An after laying test is required to demonstrate the soundness of XLPE underground cable lines with joints. AC voltage application is desirable for the test, however, the equipment for AC application must be very large because it must supply large capacitive current for cables. DC voltage is normally applied, which can be supplied with a small equipment, however, the effectiveness of the DC application is questioned because the electrical stress distribution is quite different from that with AC application and, moreover, DC application could damage insulation properties of cables.
The authors targeted Oscillating Wave (OSW) method and Very Low Frequency (VLF) method as alternative after laying tests. They first developed modeling methods of various defects on underground cable lines and applied OSW and VLF to these defect models. They found OSW and VLF methods have higher fault detection capability than DC voltage application. They also demonstrated that the defects which can be easily detected with OSW method is difficult to be detected with VLF method and vice versa. They showed that serial application of the both methods highly improve the fault detection ability for XLPE underground cable lines.

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In recent years, as one of the methods that permit effective and efficient maintenance of medium voltage XLPE insulated cables (CV cables), the DC superimposition method which is used to measure insulation resistance under hot-line condition has attracted attention, and efforts are being made to promote its practical application. However, there still remain some problems to be solved in applying this method.
The authors, therefore, considered the detection sensitivity of the DC superimposition method applied to a water-tree deteriorated CV cable, and examined the application limit of this method on system voltages. As a result, the following points were clarified: (1) this method can detect penetrated water-trees, (2) the upper limit of the residual insulation breakdown strength of a deteriorated cable should be considered 10kV, and (3) this method can be applied to system voltages below 6.6kV.

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An improved method of degradation diagnosis for medium-size distribution transformers is described. The proposed method uses refined criteria of degradation for specific application for 200-to 1000-kVA range transformer diagnostics. The new criteria were established with a 95% confidence limit of data after extensive case studies focusing on the relationship between the amounts of CO and CO₂ generated in transformer oil and the length of operating time of transformer. Another feature of this diagnostic technique is the provision of a capability to estimate the thermal history of the insulation material by examining the amounts of certain trace products of decomposition present in the transformer oil. For this purpose, a thermal history discriminant chart was devised by application of a canonical discriminant analysis method.

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This paper describes results of the discharge luminescence and discharge magnitude measurements perfomed on the insulation matelials exposed to partial discharge. PMMA with an artifical needle shape void were used as the treeing specimens. The discharge luminescence is observed by image intensifier, and the luminous image is processed by computer.
At the same time, the discharge magnitude distribution in relation to phase angle of the applied voltage was measured by a partial discharge measuring system. The mechanism of electric treeing was discussed by not only the measuremet of the discharge pulses but also the local luminescence intensity of partial discharge.
As a result, the luminescence intensity analysis showed that the relation between tree growth length and void length when maximum luminescence intensity moved from the void area to the tree area. Both the tree growth and maximum luminescence intensity at the tree area increase with constant ratio. The discharge luminescence intensity at the tree area increases significantly depending on the discharge magnitude in the specific phase angle area in positive and negative.

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A new method is presented for detecting an occurrence of the partial discharge (PD) in a high-voltage electrical apparatus due to a dielectric material defect. This method uses the phase information of electormagnetic waves propagated through space. In case the electromagnetic wave due to an internal PD is to be measured when an electrical power apparatus is operating, the signal to noise ratio poses a problem to discuss. The mean power of the background noise is normally higher than that of the signal of interest, and the signal tends to be lost in the background noise. And it is important to develop a non-contact method for detecting the insulation fault. This paper experimentally shows that our proposed “spatial phase difference method” is able to apply to identification of electomagnetic waves due to PD. This method is shown to be useful for detecting prediction of insulation fault.

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Partial discharge (PD) degradation phenomena is one of the most important factor to decide the performance and the lifetime of the high voltage facilities. During the PD degradation tests of an epoxy specimen with a closed void, PD often change to a large number of very small PD known as swarming pulsive microdischarges (SPMD). The appearance of SPMD is concerned with the degradation of the insulating materials, therefore insulation diagnosis with the detection of SPMD become to be possible. However, SPMD can not be detected with the conventional PD detector because the PD magnitude of SPMD is too small. Authors has developed the computer aided partial discharge analyzing system (CAPDAS) to detect and analyze the SPMD. Furthermore, authors introduced a new PD parameter, the SPMD ratio, for the quantitative analysis of SPMD. In this paper, the time variation of SPMD characteristics of a CIGRE Method-II specimen are investigated by using the CAPDAS. Further, the effect of the frequency of the applied voltage on SPMD are also investigated at 60Hz and the very low frequency of 0.1Hz. As the result, the comparison of PD magnitudes measured at 60Hz with those at 0.1Hz could make a quantitative analysis of SPMD characteristics possible by using only the conventional PD detector.

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This paper presents a new frequency selection method for the partial discharge detection and calibration of cable lines. By using convergence frequency of cable impedance to surge impedance, indirect calibration at Insulation Joint (IJ) can be done with quantitativeness.
For detecting small discharge in cable lines with high sensitivity, IJ detection method and indirect calibration procedure had been proposed and reported data at EHV line. But quantitative calibration condition for the IJ detection is not disclosed.
We have investigated cable impedance characteristics which converge to surge impedance at the high frequency. By selecting the convergence frequency as discharge detection amplification, quantitative insulation charactreristics of cable lines can be evaluated by partial discharge detection.

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Partial discharge (PD) and breakdown (BD) characteristics of various initial defects of 66_??_77kV or more XLPE cable joints (EMJ, TMJ, PMJ, and TJ) were investigated by empoying model electrodes, It is possible to classify various defects into following 4 types depending on each distinctive PD (including φ-q-n pattern) and BD features.
Type I: Interface (rubber/epoxy & rubber/XLPE) filled with silicone oil (no cavity)
Type II: Interface (rubber/XLPE) without silicone oil (cavity-void discharge)
Type III: Defect led to treeing breakdown (protrusion & XLPE with fiber)
Type IV: Interface with conductive path (wet fiber along interface)
To give one example for the feature depending on each type, PDIV (partial discharge inception voltage)/ BDV (breakdown voltage) ratio which can be regarded, as the tolerance time from the inception of PD to breakdown tend to be type III> I>>IV> II.
As for the effect of interface silicone oil (the difference between type I and II), the values of PDIV and BDV in type I (interface with silicone oil) are remarkably higher than these of type II (interface without silicone oil) and PD attenuation shortly after PD inception is recognized in Type I. The above effect on interface silicone oil may be thought to be similar phenomena against discharge in oil impregnated insulation systems.

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We evaluated the diagnosis of insulation performance in gas insulated switchgears (GIS). We measured the temporal change of partial discharge (PD) characteristics from initiation to breakdown in SF₆ gas with the pressure of 0.4 MPa for ac voltage application to needle-plane electrodes. As a result, breakdown occurred about 150 minutes after the voltage application of 55 kVrms. We observed that tip radius of needle electrode was enlarged due to PD during the voltage application, resulting in the shift of corona-stabilized breakdown characteristics to the lower pressure side. At the final stage close to breakdown, positive PD pulses with relatively higher magnitude began to emerge in the phase region near the applied voltage peak. This event can be interpreted by the change of PD mechanism in SF₆ gas from streamer corona to leader corona.
Moreover, we investigated the possibility of breakdown prediction. We try to seek a breakdown prediction parameter which characterize the change of PD form: the ratio R_L of the maximum charge to the average charge near the peak in positive half cycle. This suggests that positive PD form may change as R_L increase. As approaching to the breakdown, R_L rose from 5 to 11 in the approximated curve for most cases. When R_L was used as a breakdown prediction parameter, the breakdown time could be successfully predicted within the error of 20%.

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We have developed a partial discharge detection system using an external antenna that can be easily installed on the outside of the GIS. The antenna is fitted closely to the outer circumference of the insulation spacer and it can detect electromagnetic radiation of the partial discharge with high sen-sitivity.
To have high sensitivity even in noisy condition, three kinds of methods that can eliminate external noise effectively were verified. In addition, the locating method of defect based on distance atteuna-tion are confirmed. Acctually, the insulation monitoring system which employs the anntenna is used in a substation which had already been installed. At present, it is operating satisfactory.

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We have been investigating characteristics of electromagnetic spectrum for partial discharge (PD) diagnosis systems for SF₆ gas insulated switchgears (GIS). We measured noise spectrum in air at three different types of high voltage substations; 550kV open-air insulation substation, 550kV open-air GIS substation and 275kV underground GIS substation. A biconical antenna acting in a frequency range from 30 to 300 MHz was placed at a few meter away from GIS or gas circuit breaker (GCB) in the substations. From the experimental result, we obtained the average gain Ga of electromagnetic spectrum at each frequency in the substations. Next, we discussed the relationship between Ga and atmospheric conditions using an empirical formula of PD inception electric field Ec. As a result, we found the inverse proportionality of Ga against Ec which was determined by the temperature, pressure and radius of high voltage overhead conductors. We also investigated the relationship between PD pulse and the emitted electromagnetic spectrum from fundamental experiment for a needle-plane electrode configuration. Finally, we introduced “equivalent charge: qe” for substations, and concluded that the charge magnitudes of the 550kV open-air insulation substation were about 7 times larger than those in the open-air GIS substation.

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We investigated the diagnosis of insulation performance in high voltage power apparatus. We have constructed a computer-aided partial discharge (PD) measuring system which can acquire time-sequential phase-resolved PD pulse height and can derive some PD statistical parameters. Using the PD measuring system, we measured PD in air, SF₆ and oil for needle-plane electrode under the ac voltage application. Experimental results exhibited that PD pattern varied in three different mediums. Moreover, we measured time-resolved PD pulse occurring in the three mediums. As a result, observed PD pulse waveforms proved to differ each other depending on medium, voltage polarity and applied voltage. The difference in the discharge type is attributed to the difference in the PD pulse waveforms, resulting in the different frequency components. We also pointed out that the PD pattern and PD characteristics might dramatically change with the selected measuring frequency. In other words, it is necessary to properly select an optimized frequency for accurate PD detection and analysis in power apparatus.

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It is an urgent subject to establish a reliable diagnostic method for GIS. From this point, quite a few reserches have invetigated the characteristics of PD measurement in GIS such as relation between kinds of defect and PD signal, detectablity of various diagnostic means. There is, however, no data on realistic length of a particle like shorter than 10mm. This paper focused on the abave subject and a variety of characteristics have been obtained with changing kinds of defect, lengths of a particle and applied voltage levels as paramaters. The range or kind treated here is a free particle, a particle on spacer surface and a one on central conductor for the defect, 3mm, 5mm, 10mm, 20mm for the particle length, l.Opu, l.lpu, and 0.8times test voltage for the applied voltage level. Espacially, detectablity of short length particles were significant from a practical viewpoint.

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Most of the accidents of XLPE cable line are caused by defective constructions of cable accessories. So we developed a method to measure paratial discharges in pre-mold cable accessories. We provided separared shielding layers as detection electorodes in accessories, and discriminate between paratial discharge pulse and noise pulse using realtime comparation of pulse current magnitudes from both de-tection electrodes.
This paper describe the principle of noise discrimination method, portabale paratial discharge mea-surement apparatus, and the result of field test.

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The increased renewable energy utilization, especially photovoltaics using solar energy, has been expected in recent years because of the limited available fossil energy and the environmental impacts due to its consumption. The deregulation in the Japanese power industry allows transmission of surplus PV power from housing to the power grid and PV system utilization becomes more attractive. However, so for as authors know, there are few publications which analyze the economic value of the grid-connected PV system for domestic building considering running cost.
In this paper, we firstly present an operating and control algorithm for the grid-connected PV system with battery and define an economic benefit obtained through system operation. We also, define the equalized PV power generation cost considering both investment and operation costs so that we can make clear the economic effect of the amount of power generated by PV on the equalized PV power generation cost. Secondly, we show an critical condition obtained from economic view point by which we determine the critical battery capacity to be installed as power storage equipment. Finally, through numerical examples of a yearly operation of a model PV systems, we ascertain the effectiveness of the proposed approach.

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In many high voltage gas-insulated switchgears (GIS), flashover voltage is decreased in case of including conducting particles. In particular, the decrease is very severe, when the particles move to and attach on an insulator in GIS. The reason is that surface flashover voltage is usually lower than gaseous flashover voltage in comparatively high pressurized SF₆ gas. Some methods for flashover voltage prediction, one of which was proposed by authors, were already proposed of gaseous flashover. However, concerning the surface flashover, only few methods have been proposed.
In the present paper, the prediction method for gaseous flashover of SF₆ gas including conducting particles proposed by authors is extended to the prediction method for the surface flashover with particles attached on an epoxy insulator. The characteristic quantities at particle tip are reformed with two supplementary factors included in order to estimate the phenomena peculiar to the surface flashover. It is revealed through comparison with experimental results that the proposed method is useful to predict the flashover voltage along the epoxy insulator of GIS including particles.

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This paper proposes a new method to approximate a closest loadability limit (voltage collapse point or saddle-node bifurcation). A pair of multiple load flow solutions (that is, an operating solution and a low voltage solution) are used to provide the loadability margins of individual loads in MW and War at a specified operating point. Left and right eigenvectors corresponding to the zero eigenvalue of load flow Jacobian are also available, which can be used to identify weak spots of a power system and to determine an optimal control against voltage collapse. Most of the computation time of the proposed method is taken to calculate the load flow solution pair. Additional computation time is less than that of an ordinary load flow.

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With the increase of the size and capacity of electric power systems and the growth of systems interconnec-tions, the problem of power oscillations due to the reduced system damping has come to be increasingly serious. As a Super-Conducting Magnetic Energy Storage (SMES) unit with a self-commutated converter is capable to control both active (P) and reactive (Q) power simultaneously and quickly, increasing attention has been focused recently on power system stabilization by SMES control.
This paper describes a study on the power oscillation damping by the three control systems of SMES, that is P control and Q control for suppressing the power oscillation and the reactive power control (V control) for stabilizing the voltage fluctuation. The study is achieved through the analyses of the damping torque components over a wide range of parameters such as system oscillation frequency and the size and characteris-tics of system load, using a one-machine infinite-bus system with system load and SMES at the mid-point. Also, the results obtained by mathematical analysis are verified by digital simulation. The difference in the effects among the three control systems of SMES is clarified and the mechanism of damping is clearly explained.

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