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Quach, Ngoc-Thinh,Ko, Ji-Han,Kim, Dong-Wan,Kim, Eel-Hwan The Korean Institute of Electrical Engineers 2014 Journal of Electrical Engineering & Technology Vol.9 No.5
This paper presents an application of proportional-resonant (PR) current controllers in modular multilevel converter-high voltage direct current (MMC-HVDC) system under unbalanced voltage conditions. The ac currents are transformed and controlled in the stationary reference frame (${\alpha}{\beta}$-frame). Thus, the complex analysis of the positive and negative sequence components in the synchronous rotating reference frame (dq-frame) is not necessary. With this control method, the ac currents are kept balanced and the dc-link voltage is constant under the unbalanced voltage fault conditions. The simulation results based on a detailed PSCAD/EMTDC model confirm the effectiveness of the proposed control method.
Harmonic Analysis of a Modular Multilevel Converter Using Double Fourier Series
Quach, Ngoc-Thinh,Chae, Sang Heon,Ahn, Jin Hong,Kim, Eel-Hwan The Korean Institute of Electrical Engineers 2018 Journal of Electrical Engineering & Technology Vol.13 No.1
This paper presents a harmonic analysis of the modular multilevel converter (MMC) using a double Fourier series (DFS) algorithm. First, the application of DFS for harmonic calculation in the MMC is made by considering the effect of arm inductor. The analytical results are then confirmed by comparing with the simulation results of using the fast Fourier transform (FFT) algorithm. Finally, distribution of harmonics and total harmonic distortion (THD) in the MMC will be analyzed in three cases: harmonics versus number of levels of MMC, harmonics versus total switching frequency and harmonics versus modulation index. The simulation results are performed in the PSCAD/EMTDC simulation program in order to verify the analytical results obtained by Matlab programming.
Estimation of in situ permeability using a leak-off test
( Quach Quoc Nghiep ),( Yeonguk Jo ),( Chandong Chang ) 대한지질공학회 2016 대한지질공학회 학술발표회논문집 Vol.2016 No.1
Permeability plays a key role in reservoir flow characteristic assessment and reservoir description (Wong et al., 1999; Luijendijk et al., 2014; Yang et al., 2007). In this study, we attempt to determine permeability using a leak-off test (LOT) often routinely conducted to assess borehole/casing integrity as well as fracture gradient to predict in situ least principal stress magnitude (Gjønnes et al., 1998; Raaen et al., 2006; Lin et al., 2008; White et al., 2002; Allerstrofer, 2011; Addis et al., 1998). The LOT is normally conducted by injecting water at a constant rate into an open well-bore interval below casing shoe and measuring the well-bore pressures, from which we can estimate some important pressures such as leak-off pressure. It will be beneficial if we can estimate in situ permeability from this routinely conducted LOT. Thus, we derived a new theoretical equation based on Darcy``s law that gives permeability value from the pressure (P) versus time (t) as well as injected water volume versus time curves. The theoretical equation is expressed in terms of permeability as a function of various parameters such as open-hole dimensions, flowrate, porosity, pressure change and injected water volume. We applied this equation to the LOT conducted in offshore Pohang where CO2 injection test is ongoing. In this LOT, we used hydraulic pump to inject water at an approximately constant flowrate into the bottom interval through the casing, during which pressure and flowrate were recorded. Based on calculations using our permeability equation, the permeability was determined to be about 4.6E-18 m2. The comparison between the theoretical permeability, the permeability extracted from core measurement and the permeability of natural mudstones (Yang et al., 2007) gives approximately similar values. The permeability estimation from LOT can be utilized further to compare with the k values extracted from well testing such as short-term testing (DST and IMPULSE Testing and TRAP testing), conventional testing (injection test and falloff test) and advanced test (layered reservoir testing, vertical interference testing and multiwell interference testing) (Ahmed et al., 1991) to enhance the reliability and the quality of formation characteristic evaluation.
A simple, fast and highly sensitive assay for the detection of telomerase activity
Quach, Quang Huy,Jung, Juyeon,Kim, Hyeran,Chung, Bong Hyun The Royal Society of Chemistry 2013 Chemical communications Vol.49 No.59
<P>A highly sensitive and rapid PCR-free telomerase activity assay has been developed that uses SYBR Green intercalation into the G-quadruplex structures in the presence of K<SUP>+</SUP>.</P> <P>Graphic Abstract</P><P>A PCR-free, highly sensitive and rapid assay to convert telomerase activity into fluorescence signals using SYBR Green has been developed. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c3cc42571a'> </P>
Quach, Huu Luong,Kim, Ji Hyung,Hyeon, Chang Ju,Chae, Yoon Seok,Moon, Jae Hyung,Kim, Ho Min The Korea Institute of Applied Superconductivity a 2018 한국초전도저온공학회논문지 Vol.20 No.1
The thermal and electrical properties of the conductor are critical parametersfor the design and optimization of the superconducting magnet. This paper presents simulation code to analyze electrical and thermal stability characteristics of the second generation (2G) high-temperature superconductor (HTS) by varying copper stabilizer thickness. Two types of commercial 2G HTS coated conductor tapes, YBCO and GdBCO were used in this study. These samples were cooled by Liquid Nitrogen ($LN_2$) having boiling at 77.3 K and an equivalent electrical circuit model for them is choosen and analysed in details. Also, an over-current pulse test in which a current exceeding a critical current was performed. From the simulation results, the influences of the copper stabilizer thickness on the stability characteristics of these samples are presented.