Soo-Yeol Kim

A comparison of the carrier-based PWM techniques for voltage balance of flying capacitor in the flying capacitor multilevel inverter

This paper presents the analysis and comparison of carrier-based PWM techniques for the flying capacitor multilevel inverter (FCMI). The FCMI has the natural problem with voltage unbalance of flying capacitors. This problem generates distortion of output voltage and load current. It also causes serious breakdown of switching device. The FCMI has various PWM techniques for voltage control of flying capacitor to ensure the balance of flying capacitor voltage. Among them, this paper is focused on three carrier-based PWM techniques, which are the phase-shifted (PS) PWM technique, the modified carrier-redistribution (MCR) PWM technique and the saw-tooth-rotation (STR) PWM technique. They are analyzed and compared precisely in flying-capacitor 3-level inverter with respect to voltage balance of flying-capacitors and harmonic contents of output waveform. The experimental results describe performances and characteristics of them.

The analysis of STATCOM and SVC cooperation effect

This paper describes the operation effect of KEPCO(Korea Electric Power Corporation) FACTS devices in the Korean power system. At the year of 2009, KEPCO is ready for commercial operation of 345kV 100MVA STATCOM at Mi-Geum substation and 345kV 200MVA SVC at Dong-Seoul substation. In case of Mi-Geum and Dong-Seoul substation, these sites are very close. Therefore, it is necessary that the additional analysis including interaction and cooperation is implemented. To do it, a detailed EMTDC simulation model was developed and variant analysis was implemented.

Dorsal intercalated segmental instability associated with malunion of a reconstructed scaphoid

We analysed scaphoid deformity as a result of surgical treatment of scaphoid fracture nonunion and assessed the deformity associated with a dorsal intercalated segmental instability pattern of carpal malalignment. A total of 45 patients who were treated for scaphoid fracture nonunion were included in the study. The height-to-length ratio of the scaphoid was measured on computed tomographic images and used to assess scaphoid deformity. Carpal malalignment was quantified based on the radio-lunate angle. A correlation analysis between the height-to-length ratio and the radio-lunate angle was performed. Dorsal intercalated segmental instability was defined as a radio-lunate angle >15°, and a receiver operating curve analysis was used to calculate the cutoff height-to-length ratio that can be accompanied with dorsal intercalated segmental instability. Extension of the lunate increases in proportion to the flexion deformity of the scaphoid; dorsal intercalated segmental instability can occur if the height-to-length ratio of the scaphoid is >0.73. Level of evidence: IV

The thickness of the A2 pulley and the flexor tendon are related to the severity of trigger finger: results of a prospective study using high-resolution ultrasonography

We aimed to investigate the relationship between the pulley-tendon complexes and the severity of trigger finger. The thickness of the A1 and A2 pulleys, and the cross-sectional area of the flexor tendon under the pulleys, were prospectively assessed using 17 MHz high-resolution ultrasonography, in 20 patients with trigger finger (31 fingers). A control group comprised 15 asymptomatic fingers. The thickness of the A1 pulley and the proximal part of the A2 pulley, and the cross-sectional area of the flexor tendon under the A2 pulley, were significantly increased in the patient group. Clinical grade was significantly correlated with the thickness of the A1 pulley, the thickness of the proximal part of the A2 pulley, and the cross-sectional area of the flexor tendon under the proximal part of the A2 pulley. This study confirmed that the thickness of the A2 pulley and flexor tendon under the A2 pulley seems to be related to the severity of trigger finger. Level of evidences: Level III

EMTDC Simulation Model for FACTS Device

This paper describes the impact of the FACTS application on the power network using EMTDC simulation package. KEPCO (Korea Electric Power Corporation) has begun the operation of the plusmn80 MVA UPFC (unified power flow controller) from the year 2003 on the 154 kV transmission system. The UPFC consists of two voltage source inverters (VSI), STATCOM and SSSC, which are connected through a common DC link capacitor. Through a combination of both VSIs, the UPFC can control bus voltage, real and reactive power flow. But, in some cases, each VSI can be operated independently according to operation modes. After all, The UPFC can be operated 3 operation mode; SSSC, STATCOM and UPFC mode. The paper describes the fundamental operating effects and basic control approach of a SSSC mode operation of KEPCO UPFC to the power system

A test facility for large scale inverter valve and pole using resonant circuit

This paper proposes a test method for single phase inverter pole which is the basic building block for large scale three phase inverters. Inverter poles for utility application need to be tested at full voltage and current ratings which reach thousands of volts and amperes. This paper proposes a novel method for testing large scale inverter valves and poles using a resonant circuit which allows testing of inverter valves and poles at rated current and voltage with relatively low AC power levels. The configuration and layout of the test facility is introduced and actual test results are presented.

Determining wind farm locations, allocation of wind farm capacity, and sizing of energy storage for 17 GW new wind power capacity in Korea

The Korean government plans to install 17 GW of wind power capacity by 2027. However, wind farm locations, individual wind farm generation capacities, and the strategies for minimizing wind power variability have not yet been defined. In this paper, the best locations are identified based on annual wind speeds and availability, and the generation capacity of the wind farms at each selected location are allocated by using a weighted evaluation method. In addition, the strategies proposed in this paper for minimizing wind power variability are aggregation of wind farms (to take advantage of the smoothing effect) and integration of energy storage systems (to smoothen power output and provide ancillary services). Wind data was obtained from the Korea Meteorological Association, and wind power outputs were calculated using the power curve of a 2 MW Vesta wind turbine as a model for each generation unit. Sizing of ESS capacity was done by using Discrete Fourier Transform analysis and probability density analysis.

Effect Analysis for Frequency Recovery of 524 MW Energy Storage System for Frequency Regulation by Simulator

To test the effectiveness of using an energy storage system for frequency regulation, the Energy New Business Laboratory at KEPCO Research Institute installed a 4 MW energy storage system (ESS) demonstration facility at the Jocheon Substation on Jeju Island. And after the successful completion of demonstration operations, a total of 52 MW ESS for frequency regulation was installed in Seo-Anseong (28 MW, governor-free control) and in Shin-Yongin (24 MW, automatic generation control). The control system used in these two sites was based on the control system developed for the 4 MW ESS demonstration facility. KEPCO recently finished the construction of 184 MW ESS for frequency regulation in 8 locations, (e.g. Shin-Gimjae substation, Shin-Gaeryong substation, etc.) and they are currently being tested for automatic operation. KEPCO plans to construct additional ESS facilities (up to a total of about 500 MW for frequency regulation by 2017), thus, various operational tests would first have to be conducted. The high-speed characteristic of ESS can negatively impact the power system in case the 500 MW ESS is not properly operated. At this stage we need to verify how effectively the 500 MW ESS can regulate frequency. In this paper, the effect of using ESS for frequency regulation on the power system of Korea was studied. Simulations were conducted to determine the effect of using a 524 MW ESS for frequency regulation. Models of the power grid and the ESS were developed to verify the performance of the operation system and its control system. When a high capacity power plant is tripped, a 24 MW ESS supplies power automatically and 4 units of 125MW ESS supply power manually. This study only focuses on transient state analysis. It was verified that 500 MW ESS can regulate system frequency faster and more effectively than conventional power plants. Also, it was verified that time-delayed high speed operations of multiple ESS facilities do not negatively impact power system operations. It is recommended that further testing be conducted for a fleet of multiple ESSs with different capacities distributed over multiple substations (e.g. 16, 24, 28, and 48 MW ESS distributed across 20 substations) because each ESS measures frequency individually. The operation of one ESS facility will differ from the other ESSs within the fleet, and may negatively impact the performance of the others. The following are also recommended: (a) studies wherein all ESSs should be operated in automatic mode; (b) studies on the improvement of individual ESS control; and (c) studies on the reapportionment of all ESS energies within the fleet.

Comparison of ceiling effects between two patient-rating scores and a physician-rating score in the assessment of outcome after the surgical treatment of distal radial fractures

We compared the ceiling effects of two patient-rating scores, the Disability of the Arm, Shoulder and Hand (DASH) and Patient-Rated Wrist Evaluation (PRWE), and a physician-rating score, the Modified Mayo Wrist Score (MMWS) in assessing the outcome of surgical treatment of an unstable distal radial fracture. A total of 77 women with a mean age of 64.2 years (50 to 88) who underwent fixation using a volar locking plate for an unstable distal radial fracture between 2011 and 2013 were enrolled in this study. All completed the DASH and PRWE questionnaires one year post-operatively and were assessed using the MMWS by the senior author. The ceiling effects in the outcome data assessed for each score were estimated. The data assessed with both patient-rating scores, the DASH and PRWE, showed substantial ceiling effects, whereas the data assessed with MMWS showed no ceiling effect. Researchers should be aware of a possible ceiling effect in the assessment of the outcome of the surgical treatment of distal radial fractures using patient-rating scores. It could also increase the likelihood of a type II error.