Preventive maintenance of high-voltage transmission power lines is a demanding task, but the obstacles mounted on the lines have so far prevented the automation of this task. Expliner aims to overcome such obstacles by controlling actively the position of its center of mass, thus changing its configuration as needed when moving on the power lines. This work presents the concept of Expliner, its electrical design and results of field experiments performed with very high voltages.

View full abstract

Recently, total number of distributed generators (DGs) such as photovoltaic generation system and wind power generation system connected to an actual distribution network increases drastically. The distribution network connected with many distributed generators must be operated keeping reliability of power supply, and power quality. In order to accomplish active distribution network operation to take advantage of many connections of DGs, a new coordinated operation of distribution system with many connections of DGs is necessary. So far, the authors have proposed a coordinated operation of distribution network system connected with many DGs by using sectionalizing switches control method, sending voltage control method, computation method of acceptable maximum output of DG and determination method of optimal smoothing time constant of wind power generation system with storage battery. In this paper, the authors develop an experiment of scaled-down three-phase distribution system with distributed generators in order to check the validity of the proposed approach.

View full abstract

Controlled switching, taking into account the residual flux level within a transformer core, can effectively eliminate inrush currents. Many switching sequences assume the residual flux as a constant value, which can be obtained by a measurement after a transformer de-energization. However, in case of a transformer system that is switched by a circuit breaker equipped with a grading capacitor, the residual flux characteristic cannot be considered as constant. A source voltage will feed the deenergized transformer system through the grading capacitor, which will change the residual flux level and let oscillations appear. It follows that the optimal re-energization targets change and inrush currents might not be optimally minimized. Further, transient voltages based on line failures can influence the residual flux through a grading capacitor as well. At first, this paper evaluates the influence of a grading capacitor on the residual flux characteristic analytically. Further, measurements of two transformer systems at a varied de-energization instant and for different grading capacitors provide actual information for the development of future controller systems.

View full abstract

A generation fluctuation of a Photovoltaic (PV) system is one of the most important grid integration issues with dissemination of large amount of PV systems. In grid integration issues, the short time generation fluctuation of PV system such as from a few seconds to a few minutes influences the demand-supply balance of the utility grid. PV systems are, however, installed as a distributed system, and in fact residential PV systems have been dominated in the Japanese market and installation capacity. In the circumstance, the generation fluctuations of many PV systems cancel each other to yield the less-fluctuated total PV system generation in a large area. This phenomenon has been called as “a smoothing effect”. In this paper, the smoothing effect of PV systems in large area, around 100km radius area, is evaluated by means of three fluctuation parameter, maximum deviation width, deviation of moving average, and Furrier analysis to get its statistical feature.

View full abstract

That ever increasing electricity consumption, progress in power deregulation, and rising public awareness for environment have created more interest in fuel cell distributed generation. Among different types of fuel cells, solid oxide fuel cells (SOFCs) manifest themselves as great potential applications due to many advantages such as low emission, high efficiency, and high power rating. On the other hand, SOFC systems are beneficial because they can convert fuel such as natural gas (almost CH₄) which is supplied by widespread systems in many countries into electricity efficiently using internal reforming. In facts, the load demand changes flexibly and fuel cell life time decreases by rapid thermal change. Its lifetime may be extended by maintaining in appropriate temperature. Therefore, it is important to acquire the load following performance as well as control of operation temperature. This paper addresses components of the simple SOFC power unit model with heat exchanger (HX) included. Typical dynamical submodels are used to follow the variation of load demand at a local location that considers temperature characteristics using the Matlab-SIMULINK program.

View full abstract

Sensitivity of electro-magnetic contactors for voltage sags is studied by experiments and simulations. An experimental circuit with a semiconductor fault switch that controls the phase angle of system voltage at the initial point of voltage sags is developed. The phase angle is a new parameter for voltage sags. The operating characteristics of the contactor are evaluated in various conditions. It is found that shallow sags may cause faster trips of contactors compared to deep sags in some cases. The reason of this phenomenon is analyzed with simulations.

View full abstract

The purpose of this study is to verify how the swirl vane influences the acceleration performance, namely, the radial gas velocity and the static gas pressure, for the Disk MHD accelerator. A quasi-1-dimensional (Q1D) numerical program is used for the calculations. Results of the current calculations show that the static gas pressure decreases approximately 40% when using the inlet swirl vane. It is found that the MHD compression phenomena, which generates at the closest to the MHD channel inlet due to the Joule heating, could suppress effectively. The maximum radial gas velocity of 3,380m/s is successfully achieved at the channel exit when swirl ratio was -1.0, and swirl ratio was set to be 0.0 and mass flow rate was kept the same as that for the case of swirl ratio of 1.0 and -1.0. The acceleration efficiency of 40.5% and 36.7% are calculated when the swirl ratio is -1.0, and the case of swirl ratio was set to be 0.0 and mass flow rate was kept the same as that for the case of swirl ratio of 1.0 and -1.0 respectively. The difference of efficiency is due to increase the Hall parameter in the upstream and midstream of MHD channel. This current study can show and confirm the function of inlet swirl for Disk MHD accelerator.

View full abstract

When circuit parameters are known, it is easy to calculate the modified transient recovery voltage (TRV) with resister breaking or MOSA (Metal-oxide Surge Arrester) operating by numerical simulations. But, when the TRV is only known, it is very difficult to calculate the modified TRV by numerical simulations. This paper shows the theoretical analytic method of the modified TRV at such a circuit impedance modification as breaking with parallel resister or MOSA operating.
A TRV can be calculated by injecting a current from circuit breaker terminals to back impedance. TRV and injected current wave shapes can be expressed with a group of ramp waveforms in Laplace domain. By using our analytic method, the back impedance can be easily derived in the Laplace domain from the TRV and injected current waveforms. As a result, it is shown that the modified TRV at circuit impedance modification can be calculated.

View full abstract

Distribution surge arresters are often damaged by lightning strokes, in particular, winter lightning. An overhead ground wire (OGW) is one of effective measures against surge arrester failures. However, adding the conventional OGW to existing overhead power distribution lines needs the power interruption for construction as well as high costs because of installing them above phase conductors. Experimental results show that a covered conductor for distribution lines is more difficult to attract lightning than a bare conductor. Moreover, lightning strokes to distribution pole heads occupied over 90% of all lightning strokes in the observation result of lightning strokes to actual distribution lines without the conventional OGW, and lightning strokes to power lines were hardly observed. These results indicate that the pole heads shield the power lines from direct lightning strokes. Therefore the author studies the application of an OGW under the distribution lines (UGW) for reducing surge arrester failures. The lightning performance of the UGW is estimated by the Electro-Magnetic Transients Program (EMTP) and its effectiveness is demonstrated. The measure is expected to cut costs of construction and maintenance for lightning protection.

View full abstract

Introduction of clean or high-efficiency energy systems that are called eco-energy system such as photovoltaic/solar heating/wind power system and cogeneration system is important, since the rise of energy price and a scarcity of fossil fuel are feared. However, suitable design and operation of eco-energy system were not easy for energy system users, because measurement of energy demand and weather data were needed. The supporting tool “EcoTOP” that suggests suitable design and operation to the energy system users and their facilities at any location in Japan was developed and evaluated.

View full abstract