Reasons and Detection Methods of Partial Discharge in Switchgear

Reasons and Detection Methods of Partial Discharge in Switchgear

During the operation of the switchgear, there are differences in the electric field strength of each part of the switchgear. Once the electric field strength of a certain area reaches its breakdown strength, a discharge phenomenon will occur in this area, but there is no penetration between the two conductors applying the voltage. The entire discharge process, that is, the discharge does not break down the insulation system, this phenomenon is called partial discharge. The switchgear is often in the partial discharge stage and cannot be recognized by the naked eye and manual touch. If it is not treated for a long time, the area with severe discharge will reach its breakdown strength. failure will occur.

1. Reasons for partial discharge in switch cabinets

1) Influence of running state

operating overvoltage;

Lightning shock;

harmonic distortion;

2) The reason for the device itself

Insulation material is not uniform;

There are voids and impurities inside;

There are protrusions on the conductor surface;

Insufficient dielectric strength;

3) Influence of environmental factors: humidity, overheating.

2. The importance of PD online detection

1) Insulation failure can have catastrophic consequences

2) Most of the insulation accidents of electrical equipment in operation are related to partial discharge

3) The insulation condition of equipment is difficult to test in daily inspections

3. Detection method of partial discharge in switchgear

During the operation of the switchgear, the insulating material will be affected by high temperature, high voltage, oil pollution, chemical substances, vibration, etc., the insulation performance will continue to deteriorate, and the speed of partial discharge will be accelerated. In turn, partial discharge will promote the deterioration of insulation. effect. Therefore, detecting the partial discharge of the switchgear can effectively prevent the failure. For switchgear, its partial discharge detection methods include the following:

①Earth wave detection

Electromagnetic waves will be generated when partial discharge occurs in the insulation layer of the high-voltage switchgear, and the metal shell of the switchgear will shield a large part of this electromagnetic wave, but a small part will pass through the seams of the metal shell or the lining of the gas-insulated switchgear. The pad spreads out, and also generates a ground wave that travels underground through the outer surface of the metal casing of the device. Ground waves are usually in the range of a few millivolts up to a few volts, and have a rise time of several nanoseconds. The probe can be placed on the outer surface of the switchgear in working condition to detect partial discharge activity.

②Ultrasonic testing

In fact, ultrasonic detection is a kind of mechanical vibration wave. From the perspective of energy, the process of partial discharge is the process of instantaneous burst of energy. Electric energy is released in the form of sound energy, light energy, thermal energy and electromagnetic energy, and electrical breakdown occurs. It occurs in the air gap, and the discharge can be completed in an instant. At this time, the electric energy will also be converted into thermal energy in an instant. The gas in the discharge center will expand under the action of the thermal energy, and propagate outward through the sound wave. The gas in the propagation area is heated to form a In the isothermal zone, the temperature exceeds the ambient temperature; when these gases start to shrink after cooling, subsequent waves will be generated. The frequency and intensity of the subsequent waves are relatively low, contain various frequency components, and have a wide frequency band. The frequency of ultrasonic waves is greater than 20kHz . Because the area of the partial discharge is relatively small, the partial discharge sound source is a point sound source.

③Ultra high frequency detection method

In the process of time change, the electromagnetic vibration generated by the partial discharge will generate electromagnetic waves. In the solid gas and gas medium, the partial discharge pulse will produce very rich ultra-high frequency components of electromagnetic waves, which can reach several GHz. In the actual application process, the detection of partial discharge signals can be carried out by using two probes. The time sequence in which the probes detect the signals is used as the judgment basis. The closer the distance of the discharge source is, it will be detected first; Determine the approximate location of the discharge source step by step. Or through multiple probes, the time difference of the probe detection partial discharge signal is listed in the equation system, the three-dimensional space coordinates of the discharge source can be obtained, and the discharge source can be finally determined. The sensitivity of this method is relatively high, and it has strong anti-interference ability, and there are usually seams or small glass windows on the switch cabinet, so it is not necessary to consider the requirement that this method is difficult to detect under completely sealed conditions.

④Comprehensive detection technology

In fact, no matter which detection method has certain limitations, it is impossible to objectively, comprehensively and truly reflect the operating status of the switchgear, and there is a possibility of misjudgment. Due to the different discharge forms of discharge energy and the differences in the practicability and sensitivity of various detection methods, in the process of partial discharge detection of switch cabinets, the above detection methods should be comprehensively applied, mainly ground wave detection, ultrasonic detection. And UHF detection is supplemented.