Won-Kyo Lee

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.

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.

A Study of the Slim Design of Overhead Transmission Tower

Transmission and Distribution, Korea Electric Power Research Institute, Daejeon 305-380, Korea(Received May 24, 2010; Revised June 17, 2010; Accepted June 23, 2010)Abstract: This paper presents the design factor of an overhead transmission tower structure in order to reduce the tower weight. The behaviour of transmission tower structures are affected by the horizontal angle of the tower structure, the equivalent wind pressure group, the slope of the main post of the tower, the separation of the internode and the use of high-strength materials in their construction. Tower weight can be reduced by approximately 30% reduce weight by means of optimal design based on a consideration of all the above factors. In addition, the design of the foundation of the tower with the shear key installation to increase horizontal support together with a modified angle of inclination to the ground can reduce by about 37% the amount of concrete used during construction. The area of ground disturbed by the construction of the tower foundation can thus be reduced by approximately 33%. Therefore it is possible to build an environmently-friendly T/L tower with the mechanical properties of existing towers.Keywords: Slim tower, Tower slope, Horizontal angle, Angle of inclination of ground

A Study on the Environmental Effects of Compact Tower in Transmission Line

The continuous increase demand for electric power leads to the additional construction of transmission facilities, but it is not easy to acquire right-of-way for transmission facilities. Therefor, there is a need for compact tower that can be built on a narrow right-of-way the compact tower with polymer insulation arm is a solution. It can be upgrading conventional 154 kV transmission line voltages to 345 kV levels. However transmission voltage is increasing, environment interference (corona noise, radio interference, etc.) will occur gradually. This environment interference is depending on the electrical clearances of tower and configuration of conductors. Therefore the analysis of the factors of environmental interference is necessary in order to upgrading transmission voltage. This paper presents the design factor of a compact tower to meet the environmental interference standard.

Mechanical and Electrical Properties of Aluminum Wires of ACSR Conductors due to Forest Fire

Forest fire can cause a serious damage to overhead conductors. Therefore, detailed investigation on the changes of mechanical and electrical properties of damaged conductors should be carried out to understand the effect of forest fires on conductors. This is of critical importance in maintaining transmission line safely. This paper examines the changes of mechanical and electrical properties of flame exposed conductor. Tensile strength (TS) decreased according to increase of forest fire temperature and conductivity changed according to forest fire temperature. Specimens were aluminum conductors of aluminium conductor steel reinforced (ACSR) 410, 240, 480 . In this paper, the electrical and mechanical characteristics of forest fires exposed overhead conductors depending on the diameter of aluminum conductors are presented. It was possible to estimate the degree of deterioration caused by forest fires. The detailed results are given in the paper.

A Study on the Measurement of Footing Resistance of Transmission Towers with Overhead Grounding wires

Footing Resistance of a 154 kV transmission towers in korea is commonly required to be less than 15 ohm to avoid lightning back-flashover accident. The periodic measurement of Footing Resistance is important to verify that the grounding performance of the towers has been maintained good. Towers are electrically connected in parallel with overhead grounding wire, therefore footing resistance of each tower will be measured after disconnecting the overhead ground wires from the towers. however, In this paper, three direct measurement methods of footing resistance are presented. There are very useful methods without disconnecting overhead ground wires from the tower under measurement. They are compared in KEPCO 154 kV transmission towers. The experimental results describe performances of them.

A Study on the Voltage Upgrading of Transmission Lines using Polymer Insulation Arm

The large increase in the use of electricity has resulted in an ever-growing electric power demand. It has created the need for the construction of power transmission facility located close to the load centers and it also has to require wide right-of-way and large lots, that are not always available, for especially the installation of the towers. The difficulties in acquiring right-of-way have put pressure on energy companies to either upgrade a line on an existing right-of-way to higher voltage or build a new line on a narrow right-of-way. This paper presents the design of a compact tower with polymer Insulation arm, in order to reduce the separation between phases. the compact tower can be built on a narrow right-of-way. the compact tower can be designed based on 345 kV Tower regarding electrical clearances and right of way, therefore the conventional 154 kV Tower can be upgrading transmission line voltages have moved to 345 kV levels.

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.