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How to prevent corona discharge during cable partial discharge test?

by:AAA     2020-03-10
Firstly, under the action of DC voltage, both positive and negative corona electrodes accumulate space charges near the tip electrode. In the negative corona, when the electrons collide and ionize, the electrons are driven to the space far away from the tip electrode and form negative ions. When the odd positive ions gather near the electrode surface and the electric field continues to strengthen, positive ions are sucked into the electrode, and a pulse current appears at this time, and negative ions diffuse to the gap space. After that, the next ionization and charged particle motion process is repeated. Such a cycle, so that many pulse forms of corona current. If the voltage continues to rise, the pulse frequency of the corona current increases and the negative value increases, and it changes into the attached glow discharge, and the voltage rises again, and the negative streamer discharge occurs, because of its shape is also called pinnate discharge or brush discharge. When the negative streamer discharge can continue to develop to the opposite electrode, it will lead to spark discharge and breakdown of the whole gap. Positive corona also distributes positive ions near the tip electrode, but they are constantly repulsed to The Gap space, while electrons are sucked into the electrode, forming repeated pulse current Corona. When the voltage continues to rise, streamer discharge occurs and may cause gap breakdown. The characteristic of corona discharge is accompanied by the sound of 'hissing', sometimes with faint glow. When there is a small tip on the surface of the conductor, corona discharge will occur. Corona discharge may or may not point to other objects in a particular direction. When corona discharge, the strength of the electric field near the tip is very strong, the gas near the tip is ionized, and the charge can leave the conductor; However, the field strength away from the tip weakens sharply and the ionization is incomplete, so only a small current can be established. Corona discharge is characterized by a 'hissing' sound, sometimes with a faint glow. Corona discharge can be continuous discharge or continuous pulse discharge, and the energy density of corona discharge is far less than that of spark discharge. In some cases, if the potential of the tip conductor is raised, the corona will develop into a spark leading to another object. During the pressure test of finished product inspection, the conductor must be inserted into the copper joint, the fixing screw should be tightened to make it firmly fixed on the cable head, and the exposed part of the insulation should be carefully wiped clean with cotton cloth and anhydrous alcohol, if the conductor and the high pressure part are not in good contact, the tip corona discharge will also be caused. When doing the high-voltage test of three insulated wire cores, the insulating medium must be separated to prevent mutual discharge of Corona. When peeling the head, strictly follow the operating procedures, use a planer to exploit the semi-conductive shielding layer from the end of the cable, strip the insulation and conductor shielding, and exploit the length 8. More than 15 cm, 12/20KV ~ 7/15KV and below ~ 20/35KV is more than 20 cm, and the peeling thickness is required to just strip off the insulating semi-conductive shielding layer. Pay attention to the final knife closing action. The Root should be round and sharp corners are not allowed, the stripped insulation part is not allowed to have deep knife marks or black semi-conductive shielding layer debris. When using a large terminal, it is appropriate to strip the insulation and conductor shielding so that the conductor is exposed by 5cm. The small terminal only needs to strip the external shielding, and the surface is not allowed to have any tip. Corona occurs, which will gradually destroy the insulation performance of high voltage electrical equipment. The space charge of corona discharge has the effect of improving the gap breakdown strength under certain conditions. When doing withstand voltage test, we should strengthen preventive measures to prevent corona discharge and improve the inspection efficiency of finished products.
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