As Yttrium-Barium-Copper Oxide (YBCO) high-temperature superconductors have a high critical current density at high magnetic fields and high mechanical strength, they are applicable to broad areas of electric machinery. However, the YBCO magnet technology is still insufficiently developed, as the YBCO conductors are only recently commercialized. From this point of view, the “Investigation Committee of YBCO High Temperature Superconducting (HTS) Magnet Technology” was founded in 2013 to investigate the YBCO-HTS magnet technology. This report summarizes their findings as follows: (a) basic research results on “magnet degradation on cooling”, “screening-current induced magnetic field”, “magnet protection”, and “persistent joints”; (b) technical problems being encountered during magnet fabrication; (c) developments of YBCO superconducting apparatus. The authors are confident that the report has highly original aspects and will arouse much interest among researchers in this field.
This paper proposes a reduction method of subtransmission system including various distributed generators (DG) mainly for the efficient transient stability studies. The proposed method is based on the concept of expressing the original branch impedances consisting of the reduced system as equivalent branch impedance. The reduced system consists of two nodes and two branches with two equivalent impedance, those are obtained respectively for loads and DG's of the original system. Each node in the reduced system represents either load or DG, and both the value of two reduced branch impedance can be kept in the reasonable positive range. The reduction system is configured so that the operating condition at boundary node after reduction is matched to that of the original network, also the DG terminal voltage magnitude at the reduced system is maintained as the average DG voltage magnitude in the original system. Demonstrative result to the application in the transient stability simulation is presented in order to verify the practicability of the proposed method.
In general, conventional power generators, which are scheduled to operate in unit commitment (UC) planning based on power generation and demand forecasts, have their output regulated to maintain the power balance and to minimize their operational cost for demand and supply operation in power systems. The number of operating (connecting) generators determines whether the power balance can be maintained or not. During load dispatch, it is difficult to start-up or stop generators at once for securing regulating capacity. A power surplus occurs if outputs less than the minimum outputs are requested of the connected generators, whereas a power shortfall occurs if outputs greater than the maximum outputs are requested. In this study, we analyze the relationship between the supply-demand balance and the domain of the existence of the number of connected generators for determining UC in a power system with a large integration of renewable energy sources. Moreover, we discuss quantitatively the impact of the power system conditions on the supply-demand balance using two-dimensional diagrams and numerical simulations.
Real-time pricing will be one of the realization methods of demand response scheme for the future smart grid. Assuming an advanced building energy management system for multi-type packaged air-conditioning facilities in commercial buildings, simulated annealing optimization that minimizes an evaluation function consisting of power cost and comfort degradation terms was studied. Experiments using an actual office building showed the result of reducing power consumption by 39% while keeping the room temperature degradation by only 0.16 degrees on average for an example case of ten-minute interval real-time pricing.
A significant increase of renewable energy generation such as photovoltaic and wind power causes fluctuations in frequency and voltage in power grid. Battery energy storage systems (BESSs) are expected as effective measures to mitigate these fluctuation problems. Among many of technical parameters which specify or characterize BESS performance, the paper focuses on “efficiency” as a key performance indicator for BESSs. The authors propose a comprehensive calculation model to evaluate efficiency of a BESS, where losses in its interconnecting transformer, power conditioning system (PCS), auxiliary equipment and energy storage device are taken into account. The paper demonstrates seasonal field results of the efficiency measured in an actual 100kW-300kWh BESS. The measured results are compared with the calculation results based on the designed values of the BESS, so that the proposed evaluation model for BESS efficiency is verified to be a useful tool for the comparison of BESSs.
Overhead transmission lines are exposed to the natural environment, and many electrical faults are caused by influences of the external environment such as lightning and snow. In addition, birds such as crows and animals such as monkeys, snakes, and bears also cause electrical faults. The material of a crow's nest, which is often made of metallic materials such as wire or hangers, has been one of the causes of electrical faults. As a countermeasure against the crow's nesting, warning sounds and flashes that crows must dislike, objects that move irregularly by springs, artificial birds that imitate the natural enemies of crows such as falcons, and the corpses of crows are have been experimentally installed to transmission towers, however, there are few quantitative evaluation about these, and the effect has been limited. Moreover, it has been difficult to evaluate the effectiveness of these countermeasures clearly. Therefore, this paper focuses on the decreasing of the electrical faults by decreasing of the crow's nesting and describes the characteristics of nesting behavior of crows at 66-154kV transmission towers and effectiveness of crow nesting prevention products.
Distributions of transient electric field and current density in and around a metal oxide varistor, which has a diameter of 42mm and a thickness of 36mm, have been studied using the finite-difference time-domain (FDTD) method. The current injected into the varistor is a lightning impulse current having a peak of 100kA and a risetime of 4-µs. The varistor is represented with many nonlinear cubic cells of 3mm × 3mm × 3mm. The cell has a nonlinear resistivity—electric field characteristic in each of x, y and z directions, which is based on the measured nonlinear voltage—current characteristic. It follows from the FDTD analyses that the skin effect of the 42-mm-diameter varistor is insignificant for a lightning current having a rise time of 4-µs. However, if there is small difference of the nonlinear characteristic of ZnO element between its periphery and other parts, the distributions of electric field and current density are considerably affected, which may result in enhanced degradation or failure of the varistor.
The estimation of the expected energy generation of the PV power system by using the PV mini-module was demonstrated. Its reflection not only of the irradiance but also of the panel temperature on the output power resulted in the small difference below few % between the expected and the actual energy generation especially on the relatively high energy yield days.