How to Eliminate Background Noise Interference During Series Resonance Withstand Voltage Test Device Process
During the daily factory test process of the series resonance withstand voltage test device, sometimes the test background suddenly increases. The main manifestation is that after the test system is connected to the cable under test, before the test voltage is applied, the elliptical baseline of the partial discharge oscilloscope suddenly becomes wider and blurred. At the same time, large external interference pulse glitches appear at fixed phases of the elliptical baseline. When the interference glitches are large and numerous, exceeding the background requirements for cable partial discharge testing, the test cannot proceed normally. This seriously affects the normal delivery of cables and timely delivery. How to eliminate and troubleshoot various background noise interferences during the test process is one of the headaches and tricky problems faced by every on-duty testing personnel.
Through long-term test practice and summary, we have found and discovered some practical methods to eliminate and reduce background noise of test equipment. The increase in background interference noise after connecting cables to the series resonance withstand voltage test device may be caused by the following reasons.
1. The high-voltage lead quick connector part of the test system may have loose connections.
The length of the high-voltage lead is generally about 10 meters, and the outside is covered with a metal hose to evenly distribute the surface electric field of the lead. Both ends of the metal hose are reliably connected to the cable lug and quick connector by the test equipment manufacturer. However, due to frequent movement during the test and the tension from the test terminal (mainly because the 35kV cable test terminal is heavy after oil filling, causing significant tension on the lead), over time, the connection between the quick connector and the metal hose may become loose and have poor contact. After this phenomenon occurs, on one hand, it will cause the elliptical baseline of the partial discharge oscilloscope to become wider and blurred, and the background noise will increase exponentially. On the other hand, it may cause frequent false breakdown phenomena during the test due to poor contact. By disassembling and re-tightening the quick connector, the background noise can generally be greatly reduced, quickly lowering the test background of the cable to below 5PC, thus meeting the test requirements.
2. There may be repeated grounding during the cable test, causing the background noise displayed by the partial discharge instrument to increase exponentially.
This phenomenon is especially obvious when performing partial discharge tests on shielded semi-finished products mounted on iron fixtures. This is because the exposed copper tape shielding layer is connected to the system ground through the iron fixture, cable moving trolley, and track, causing repeated grounding of the semi-finished cable under test, thereby introducing a large amount of external interference signals. This increases the background noise of the test system. By placing insulating materials (such as cable finished plastic sheath or thick epoxy insulating material) between the iron fixture and the moving trolley to isolate them, the interference can be greatly eliminated. Sometimes, if the copper tape at the end of the finished test cable is left too long and overlaps with the iron or wooden fixture angle iron, it can also cause repeated grounding and introduce large interference. The above phenomena can be greatly reduced by taking insulation isolation measures, thus meeting the requirements for factory partial discharge testing.
3. There may be poor contact at the connector of the coaxial cable signal connection line in the system.
The signal lead end sealing heads of the partial discharge instrument and detection unit may have poor contact after long-term use, causing increased background noise in the partial discharge detector. In severe cases, when marking, the elliptical oscilloscope may not receive the marking return pulse signal. However, after pressing the "high voltage close" button or during the high voltage boosting process, the elliptical baseline suddenly becomes wider and blurred, and the background also changes suddenly. At this time, when marking under high voltage, the marking return pulse signal appears on the oscilloscope, and everything seems to return to normal under high voltage. This phenomenon is caused by the current flowing through the signal circuit after high voltage is applied, which connects the poor contact part through an arc. By re-plugging and checking the sealing heads at both ends of each coaxial cable signal transmission line, or performing continuity checks, this phenomenon can be quickly eliminated, ensuring the normal progress of the test.
