In the switchgear insulation system, the electric field strength of each part is different. Once the electric field strength of a certain area reaches its breakdown electric field strength, the area will be discharged. However, the two conductors applying voltage do not run through the entire discharge process, that is, the discharge does not break through the insulation system. This phenomenon is called partial discharge. The electric field distribution in the insulating medium and the electrical and physical properties of the insulation determine the conditions under which partial discharge occurs. Generally, partial discharge is easy to occur under the conditions of high electric field strength and low electrical strength. Although partial discharge usually does not break through the insulation, it may damage the dielectric locally. If partial discharge exists for a long time, the electrical strength of the dielectric will be reduced under specific conditions. It can be seen that partial discharge is a hidden danger in electrical equipment, and its destruction process is slow and long-term. Generally, the characteristics of partial discharge can better confirm the insulation defects, and the partial damage degree of insulation can be analyzed through the characteristics of partial discharge. To a large extent, comprehensive measurement of various partial discharge characteristics can objectively evaluate the insulation level of products.

2 Type and characteristics of partial discharge

2.1 Corona discharge

Corona discharge usually occurs around high-voltage conductors surrounded by gas, such as high-voltage transmission lines or high-voltage transformers. The high-voltage terminals of these high-voltage electrical equipment are exposed to the air, so the probability of corona discharge is relatively high. Corona discharge reflects the characteristics of typical and extremely uneven electric field, and is also a unique self-sustaining discharge form under extremely uneven electric field. Many external factors will affect the corona inception voltage, such as electrode shape, applied voltage, gas density, distance between electrodes, air humidity and flow velocity, etc.

2.2 Surface discharge

Generally, surface discharge occurs on the surface of insulating medium. This type of partial discharge belongs to a special gas discharge phenomenon. Surface discharge is common at the ends of power cables, motor windings, insulating bushings, etc. Once the electric field strength inside the medium is lower than that of the air gap at the electrode edge, and the dielectric breakdown voltage along the surface is relatively low, the surface discharge will occur on the surface of the insulating medium. Generally, voltage waveform, distribution of electric field, air quality, surface state of medium, climate conditions, etc. will affect the surface discharge voltage, so the surface discharge shows the characteristics of instability.

2-3 Internal discharge

Internal discharge is common in solid insulating medium. During the production and processing of insulating media, it is inevitable that there are material and process defects, which lead to internal defects in the insulating media, such as adding a small amount of air or impurities. Once the insulation is subjected to high voltage, local breakdown or repeated breakdown may occur to internal defects. In general, the characteristics of the medium itself, the size of the air gap, the location and shape of defects, and the type of air gap gas will affect the occurrence conditions of internal discharge.

2.4 Suspended potential discharge

This form of partial discharge refers to that a conductor part in the high-voltage equipment has structural design defects, or is disconnected due to poor contact due to other reasons, which finally causes the part to be located between the high-voltage electrode and low-voltage electrode and obtain partial voltage according to the impedance ratio of its position to discharge. The potential on the conductor part to the ground is called the floating potential. When a conductor has a levitation potential, the electric field near it is usually concentrated and will destroy the formation of the surrounding insulating medium. Generally, suspended potential discharge is easy to occur on metal parts with high potential in electrical equipment or metal parts with ground potential.

3 Partial discharge detection method of switch cabinet

For the switchgear, its partial discharge detection methods include the following:

3.1 Ground wave detection

When partial discharge occurs in the insulation layer of high-voltage switch cabinet, electromagnetic wave will be generated, and the metal shell of the switch cabinet will shield a large part of this electromagnetic wave. However, a small part of this electromagnetic wave will be transmitted through the joints of the metal shell or the gas insulated switch pad, and a ground wave will also be generated and transmitted to the ground through the external surface of the metal shell of the equipment. The range of geoelectric wave is usually from several millivolts to several volts, and the rise time is several nanoseconds. The probe can be set on the external surface of the switch cabinet in the working state to detect the partial discharge activity.

3.2 Ultrasonic testing

In fact, ultrasonic testing belongs to a kind of mechanical vibration wave. From the perspective of energy, the process of partial discharge is the process of instantaneous energy explosion. Electric energy is released in the form of sound energy, light energy, heat energy and electromagnetic energy. Electrical breakdown occurs in the air gap, and the discharge can be completed in an instant. At this time, electrical energy will also be converted into heat energy in an instant, and the gas in the discharge center will expand under the effect of heat energy, The sound wave propagates outward, and the gas in the propagation area is heated to form an isothermal zone whose temperature is higher than the ambient temperature; When these gases begin to shrink after cooling, subsequent waves will be generated. The frequency and intensity of the subsequent waves are relatively low, including various frequency components, with a wide frequency band, and the ultrasonic frequency is greater than 20kHz. Because the area of partial discharge is relatively small, the local emission sound source is the point sound source.

3-3 Ultra high frequency detection method

In the process of time change, electromagnetic vibration generated by partial discharge will generate electromagnetic wave. In solid gas and gas medium, partial discharge pulse will generate very rich ultrahigh frequency components of electromagnetic wave, up to GHz. In the actual application process, the partial discharge signal can be detected by two probes. The time sequence of the signals detected by the probes is used as the judgment basis. If the discharge source is close, it will be detected first; The probe position is constantly changing, and the general position of the discharge power can be determined step by step. Alternatively, the three-dimensional space coordinates of the discharge source can be obtained by using the time difference equation set of the probe to detect the partial discharge signal through multiple probes, and the discharge source can be finally determined. The sensitivity of this method is relatively high, and it has strong anti-interference ability. Moreover, there are usually seams or small glass windows on the switchgear, so the requirements that this method is difficult to detect under completely sealed conditions can be ignored.

3.4 Comprehensive testing technology

In fact, no matter which detection method is used, it has certain limitations. It can not objectively, comprehensively and truly reflect the operation state of the switchgear, and it may lead to miscalculation. Due to the different forms of energy release of the discharge types and the differences in the practicality and sensitivity of various detection methods, the above detection methods should be comprehensively applied in the process of partial discharge detection of the switchgear, with the ground wave detection as the main, ultrasonic detection and ultra-high frequency detection as the auxiliary.

4 Partial discharge analysis technology

Specifically, the commonly used partial discharge analysis techniques include the following: first, the horizontal analysis method, that is, to make a horizontal comparison of the test results of the switchgear in the same switch room. If the test results of one switchgear are greater than the field background value and the test results of other switchgear, it can be determined that the equipment may have defects; Second, the trend analysis method is used to analyze the test results of the same switch cabinet at different times, and make a longitudinal comparison to judge the operation trend of the switch cabinet. Detect the switch cabinet in the switch room according to a specific period, keep the test results every time, and then analyze the trend of partial discharge state change of equipment according to the test results; Third, threshold comparison, that is, provide a judgment threshold, compare it with the detection results of the switch cabinet, and analyze the results to judge the operation status of the switch cabinet. It can be judged according to the following: when both the background value and the test value in the switch room are below 20dB, the switchgear is normal, and the patrol inspection will be carried out again next month; The background value in the switch room is below 20dB, while the test value of some switch cabinets is 20-30dB. Pay more attention to this switch cabinet, shorten the test cycle, and observe the change trend of the test amplitude;; If the background value in the switch room is below 20dB, and the test value of some switch cabinets is greater than 30dB, the switch cabinet has partial discharge, and the discharge point should be located using the positioning technology.

In a word, partial discharge shows a certain complexity. Generally, partial discharge is easy to occur in the part with relatively low breakdown field strength inside the insulation, and the electric field distribution inside the insulation medium and the electrical performance of the insulation play a decisive role in the condition of partial discharge. In the actual detection process, reasonable and applicable detection methods and analysis methods should be selected, and faults should be eliminated in time to ensure that the switchgear is in good operating condition.