An application guide for evaluation of harmonic currents emitted by consumers of middle or high voltage power supply was issued in 1995 but it has not been amended since then. It was amended for the first time and a new version was issued this year. This article explains some important points of the amendments.
In recent years, the number of connection of renewable energy sources (RESs) such as the photovoltaic generation (PV) and wind power generation is increasing to reduce the influence on the environment. The outputs of these RESs vary rapidly with the influences of the weather, conditions of location, etc. Therefore, the point voltages of distribution system are worried that the voltages vary drastically and the voltages may deviate from the proper range of voltage by the influence of RESs connected to the distribution system or diversification of loads. Moreover, it is apprehended about the adverse effect on the electric power quality such as voltage imbalance and harmonics. In this paper, the authors propose a cooperative voltage control method of a distribution system by the system voltage control equipments in consideration of reducing capacity of the static synchronous compensator (STATCOM). In order to verify the validity of the proposed method, the numerical calculations were carried out.
Recently, there are problems with environmental issues such as global warming and depletion of natural energy resources. Photovoltaic systems as renewable energy have been attracting attention. The output power of the photovoltaic system changes steeply, because solar radiation intensity and temperature depend on the weather. In the conventional photovoltaic systems, the surplus power excluding the power consumed in the load flows back to the grid. Therefore, the fluctuation in the output power of the photovoltaic system have an bad influence on the grid. This paper proposes a novel hydrogen-storage distributed generation system using unitized reversible cell integrating the fuel cell and water electrolyzer for smoothing the reverse power flow. The hydrogen-storage distributed generation system is modeled and the computer simulation is implemented. It is clarified that the proposed system is valid for leveling the power flow back to the glid by two evaluation methods.
This paper proposes a new control method for variable speed wind turbine with permanent magnet synchronous generators (PMSG) in the grid connected wind farm to damp load frequency oscillation of power system. In the wind farm, fixed speed wind turbine with induction generators are also considered to be installed together with the PMSGs. The control system of PMSG is developed in such a way that PMSGs can give contribution to control frequency oscillation of the grid system. The method is based on the control of the kinetic energy stored in rotational mass of the wind turbine and it can be achieved by controlling pitch blade angel and power output reference of the PMSG. Simulation study using multi machine power system model is performed by using PSCAD/EMTDC to demonstrate the effectiveness of the proposed control method.
It is important that PV systems detect islanding caused by power system fault to ensure electrical safety. So, we propose a novel islanding detection method which detects harmonic impedance by injecting the harmonic currents synchronized with exciting current harmonics of the pole transformer. In case that an induction motor load is included in the load system, the basic experiments of islanding detection and simulation analysis are conducted to verify the effectiveness of the proposed method.
Distribution surge arresters with an internal series gap have been installed on Japanese distribution lines in high density for reducing lightning-related overvoltages. In contrast, surge arresters are sometimes damaged by direct lightning strokes, particularly in winter. Measurement of insulation resistance between terminals of the surge arrester and measurement of DC discharge voltage of the surge arrester are generally adopted to detect damaged arresters. However, if metal-oxide (MO) varistors are damaged and the internal series gap is not damaged, it is difficult to detect damaged arresters by two conventional methods. Therefore, this paper presents the proposed detection method which can accurately detect damaged arresters. In the proposed method, first, the voltage between terminals of the surge arrester is measured when the output of an impulse generator is applied to the surge arrester. Secondly, the damaged arrester is detected by the comparison between its voltage waveform and the voltage waveform of the non-damaged arrester. The detection rate of damaged arresters by the proposed method is remarkably higher than those by two conventional methods.
The aerial insulated wires are supported by post insulators and binding wires on the reinforced concrete pole. When a lightning flash strikes near by the wires, the overvoltage due to the indirect lightning surge induces to the conductive wire core. The creeping discharge can develop along the wire surface from the free end of the binding wire through post insulator and reinforced concrete pole compound when a flashover occurred at the post insulator and it also leads to damages on the wire such as melting or snapping. It could be occurred under both rainy and thunderstorm condition. In this case, it indicates that the wire surface is stressed by lightning impulse under wet condition. Consequently, it is an important subject to prevent the wire damages and to clarify the characteristics of creeping discharge along the wire surface under dry and wet condition. In the previous studies, we had observed the characteristics of creeping discharge along the wire surface under dry condition. In this study, we examined creeping discharges under both the impulse voltages with various durations of wave front and wet condition on the wire surface. Then, these voltages were applied to the conductive wire core as the inductive lightning surge. The length and phenomena of negative creeping discharge developing along the wire surface were measured using a still camera with an image intensifier. We report the developing characteristics of negative creeping discharge along the wire surface under wet condition.
When a fault arc occurs in oil-filled electrical equipment such as transformers in transmission and distribution systems, it generates a flammable gas and a dynamic pressure fluctuation that may trigger an explosion. The purpose of this fundamental study is to investigate the effects of an underwater arc on pressure fluctuation in a closed vessel containing air and water before examining the arc phenomena in the electrical insulating oil. In particular, this paper is intended to investigate the influence of the depth of the underwater arc on the peak pressure amplitude in air and water. The pressure fluctuations in both air and water are measured under experimental conditions involving an arc current ranging from 2.3 to 6.7kA, an arc duration of 20ms and a depth of underwater arc Darc from 0.255 to 1.135m. Experimental results show that the depth of underwater arc influences not only the waveform of pressure fluctuation in air and water but also the peak pressure amplitude though the same electrical condition of the arc. In addition, the maximum pressure rise in water has a depthwise distribution due to a water movement.
A concentrator photovoltaic (CPV) system which can reduce the power generation cost of PV system has been developed and assessed for commercialization. An early-type CPV module consists of 20 pairs of dome Fresnel lenses the geometrical concentration ratio of which is X550 and three junction solar cells (InGaP/InGaAs/Ge). We had carried out a field test of the early-type CPV module to evaluate the power generation, the efficiency, and other characteristics from 2004. The Fresnel lenses had never been washed until 2012 were washed and some characteristics of the CPV module were compared in before and after cleaning the lenses. Before the cleaning, many micro particulate dust included 46wt.% of C was on the lenses. Its number density decreased from 600mm-2 to 230mm-2 with cleaning the lenses. However, some characteristics of the CPV module such as I-V curve, FF and electricity efficiency were not changed by cleaning the lenses. The influences of yellow sand (Asian dust) which decreased the direct irradiance and covered on the lens surface from some deserts in interior of East Asia did not continue persistently. This is because, any dirt would not cover easily on the lenses surface, since the CPV module was moved to track to the Sun, and it had the dome-shaped Fresnel lenses which had smooth surface.
Water trees in insulator of XLPE cable may considerably reduce the dielectric breakdown voltage, thus being an important lifetime-governing factor. AC superposition method we have investigated is a new technique for hot-line diagnosis of 6.6kV XLPE cables, and the diagnostic apparatus using this technique is now widely used in the field of distribution line. In order to study an application of the AC superposition method to degradation diagnosis of 22/33kV class XLPE cables, we measured deterioration signals of the cables with water trees by modified above diagnostic apparatus. The deterioration signals, hereafter called “AC superposition current”, were generated by an amplitude modulation effect due to non-linear resistance of water tree. Moreover, we evaluated the relationship between the AC superposition current and the AC breakdown voltage. It is difficult to judge whether the correlation of them is present or not because of uneven distribution of the measured data. However, the AC superposition current tends to increase linearly with a decrease in the residual thickness of the insulator. For example, the AC superposition current was about 40nA when the residual thickness decreased to 3mm. Thus, we consider that the AC superposition method is available for degradation diagnosis of 22/33kV class XLPE cables.
Tokyo Electric Power Company (TEPCO) introduced the Lightning Position And Tracking System (LPATS) Version III in 1990. The system uses the time-of-arrival technique for lightning location. TEPCO had studied the operating characteristics of the system, and had confirmed its effectiveness. However, further improvement of the functions and performance was necessary from the viewpoint of utilities. TEPCO, therefore, began development of the LPATS-T (version TEPCO) in cooperation with Global Atmospherics, Inc. and formally put the system into operation in 1996. Performance of LPATS-T at TEPCO was evaluated based on comparison with actual lightning current measurement data at transmission towers. Forty negative first return strokes observed at instrumented transmission towers were detected by LPATS-T from 1996 to 1999. Location error was estimated to be about 600m on average. Estimated peak currents were compared with directly measured values.
In Japan, electric power companies have been spending a large amount of money on the construction cost and maintenance cost of low grounding resistances connected to surge arresters (Arrs) that is basically maintained below 30Ω. In a recent study, it has been clarified that the rate of protection against direct lightning hits on distribution lines with an overhead ground wire (GW) hardly decreases, even if the grounding resistance of Arrs is high. However the installation of Arrs on a pole where a pole transformer (Tr) is mounted has not been considered, and in the case of a high grounding resistance, the lightning surge behavior observed in customer's equipment has not yet been clarified. To examine the grounding resistance of Arrs, it is necessary to understand such behavior in the case in which the grounding resistance of Arrs is varied. In this paper, we clarified the relationship between the grounding resistance connected to Arrs and lightning surge behavior in customer's equipment. The result shows that the grounding resistance of Arrs installed on a pole with a Tr can be increased to some extent without decreasing the lightning protection level of the customer's equipment by the installation of a GW and the shortening of the interval between concrete poles with Arrs. A reduction in the cost of grounding construction can be expected.
The plasma fluid flow behavior and power generation characteristics in a Faraday-type magnetohydrodynamics (MHD) generator using high temperature inert gas (argon) plasma has been examined by a time-dependent two-dimensional numerical simulation. The inhomogeneous and unstable plasma at an inlet total temperature of 7000K results in the reduction and fluctuation of the output power. The plasma changes to homogeneous and stable one with the increase in the inlet total temperature to 9000K. One of the reasons of the suppression of the ionization instability can be due to the weakness of the dependency of the electrical conductivity on the electron number density because the coulomb collision of electrons becomes dominant under the departure from Saha equilibrium.
In this letter, a lightning observation camera system and an observation result are represented. The system consists of a slow-speed CCTV camera, a consumer high-speed digital camera EXILIM EX-F1S and a personal computer for controlling the camera system. Its total cost is a fraction of standard high-speed cameras used for the lightning observation. Observation of downward lightning strokes to the top of TOKYO SKYTREE was succeeded by the camera system.