In-Hyuk Choi

Effective Combination of Soft Magnetic Materials for Magnetic Shielding

This study proposes an effective method in the arrangement of layers of high permeability materials for shielding power frequency magnetic field. The side of a neutral ground reactor was blocked with one or two layers of grain-oriented electrical steel (GO), nonoriented electrical steel (NGO), and permalloy (PC). In a weak magnetic field, PC showed the highest shielding performance, whereas GO was the best in a strong magnetic field. NGO had a lower shielding performance than GO, but was better than PC in a very strong magnetic field. When shielding with two layers directly adjacent to each other, the performance level of GO/PC combination (GO is closer to the field source) was between that of GO/GO and PC/PC combinations. When the two layers were slightly separated, on the other hand, GO/PC was most effective in a wide range of magnetic fields. These results are due to the shunt effect of high permeability materials and change of rank of permeability with magnetic field strength between different magnetic materials. Closely adjacent layers act as one body under the regime of shunt mechanism of magnetic shielding. Once the layers are separated, the GO sheet effectively reduces the strong magnetic field first, and then the PC sheet effectively shields the weakened field.

Failure characteristics of suspension-type porcelain insulators on a 154 kV transmission line

The paper reports on the failure characteristics of porcelain suspension insulators removed from a 154 kV transmission line in Korea. It is found that it is very important to choose the test data describing the homogeneity of porcelain, which is related to the micro-crack propagation in the porcelain body, for leveling the ageing state of the porcelain insulators. Tests such as the Rockwell hardness, is found to be effective in calculating the statistical values on the micro-crack fracture of the porcelain body. The combined thermal and mechanical tests have a close relationship to the dielectric strength of the porcelain body. The use of the Weibull distribution function is shown to be a very reasonable way of calculating the failure probability of aged porcelain insulators

A study on the influence of forest fire on polymer insulators

The forest fire simulation tests were performed with polymer and porcelain insulators at Gochang Power Testing Center. These tests consisted of open flame rising up to 600~820degC as being measured at insulator surface. Mechanical and electrical characteristics such as specific mechanical load, low frequency dry flashover voltage and impulse flashover voltage were analyzed for the polymer insulators before, during and after simulation tests compared with porcelain insulators. At the end of fire simulation tests, there was no detrimental deterioration of any insulators. All insulators passed the criteria of KEPCO specification. This study showed that forest fire simulation had no impact on polymer insulators.

Accelerating aging of transmission line porcelain suspension insulators by autoclaving

Porcelain suspension insulators aged for 1, 5 and 10 years on Korean transmission lines and new insulators are tested for autoclave expansion. The compressive strength of the insulators aged in an autoclave with conditions of 250degC and at 20 atm for 30 minutes, was about 7.6 kgf/mm2, which is close to that measured on insulators aged for 10 years in the field. From simulation results, the cement displacement changed linearly with temperature. At a temperature of 200degC, the shear stress was approximately 7 kgf/mm2; a stress that is brought about by a 0.07% expansion of the cement. It is evident that the cement would fracture at a 0.07% expansion, because the cement has about 7 to 9 kgf/mm2 flexure strength. A turning point in the shear stress with mechanical load occurred at 0.02% cement expansion. From an analysis of the porcelain body it is shown that there is sufficient margin of strength to guard against fracture of the porcelain body even for a cement expansion more than 0.12%.

Thermal impact characteristics by forest fire on porcelain insulators for transmission lines

In this study the thermal impact characteristics by forest fire are extensively investigated using temperature controlled ovens. The test conditions for thermal impact damage are simulated according to the characteristics of natural forest fire. The test pieces are suspension porcelain insulators made by KRI in 2005 for transmission lines. In the thermal impact cycle tests with 300degC thermal impact gradient (-70 to 230degC), cycling in 10 minute periods, no critical failures occurred in the test samples even with long cycle times. But in tests with thermal impact gradient from room temperature to 200-600degC, cycling in 10 to 30 minute periods, there were critical failures of the porcelain insulators according to the thermal impact gradient and quenching method. In the case of thermal impact by forest fire, it was found of that duration time is more important than the cycling time, and the initiation temperature of porcelain insulator failures is about 300degC, in the case of water quenching, many cracks and fracture of the porcelain occurred. It was found that the thermal impact failure is closely related to the displacement in the cement by thermal stress as confirmed by simulation. It was estimated that the initiation displacement by the thermal impact of 300degC is about 0.1%. Above 1% displacement, it is expected that the most porcelain insulators would fail.

Switching Impulse Flashover Tests and Analysis for 765kV Jumper V-strings with Damaged Insulators

The 765kV transmission line will be maintained by live-line works for efficient operation. In order to maintain the 765kV transmission lines safely by live-line works, lineman has to know switching impulse flashover characteristics of the jumper V-strings with damaged insulators in advance. In order to know the flashover characteristics this paper carried out experimental flashover mockup tests for jumper V-string with damaged insulators in the outdoors. And it suggests flashover characteristics of the 765kV jumper V-strings also. The results will be used in estimating the safety of live working for 765kV transmission lines.

A Development of Reduced-scale Model to Predict of Environmental Characteristics of AC/DC Hybrid Overhead Transmission Line

To review application of HVDC (High Voltage Direct Current) transmission line to HVAC T/L in operation, reduced-scale model was designed and manufactured. The arms of model were designed to change height and interval of conductors. Electrical environmental interferences were estimated by various configuration of AC 345kV and DC 250kV T/L. The interferences such as electric field intensity and ion current density were measured and converted reduced-scale factor to full-scaled. Additionally, effects between AC and DC T/L were studied.

Accelerating Aging of Transmission Line Porcelain Suspension Insulators by Autoclaving

Porcelain suspension insulators aged for 1, 5 and 10 years on Korean transmission lines and new insulators are tested for autoclave expansion. The compressive strength of the insulators aged in an autoclave with conditions of and at 20 atm for 30 minutes, was about , which is close to that measured on insulators aged for 10 years in the field. From simulation results, the cement displacement changed linearly with temperature. At a temperature of , the shear stress was approximately ; a stress that is brought about by a 0.07 % expansion of the cement. It is evident that the cement would fracture at a 0.07 % expansion, because the cement has about 7 to flexure strength. A turning point in the shear stress with mechanical load occurred at 0.02 % cement expansion. From an analysis of the porcelain body it is shown that there is sufficient margin of strength to guard against fracture of the porcelain body even for a cement expansion more than 0.12 %.

Thermal Impact Characteristics by Forest Fire on Porcelain Insulators for Transmission Lines

In this study the thermal impact characteristics by forest fire are extensively investigated using temperature controlled ovens. The test conditions for thermal impact damage are simulated according to the characteristics of natural forest fire. The test pieces are suspension porcelain insulators made by KRI in 2005 for transmission lines. In the thermal impact cycle tests with thermal impact gradient (-70 to ), cycling in 10 minute periods, no critical failures occurred in the test samples even with long cycle times. But in tests with thermal impact gradient from room temperature to , cycling in 10 to 30 minute periods, there were critical failures of the porcelain insulators according to the thermal impact gradient and quenching method. In the case of thermal impact by forest fire, it was found of that duration time is more important than the cycling time, and the initiation temperature of porcelain insulator failures is about , in the case of water quenching, many cracks and fracture of the porcelain occurred. It was found that the thermal impact failure is closely related to the displacement in the cement by thermal stress as confirmed by simulation. It was estimated that the initiation displacement by the thermal impact of is about 0.1 %. Above 1% displacement, it is expected that the most porcelain insulators would fail.

The Influence of Forest Fire on the Polymer Insulator for Transmission Lines

To understand the effects of forest fires on polymer insulators for transmission lines, the forest fire simulation tests were performed with polymer and porcelain insulators at Gochang testing center. These tests consisted of energizing 90 kV at line-to-ground voltage of 154 kV lines and open flame rising up to as being measured at insulator surface. Mechanical and electrical characteristics such as specific mechanical load, leakage current, low frequency dry flashover voltage and impulse flashover voltage were analyzed for the polymer insulators before, during and after simulation tests compared with porcelain insulators. At the end of fire simulation tests, there was no detrimental deterioration of any insulators. All insulators passed the criteria of KEPCO specification. This study showed that forest fire simulation had no impact on polymer insulators.