Small-Signal Modelling and Analysis of Doubly-Fed Induction Generators in Wind Power Applications

The worldwide demand for more diverse and greener energy supply has had a significant impact on the development of wind energy in the last decades. From 2 GW in 1990, the global installed capacity has now reached about 100 GW and is estimated to grow to 1000 GW by 2025. As wind power penetration increases, it is important to investigate its effect on the power system. Among the various technologies available for wind energy conversion, the doubly-fed induction generator (DFIG) is one of the preferred solutions because it offers the advantages of reduced mechanical stress and optimised power capture thanks to variable speed operation. This work presents the small-signal modelling and analysis of the DFIG for power system stability studies. This thesis starts by reviewing the mathematical models of wind turbines with DFIG convenient for power system studies. Different approaches proposed in the literature for the modelling of the turbine, drive-train, generator, rotor converter and external power system are discussed. It is shown that the flexibility of the drive train should be represented by a two-mass model in the presence of a gearbox. In the analysis part, the steady-state behaviour of the DFIG is examined. Comparison is made with the conventional synchronous generators (SG) and squirrel-cage induction generators to highlight the differences between the machines. The initialisation of the DFIG dynamic variables and other operating quantities is then discussed. Various methods are briefly reviewed and a step-by-step procedure is suggested to avoid the iterative computations in initial condition mentioned in the literature. The dynamical behaviour of the DFIG is studied with eigenvalue analysis. Modal analysis is performed for both open-loop and closed-loop situations. The effect of parameters and operating point variations on small signal stability is observed. For the open-loop DFIG, conditions on machine parameters are obtained to ensure stability of the system. For the closed-loop DFIG, it is shown that the generator electrical transients may be neglected once the converter controls are properly tuned. A tuning procedure is proposed and conditions on proportional gains are obtained for stable electrical dynamics. Finally, small-signal analysis of a multi-machine system with both SG and DFIG is performed. It is shown that there is no common mode to the two types of generators. The result confirms that the DFIG does not introduce negative damping to the system, however it is also shown that the overall effect of the DFIG on the power system stability depends on several structural factors and a general statement as to whether it improves or detriorates the oscillatory stability of a system can not be made

Teaching Competency of Teacher of Building Drawing Skill In SMK N 1 Sedayu Bantul

Teacher is an important element for the improvement of Indonesia’s education because teacher has a role to delivery subject matter directly to the student. The teacher must have good teaching competency so the aim of learning can be achieved. This research is explain how teaching competency in productive subject matter, building drawing course in SMK N 1 Sedayu Bantul. In this research the writer take research object was 6 teachers of building drawing course in SMK N 1 Sedayu Bantu. Data collection technique was indirect observation in 3 kinds of questionnaire. First questionnaire was for student participant, the second was for head master participant. The questionnaire consisted of 4 answer choice, (BS) very good, (B) good,(K) minus,(KS) very minus. The third was portfolio questionnaire for teacher participant. Data of this research was analyzed with descriptive statistics technique and descriptive quantitative. Result of the research show that in general teaching competency teacher in productive subject matter, building drawing course SMK N 1 Sedayu Bantul can be categorized in good category. From 6 indicators of teaching competency 83,3% were good and 16,66% was minus. The indicators categorized as good category were indicator of teaching instrument usage, giving motivation, indicator of assessment, the use of teaching method, interaction between teacher and student, However, for presentation matter was categorized as minus. Second, result of this research indicated that there was correlation between head master assessment and student assessment for teaching competency. The student assessment was more objective than head master’s assessment because the number of student was bigger than the number of head masters that just one person. The student had enough time to fill up the questionnaire so the result was more accurate. The student was more understand because the teacher aught them every day. Third, this research indicated that there was no correlation between assessment of teacher portfolio and their teaching competency, it means, for the student, high result of portfolio assessment didn’t increase the teachers’ teaching competency. So, portfolio assessment can’t be absolute reference to assess the teacher competency. Keyword: Teaching competency, Productive subject matter, Pedagogic competency, Personal competenc

Intermediary's Elicitation and Patron's Retrieval Satisfaction

[[abstract]]An elicitation is a verbal request for information reflecting one's interests, concerns or perplexities in conversation. Elicitation behavior in studies of information retrieval interaction is, in fact, the micro-level of information-seeking behavior in which the user and the intermediary exchange information to fill the gaps in one's internal state of knowledge. This study aims to understand the intermediary's elicitation behavior in terms of linguistic forms, communicative functions (illocutionary force) and utterance purposes (semantic contents) and further to identify the relationship between intermediary's individual differences and search results satisfaction. Research methods include participatory observation, conversation analysis, content analysis and statistical analysis of elicitation frequencies and questionnaires. Our research results successfully identify the three dimensions of intermediary's elicitation behavior and characterize intermediary's inquiring minds and elicitation styles. Further analysis shows that there exists a significant relationship between inquiring minds/elicitation styles and user's relevance judgment of search results.

Sequential change-point detection when unknown parameters are present in the pre-change distribution

In the sequential change-point detection literature, most research specifies a required frequency of false alarms at a given pre-change distribution $f_{\theta}$ and tries to minimize the detection delay for every possible post-change distribution $g_{\lambda}$. In this paper, motivated by a number of practical examples, we first consider the reverse question by specifying a required detection delay at a given post-change distribution and trying to minimize the frequency of false alarms for every possible pre-change distribution $f_{\theta}$. We present asymptotically optimal procedures for one-parameter exponential families. Next, we develop a general theory for change-point problems when both the pre-change distribution $f_{\theta}$ and the post-change distribution $g_{\lambda}$ involve unknown parameters. We also apply our approach to the special case of detecting shifts in the mean of independent normal observations.Comment: Published at http://dx.doi.org/10.1214/009053605000000859 in the Annals of Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical Statistics (http://www.imstat.org