Application of the hottest UHF method in partial d

Application of UHF method in partial discharge detection of GIS equipment

Qu Haitao 1, Guo Lin 2, shaofanghua 3, Xu Hui 4 (1. Sanmenxia Electric Power Bureau, Henan Sanmenxia 472000;

2. Zhengzhou Electric Power College, Henan Zhengzhou 450004;

3. Hebi Wanhe Power Generation Co., Ltd., Henan Hebi 458000;

4. Luoyang Yugang Electric Power Development Co., Ltd., Henan Luoyang 471009) 1 overview

in recent years, With the development of urban power construction, the number of GIS substations is increasing. At present, there are nearly 40 GIS substations in Henan Province, which is expected to continue to increase in the next few years. Due to the high operating voltage of GIS equipment and the extremely limited internal space, the working field strength of GIS equipment is very high. On the other hand, the insulation margin in GIS equipment is relatively small. Under strictly controlled environmental conditions, the breakdown strength of SF6 gas in GIS equipment is expected to reach a very high level, but in fact, it can only reach half of the expected value, or even lower. Once there is some defect in the GIS equipment, it is very easy to cause equipment failure. With the increase of the number of GIS substations, the probability of GIS equipment failure is also increasing. In the past, people believed that GIS equipment was maintenance free equipment, and the number of GIS substations was small, so the means and measures to monitor equipment failure were relatively limited. Therefore, it is of great significance to strengthen and improve the handover test of GIS equipment and the operation monitoring of GIS equipment to ensure the safe operation of GIS equipment

the research shows that the internal faults of GIS equipment are mostly insulation faults. In 2001, three cases of GI vehicle internal environment, especially the smell and atomization in the vehicle, occurred in Henan Province, and have been listed as a new standard to judge whether it is comfortable or not. S equipment failures are all insulation failures. Other provinces in China also have similar situations. Partial discharge of GIS equipment is often the precursor and manifestation of insulation fault. It is generally believed that the discharge in GIS equipment decomposes SF6 gas, seriously affects the electric field distribution, leads to electric field distortion, insulation material corrosion, and finally leads to insulation breakdown. Practice has proved that PD detection can effectively avoid GIS accidents. At present, the relevant departments of Henan Province are studying and formulating the specific implementation plan of GIS equipment partial discharge measurement, and it is proposed to include GIS equipment partial discharge measurement in the GIS equipment handover test and operation monitoring project

this paper describes the principle of ultra-high frequency method (UHF method) and its application in GIS partial discharge detection

2 partial discharge detection method of GIS equipment

partial discharge of GIS equipment is not only a precursor of internal insulation fault, but also a typical manifestation of insulation fault

2.1 common defects inside GIS equipment

a. defects inside solid insulating materials in GIS equipment, such as the residual air gap in the basin insulator or at the junction with the conductor in the production process

b.the residual free conductive particles in GIS equipment, such as metal debris or metal particles, are common defects, which are generally caused by manufacturing, installation and other reasons

c.there are protrusions on the conductor surface in GIS equipment, such as burrs, sharp corners and other defects, which are prone to corona discharge. Generally, insulation breakdown will not be caused under stable operating voltage, but insulation breakdown may be caused under impulse voltage

d.poor contact of conductors in GIS equipment, etc

the above defects can often cause partial discharge of GIS equipment

2.2 the internal defects of GIS equipment produce the characteristics of the sample discharge used in the local metal material compression failure experiment

a. the corona discharge occurs around the conductor. Because the molecules in the gas move freely, the corona discharge process in GIS equipment is similar to that in the air

b.the air gap discharge inside the insulator in GIS equipment is basically the same within the positive and negative half cycles of power frequency, that is, the positive and negative half cycle discharge fingerprints are basically symmetrical. The discharge pulse generally appears in the rising part of the absolute value of the test voltage amplitude, and the discharge frequency depends on the applied voltage, so it can maintain its shape voltage for a long time. Only when the discharge is strong, it will expand to the phase of the falling part of the absolute value of the voltage, and the magnitude of each discharge is different. Insulator defects may not appear when leaving the factory, but may cause damage during transportation and installation. Some defects may be harmless at first, but may move slightly under the action of mechanical vibration and electrostatic force, forming potential hidden dangers

c. the defects on the insulator surface (such as dirt, etc.) contribute to the increase of surface charge, which may form surface discharge, leading to insulation deterioration and even breakdown on the insulator surface

d. the phase distribution of metal protrusion discharge on free conducting particles and solid conductors is significantly different. This feature can usually be used to distinguish the types of defects. Free conductive particles in GIS equipment have the ability to accumulate charge. Under the action of AC voltage, electrostatic force can make conductive particles jump in GIS cylinder, such as vertical rotation, dancing movement, etc. The occurrence of this movement and discharge is largely random, and this process is related to the applied voltage and the characteristics of particles. If a bouncing particle approaches or moves to the high field strength area in GIS equipment, the accompanying local standard introduces the concept of forming shrinkage difference △ vs. partial discharge may form a conductive channel and cause insulation breakdown. Relatively speaking, various effects caused by residual metal debris or metal particles in GIS equipment are the most serious. Therefore, the discharge of metal particles is relatively harmful to GIS equipment

2.3 detection method

partial discharge in GIS equipment will generate flowing electromagnetic waves on the GIS equipment shell, causing high-frequency discharge pulse current to flow on the grounding wire, resulting in high-frequency voltage to the ground and spreading to the surrounding space. Partial discharge will also cause a sudden increase in the channel gas pressure, generate longitudinal waves or ultrasonic waves in the gas of GIS equipment, and produce various sound waves on the metal shell, such as longitudinal waves, transverse waves and surface waves. Partial discharge in GIS equipment can also cause decomposition or luminescence of SF6 gas. The characteristics of these physical and chemical changes are helpful to the detection of partial discharge in GIS equipment. At present, the following methods are widely used at home and abroad

a. shell electrode method lay insulating film and metal electrode on the shell of GIS equipment to form a small capacitance between the shell and metal electrode. The discharge signal is coupled to the detection and processing unit through capacitance, and the partial discharge level of GIS equipment is finally displayed. The principle of this method is similar to the traditional partial discharge detection method. Its advantage is high detection sensitivity, but its fatal disadvantage is that it is vulnerable to external interference, especially the interference of various pulse signals naturally existing in electricity (such as corona interference in electricity)