We discuss the charge developments in polymer specimens under a DC high electric field utilizing a direct current integrated charge method (Q-t method). It has been suggested but not clearly discussed that the current depended on time, even at long times, showed a power-law time dependence, proportional to t beta, and did not reach a steady state current corresponding to the conducting current. Therefore, we analyzed the data phenomenologically with the aid of an analogy between viscoelasticity and dielectric properties. We examined the effect of temperature and the magnitude of the electric field and found that the time-temperature and time-electric field superposition principles work well for low-density polyethylene (LDPE) and polystyrene (PS). We also discuss the physical meaning of the shift factors. The horizontal shift factors are re-interpreted as the strength of anomalous conduction at a fixed time. The electric field dependence of shift factors can be explained by the charge injection. The microscopic origin of anomalous conduction is discussed in the frame of fractional friction.

It has been reported that the electrical conductivity properties of insulating materials for cables and capacitors can be measured by a Q(t) meter. It is considered that the Q(t) data contains information of transient charge behavior in the sample. If Q(t) meter can be used to measure not only the conduction properties but also the transient charging behavior in the sample, it will be useful for product inspection and product deterioration diagnosis. For film samples, both Q(t) and space charge distribution p(x,t) can measure. In this report, Q(t) and the p(x,t) are simultaneously measured in a polypropylene (PP) film, and the relationship between Q(t) and p(x,t) is discussed.

Polymer nanocomposites (PNCs) exhibit excellent electrical properties owing to charge trapping provided by nanofillers. However, the role of nanofillers in trap formation at the microscopic level is poorly understood. In this study, we propose a method to determine the charge trap depth of nanofillers in PNCs using x-ray photoelectron spectroscopy (XPS) measurements and first-principles calculations. The low-density polyethylene (PE)/TiO 2 nanocomposite is selected as the measurement target as it was previously reported the charges are trapped by TiO 2 loading to PE. We observe TiO 2 can serve as a trap for holes, and the trap depth is determined to be 0.9 eV. Furthermore, the computed charge trap depth calculated by G 0 W 0 calculation, which reproduce the experimental band gap, is comparable to the XPS result, which strongly supports the validity of our method. In addition, owing to the quantitative evaluation of the electronic structure, it was shown that the charge trap depth of the nanofiller can be controlled by tuning the surface dipole with surface modification of the nanofiller. The approach proposed in this study to determine the charge trap depth of nanofillers provides the prospect of designing PNCs with desirable properties from the atomic or molecular level.

Applying a DC voltage to an electrical insulating material such as polyethylene, various kinds of current flow within a sample due to varying physical phenomena. Among them, a conduction current (leakage current) flows in the final steady state which is usually measured by a picoammeter. As the conduction current is so small, noise reduction is a very big issue in measuring the current accurately. We tried to measure the conduction current using the “Direct Current Integrated Charge Method” (Q(t) method) which measures the current as an integrated charge using an integrating capacitor. Integration of the current leads to the accurate measurement of the current as random noises are canceled; in addition, parameters such as Q(t)IV or Q(t)/CV reveal similar figures for voltage or temperature. From this, using the converted time in which a shift factor is multiplied by real time, Q(t) data in different temperatures or electric fields can be drawn by a single curve, namely time-temperature and time-electric field correlation are shown. In this paper, based on the cross-linked polyethylene (XLPE) data, a potential time-temperature and time-electric field correlation is revealed, and the physical meaning of the shift factor is discussed.

Recently, as a solution for earth-warming problem, the high voltage direct-current (HVDC) technology is growing in the world. An important issue of insulation for HVDC is space charge phenomena that would cause a severe influences on cable performance, then measurement methods of space charges also become more and more important. Here, space charge phenomena and their measurement techniques are looked over. About measurement techniques, not only recent progresses but international standardization of them are described.

A Q(t) meter is used to measure electrical properties in insulating materials. Here, Q(t) is the integral value of circuit current with time t. It is possible to measure the Q(t) of the insulation material of a capacitor and a cable by using Q(t) meter. The current through an insulating material is divided into an instantaneous charging current, an absorption current and an equilibrium leakage current. It is considered that the absorption current is related to charge accumulation in the insulating materials. In this paper, an enameled wire (1.6 PEW) is used to prepare a cable sample, the Q(t) of the coated polyester (PEs) is measured, and extraction of absorption current components from measured Q(t) are reported.

In recent years, growing awareness of global environmental conservation has increased the demand for power leveling through inter-grid connections between nations or islands, and the introduction of renewable energy raises more necessity of energy leveling and remote connections between power-generation facilities and power demand area. There is a lot of market interest in high-voltage direct current (HVDC) technology, which is advantageous for long-distance power transmission. HVDC power cables are playing an important role in this to connect HVDC-generation facilities and the existing power network efficiently. Solid insulation power cables, in particular, are attracting attention worldwide and various insulation materials have been proposed. Though cross-linked polyethylene (XLPE) cable is widely used for alternative current (AC) transmission, if attempts are made to adapt AC-XLPE cables for DC transmission as significant electric field enhancement by space-charge accumulation may occur, then breakdown would take place especially during surge voltage in opposite polarity and polarity reversal. Therefore, it is desired to innovate a special insulating material for DC transmission to suppress the accumulation of space charge. In this chapter about such insulating material for DC transmission cable, nanocomposite that is composed from polymer materials as a matrix and nanofiller, polymer blend, and copolymer as kinds of composites in a broad sense are described. And for the real production of the nanocomposite materials in the near future, to increase the performance and reliability of the production and a deeper understanding of insulation and dielectric phenomena, some issues and perspectives are shown.

The direct current integrated charge method (Q(t) method) is a useful method to understand dielectric properties of insulating materials with a simple treatment. As total currents flowing through the insulating material are integrated by a capacitor, Q(t) data include various information about the material in them. To understand the Q(t) data, especially from the view point of electric charge behaviors, a parameter “charge ratio Q(tm)/Q(0)” which is the ratio of the initial charge amount Q(0)=CV and the Q(t) after time tm is introduced. An algorithm for classifying the electrode charge amount Q(0), the space charge accumulation amount Qspac(t), and the leakage current charge amount Qleak(t) using charge ratio is proposed and evaluated.

In this work, with the aid of first-principles calculation, we attempt to clarify whether nanoparticles itself can trap electrons and holes. Polyethylene (PE)/MgO nanocomposite is chosen as a model system. According to the computed adsorption energies, pristine and fully terminated surfaces are stable for the MgO (100) and MgO (111) surfaces, respectively. The electronic structures of several energetically favorable surface configurations are then studied. To probe the behavior of the excess charge, we examined the change in the charge distribution upon electron and hole doping. In the case of ideal MgO, holes are localized within the MgO slab, whereas electrons reside at the PE layer. We also show that the band alignment at the MgO/PE interface and, therefore, the carrier trapping property of MgO, is mainly determined by the intrinsic band edge positions of PE and MgO and the surface dipole of MgO, which can easily be tuned by surface modification.

Space charge distribution p(x,t) and current integrated charge Q(t) at high electric field (up to 130 kV/mm) in a 250 μm thick low density polyethylene (LDPE) film are measured from 30 to 85 °C by using a assembled device. The electric field and temperature dependences of the resistance component R s and the capacitance component C s of the sample are calculated and the relation of measured space charge distribution is discussed.

XLPE (Cross-linked polyethylene) has been used as an insulating material of solid insulation cables for long years, because of its excellent insulation performance under AC high voltage and advantage on practical use without maintenance. However, a concern that a dielectric breakdown occurs especially under a high DC electric field at a high temperature comes up due to an increase in electric field caused by space charge accumulation. To solve the problem, a modification of XLPE and usages of non-cross-linked materials such as modified polypropylene are studied worldwide recently. We have been studying possibilities of modified polypropylene as new cable insulating materials, and we have evaluated insulation performances of some polypropylene based copolymers by measuring space charge distributions and external circuit currents at high temperature under high DC electric field. In this research, a block and a random copolymerized polypropylene with polyethylene, which show good flexibilities compared to isotactic polypropylene, are investigated. It is found while a space charge injection and its accumulation was observed in the materials as they are, the accumulation characteristics were improved by introducing a polar group to both of the block and the random copolymers under an applied dc electric field of less than 100 kV/mm even at a temperature of 80 °C.

Relaxation time τ (=ε/k) is one of the important dielectric properties, and the τ can be obtained by Q(t) method with an relatively simple treatment. The procedure of the consideration to obtain τ, and measurement results of AC- and DC-XLPE are shown in this article. To consider τ efficiently and effectively, a parameter “charge ratio” was introduced. The charge ratio means a ratio of Q(tm) which is a charge amount Q(t) at a certain measurement time tm and Q(0) which is an initial rise of voltage application, that is Q(tm)/Q(0). The obtained τ is 300 s for AC-XLPE and more than 28800 s (8 h) for DC-XLPE at 60°C under 60 kV/mm.

We have investigated a space charge distribution and a conduction current characteristics in copolymerized materials of propylene and ethylene under high DC electric field at various temperatures, which are considered as one of candidate insulating materials for HVDC cable. The materials show excellent mechanical flexibility and relatively low conductivity at room temperature. However, at high temperature under very high DC applied electric field, electric field distortion induced by accumulated space charges in the bulk was observed. We attempted to improve the insulating properties of them by adding inorganic fillers into them. In this report, we describe measuring results of space charge distribution and conduction current in the composite materials under high DC electric field at high temperature.

One of the recent lively spot in the power technology is a High Voltage Direct Current (HVDC) market, thus development of insulation technology for HVDC has been in an active state from the beginning of the century. As the electrical performances of insulation materials under AC voltage and that under DC voltage are relatively different each other, an advanced material for AC voltage does not always show superior property under DC voltage. In order to understand the insulation technology of AC and DC cables more deeply, history and recent trend of R&D in insulation technology are investigated and summarized.

1. 緒 言 近年、高圧直流送電や直流機器類の需要拡大に伴い、直流 電圧印加時における誘電・絶縁材料の評価の重要性は確実 に高まっており、空間電荷現象や電気伝導現象について、 これまで以上に考察を深める必要性が増している。これら の現象の鍵となるのが材料中を流れる微小電流である。方 形波電圧の印加を考えると、誘電・絶縁材料における電流 は、課電の初期における瞬時充電電流(電極と試料をコン デンサと見立てた時の充電電流)、空間電荷蓄積・移動に伴 う吸収電流、定常状態に達した後の漏洩電流に分類され、 通常はそれぞれの物理現象と対応した個別の測定手法によ り計測が行われる。例えば、課電による材料の劣化の前駆 現象として重要な空間電荷蓄積挙動は、パルス静電応力法 (PEA法)により1980年代から広く評価が行われている。 PEA法は蓄積電荷の位置情報がわかる優れた手法である が、対象となるのは比較的移動速度の遅い電荷に限られて おり、動きの速い電荷や伝導現象には対応できない。 電流積分電荷法は、試料に対して直列に接続した測定コ ンデンサに、電流の積分値である電荷量(Q(t))を蓄積し て測定・解析する手法である。コンデンサを使用するた め、全ての電流成分を網羅的に捕捉でき、また測定が簡便 であるため、材料の誘電・絶縁特性を短時間で俯瞰的に把 握するのに有効である 。ここでは、電流積分電荷法に ついて簡単に解説するとともに、いくつかのポリマー材料 に適用した結果を示した。また、最後に今後の考えられる 展開についてまとめた。

Recently, as quantum chemical methods such as DFT method have widely investigated on the dielectric and insulating materials, electronic and orbital structures of the materials which contain aromatic groups in the molecular chain have been gradually becoming apparent. From the quantum chemical view points, the styrene group has higher HOMO and lower LUMO levels compared to those of polyethylene, and this means the styrene groups may form a new molecular orbital between the HOMO/LUMO levels of polyethylene as a trap site of electric charges. The influences of styrene groups on the molecule of polyethylene were investigated in 30 years ago as the ethylene-styrene random copolymer manufactured by high pressure method. The content of styrene in the copolymer was said to be less than 1 wt % which gave a most appropriate dielectric and insulating properties, but their properties which relate to molecular orbital such as space charge behavior were less studied then. We have obtained some of commercially available styrene modified polyethylene, and investigated the influences of styrene groups on dielectric and insulating properties theoretically and experimentally. As the results, even more contents of styrene than that of previous studies show a more excellent dielectric and insulating properties than polyethylene.

Both the space charges and the currents in the solid dielectrics are important properties to describe the insulating and dielectric behaviors of insulation materials. Usually, they are measured by the individual appropriate method, such as PEA method for space charges and pico-ammeter for leakage currents, however, as weak signals should be treated with, the information obtained from the measurements is that of bulk samples in general, and thus it is difficult to find the properties in a small area. We have been investigating “direct current integrated charge (DCIC-Q(t)) method” which the charges can be estimated as a time integration of the whole current in. As the current is integrated in the method, compared to the pico-ammeter, the measuring accuracy of a small current is expected to be improved. From the view points, applying DCIC-Q(t) method, we have tried to decrease the dimensions of measuring electrodes, and have arranged the electrodes in the square area, and then have made Q(t) meter with “the multiple electrodes”. Some of plaque samples of oil-impregnated papers are measured by the method, and space charges and currents distributions were evaluated.