전력용 피뢰기의 열화진단기술 개발에 관한 연구

Abstract

In this dissertation, the changes of electrical properties and new diagnostic techniques for lightning arresters have been studied. Generally, lightning arresters deteriorate as the result of a repetitive application of AC over-voltages or lightning impulse currents. The resistive leakage current of arresters increases as the deterioration of the arrester progresses, showing an increase in the 3^rd harmonic component of the leakage current. The deteriorated lightning arrester is brought to a thermal runaway, where it can cause a line-to-earth fault. It is, therefore, important to estimate the change of electrical properties and the expected lifetime of lightning arresters.

To study the change of electrical properties in lightning arresters, a multiple-lightning impulse generator which can produce a quadruple with impulse current of 8/20[㎲] 5[kA] at an interval of 30~120[ms] has been designed and fabricated. The generator generates almost same thing as natural lightning ground flash. The total energy applied to the arrester at each pulse is about 1,200[J]. In the experiment, various parameters such as leakage current, reference voltage, and temperature were measured with the number of applied impulse currents. Also, micro-structures of the ZnO blocks were investigated 200 times after applying the single- and the multiple-lightning impulse currents.

The experimental results indicated that the arrester blocks are more vulnerable to deterioration or damage from the multiple-lightning impulse currents. Therefore, it is necessary for lightning arresters used in power systems to be tested by applying not only a single-impulse current but also a multiple-impulse current. Also, to deduce the parameters necessary for diagnosing arrester deterioration, the accelerated deterioration tests were carried out.

In the test, leakage current components were measured, and the wave height distributions of leakage current were analyzed according to the progress of deterioration in the arrester.

The wave height distributions of the leakage current showed conspicuous differences even though the deterioration of arrester was progressed a little. Consequently, the use of wave height distributions in the leakage current for a deterioration diagnostic technique is valuable because it makes a more accurate diagnosis than the conventional method of using only a leakage current value.

The expert system for diagnosing the deterioration of arresters has been designed, and the system can diagnose lightning arresters by measuring the leakage current magnitude and its wave height distribution.

Today, arrester diagnostic techniques based on total leakage current and/or the 3^rd harmonic leakage current as a reference parameter of deterioration are widely used. The technique, however, includes an error due to the harmonics of power system voltage. Therefore, the influence of the harmonics on arrester diagnostics should be considered.

In this dissertation, an arrester model was designed to simulate the influence of the voltage harmonics described above. A pure sinusoidal voltage and its the 3^rd harmonic voltage were applied to the model, and the leakage current components were analyzed.

From the simulation results, it was confirmed that the peak value of resistive leakage current depends not only on the phase of the 3^rd harmonic voltage but also on the magnitude of it. Consequently, the errors caused by the harmonic voltage could be minimized by correcting the magnitude of leakage current upon analyzing the harmonics.