems which are able to convert rotational motion into linear motion by torque were devised for changing and subjecting the tensile stress to PPLP. From the experiments, it was confirmed that the DC surface flashover strength of PPLP decreased over 40% due to tensile stress. In the case of the interface, tensile stress had no effect on the DC surface flashover strength. Consequently, tensile stress was an important design factor in a single insulation structure with PPLP but was not essentially considered in the composite insulation structure with PPLP and epoxy.

Influence of Water Content on the Dielectric Characteristics of LN2-Impregnated PPLP for HTS Power Cables

High-Temperature Superconducting (HTS) power cables operate at low, medium, and high voltage levels for electrical power transmission. Polypropylene Laminated Paper (PPLP) is wrapped helically around the HTS layer and it is impregnated with Liquid Nitrogen (LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) to form a composite dielectric layer. During the installation and maintenance process of HTS cables, any leak of moisture in the cryostat will lead to immediate ice formation. Once the cable warms up, the ice will melt into water, which will be absorbed by the PPLP Kraft paper layer and is similar to the PPLP paper soaked with water. When the moist PPLP is cooled with LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> it will form ice and forms a composite material during operation. The objective of this paper is to investigate the influence of water content on the breakdown strength, relative permittivity (ϵ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> ) and dielectric losses of LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> impregnated PPLP samples for different number of layers. Considering the worst-case scenario of moist samples, wet PPLP samples were tested for breakdown strength and dielectric properties at Room Temperature (RT) and LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> temperature and was compared with dry PPLP samples. Two different experimental setups were developed to measure the breakdown strength and dielectric properties respectively for stacked PPLP layers. The wet and dry samples were tested for the AC V-I characteristics, the effect of PPLP stack thickness on dielectric strength, statistical analysis using two parameter Weibull distribution and computation of relative permittivity and loss factor from capacitance measurement.

Effect of Moisture Content on the Performance of PPLP as a Dielectric for HTS Power Cable

For the operation of High Temperature Superconducting (HTS) power cables at high voltage levels, the electric field distribution depends on the relative permittivity of dielectric materials. In HTS cables, PolyPropylene Laminated Paper (PPLP) is used as a cold dielectric material and is wrapped helically along the length of the conductor thereby electrically insulating it. During the installation and maintenance process of HTS cables, termination units and joint boxes, the PPLP is often exposed to moisture. The presence of moisture affects the dielectric breakdown strength and dielectric losses. The effect of moisture content in PPLP at ambient and liquid nitrogen temperature has been studied to determine the breakdown strength, relative permittivity (εr) and dissipation factor (tan δ). Effect of moisture content on the performance of PPLP via the measurement of dielectric breakdown strength and relative permittivity for various temperatures and moisture content are discussed in this paper.

압력 변화에 따른 PPLP 미니케이블 모델의 전기적 특성 변화에 관한 연구

PPLP(polypropylene laminated paper) is used as an insulation material for an OF(oil filled) cable system because of its good dielectric characteristics in oil temperature. In an HTS(high temperature superconductor) cable system, PPLP used in an OF cable system is also used as an insulation material because of its excellent electrical breakdown performance in LN₂(liquid nitrogen). Studies on the dielectic characteristics of PPLP in oil have been actively conducted, but in contrast, those of PPLP in LN₂ have been relatively less conducted. Moreover, studies on V-t characteristics including dielectric characteristics according to pressure of mini cables wound by PPLP have been insignificant. In this study, a mini-cable is fabricated using PPLP with different PP ratios, and V-t characteristics are analyzed with an electric field intensity at breakdown according to pressure to derive an index n. It is found that dielectric characteristics of a mini cable model in LN₂ considering long term operation are affected by PPLP type. Also, it is revealed that the index n of a mini cable model decreases as pressure increases.

PP 비율에 따른 PPLP의 절연파괴 특성에 관한 연구

Polypropylene laminated paper(PPLP) has a structure in which polypropylene(PP) film is inserted between two sheets of Kraft paper widely used as an insulation material for conventional power cables as well as superconducting cables. In order to develop a high-temperature superconducting(HTS) cable system, it is important to establish insulation technology at cryogenic temperatures as well as enhancing current capacity and lowering loss of conductor systems. To develop HTS cables, a composite insulation method using liquid nitrogen(LN₂) and solid insulation materials is widely adopted. In particular, dielectric characteristics of materials in cryogenic condition should be collected and analyzed to improve the reliability of HTS cables. In this paper, two kinds of PPLP with different ratio of Kraft paper and PP are used to conduct electrical breakdown experiments under AC and Lightning Impulse(Imp.) voltage in LN₂ according to pressure and temperature.

Effect of Electrical Aging On PPLP Insulation Characteristics in LN2

The high-temperature superconductivity (HTS) DC cable has brought huge economic benefits and has been widely used in the system with its low line loss. The Polypropylene laminated paper (PPLP) tape is wrapped around the core due to its excellent insulation properties, which produces butt gaps. Since cables in HTS DC transmission system often withstand DC and AC composite voltage, these gaps make partial discharge (PD) easier to occur. Meanwhile, degradation occurs in PPLP under long-term electrical stress, which could threaten the safe operation of the power system. Therefore, this article conducted an electrical aging test in liquid nitrogen (LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) to simulate the actual operating environment of PPLP, and also measured the conductivity, relative permittivity, PD, and breakdown strength of PPLP with different aging times in LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . The results show that the conductivity basically increases with the increase of aging time and relative permittivity has a significant increase at the end of the aging stage. As the aging time increases, the PD activity of PPLP becomes more intense and the DC breakdown strength shows a downward trend. The reason for the deterioration of PPLP insulation performance after aging was analyzed. Besides, the effect of different alternating components on PD was studied in the case of constant effective voltage. When the alternating component decreases, the number of PD is significantly reduced.

Breakdown Characteristics of Composite Insulation for High Temperature Superconducting Cable

Polypropylene laminated paper (PPLP) and Kraft paper have been studied as multi-layer insulation for high temperature superconducting cable. The experiment was examined with AC and impulse voltages. Short-term and long-term breakdown of insulation samples were investigated. It was found from the experimental results that the breakdown voltage becomes smaller in the order of PPLP, PPLP/Kraft and Kraft, but the lifetime index increases with the presence of Kraft layer in the multi-layer insulation. In addition, the position of PPLP layers has a significant effect on the breakdown voltage of PPLP/Kraft samples under positive impulse voltage. Finally, breakdown mechanism of PPLP/Kraft composite insulation was suggested. Partial discharges in butt-gaps and in the porous structure of insulation layers combined with electric discharges on the surface of these layers are considered to be main factors causing the puncture of multi-layer insulation.

초전도케이블용 Stop Joint Box 설계를 위한 연면방전 특성에 관한 연구

A study on the development of a superconducting cable has been conducted actively due to its ‘0’ resistance characteristics. A superconducting cable consists of three parts: termination, core, and joint box. A joint box could be classified into normal joint box (NJB) and stop joint box (SJB) according to its structure. In this paper, a study on the development of the design technology of SJB where a bulkhead is installed to maintain a constant pressure and temperature of a cryogenic system is conducted. In an SJB system, a bulkhead is installed at a superconducting core system and impregnated with epoxy resin, and then wound with polypropylene laminated paper (PPLP) in order to insulate the superconducting core system. The creepage discharge characteristics along the surface of a solid insulation material in liquid nitrogen (LN₂) are investigated through dielectric experiments and the criterion to perform electrical insulation design of an SJB system for a distribution superconducting cable is proposed.

Dielectric Characteristics of Liquid Nitrogen/Synthetic Paper Composite Insulation System for Extra High Voltage HTS Cable

For extra-high-voltage high-temperature superconducting (HTS) cables, not only high electrical insulation performance but also low dielectric loss is required. Therefore, we focus on a porous synthetic paper with lower dielectric loss, e.g., Tyvek polyethylene nonwoven fabric, than that of polypropylene laminated paper (PPLP), which has been used for a conventional HTS cable insulation system. In this paper, we compare partial discharge inception characteristics for different liquid nitrogen/synthetic paper composite insulation systems. The partial discharge inception electric field strength in the composite insulation system depends on the electric field strength in the porous area of synthetic paper.

Interfacial space charge characteristic of PPLP insulation for HVDC cables

High-voltage DC (HVDC) cables show more efficiency than high-voltage AC cables over the long distance power transmission and have attracted significant attention recently due to rapid developments in off-shore wind farm. Polypropylene laminated paper (PPLP) insulation based HVDC cables have provided a reliable operation for many years and are the highest voltage level in operation at the moment. In this study, the authors intend to understand the electrical performance of PPLP insulation system by examining space charge in detail, especially at the interfaces. It has been found that the interface between polypropylene and Kraft paper acts as trap for charge carriers, resulting in an optimal electric field distribution in PPLP insulation system. The optimal electric field distribution enhances overall electrical performance of PPLP insulation, including higher dc breakdown strength and lower electrical conductivity.

Insulation Design for 10 kV Three-Phase Concentric High-Temperature Superconducting Cable

This paper introduces a insulation design for a 10 kV three-phase concentric high temperature superconducting (HTS) cables with cold dielectric mode. Insulation properties of PPLP (Polypropylene laminated paper) and its dielectric strength under AC voltage endurance test and the lightning impulse test in liquid nitrogen have been performed. According to test results and the standard for 10 kV paper insulation cable, the main insulation and the stress cone for the 10 kV three-phase concentric HTS cables have been designed. The voltage endurance test under power frequency and the lightning impulse test have been carried out, indicating that the cable is complied with the insulation requirements.

Insights on the Breakdown Characteristics of PPLP Under Different Test Protocols and Temperatures

The electrical breakdown strength of insulation material is very important for the design of a high-temperature superconducting (HTS) cable. This article investigates the breakdown characteristics of polypropylene laminated paper (PPLP) film samples, which are widely used in an HTS cable. Various test conditions, such as different modes of increasing voltage, voltage types, and ambient temperatures, are used. It is found that the breakdown strength of the PPLP film samples varies greatly with the aforementioned experimental conditions. Based on the existing research, the key reasons for the experimental phenomenon are analyzed. The experimental results provide the reference experimental data for the insulation design of the HTS cable.

Experimental Studies on the Partial Discharge of Insulation for High Tc Superconducting Cable

Partial discharge (PD) is the main factor that can cause failure of the cold dielectric insulation system of superconducting cables. Nine insulation model cables with different insulating materials and wrapping styles are prepared. PD experiments under liquid nitrogen and room temperature are carried out on these model cables. Assuming that the lifetime of superconducting cable is 35 years, 40 thermal cycles from 77 –323 K are carried out on three model cables. Experimental results show that the PD is 100 times lower in liquid nitrogen than at room temperature for both polypropylene-laminated paper (PPLP) and polyimide (PI). Compared with model cables that used PI, the ones that used PPLP show much higher inception electric field. And no decay in partial discharge inception electric filed (PDIE) is found during the thermal cycles. The study will be helpful to the insulation system design for superconducting cable.

THE INFLUENCE OF POLYPROPYLENE LAMINATED PAPER ON THE BREAKDOWN STRENGTH OF INSULATION FOR HIGH TEMPERATURE SUPERCONDUCTING CABLE

Polypropylene Laminated Paper (PPLP) and Kraft paper has been used as ac power insulation for conventional cable as well as high temperature superconducting (HTS) cable because of its prominent insulating characteristics. However, research on the use of PPLP/Kraft insulation for HTS cables are thinly scattered. In this paper, the effect of PPLP on the breakdown strength of PPLP/Kraft multi-layer sample impregnated with liquid nitrogen (LN2) under ac and impulse applied voltage was studied. In addition, the breakdown strength characteristics of PPLP and Kraft samples were also investigated in order to determine breakdown strength characteristics of PPLP/Kraft insulation directly. It was found from the experimental data that the breakdown strength increases as the component ratio of PPLP in the PPLP/Kraft sample increases and is slightly affected by the inserting position of PPLP but in impulse case, the breakdown strength strongly depends on the number of PPLP and the relative position of PPLP.

Novel Diagnostic Method of DC Void Discharge in High Temperature Superconducting Cable Based on Pulse Sequence Analysis

In a high voltage dc (HVDC) high temperature superconducting (HTS) cable composed of polypropylene laminated paper (PPLP), butt-gaps are unavoidable during fabrication, which may lead to formation of voids. When dc voltage higher than partial discharge inception voltage (PDIV) is applied, partial discharge occurs in the void. If void discharge sustains, the electrical ageing of PPLP accelerates, and eventually dielectric breakdown may occur due to the lowering of dielectric strength. Thus, for timely maintenance and repair of HVDC HTS cable, identification technology for void defects is required. However, there is no previous research and technologies for defect identification and insulation diagnosis on HVDC HTS cables. Therefore, in this paper, in order to suggest the insulation diagnosis method for void defects of the HVDC HTS cable based on the pulse sequence analysis (PSA) method, void discharges were measured according to the size of void and magnitude of applied voltage. Also, patterns of the void discharge were obtained using PSA methods and PYTHON program. Experimental results indicate that, although the magnitude, time difference and repetition rate of partial discharge are different depending on the configuration of the void and the applied voltage, but the patterns of void discharge are consistent. Therefore, it is considered that identification and insulation diagnosis for void defects in HVDC HTS cable is possible through the obtained DC void partial discharge patterns.

Insulation Characteristics of PPLP in GHe and Design of 10 kV Bipolar Coaxial HTS DC Cable

Electrical insulation is an essential component of high temperature superconducting (HTS) dc cable systems and polypropylene laminated paper (PPLP) and has been widely used as insulation material for cold dielectric (CD) HTS cables due to its excellent dielectric mechanical properties. In this paper, mini-model cables were fabricated to simulate CD-type HTS cables and the insulation tests were carried out. DC and the lightning impulse breakdown strength of the 1% Weibull distribution of PPLP at 0.3 MPa helium gas (GHe) were investigated. Based on these results, the insulation design of a 10-kV bipolar coaxial HTS dc cable was performed. Furthermore, a model cable was manufactured according to the insulation design and tested for dc and impulse withstand voltage, which verified that the insulation design was qualified.

Improving the Electric Field Distribution in Stress Cone of HTS DC Cable Terminals by Nonlinear Conductive Epoxy/ZnO Composites

Nonuniform electric field distribution is one of the key issues for the superconducting cable terminals. This paper tries to modify the electric field distribution in HTS dc cable terminals by the nonlinear conductive epoxy/ZnO insulation, instead of the insulation with a fixed conductivity. The nonlinear conductivity of epoxy/ZnO composites is measured, and a finite element simulation model of the +100 kV HTS dc cable terminal is established to analyze the electric field distribution at the steady and transient state. The results show that the field-dependent conductivity has a great effect on the dc electric field distribution. If the nonlinear coefficient is excessively large, the electric field in the polypropylene laminated paper (PPLP) will be extremely strong; otherwise, the electric field in the terminal insulation will be extremely strong. The epoxy/ZnO composites with an appropriate nonlinear conductivity can effectively uniform the electric field distribution at the triple-junction and the stress cone surface. When the switching overvoltage impulse superimposed dc voltage is applied, the maximum field is always located in the cable insulation of PPLP, which is because that the electric field distribution in the stress cone is mainly dependent on the capacitance characteristics of the insulation instead of the conductivity. The nonlinear conductive epoxy/ZnO composites could play an important role in promoting the development of stress cone of HTS dc cable terminals.

Insulation Characteristics of Dielectric Material for CD HTS Cable

The electrical insulation performance of dielectric materials is an important parameter in the insulation design of a cold dielectric (CD) high-temperature superconducting (HTS) cable. Polypropylene laminated paper (PPLP) and Kraft paper are suitable for the insulation design of the CD HTS cable. The experimental study on the cryogenic insulation properties of PPLP and Kraft paper at 0.1 MPa liquid nitrogen were carried out, and the dielectric strength of a mini-model cable, which were assembled as the same structure with CD HTS cable in dc breakdown, lightning impulse breakdown, and ac breakdown, were obtained according to the three-parameter Weibull distribution based on grey model GM(1,1). The results have an important practical value for the insulation design of the CD HTS cable.

Dielectric Characteristics of PPLP with various Pressures and Intervals between Butt Gap

Superconducting cables are one of the most promising applications among several high voltage superconducting power apparatuses owing to its low energy loss and high power capacities. Superconducting cables system consists of four parts: the termination, the joint box, the core, and the cooling system. The core is responsible for power transport, and is insulated with polypropylene laminated paper (PPLP) to ensure a high dielectric strength in the presence of liquid nitrogen (LN 2 ). The insulation layer of the core is fabricated by winding PPLP in multiple layers. It is inevitable that butt gap filled with LN 2 is generated in this process. Currently, many studies on the dielectric characteristics of PPLP according to various intervals between butt gap under 0.1 MPa and those with fixed butt gap according to various pressures have been conducted. A study on determining a proper interval between butt gap according to pressure in order to develop reliable superconducting cables under high pressure should be performed because superconducting cables system in power grid is operated under high pressure. This study focuses on the evaluation of the dielectric characteristics of PPLP with interval between butt gap at various pressures. In addition, dielectric experiments on V-t characteristics for a mini-model cable are conducted and experimental results are analyzed by the inverse power model.

Surface Flashover Characteristics of PPLP According to Its Orientation in Cryogenic Environment

Polypropylene laminated paper (PPLP), which is used as the main insulation material of the stop joint box (SJB), has excellent dielectric characteristics in cryogenic environment. Despite excellent dielectric characteristics of PPLP, the interface between liquid nitrogen (LN2) and PPLP of the SJB is very vulnerable to the surface flashover. Therefore, it is necessary to investigate the surface flashover characteristics of dielectric materials including LN2 and PPLP for the insulation design of the SJB. PPLP has two orientations according to the direction of fiber in Kraft paper that are cross direction (CD) and machine direction (MD). The research on the mechanical properties of PPLP has been reported since there are differences in physical properties depending on its orientation. However, the study of the surface flashover characteristics of PPLP in the cryogenic environment according to its orientation has not been reported yet. Thus, the surface flashover characteristics of PPLP according to its orientations should be examined to ensure the reliability of the SJB connecting the High-temperature superconducting (HTS) dc cable. In this paper, the surface flashover characteristics of the PPLP in cryogenic environment in accordance with its orientation was fully investigated. The dc surface flashover tests were performed with different orientations of PPLP and various gap distances. In addition, the surface leakage currents were measured by increasing voltage at fixed gap distance. From the test results, the dc surface flashover voltage of CD was found to be 15-20% higher than that of MD. In addition, the breaking point of CD was three times higher than MD, and beyond the breaking point, the surface leakage current of CD was lower than that of MD. Consequently, the surface flashover characteristics of PPLP are significantly affected by fiber orientations of PPLP.