If we rely only on the conventional technology, it may be difficult to meet the recent requirements to electric power system such as further improvement of the quality of electric power supply, environmental compatibility and so on. Some of the limitations of comventional technology can be broken through by the application of superconductivity technology. Roughly speaking, superconductivity-applied components which have relatively high feasibility are superconducting generator, small capacity SMES and liquid nitrogen cooled superconducting cable. To use them practically, reliablilty, maintenability, controlability and economy have to be improved.

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Electrical breakdown mechanism of liquid nitrogen or liquid helium is investigated in the presence of thermally induced bubbles to simulate the quemching state in superconducting magnets. The results show that the mechanisms are classified into three categories which are related to the slope of the applied voltage: breakdown without deformation of suspended bubble at fast rising voltage, breakdown with elongation of suspended bubble at medium rising voltage and breakdown through locked vapor at slow rising voltage. The investigated subjects may relate closely to the phenomena in superconducting pulsed magnets or AC superconducting apparatus in which AC loss is produced together with a high voltage.

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It is generally understood that a mechanical disturbance due to an abrupt conductor motion is one of main causes of premature quenches in a high-performance and high-current density superconducting magnet. It is considered that this conductor motion is caused by insufficiency of force supporting the conductor. In an usual magnet of significant scale, the conductors are supported by frictional force between the spacers and the conductors. Generally, there are irregularities in contact forces at the interfaces between the conductors and the spacers, which are caused by local bent of the conductor and fluctuation in the dimensions of the spacer. Therefore, there are probably the interfaces where the contact forces are not enough to support the conductor subject to the electromagnetic force.
In the paper, we assume that local bent of the conductor and fluctuation in the dimensions of the spacer are gaussian stocastic processes and calculate the probability of the conductor motion and the expectation of the number of premature quenches.

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The superconducting field winding of 70 MW class high response excitation superconducting generator requires the maximum operation current of 4.5 kA at the field of 6 T and needs to allow the field change of 10 T/s. Besides these, superconductors have to meet the requirement of saddle shape winding in the slot of the rotor and require the mechanical strength withstanding the centrifugal force as well as electromagnetic force.
The developed conductor has the configuration of double stranded cable, consisted of NbTi, Cu and CuNi. An optimization was made, taking into account the requirement of low AC loss and high current density. The critical current of 13.6 kA was achieved at the field of 6 T and the AC loss was 6.9 kW/m³, small enough for the aim of 10 kW/m³. The developed low loss high current density superconductor will be able to be applied to the field winding of 70 MW class high response excitation superconducting generator.

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This paper describes experimental results of investigations on electrical and mechanical characteristics of armature coil bars to develop the insulation and cooling structure of double transposed conductors for 70 MW class superconducting generators. Four kinds of bar strands consisted of transposed small rectangular wires with different insulation structure and short armature coil bar samples with those strands were trially produced. Electrical and mechanical characteristic tests verified that epoxy resin impregnated single fiber glass cover insulation on a polyamide-imide resin coating was most available for armature coils of air gap windings, and this structure had a sufficiently high durability for its use in 70 MW class machines.

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We are developing a double layer damper which will consist of a squirrel-caged warm damper and a cylindrical cold damper as a part of our 70MW superconducting generator development project. To evaluate the characteristics of unbalance current capability, magnetic shielding and damping for rotor swing, we have constructed a 1/2 scale rotor model and conducted an experimental study. From analysis of the experimental results, we have developed design guidelines and refined our FEM magnetic field analysis technique.
The main results are summerized below:
(1) The squirrel-caged warm damper has sufficient capability against negative sequence current.
(2) Damping characteristics can be designed to be effective near the rotor swing frequency.
(3) Flux shielding at low frequencies corresponding to quick response excitation is sufficiently small so as not to prevent field flux change.
(4) Magnetic field can be analyzed by the refined FEM field analysis technique which now takes into account the effects of the rotor end region by estimating the end resistance based on eddy current flow path.

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Superconducting generators have many advantages, one of which is their capability of improving the stability of power systems because of the low synchronous impedances. The advantage may be increased by adoption of high-response excitation. This led us to design and manufacture high-response excitation superconducting generator of 100kVA, which is world's first one of this type. One of the problems to solve on superconducting generator of high response excitation is the characteristics of the field circuit, which means the field current, the field magnetic induction, the flux linkage of armature winding, the loss of component material, the loss of superconducting field winding and so on. In this paper, we discuss the characteristics of the field circuit by use of 2-dimensional electro-magnetic field analysis.

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In order to define problems on superconducting generators (SCG) in power systems, we have been studying characteristics of SCG by use of the 20kVA SCG. In this paper, we describe one result of our studies, that is, Parallel running experiments of the 20kVA SCG and a conventional generator (CG). In the experimental system, the SCG is connected to regional power system (infinite bus) through reactors (artificial transmission lines) and the CG is connected to the terminal of the SCG. The system becomes unstable by increasing the output power of either generator. We pay attentions to processes where the system is going to an unstable state. We observe two types of the processes, that is, the slip of SCG is positive or negative. After isolating the CG at unstable states, two cases are observed, that is, the SCG returns to stable operation or not. By simulation studies, we discuss, in detail, the above mentioned characteristics given by experiments.

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AC Supercoducting wire is being developed for electrical equipment such as superconducting transformers and superconducting generators. AC loss reduction is of primary concern in the development of such high-efficiency equipment. In reducing AC losses, it is necessary to develop assessment methods for AC loss in test samples which have shapes similar to the end-product equipment.
This paper describes a least squares calculation of AC losses of superconducting wire as a function of frequency and magnetic field strength measured in test coils.
Two sample solenoid coils were made to test the influence of different capacities and winding methods on AC losses in AC superconducting coils with rated capacities of 500kVA and 20kVA, and impregnated (epoxy resin) and non-impregnated windings.
The AC losses in the superconducting coils were measured by a calorimetric method using the evaporating rates of liquid helium. Estimated AC losses in the superconducting wire of the two coils were compared with joule losses of copper conductors at ambient temperature. As a result of this comparison, a low-loss AC superconducting wire winding can be made for electrical equipment rather than using conventional copper winding when used under low magnetic fields under 0.5 T.

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The development of ultra-fine filaments superconductors for 50/60Hz use opens a door to a fully superconducting generator which has superconducting field windings and armature windings. Omitting dampers and narrowing gaps between the field and the armature windings, the size and the weight of the fully superconducting generator can be drastically decreased. However, the damperless generator can not be stably operated and needs to be stabilized by an appropriate method. We have investigated a method to stabilize the generator by controlling excitation voltage of the field winding using state-value field-back technique. The state values of the system are estimated by a state-value observer and the feed-back constants are determined according to the optimal control theory. It is shown that this method is effective to stabilize the fully superconducting and damperless generator.

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The advantages of synchronous generator with superconducting field winding can be stated as: (1) Reduction in losses, (2) Improved system performance, (3) High-Voltage armature feasibility, (4) Small size and weight. Some feasibility studies have been carried out, and currently some model machines for demonstration are under construction.
The superconducting field winding which produces strong magnetic flux can be viewed as an energy storage and its application to power system stabilization has been proposed and the damping effect of power swing has been confirmed numerically.
In this paper an improved control scheme for power system stabilization which has the function of voltage regulation by the control of field winding current, is proposed. Some numerical examples demonstrate that the proposed system is capable of suppressing the voltage fluctuation as well as damping power oscillation, significantly. Furthermore, the specifications of the superconducting generator necessary for the stabilization of a model power transmission system are estimated in detail. In the case of 1, 120MVA generator with 24MJ stored energy at 6kA rated field current, the maximum DC voltage is 3, 300V, the maximum rate of field current change (dI/dt) is 3, 500A/s and the maximum power exchanged between the field winding and the power system is 20MVA.

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The intension of large scale power system interconnection is to achieve extremely economical and reliable power generation and transmission. It has established the present power systems of high quality. On the other hand, in the large power systems with complex configuration, undamped power swing with low frequency caused by the synchronous power between interconnected systems tends to occur as well as undamped power swing caused by the synchronous power of specific generators. Several coordinated stabilizing control schemes for the power systems by sets of power system stabilizers (PSS's) have been investigated so far. PSS is very effective for the stabilization of power swing among a few specific generators because the function of PSS is achieved by the voltage control using the generator field winding circuit. However, it seems that PSS do not have sufficient ability to stabilize the power swings between interconnected systems. In this paper, the superconducting magnetic energy storage (SMES) which is significantly effective for the power swings between interconnected systems, is introduced and a coordinated power system stabilizing control by SMES and PSS's is proposed. The advantages of the proposed control scheme are: 1. high efficiency of the controller by the distribution of functions, 2. independency of the control design for PSS and SMES, 3. robustness of the controller and so on.

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Large area interconnection of power systems are increasing year by year to improve reliability and economy of the interconnected system, and its configuration has become very complex. In such a system undamped power swings with low frequency tend to occur. For the stabilization of the power systems, it is important to detect the unstable mode efficiently and to operate a power system stabilizer adequately according to the detected signal. A system observer is often used for this purpose. On the other hand, the complexity of power system configuration may make it difficult to construct the observer.
In this paper a simple observer is constructed by using the order reduction method for power system models and is applied to the power system stabilization by superconducting magnetic energy storage. Some numerical and experimental studies demonstrate the significance of the state observation and its power stabilization capability.

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Superconducting Magnetic Energy Storage (SMES) has many advantages as a storage element in power systems. It is necessary to study problems on power system including SMES. For example, how the power system behaves during operations of SMES or at faults of SMES. On the other hand, how SMES will respond or should operate when some faults occur in power system. In order to investigate these problems, simulation study is necessary and effective. In this paper, a simulation program for such purposes is discussed and developed. The features of the simulation code are: (1) mode changes of commutations of thyristor converters of SMES are taken into account, (2 ) it is easy to obtain codes for changes of power system configuration. The power system characteristics of SMES in one-machine infinite bus system are analyzed by use of the simulation method for the case of SMES faults and power system faults. The simulation results agree well with the experimental results.

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With the recent abvent of high-performance AC superconductor with very small AC loss and large current carrying capacity, the possibility of air-core superconducting transformer is being studied. Though the exciting current is very large, the air-core transformer has merits of (1) no iron loss, (2) no insulation care to the core, (3) no harmonics, and no inrush current induced by iron saturation, etc.. So, the authors fabricated and tested a small experimental transformer (2.5kVA, 330/150V). As easily predicted, the exciting current becomes very large. It occupies about 45% of the rated current. Meanwhile, the leakage reactance increases considerably. The %IX becomes about 28%. These experimental results show good agreement with the theoretical analysis based on a equivalent circuit.

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A superconducting power cable is one of the promising ways of underground transmission of huge electric power in the future. We have long proposed the idea of the extruded polymer insulation for superconducting cables. The prominent features of the design are to exploit the excellent electrical properties of polymer in the cryogenic temperatures and to separate the helium coolant from the electrical insulation. Although the extruded cross-linked polyethylene cable has proved an ability at the liquid nitrogen temperature, the cable insulation cracked due to mechanical stress during cooling to the liquid helium temperature.
To overcome this problem, ethylene propylene rubber (EPR) was selected as a new insulating material considering the good results of mechanical and electrical tests of EPR pieces at cryogenic temperatures. An extruded EPR insulated superconducting cable in 15m length was fabricated and cooling test down to the liquid helium temperature and voltage test at the liquid helium temperature were carried out with fair success. This is a breakthrough in terms of the electrical insulation design of cryogenic cables.

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The present study describes about superconducting magnets coupled with two kinds of disk type MHD generators. One is coupled with a disk generator in the closed cycle MHD experimental facility FUJI-1. The other one is for a full scale disk MHD generator. These are split-pair magnets.
In the magnet for FUJI-1 facility, a unique structure which supports the coils against the electromagnetic force has been fablicated and the magnet has been operating stably. During MHD power generation experiments, an induced voltage across the terminals of the coils were measured. A magnitude of the Faraday current in the generator was calculated from this induced voltage.
A possible construction of magnets for a full scale disk MHD generator is indicated. It is suggested that a high performance of the generator (output power density of 0.3_??_1GW/m³) can be obtained with high magnetic field up to 10 T.

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This paper presents a new algorithm to solve an optimal power flow problem which can take into considerations discrete nature of some facilities in power systems. The optimal power flow problem is formulated as a nonlinear mixed-integer programming problem in which the number of transformer taps and the number of shunt capacitor units and reactor units are treated as discrete variables. This paper attempts to solve such a large-scale nonlinear mixed-integer problem by some effective programming techniques. The optimization procedure of the algorithm is that the nonlinear mixed-integer programming problem is iteratively linearized and solved by an approximation method for linear mixed-integer programming. A fundamental feature of the algorithm is that it can guarantee a solution which is both discrete feasible and near-optimal. The validity and efficiency of the algorithm is demonstrated by the numerical results of real-scale optimal power flow problems.

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Lightning is one of the most important reasons for proper insulation design of power transmission apparatus. It has been known that lightning progression features in winter and summer seasons differ in the occurrence process and a scale and so on. There are unknown lightning parameters though a lot of records of lightning observation were obtained up to this time.
This report deals with a new lightning progression model and the estimation methods of the various parameters of the lightning phenomena quantitatively based on the data of long gap tests and lightning observations. The new progression model is applicable to not only winter lightning but also summer lightning, and the values of various parameters such as the charge density of a stepped leader, the leader development velocity, the time interval between steps, the time-to-discharge of first stroke, the final striking distance and so on are estimated by a new calculation method. The new model is useful for a computer simulation of lightning progression and development of a new lightning shielding theory.

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Grounding equipment will be set up after general consideration required earth resistance and area of equipment. Generally, long distance is required to obtain the low resistance by equipment such as buried parallel conductors. It is successful method that the long parallel conductors are buried in the earth because of following reasons: (1) it is enable that the long conductors are buried along the road; (2) earth resistivity and earth resistance are easily estimated for the equipment buried near the surface; (3) the cost of grounding works are relatively less than the boring electrodes. It is very important to estimate accurately the earth resistivity because earth resistance in homogeneous earth is directly proportional to resistivity.
In this paper, the equation was presented in order to correct the apparent earth resistivity considering the length of the current and potential electrodes in earth. Subsequently, the equation was derived to obtain the earth resistance of buried short length conductor, and earth resistance and distribution of flowing current are investigated. When the grounding conductors are long, it is necessary to measure the accurate earth resistance paying attention to the position of the current and potential electrodes. Then, it was proved the range of the measurement errors due to the position of the potential electrode.

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