What Are The Pre-Service Physics Teachers' Opinions About Context Based Approach In Physics Lessons?

It is known that the new teaching program based on the context based approach has been performed gradually from the 2005-2006 academic year in Turkey to facilitate the students' understanding of the science and technology subject matter. Therefore, it is aimed to teach the students according to the new program supported with daily life activities. Thus, the students can establish a link between physics and everyday life. In this sense, the preliminary knowledge of the pre-service physics teacher about the context based approach is important before they became in-service teachers at schools. From this point on, the purpose of this study is to determine the pre-service physics teachers' prior knowledge about the subject of context based learning. The study was conducted with 16 senior students who have taken the major part of the physics and physics education courses. The collected data was analyzed by using the content analysis method. The results of the analysis showed that the pre-service physics teachers have some lack of knowledge and lack of applications about the context based approach used in physics lessons. According to the results, the pre-service physics teachers' knowledge on this subject should be improved by using the context based approach activity in courses at university level. (C) 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license.Wo

Elimination of Coupling Transformer Core Saturation in Cascaded Multilevel Converter-Based T-STATCOM Systems

This paper deals with coupling transformer core saturation problem in cascaded multilevel converter (CMC) based transmission-type static synchronous compensator (T-STATCOM) systems. The effects of nonlinear characteristics of the coupling transformer and CMCs on the core saturation phenomenon are investigated, and the associated mechanisms are described. In order to prevent the transformer core from being saturated, dc current components in the coupling transformer secondary lines are eliminated by modulating one or more than one voltage steps of staircase CMC voltage waveforms via the electronic gating circuitry. The proposed dc current elimination method is implemented on a 154-kV, +/- 50-MVAr CMC-based T-STATCOM system, which is operating either in interarea oscillation damping or terminal voltage regulation modes at any time in a 400/154-kV transformer substation since March 2010. The effectiveness of recommended dc current elimination method is verified in the field

Design, Implementation, and Operation of a New C-Type 2nd Harmonic Filter for Electric Arc and Ladle Furnaces

In this paper, the transient overvoltage suppression capability and harmonic filtering performance of C-type 2nd harmonic filters (HFs) are optimized by using two-stage damping resistors; one is permanently connected to the filter circuit, while the other one is switched on by back-to-back connected thyristors during furnace transformer and HF energization periods. However, in conventional C-type 2nd HFs, there is only one damping resistor, which is permanently connected to the filter circuit. In conventional designs, either the filtering performance is maximized or transient overvoltage suppression capability is enhanced or a compromise is made between these two objectives. This new configuration of C-type 2nd HFs has been applied to a sample iron and steel plant in which two ladle refining furnaces are in operation. For this purpose, an static var compensation system has been designed and installed, which is composed of a thyristor-controlled reactor, a 3rd HF, and the new C-type 2nd HF configuration proposed in this paper. The results of field tests and simulation studies show that the proposed C-type 2nd HF configuration gives much better results than conventional designs

Views of Biology Teacher Candidates about Context Based Approach

Biology is one of the courses that explain natural events. However, in literature there are some studies claiming that the association level of students between daily life events and biology subjects is very low. Contest-based approach is described as the starting point for the development of scientific ideas in science teaching. In this approach real-life contexts are used to introduce concepts. The purpose of this study is to determine biology teacher candidates' view about context based approach. In this study the qualitative research method was used and the participants of this study consist of 14 volunteer senior teacher candidates from the division of biology education in Hacettepe University. A questionnaire was used as a data collection tool. The collected data was analyzed by using the content analysis method. In the light of the results, it is determined that the biology teacher candidates has some lack of knowledge about context based approach and it should be improved by using the context based approach activity in courses at university level so that the teachers candidates should be aware of with guides about context based approach. (C) 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Academic World Education and Research Center.Wo

Suppression of the Second Harmonic Subgroup Injected by an AC EAF: Design Considerations and Performance Estimation of a Shunt APF

This paper proposes a design methodology for an active power filter (APF) system to suppress the second harmonic subgroup injected by an AC electric arc furnace (EAF) to the utility grid. The APF system is composed of identical parallel units connected to the utility grid via a specially-designed coupling transformer. Each APF converter is a three-phase three-wire two-level voltage source converter (VSC). The number of parallel APF units, coupling transformer MVA rating, and turns ratio are optimized in the view of the ratings of commercially-available high voltage (HV) IGBTs. In this research work, line current waveforms sampled at 25.6-kS/s on the medium voltage (MV) side of a 65-MVA EAF transformer are then used to extract the second harmonic subgroup, 95-, 100-, and 105-Hz current components, by multiple synchronous reference frame (MSRF) analysis, which was previously proposed to decompose EAF current interharmonics and harmonics in real-time. By summing up this digital data of the second harmonic subgroup, the reference current signal for the APF system is produced in real-time. A detailed model of the APF system is then run on EMTDC/PSCAD to follow the produced reference current signal according to hysteresis band control philosophy. The simulation results show that the proposed APF system can successfully suppress the second harmonic subgroup of an AC EAF

Multi-DSP and -FPGA-Based Fully Digital Control System for Cascaded Multilevel Converters Used in FACTS Applications

In this paper, a fully digital controller based on multiple digital signal processor (DSP) and field-programmable gate array (FPGA) boards has been proposed for parallel-operated cascaded multilevel converters (CMC) used in flexible AC transmission system (FACTS) applications. The proposed system is composed of a DSP-based master controller in combination with a multiple number of slave DSP boards, FPGA boards, microcontrollers, a programmable logic controller (PLC), an industrial computer, and their peripherals in interaction. Inter-communication of these digital controllers is achieved mainly through fiber-optic links, via synchronous serial data link wherever a high-speed, full duplex communication is needed, and via asynchronous serial communication interface wherever relatively slow communication speed is required. The proposed fully-digital control system has been implemented on a sample 11-level CMC-based 154-kV, +/-50-MVAr transmission type static synchronous compensator (T-STATCOM). Field test results have shown that the proposed fully digital control system provides good transient response and steady-state characteristics for the oveall system including protection and monitoring functions

Students' Mental Models Of Light To Explain The Compton Effect

Light is one of the interesting concepts used for explaining the important processes in modern physics. Because of this reason the students have some misunderstandings and difficulties about light and related phenomena. The present study aims to determine pre-service teachers' understanding of and difficulties with light concept via mental models of light utilized by the students for the explanations of Compton Effect. This research has been carried on the group of 25 students studying in the departments of physics education at a Government University in Turkey. As a data collection tool open - ended questions pertaining drawings and explanations were used. The participants consist of 15 female and 10 male students and they ranged from 22 to 24 years of age. According to the results of this study three mental models which were named as hybrid model, ray and particle model were determined used by the students while explaining the Compton Effect. Implications of our findings for teaching are discussed. (C) 2015 The Authors. Published by Elsevier Ltd.Wo

Design and Implementation of a 154-kV +/- 50-Mvar Transmission STATCOM Based on 21-Level Cascaded Multilevel Converter

In this research work, the design and implementation of a 154-kV +/- 50-Mvar transmission static synchronous compensator (T-STATCOM) have been carried out primarily for the purposes of reactive power compensation and terminal voltage regulation and secondarily for power system stability. The implemented T-STATCOM consists of five 10.5-kV +/- 12-Mvar cascaded multilevel converter (CMC) modules operating in parallel. The power stage of each CMC is composed of five series-connected H-bridges (HBs) in each phase, thus resulting in 21-level line-to-line voltages. Due to modularity and flexibility of implemented HBs, each CMC module has reached a power density of 250 kvar/m(3), thus making the mobility of the system implementable. DC-link capacitor voltages of HBs are perfectly balanced by means of the modified selective swapping algorithm proposed. The field tests carried out at full load in the 154-kV transformer substation where T-STATCOM is installed have shown that the steady-state and transient responses of the system are quite satisfactory