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Brute-Force Sentence Pattern Extortion from Harmful Messages for Cyberbullying Detection
Get PDFCyberbullying, or humiliating people using the Internet, has existed almost since the beginning of Internet communication.The relatively recent introduction of smartphones and tablet computers has caused cyberbullying to evolve into a serious social problem. In Japan, members of a parent-teacher association (PTA)attempted to address the problem by scanning the Internet for cyber bullying entries. To help these PTA members and other interested parties confront this difficult task we propose a novel method for automatic detection of malicious Internet content. This method is based on a combinatorial approach resembling brute-force search algorithms, but applied in language classification. The method extracts sophisticated patterns from sentences and uses them in classification. The experiments performed on actual cyberbullying data reveal an advantage of our method vis-à-visprevious methods. Next, we implemented the method into an application for Android smartphones to automatically detect possible harmful content in messages. The method performed well in the Android environment, but still needs to be optimized for time efficiency in order to be used in practicejournal articl
INVESTIGATION OF ICE MELTING PROCESS IN A ICE-COVERED RIVER AND PREDICTION WITH SIMPLIFIED MODEL OF ICE MELTING
Get PDF結氷河川において解氷期に発生するアイスジャム洪水は，河川の防災および維持管理上の懸案である．気候変動により，解氷期のアイスジャム発生状況が将来的に変化する可能性もある中，解氷期の河氷挙動把握が求められている．本研究では過去にアイスジャムが発生した結氷河川において，解氷過程の把握のため，氷温，水位，河氷画像の同時縦断観測を行った．その結果，氷温変化から解氷時期を解明するとともに，画像および水位観測から縦断的な解氷プロセスを把握できた．また実務利用を考慮した河氷厚変動簡易予測モデルを適用し，河氷厚変化から解氷進行現象を考察するとともに，解氷時期の予測に対するモデルの有用性を確認した．Ice jam floods under spring thaw is a problem for disaster prevention and maintenance of ice rivers. And it is required to understand the possibility of changing behavior of river ice caused by climate change. In this study, we conducted simultaneous longitudinal observation of ice temperature, water level and timelapse camera in order to understand the deicing process in an ice-covered river. As a result, it was possible to understand the deicing time from the ice temperature change, and grasp the longitudinal deicing process from the image and water level observation. Next, we applied a simplified prediction model of river ice thickness and discussed the deicing progress phenomenon from the river ice thickness change. We confirmed the usefulness of the model for future ice melting prediction.journal articl
可変速風力発電機によるウィンドファームを含む複合電力系統の安定度改善
Get PDF北見工業大学博士（工学）This thesis deals with the stability enhancement of grid-connected fixed speed wind turbines with squirrel cage induction generators (FSWT-SCIGs) based wind farm (WF) and conventional synchronous generators (SGs) by using variable speed wind generators. The penetration level of grid-connected large-scale WF into the existing grid system has been increasing significantly since the last decade. This increasing rate of wind power impacts on the power system transient stability, for example, the power system can be unstable during severe network fault. During network disturbance situation, voltage stability of WF and frequency stability of entire power system are the important concerns. The modern grid codes demand that the WF should stay online during network disturbances and support the power system in the same way as conventional units. New studies must be performed in order to maintain the stable and reliable operation of WF and the entire power system. Therefore, from the view point of transient and dynamic characteristics, interaction of large-scale WF with the existing power system is an important issue to be analyzed in order to enhance the stability of WF and power system.
High penetration of wind power into an existing grid can be attributed largely to the FSWT-SCIG due to its robust construction, low cost, and operational simplicity. However, SCIG-based WF is sensitive in nature to the grid faults, and thus, big concern has created for their mass integration into the existing power system. WF grid codes require wind generators to have the low voltage ride-through (LVRT) capability, which means that normal power production should be resumed quickly once the nominal grid voltage has been recovered and they are also supposed to participate in voltage build up process. But SCIG-based WF has failed to fulfil the LVRT requirements, which has great impact on the power system stability.
On the other hand, variable speed wind turbine with permanent magnet synchronous generator (VSWT-PMSG) has suitable LVRT capability. In addition, the PMSG system can control the active and reactive power injected to the power system independently. The only drawback of this system is expensive cost because it requires full rating of power converter. By taking the advantage of reactive power control capability of PMSG, it is possible to stabilize the large-scale SCIGs. Thus, combined installation of large-scale FSWT-SCIGs along with smaller rating of VSWT-PMSGs in a WF can decrease the overall installation cost and also ensure LVRT capability. The power converter of PMSG consists of machine side converter (MSC) and gird side converter (GSC). The GSC is responsible for injecting reactive power during voltage dip situations. Normally, conventional cascaded proportional-integral (PI) controller is used to design the GSC controller. But conventional PI controller is not sufficient to ensure LVRT capability of large-scale SCIGs. In this thesis, a novel fuzzy logic controller (FLC) in the GSC controller of VSWT-PMSG is proposed to improve the LVRT capability of WF during voltage dip situation due to severe network fault. The simulation results in this thesis show that the proposed control strategy is more effective to enhance the transient stability of WF during fault condition compared to conventional PI based controller.
Also, variable speed wind turbines with doubly fed induction generators (VSWT-DFIGs) have sufficient LVRT augmentation capability and can control the active and reactive power delivered to the grid independently. However, in the same way as PMSG, the DFIG is more expensive than the SCIG due to its partial rating of AC/DC/AC converter. Therefore, the combined use of SCIGs and DFIGs in a WF also could be an effective solution. The design of the rotor side converter (RSC) controller of DFIG is crucial because the RSC controller contributes to the system stability. The cascaded control strategy based on four conventional PI controllers is widely used to control the RSC of the DFIG, which can inject only a small amount of reactive power during fault conditions. Therefore, the conventional strategy can stabilize only the lower rating of the SCIG. Therefore, in this thesis, a new control strategy based on fuzzy logic is proposed in the RSC controller of the DFIG in order to enhance the LVRT capability of the SCIG in a WF. The proposed FLC is used to control the reactive power delivered to the grid during fault conditions. Moreover, reactive power injection can be increased in the proposed control strategy. Extensive simulations analyses for both the proposed and conventional PI controllers of the RSC of the DFIG reveal that the proposed control strategy can stabilize the higher rating of the SCIG.
Another salient feature of this thesis is the development of virtual inertia controller (VIC) for DFIG to enhance the transient stability of hybrid power system. Large integration of renewable energy sources (RESs), such as WF and solar photovoltaic (PV) plant, into the power systems, impacts on the system frequency stability. Normally, a WF and PV system do not provide frequency support because of the uncontrollability of the input energy. Moreover, overall system inertia will be reduced due to massive integration of RES, because conventional generation units that provide reserve power need to be decreased. To overcome the problems of frequency stability as well as power system transient stability resulting from the insufficient inertia response, this thesis proposes a new method to enhance the transient stability of the power system with RESs introduced, in which VSWT-DFIG supplies its kinetic energy (KE) during generation outage to stabilize conventional SGs. A suitable fuzzy logic based VIC is proposed to release the KE efficiently during transient period. This FLC can continuously adjust the VIC gain depending upon the incoming wind speed. To verify the effectiveness of the proposed VIC, simulation analyses are performed on a multi-machine hybrid power system model.
Simulations are carried out by PSCAD/EMTDC software. Real wind-speed data measured in Hokkaido Island, Japan are used in the dynamic analysis. Three-line-to-ground (3LG) fault is considered as network disturbance in this study. The standard IEEE nine-bus model is used to evaluate the performance of the proposed control strategies.
Considering all the aspects, it is concluded that the stability of the WF and power system can be ensured by the control strategies for PMSG and DFIG proposed in this thesis.doctoral thesi
UNRAVELLING THE BREACH MECHANISM OF TOKORO RIVER TRIBUTARIES DURING HEAVY RAIN IN AUGUST 2016
Get PDFIn recent years, bank breaching occurred high frequency. In August 2016, the east pan of Hokkaido in Japan was attacked a heavy rainfall and the tributaries of Tokoro River (Shibayarnasawa River and Toua River) had a bank breach caused by the back water of Tokoro River. Although both tributaries were almost the same size and both junctions were very close, the situation of inundation were largely different. At the breaching point of the Sibayarnasawa River, the inundated flow toward to the upstream side of the inland, while at the Toua River it was perpendicular to the channel. In order to understand the cause of the difference in phenomena, it was tried to reproduce the breaching situation in both tributaries using the numerical simulation model “iRIC Nays 2D Breach”. From the result, it was revealed that the direction of the inundated flow changes greatly depending on the timing of the discharge change at the tributaly when the bank failure occurs. At the time of bank breaching, when the discharge of the tributary is small, the inundated flow towards to the upstream direction. At the time of bank breaching, when the discharge of the tributary is large, the inundated flow towards to the perpendicular direction of the channel.journal articl
Variety of clustering in 18O
Get PDFWe investigate excited states of 18O by using the antisymmetrized molecular dynamics. It is found that five different types of cluster states exist which we call 14C+α, higher-nodal 14C+α, two molecular states, and 4α linear-chain. The calculated α-decay widths are compared with the observed data. The higher-nodal 14C+α cluster states reasonably agree with resonances reported by the recent experiments. We predict that the α-particle emission is dominant for the 14C+α cluster states while the molecular states prefer the 6He emission.journal articl
The causes of damage to Kougen Ohashi Bridge during 2016-flood in Hokkaido and effects of the countermeasures
Get PDF2016年8月，石狩川上流で洪水が発生した。この洪水において，石狩川上流に位置する上川町の高原大橋は，橋台背面盛土の浸食や，橋脚の沈下など甚大な被害を受けた。本研究では，平面二次元河床変動計算ソフトiRIC Nays2DHを用いて，洪水における河道変化の過程の数値計算を行うこととした。その結果，被災要因が，流れの集中と，河道の平面形状に伴う河床変動によるものであることが明らかになった。また，出水時の流れの集中の解消を図るための河道平面形状を修正することを目的とした橋長の伸長が，極めて有効な対策であることが確認された。In August of 2016, a huge flood attacked the Ishikari River upstream basin. The Kougen Ohashi Bridge in Kamikawa town over the upstream of Ishikari River suffered great damage. The collapse of the abutment and the sinking of the piers occurred during the flood. We conducted a numerical simulation of the river channel processes during the flood using a numerical simulation software iRIC Nays2DH. It was clarified that the causes of the damage were the concentration of flow and the riverbed fluctuation accompanying the plane shape of the river course. And it was clarified that the extension of the bridge length for the purpose of changing the shape of the river course is a very effective measure.journal articl