A new type non-flammable large power transformer system is proposed in which the excellent cooling characteristics and insulation properties of non-flammable perfluorocarbon (PFC) Liquid is utilized. In the system, transformer windings are put in an insulating cylinder filled with PFC liquid. The core includes cooling panel through the liquid. The space between the insulating cylinder and tank is filled with compressed SF₆ gas. This paper descrides the insulation characteristics of main insulation and inter-winding insulation models which simulate the transformer. It is found that: (1) higher withstand voltage is expected by means of insulation barriers in main insulation model; (2) partial discharge in inter-winding insulation model is beginning to behave in a constant electric field strength on the point of 0.1mm liquid gap length.

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A relationship between the repetitive impulse voltage and the number to breakdown becomes matter of great concern since the lightning impulse withstand voltage of UHV transformers will be remarkably reduced owing to the low protective level by high performance surge arresters. It is shown that the inclination of V-N characteristics is rather flat up to 1000 shots of lightning impulse and switching impulse for inter-coil and turn insulation models by experimental results. Finally, it is concluded that the modification of lightning impulse withstand test voltage based on V-N characteristics is not necessary.

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In recent years, power electronics technology has been developed rapidly and semiconductor power converter equipments are widely used. At the same time, new problem due to the magnetic nonlinearity is appearing. It is generation of excessive magnetizing currents of transformer and saturable reactor caused by DC magnetic deviation.
As a method of numerical analysis of magnetical nonlinear circuit, it was proposed to describe the nonlinearity of iron core coil as the relations between current and differential inductance instead of the traditional relations between magnetic flux and current, to measure it directly, and to approximate by suitable functions to appropriate over the measured range. And current waveforms of nonlinear inductor circuit computed by the method is in good agreement with experimented was reported.
This paper reports the numerical analyzed results computed by the method considering the hysteresis characteristics and experimental results of the magnetizing currents increasing in transient state caused by the growing of DC magnetic deviation.
These results reasonably well agreed.

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A 3000/6000V, 1000kVA class superconducting transformer (SC-Tr) was developed for a Prospective Power Transmission Model System Integrated under Superconducting Environment (PROMISE). In this transformer, the core is immersed in liquid helium together with superconducting windings, and the major insulation is provided by the liquid helium. This paper describes the design feature and measured characteristics of the SC-Tr. Fundamental characteristics of the SC-Tr are obtained through no-load, short-circuit tests and quench experiments. The results of the no-load test have verified that the SC-Tr has the capability to withstand a.c. voltage of 3000/6000V of 60Hz without any partial discharge. The short-circuit tests have proved that the SC-Tr is capable of carrying a.c. current of 170Arms without quench in the superconducting windings. Furthermore, in a real-load experiment with the PROMISE, electric power of 3800V-460kVA of 50Hz in high-voltage side is transmitted through this SC-Tr.

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Because the gas forced cooling type gas-insulated transformer was considered to be available up to approximately 60MVA, several types of liquid cooling large-capacity gas-insulated transformer have been developed. But liquid cooling type has a disadvantage of complex structure for liquid cooling.
In this reason, authors have been studying to develop the simple design of gas forced cooling large-capacity gas-insulated transformer. This paper deals with reseach and development of cooling and insulation technology of large-capacity gas-insulated transformer. Cooling characteristics of high pressure SF6 gas have been studied for cooling design. Turn-to-turn and section-to-section insulation of gas-insulated transformer winding have been studied for insulation design. Furthermore, development of 275kV 300MVA gas-insulated transformer is described.

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Recently SF6 gas insulated equipments are equiped more and more in cities in the view point of disaster prevention. These equipments must be expected to work safely and reliably for several ten years. The SF6 gas insulated transformer is one of these equipments.
In SF6 gas insulated transformers polyethylene terephthalate(PET) film and transformer boards are used as main insulating materials.
Because the transformer is filled with SF6 gas that is chemically and physically stable, the life of the transformer depends on the thermal stability of insulating materials.
Therefore, it is important to clarify thermal degradation properties. of insulating materials in gas insulated transformers. This paper deals with the thermal degradation of insulating materials of SF6 gas insulated transformers, and relations between changes in characteristics and the amount of evolved gas from insulating materials due to thermal degradation in SF6 gas are described.

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Three phase core type power transformer core is usually made that core laminations are lapped at so called T-joint of legs and yokes, And yokes are magnetically connected over three phases to make the magnetic flux be able to flow smoothly between legs through yokes. It has been hardly believed that split core which has a full separation at the center of the leg including yokes may be used for power transformers from the points of core loss, local heating and noise by the imbalance and distortion of magnetic flux though it is useable for distribution transformers with wound cores, Split core has so many advantages in core manifacturing though that magnetic characteristics were stadied in detail for small core models to make the practical applicability clear and was checked with less degradation of magnetic characteristics compared to conventional cores using a full scale 1500/3 MVA model transformer core.

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Load current of a converter transformer is nonsinusoidal and has many harmonic currents in it. Because of harmonic components, load loss of the converter transformer on site becomes greater than that of the converter transformer by factory test. Load loss consists of joule loss, winding eddy current loss and other stray loss of which quantity is difficult to estimate with accuracy due to the complicated frequency dependence. Few papers were reported on the frequency dependence of stray loss and the characteristic is not cleared yet. This paper describes the frequency dependencies of the winding eddy loss and other stray losses such as loss in tank vessel, loss in core clamps and loss in surface laminations of core leg derived from the numerical analysis of FEM.

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Partial discharge inception voltage-time (V-t) characteristics of transformer insulation partial model were investigated systematically to contribute in deciding the AC test voltage for the UHV transformer. This research covered the wide time range from several tens of milliseconds to several months. As a representative insulation model of the the transformer, turn-to-turn, coil-to-coil and barrier insulation ones were used.
Further characteristics towards trapezoidal voltage waveforms were studied as well as the standard AC waveform, when attention was put on the overvoltage occuring in load rejection As a result, quite precise data on V-t characteristics were obtained and got available to discuss the AC test voltage.

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Liquid cooling type gas-insulated transformer utilizes perfuruorocarbon (C₈F₁₆O) and SF₆.If the faults in transformer occur, insulating materials generally are decomposed by overheat, Recently, we found that the decomposed products from each insulating materials slightly dissolved in perfuruorocarbon.
Analysis of decomposed products in perfuruorocarbon offers a new diagnostic technique for liquid cooling type gas-insulated transformer.Decomposed products characteristics to each insulating materials were identified by the decomposed experiments.
Decomposed products dissolved in perfuruorocarbon were analyzed by high-performance liquid chromato -graphy with ultraviolet detector.

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Refining method of insulating oil was changed from acid-refining to hydro-refining in the 1970's. And addtion of benzotriazole (BTA) in insulating oil was recognized in Japanese Industrial Standard in 1988. These changes are preferable to suppression of oxidative degradation and cupric corrosion phenomena caused by corrosive sulfur.
We have studied oxidative degradation and cupric corrosion caused by corrosive sulfur.
It is known that tanδ changes like N-character pattern in process of oxidative degradation.
The maximum value of tanδ is proportional to amount of soluting copper, sulfur contents and inversely contents of aromatic compound in the insulating oil.
The amount of corrosive sulfur in the insulating oil is at small level recently, but cupric corrosion caused by corrosive sulfur increases with amount of oxigen in the insulating oil.
Cupric corrosion caused by corrosive sulfur is related to soluting copper and sulfuric ion in the insulating oil, therefore degree of cupric corrosion could be measured by soluting copper and sulfuric ion.
We found that BTA is effective for both prevention of oxidative degradation and cupric corrosion caused by corrosive sulfur in the insulating oil.

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In order to evaluate an appropriate level of AC test voltage for 1000kV transformers, it is nece-ssary to make an accurate estimation of AC V-t characteristics. For the level of impulse test volt-age, V-N characteristics are important to evaluate effects of impulse stress repetition. Since both of the characteristics are dependent on insulation structures and manufacturing processes of trans-formers, it is important to make accurate simulations on transformer insulation for reliable insu-lation data.
To obtain some of the basic data for 1000kV transformer test levels, long time AC V-t tests and impulse V-N tests of up to 1000 times voltage application were carried out on an insulation model simulating fundamental structure of shell-form transformers. The model is composed of a para-llel electrodes system with a compound structure of pressboard barrier and oil gap.
The tests showed the following characteristics: AC V-t characteristics of up to several ten days could be described by two lines, each representing short time and long time characteristics. V-N characteristics of lightning and switching impulse were expressed by relatively flat lines, which showed decrease in breakdown voltage by less than 10% after 1000 times impulse application.

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A perfluorocarbon (PFC) immersed 275kV transformer with compressed SF6 gas insulation has been under development. This paper clarifies the basic characteristics about cooling and insulation of PFC and the AC partial discharge inception voltage and time characteristics of PFC immersed insulation. It is shown that a prototype 275kV 100MVA three phase transformer can be operated without any trouble during the long-term overvoltage test. This prototype has the AC partial discharge inception strength of higher than 1.5 times the AC test voltage and the lightning impulse breakdown strength of 1.5 times the test voltage. A 275kV 250MVA three phase transformer was developed and is being operated at Abe substation of Chubu Electric Power Co., Inc. The transformer have been operating satisfactory.

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Recently, the power demand has been increasing in Nagoya city. To supply the power stably, large and extra-high voltage substations should be installed at the central region of Nagoya city. But the harmonization of environment is required and the escalation of land price has become big problem. They are the reason why it was determined that the substation would be installed underground.
Usually the power system of substation has two independent routes connected to a power source for the supply reliability. But there is a phase difference between the routes because of a difference in power system construction and line length. In case of a loop system operation, a current caused by this phase difference influences the power system and may exceed the supply capacity of the line. A phase shifter was installed between the lines to control the current by adjusting the phases of both power sources.
The features of the phase shifter are; First, the phase shifting method that the output voltages always equals to the input ones without influence of phase angles is adopted. Secondly, the two-phase shell-form core and lay-down structure to fit a limited space in an underground substation is adopted. Finally, for underground substation, both passing and regulating capacities are the largest in Japan.

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The authors proposed a nonlinear adaptive generator control system with neural networks for improving damp-ing of power systems and showed its effectiveness in a one-machine infinite bus test power system in their previous paper. The proposed neuro-control system has a similar system structure to that of self-tuning regulators based on adaptive control theory and it works adaptively with a performance index similar to that of linear optimal regulators based on optimal control theory.
In this paper, a self-tuning regulator for generator excitation control is designed in order to be compared with the proposed neuro-control system. Through digital time simulations in a one-machine infinite bus test power system, the control performance of both methods is discussed. As a result, the proposed adaptive neuro-control system improves the system damping more effectively than the self-tuning regulator. It is mainly because the nonlinear characteristics of neural networks in the proposed neuro-control system work effectively while the self-tuning regulator is linear in nature.

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We studied partial discharge (PD) for assessing the condition of insulation performance in gas insulated switchgears (GIS). We measured PD characteristics from its inception to breakdown in SF₆ gas as a function of gas pressure from 0.1 to 0.4 MPa for a needle-plane electrode configuration. Experimental results revealed that PD began to occur in the negative half cycle as ac applied voltage rose. Above the PD inception voltage, the repetition rate _??_₊ of positive PD pulses occurring in the positive half cycle increased more rapidly with enhancing the voltage than _??_ in the negative half cycle. We proposed new PD parameters “he unbalance ratio U_r” of repetition rate and “the average PD inception phase _??__i+, ” defined as the phase at which the first PD pulse takes place after the polarity reversal. It was found that U_r changed from -1.0 to +1.0 over the applied voltage range from the inception to breakdown. As the applied voltage approached to the breakdown voltage, U_r proved to fall down from +1.0 to +0.5. We also measured change of PD characteristics with time to breakdown with ac long time voltage application. U_r and _??_ⁱ⁺ were found to decline with approaching to breakdown. Thus, we suggested that U_r and _??__i+ could be used as breakdown prediction parameters in GIS.

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One of the most effective methods for utilizing solar energy is to use the sunshine and the solar thermal energy simultaneously. For this purpose, various kinds of photovoltaic-thermal panel (PV/T panel) which include both PV cells and solar collectors were constructed.
In this study, we evaluate the photovoltaic and the solar thermal energy of a PV/T panel in terms of exergy. The solar collector has the maximum exergy temperature desirable to be operated. However, the PV cell conversion efficiency decreases at the temperature because of the increase of the cell temperature by the heat of the collector. Therefore, cooling of the PV cell is important to use the cell under favorable condition.
On the basis of the exergy evaluation for the PV/T panel, we propose a new construction of a PV/T panel and discuss the cooling of the PV/T panel by natural convection induced by buoyancy effect. By this cooling effect, the PV cell conversion efficiency is not decreased, while the collector is operated at the maximum exergy temperature. As a result, the proptosed panel would be a significant improvement in the utilization of solar energy.

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