By hvhipot 
The partial discharge phenomenon inside the insulation of electrical equipment is periodic, and it goes on repeatedly with the change of the AC voltage, and discharges when the amplitude of the AC voltage is maximum. When a partial discharge occurs inside the insulator, energy in the form of light and sound waves will also be generated.
For electrical equipment operating in the power grid, such as high-voltage electrical equipment such as cables, capacitors, transformers, transformers, and motors, the insulation withstand voltage level is designed according to its operating voltage level. Under normal conditions, its insulation performance can withstand operating voltage. Due to poor manufacturing processes, the insulation of electrical equipment may leave bubbles, impurities, cracks, etc. inside. Under the action of a high-voltage alternating electric field, this kind of insulation will cause periodic partial discharges inside the insulation. Because its discharge energy is very small, it will not break down the entire path at once, but it can reduce the insulation performance and even lose the withstand voltage performance. Over time, this partial discharge will cause the entire insulation to break down and explode, greatly affecting the safe operation of the power grid and power supply reliability, and causing great economic losses to users and society. Therefore, attention should be paid to the partial discharge of electrical equipment.
Partial Discharge Measurement methods are generally divided into non-electrical measurement methods and electrical measurement methods.
The non-electric measurement method is mainly the ultrasonic method, whose purpose is to detect the location of the partial discharge site. The electrical measurement method is a measurement method that uses high-frequency pulse currents. It is widely used due to its high sensitivity and ease of use. The partial discharge tester is made according to the principle of the high-frequency pulse current method.
Through the detection of partial discharge, the insulation defect of electrical equipment can be reflected. This kind of test is non-destructive and replaces the destructive withstand voltage test to a certain extent, and its test voltage should ensure that it will not cause penetrating discharge. When testing the partial discharge capacity of a certain device, the pulse current generated by the partial discharge of the test object is amplified and sent to the measuring instrument. After the discharge capacity is corrected, the discharge capacity value is measured. For different types of equipment and insulation. The discharge value is also different. Manufacturers of high-voltage electrical products have ex-factory test standards, which can be evaluated with reference to the standards.
In order to ensure the safe operation of the power grid, it is necessary to conduct technical testing of partial discharge for newly put into operation equipment, and unqualified products shall not be put into operation. Equipment that has been installed in the power grid for operation should also be tested according to regulations, and those that fail to pass should be withdrawn from operation. In this way, major accidents such as explosions and power outages caused by partial discharge of equipment can be avoided.
Attention should be paid during the test: the wiring of the test loop must be correct, the contact part of the wiring must be firm, and the entire test system must be properly grounded. Before the test, set the sensitivity of the instrument to the required minimum value, and gradually increase the pressure. Pay attention to the indicator value of the instrument at any time. If an abnormality is found, the power should be cut off.
In short. Partial discharge test is one of the important detection methods to detect the insulation defects of electrical equipment of solid insulation structure and oil-immersed insulation structure.