Phasor Measurement Unit (PMU) is an apparatus which detects the absolute value of phase angle in sinusoidal signal. When more than two units are located distantly apart from each other, and they are synchronized with GPS signal which tells us the information on exact time, it becomes ready to get phase differences between two distant places. Thus, PMU with GPS receiver is applied to the monitoring of AC power system dynamics and usually installed at substations of transmission lines. The states of power network are uniquely determined by the active and reactive power and the magnitude and phase angle of voltage in each node. Among these values the phase angle had not been easily obtained until the scheme of time synchronism with GPS appeared. In this report, the history of GPS and PMU, and the current status of the applications in power systems in the world are presented. In Japan we are developing a power system monitoring system with PMUs installed at University's campuses with 100V outlets, which is called Campus WAMS. This report also introduces some results from the Campus WAMS briefly.

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In this paper, high speed searching of stability limit output for preventive control of a power system is proposed. It is necessary to understand a range of stability limits of a generator in various possible operating conditions to execute preventive control. The possibility of an accident increases as a power system becomes large. Therefore, it takes a certain amount of time to conduct a stability analysis. Then, we propose three kinds of searching techniques (straight line method, quadratic curve method and spline curve method) to search for a stability limit in the power system at high speed, and the most effective method is identified through comparison of these methods. In this case, the mode coupling method (MC method) was used for the presumption of the eigenvalue that evaluated stability in the steady state stability analysis of the power system. Moreover, the MC method used this time selects major modes for that we pay attention and highly related modes. Next, these two or more selected modes are coupled and new eigenvalues of the coupled matrix are calculated. The size of the coupled matrix is very small. As a result, it found to be an effective method in which non-liner estimation of the eigenvalue is possible. Also, it became possible to search for the limit of the output in high accuracy by setting the estimated error as the situation demands.

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Pulsed electromagnetic waves causing interference may be emitted from power equipment owing to a discharge phenomenon, which may occur at a faulty point, such as an unintended narrow gap between metal supports or a deteriorated insulator. Therefore, it is necessary to localize the electromagnetic source or estimate its direction of arrival (DOA) effectively to reduce maintenance and eliminate its effect on communication systems.
There is also a possibility that multiple sources of electromagnetic wave and multiple waves exist in power equipment. In this study, we apply the signal subspace method to the DOA estimation of the electromagnetic wave. The signal subspace method is developed to estimate the DOAs of continuous signal waves. This method can handle multiple waves and is based on the eigenvalue decomposition technique.
When electromagnetic waves are emitted from spherical gaps attached to a test transmission line, We can estimate those DOAs within the errors of 2 degrees, and confirm the applicability of the signal subspace method to the DOA estimation of the electromagnetic wave due to the discharge phenomenon.

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A steep-front square pulse voltage generator, which has a rise time of 16ns, a dumping factor less than 2.5% after 10μs duration and a peak value of 200kV, is successfully used to survey the V-t characteristics in nanosecond range. The V-t characteristics of CF₃I, SF₆, CF₃I/N₂ and SF₆/N₂ gas mixtures under non-uniform field gaps by using the steep-front square pulse voltage. When the field utilization factor are measured less than 0.38, the sparkover voltage of CF₃I is lower than that of SF₆. On the contrary, when the field utilization factor is more than 0.38, the sparkover voltage of CF₃I is higher than that of SF₆. The corona inception voltage calculated by using Schumann's criteria and compared with measured one. The V-t characteristics for a uniform field gap in CF₃I/N₂ gas mixture containing N₂ gas of 40% are equivalent to those in pure SF₆ gas. The synergism for appears when CF₃I gas is mixed with N₂ gas.

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63kA-50Hz 90% short line fault (L90) is severer than 50kA-60Hz L90 from the points of view of electric endurance and interruption window. Interruption criteria and an optimization method of the chamber design are discussed to improve thermal interruption capability with it. The combination of the SA (Simulated Annealing) and MMFD (Modified Method of Feasible Directions) is applied to the optimization procedure. The optimized chambers with superior electric reliability are designed at 63kA-50Hz to improve gas density at current zero in thermal volume. 63kA-50Hz L90 interruptions are performed with the new chamber. In addition, influence of gas blast angle of nozzle on L90 interruption is clarified.

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No load loss of amorphous transformer (i.e. amorphous metal-based transformer) could be reduced by about 70% compared with traditional transformers (e.g. silicon steel-based transformer). Amorphous transformers have the potential to realize large amount of energy savings and reduction of CO₂. However, amorphous transformers have disadvantages of high initial costs and high load loss parameters compared with traditional transformers. In Japan, JIS (Japanese industrial standards) set two regulations about efficiencies of transformers. One is a regulation about amount of total loss at equivalent load factor P_e = 40%, which is made to improve efficiency in actual usage. The other is a regulation about efficiency of rated load (P_e = 100%).
Authors propose an optimal design of transformers considering trade-off between the material cost and power loss cost with taking into account the standard in the rated load. Economies of transformers are evaluated in the paper by the sum of the initial cost and the power loss cost.
Simulation results show that the two regulations about efficiency of JIS are not reasonable. If there were not the regulation in the rated load, we could design low initial cost and environmentally compatible transformers.

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The application of externally gapped line surge arresters (EGLAs), which have been developed and established in Japan, is now expanding into many countries. Therefore, the maintenance team 4 (MT4) in the international electrotechnical commission (IEC) technical committee 37 (TC37) for surge arresters advances the standardization works to specify the minimum criteria for requirements and testing methods of EGLAs. EGLAs are effective lightning protection of overhead transmission lines, and have unique required performances originated from the external series gap. The unique required performances of EGLA are insulation coordination performance of EGLA sparkover voltage for lightning overvoltage with the insulator assembly to be protected, withstand voltage performance for switching surge overvoltage and TOV, and follow current interruption performance.
This paper discusses the specific issues to be considered for the standardization, such as classification of lightning discharge current rating and a test procedure for follow current interruption performance, based on the Japanese technology through more than twenty years of experience with a large numbers of EGLAs in 22kV to 500kV systems.

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Pressure measurement with insulation tubes is successfully performed at the nozzle throat, in the upper flow path and in the thermal room for the two types of tandem-puffer (self-blast chamber) adopting different gas blast angle of nozzle. The pressure rise mechanism with auto-expansion effect of arc is discussed. The pressure rise in the upper flow path and the thermal chamber is driven by propagation of pressure wave from the arc to the thermal chamber. And several types of oscillation caused by rarefaction wave after the pressure wave and multi-reflection of the pressure wave are superposed on the pressure profile. Finally, an influence of the gas blast angle of the nozzle on cooling of stagnation point (thermal interruption capability) is explained based on the results of these measurement and 2-dimensional thermo-fluid analysis. A little larger gas blast angle of the nozzle leads to stronger gas flow to the stagnation point caused by a little larger resistance to the pressure wave and the gas flow.

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