In longitudinal power systems, there is a possibility that a low-frequency oscillation mode gets unstable because of autoparametric resonance. The resonance occurs through interaction between two oscillation modes. In this paper, we calculate the stable region for the resonance by considering the interaction of the modes. First, we calculate steady-state solutions by the harmonic balance method. The steady-state solutions are stable or unstable. If we decrease the amplitude of one mode at an unstable solution, the modes decay. Conversely, if we increase the amplitude, then the modes diverge. Namely, the unstable solution is located on the boundary of the stable region. The amplitudes of the modes are rarely the same as those of the steady-state solution. However, the amplitudes approach to a steady-state after some transients. If the state is in the stable region, the system is stable. If in the unstable region, it diverges. Lastly, we estimate the amount of damping torques necessary to stabilize the system with the newly calculated stable region. Similar results as those by the Mathieu diagram are obtained, for example, certain amount of damping torques can stabilize the system irrespective of its size, AVRs can substantially reduce the amount of damping torques, etc.

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The Single Machine Equivalent (SIME) is a hybrid method resulting from the coupling of a time-domain transient stability program with the equal-area criterion. Its aim is to set up a software as general as the time-domain program with which it is coupled but much more powerful and faster than it. The paper shows how the method reaches the threefold objective: to properly and readily assess transient stability limits (such as critical clearing times and power limits) under any power system modelling and stability scenario, to identify the relevant system machines and to appraise stability margins. In turn, these latter pieces of information open possibilities towards handy sensitivity analysis and transient stability monitoring. In short, they pave the way for real-time transient stability preventive monitoring and control. A variety of sample simulations highlight the method and its specifics.

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The new thermodynamic cycle using hydrogen energy is now under investigation by many engineers. The Japanese World Energy Network research program is also hydrogen technology development program. One of the target of the program is to develop the high thermal efficiency and emission free power plant. The H2-O2 fired gas turbine is the key technology of the program and the advanced RANKINE cycle is suggested as one of the most effective cycle.
The advanced RANKINE cycle is based on the direct steam expansion cycle. The performance calculation has been examined to find the optimal operating point. For the cycle development, further investigations of the component development, the operational ability, and the cost competitiveness are important. Among these investigations, this paper reports the operational ability, especially the start up performance.
In this analysis, the algorithm and flow line for start up is developed. And the investigation finds that the advanced RANKINE cycle has the good possibility for the practical use.

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For estimation of earth parameters of multi-layer earth structures, in many cases, Wenner's four electrodes arrangement is employed. An apparent resistivity curve (ρ-a curve) from results measured with the arrangement has been analysed by using Sundberg's standard curves and Hummel's auxiliary curves. However, in this paper, a ρ-a curve for four-layer earth structure is analysed by using three-layer standard curves and one of three kinds of auxiliary curves of parallel (Hummel's), series and Ono's displacements.
First, the analysed ranges of ρ-a curves and the correlated ranges of three-layer standard curves are discussed for eight ρ-a curve types of four-layer earth structures. Then ρ-a curves are analysed to get earth parameters. For estimating precisions of analysed earth parameters, the so;-a curve is drawn from analysed parameters and compared with the ρ-a curve from measured results. When there is considerable difference between two these curves, corrections are made for each earth parameter step by step. Errors of corrected earth parameters are less than about 15%.

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Numerical simulations of detailed time-dependent behavior are performed on a simulation model in which a coupled system of nonequilibrium disk MHD generator and line-commutated inverter is connected to the infinite bus through the transmission line, showing that continuous operation of the system is usually possible even when the power line fault occurs near the system. Then, time-dependent swing simulations are carried out on another simulation model in which the MHD/inverter system and a synchronous generator are connected in parallel to the infinite bus through the common transmission line, revealing that the rotor angle swing of the synchronous generator caused by the power line fault can be effectively suppressed by the fast output power control of the MHD/inverter system.

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A partial discharge (PD) inside a power apparatus is a symptom of an insulation breakdown and machine trouble and the electromagnetic signals emitted from it interferes with a communication system as a noise. From above, detection and location PD are very important. The method of location PD based on several time delays of arrival of electromagnetic wave emitted from PD among antennas is well known. However, it is difficult to locate PD as a one point. It is better to locate as a space, but as a point. In this paper we propose “Superimposed Positioning Optimization on the Time delay of arrival(SPOT)”as a new method to locate PD. We discuss an effectiveness of “SPOT” and properties of the spaces located.

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Electric field changes associated with lightning discharges were observed by a network of fast antennas at Hokuriku area. These fast antennas were time-synchronized by GPS (Global Positioning System), which enabled location of individual return strokes. It turned out, that at least about. a half of negative multiple-stroke flashes had multiple terminations on the ground regardless of the seasons, and the distances between the terminating points were mostly within km. In winter, all the return strokes in positive multiple-stroke flashes had different terminations, and about 70%, of them were separated for more than 10km. The time intervals between successive return strokes in negative multiple-stroke flashes in winter were generally much shorter than those in summer.

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Lightning-triggered experiments have been conducted both in the north and southeast of China since 1989. Twenty five events of artificial lightning were triggered by using rocket-wire technique. The experiments and characteristics of triggered lightning are briefly reviewed and most of results of these experiments are outlined in this paper. The characteristics of triggered lightning appear to be significant different in the north and southeast of China.

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The criterion of artificially triggering lightning is studied by using the field at triggering height calculated with one-dimensional model and the data from 7-year experiments in China. Results show that the ambient electric field at triggering height is about -60_??_70kV/m in Heilinzi, northwestern China, where there are large amounts of positive charges in the lower part of storm, while that is 15_??_20kV/m in Nanhui, southeastern China, where the thunderstorm has a dipole charge structure. The increase of electric field with altitude will be helpful to supply energy to corona streamer produced at rocket front. The space charge density below 100m can be as high as -1.1nC/m³ in northwestern China and 0.8nC/m³ in southeastern China.

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The insulation recovery characteristics of 500kV class DC insulator assemblies were measured. Switching impulse voltage was applied to a tension insulator assembly and three kinds of suspension insulator assemblies (V, Y and I-arranged) following arc extinction. Arc currents ranged from 400 to 4, 000 A and arc duration ranged from 0.1s to 0.4s.
(1) All insulator assemblies show slow insulation recovery after arc extinction up to an impulse withstand voltage of 100-200 kV. The insulation recovery rates increase at near the impulse withstand voltage of 500kV.
(2) At an arc current of 400 to 4, 000 A, for a duration of 0.1s, the recovery rate for all insulator assemblies decreases with increases in arc current. This is believed to be due to the increases in the temperature of post-arc gas caused by the increased arc currents.
(3) When the arc duration is increased from 0.1s to 0.2s at an arc current of 4, 000 A, the insulation recovery of the V and Y arranged assemblies is delayed by 0.05s. This is believed to be due to the increases in the temperature of post-arc gas caused by the longer arc duration.
(4) By measuring the insulation recovery characteristics of the tension insulator assembly and three kinds of the suspension insulator assemblies with an arc current of 4, 000 A and an arc duration of 0.1s, the minimum value of non-voltage time for a 500kV DC transmission line is calculated to be 0.17s.

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High field is very favorable for magnetically confined fusion devices such as aa tokamak, but electromagnetic force derived from the field becomes aa fatal problem. Especially, the largest in--plane centering force hinders toroidal field coils from operating at higher fields. Variable pitch multi-helical coils, we are calling force-balanced coils (FBCs), by which this force is drastically reduced, are proposed for the coil system of tokamak. FBG can also provide poloidal flux swing for plasma breakdown and current induction during the ramp-up phase of the coil current. In this paper, it is indicated how to design FBG for tokamak. The fusion reactor-size FBCs are designed and compared with that of the conventional toroidal field coil system.

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A preliminary analysis of two-phase flow in the pulsed MHD generator Pamir-3U is carried out. The two-fluidmodel for dusty gas flow is applied to treat the two-phase working body which consists of combustion gas and liquidparticles of Al₂O₃. One-dimensional time-dependent calculations show that the velocity lag and the thermal lag between the two phases are large when the particle diameter is 15μm. The lags become small when the diameteris small because the decrease of the diameter increases the momentum transfer and the heat transfèr between thetwo phases. When the large Lorentz force develops a shock wave, the interaction between the two phases relaxesthe shock wave. The increase of particle diameter decreases the channel current and the the power output becausethe increase of the diameter results in the decrease of the energy conversion from the liquid phase.

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The use of AC arc plasma heating involves a problem that the arc plasma is apt to be unstable at current zero. This paper describes the stability of the transferred type AC arc plasma with the arc current increased and the effect of superimposing a pulse current on an AC current only at each current zero point on the arc plasma stability. The argon arc plasma was generated in the air between the tungsten electrode in the plasma torch and the copper counter electrode. The distance between the two electrodes was 40mm. The AC power supply was single phase and the voltage was 180-330Vrms, the current, 100-500Arms, the frequency, 50Hz. The superimposed pulse current was 5-15A, the pulse voltage, 100V, the pulse rise/fall time, 2, μs, the pulse width, 300, μs. Main results are as follows. As the arc current increased (100→500A), the re-ignition voltage decreased (290→240V), and the required power supply voltage for the arc plasma to stabilize was decreased (330→270Vrms), and through the investigation it was guessed that the required voltage would not decrease so much even if the current increased more than 500A. The superimposing pulse current caused the required power supply voltage to decrease farther (270→180Vrms). As the arc current increased and a pulse current was superimposed at each current zero point, the temperature and the electric conductivity between electrodes at current zero point would increase and the transient recovery voltage between electrodes would rise rapidly. Therefore, the required energy for the arc plasma to re-ignite and stabilize would decrease and the required power supply voltage would decrease.

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In order to study lightning problems of low-voltage power distribution lines, lightning overvoltage waveform were observed inside the house of customers. And the cause of lightning overvoltages was examined from observation of striking points by still cameras. Lightning overvoltages of 62 waveforms were recorded by observation for about four years. Observed waveforms can be classified into three types of single polarity (positive or negative), both polarity (which changes from positive to negative or negative to positive), and pulsive waveform. The causes of these lightning overvoltages which were estimated from striking points are shown as follows:
(1) Induced lightning overvoltages on low-voltage distribution lines.
(2) Electric potential rise due to discharge of surge arresters or current of overhead ground wire.
(3) Shift of lightning overvoltages from high-voltage side of transformer to low-voltage side, which due to electro-magnetic induction.

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