K. Tohyama

High field dissipation current waveform of polyethylene film obtained by new method

Non-polar polymers, such as polyethylene (PE), polypropylene (PP) and so on, are widely used as insulators for power cable and capacitor. In ac high field region, it is known that carrier injection from electrodes is getting dominant, and then the dissipation current waveform shape begins to show a nonlinear property due to complex conduction mechanism, that is, charge injection, space charge effect, recombination and so on. To study the conduction mechanism under ac high field, a new dissipation current waveform observation method which enables detection of the harmonies with high accuracy is developed. By using low density polyethylene (LDPE) film, high field dissipation current waveform observations from room temperature to 90/spl deg/C are carried out. Through these series of experiments, very unique properties which indicate the space charge effect under ac high field in high temperature are obtained.

High-field dissipation current waveform in e-beam-irradiated XLPE film at high temperature

We developed a detection method of ac dissipation current waveforms combined with a fast rising ac ramp voltage application as a new technique to evaluate the high-field dielectric properties and the dissipation current waveform over a very short time range. This measurement was carried out on electron beam irradiated crosslinked polyethylene film. At high temperature the high field tan /spl delta/ and ac dissipation current show nonlinear electric field dependence and the dissipation current during decreasing field becomes larger than that during increasing field, suggesting the contribution of carrier injection from electrodes. The dissipation current waveforms in the high temperature region show the peak in the first part of each half cycle of a full waveform, and fast decrease in the latter part. This result is probably due to the hetero and home space charge formation caused by the injected carriers from the electrode.

Electroluminescence due to impulse voltage in cable-grade XLPE

Underground power cables in service are inadvertently subjected to impulses generated by lightning and switching surges that are superimposed on the ac voltage at which they operate. HV transients caused by lightning and switching operations not only radiate large electromagnetic fields but also impose additional stresses on the insulation and could initiate deterioration which can continue under normal operating conditions. Electrical breakdown of HV cables is a local phenomenon and electrical aging at local sites in the polymeric insulation occurs by molecular dissociation of the polymer and formation of new chemical bonds. This process usually involves the electronically excited states of the molecules that give rise to radiative phenomena, such as electroluminescence (EL). Analysis of EL can help to clarify the degradation mechanisms that occur at points of electrical stress enhancement and lead to cable breakdown. The EL technique is several orders of magnitude more sensitive than the commonly employed partial discharge detection and can provide a better understanding of the various time dependent mechanisms such as space charge injection, trapping and decay that can lead to insulation failure. The characteristics of EL in crosslinked PE subjected to impulse voltage, are described in this paper. Impulses having the same polarities as the half cycles of the ac voltage on which they are superimposed give rise to the largest number of EL pulses. Also, more EL pulses are emitted when impulses are applied at the peaks of the positive and negative half cycles than at the zero crossing of the ac voltage. This suggests that the amount of charge injected and trapped into the polymer plays a crucial role for EL emission.

Simultaneous measurement of Electroluminescence and dissipation current in cable insulation

Polymeric materials are widely used in high voltage devices such as capacitors, cables and transformers. Electroluminescence (EL), the emission of light in insulating materials subjected to high electric stress, is known to occur much before partial discharge inception. EL emission has been linked to the presence of space charge in the polymeric insulation. At high ac fields, charge injection from the electrodes into the insulation becomes dominant and the dissipation current becomes nonlinear. This paper describes the simultaneous measurement of EL emission and dissipation current in LDPE subjected to uniform ac field. It is shown that at the onset of EL emission the dissipation current waveform deviates from its sinusoidal characteristics and starts to show harmonics. The increase in the harmonic distortion of the dissipation current could indicate a change in the ac conduction mechanism.

Development of high accuracy measurement system for dissipation current of nonpolar polymers

In order to clarify the high field dielectric properties, high accuracy measurement system for dissipation current of nonpolar polymers without Capacitance Bridge was developed. In high temperature and high field region, it is known that the dissipation current is getting large due to charge injection from the electrode. Furthermore the shapes of dissipation current waveform deviate from sinusoidal waveform and it shows nonlinear property. Therefore, it is important to measure the dissipation current with high accuracy, which include harmonics of dissipation current and to investigate the space charge effect under ac field. In this paper, we will discuss the technique, which enables the observation of the high field dissipation current under ac high field.

Increment of capacitive current and space charge formation of PP film near breakdown field

We studied the dielectric properties of biaxially oriented polypropylene (BOPP) film near AC breakdown field at room temperature. As a result, the AC dissipation current waveform has a distortion suggesting that this distortion is caused by the space charge formation under the AC high field.

High-field AC conduction in electron beam irradiated crosslinked polyethylene films in high temperature region

High-field AC conduction was investigated in electron-beam irradiated cross-linked polyethylene (IR-XLPE) film in the high-field and high-temperature region. Measurements of the electric field dependence of tan /spl delta/, dissipation current waveform, and DC absorption current were carried out. In the high-temperature and high-field region where the increment tan /spl delta/ of IR-XLPE with electric field becomes large, the dissipation current waveforms have triangular shapes and the DC absorption current show a gradual increase after voltage application, probably due to space-charge-limited current. The high-field AC conduction of IR-XLPE in the high-temperature region is considered to be dominated by carrier injection from the electrode associated with the Schottky model.<<ETX>>

Improvements of high accuracy AC dissipation current waveform observation system

Nonpolar polymers, such as polyethylene, polypropylene, are widely used as the insulation materials for power cables and power capacitor. It is very important to know high field AC conduction mechanisms for designing the high performance insulator. In our previous study, dissipation current under AC high field utilizing Capacitance Bridge was developed. As the results of these experiments, possibility of space charge effect under AC high field is pointed out and reported. Normally, under AC high field and in high temperature, the dissipation current becomes nonlinear property. To study space charge effect, it is important to measure harmonics of dissipation current with high accuracy. Then, it is developed new dissipation current observation method, which makes possible to measure harmonics with high accuracy. In this paper, the improvement of this new measuring system is discussed.

Effect of frequency on XLPE cable insulation at high electric field

Electroluminescence (EL), the emission of light in dielectrics subjected to high electric stress, is a very sensitive technique to detect space charge injection and the very early stages of insulation degradation. In this paper the EL technique is employed to determine the effect of frequency in the range of 5 to 600 Hz on unaged and aged crosslinked polyethylene (XLPE) insulation of underground high voltage cables. The number of EL pulses emitted from XLPE and the area of the space charge region simultaneously obtained by the PMT and CCD camera are studied as the frequency of the ac voltage is varied between 5 and 600 Hz. The implications of the frequency range on diagnostic tests performed on the polymeric insulation of underground power cables is discussed.

Variations of high-field dissipation current waveform under AC ramp voltage application

To study the high-field AC conduction mechanisms of nonpolar polymers, the AC dissipation current waveforms are observed. The shapes of the AC dissipation current waveforms in the high-held and high-temperature region become the triangle shape in spite of the sinusoidal voltage application, suggesting the nonlinear electric field dependence of instantaneous AC conductivity. The fast rising AC ramp voltage application method is developed as the new technique to examine the dielectric properties in the very short time range. This method enables the measurement of the variations of the high-field dielectric properties and the dissipation current waveform immediately after the voltage application. The new measurements were carried out on electron beam (EB) irradiated crosslinked polyethylene (IR-XLPE) film. The dissipation current waveforms in the high temperature region show the peak in the former part of each half cycle and quickly decrease in the latter part of each half cycle of a full waveform, which seems to show that the carrier injection from the electrode is assisted by the heterospace charge in the region near the electrode.<<ETX>>