How to Eliminate Background Noise Interference During Series Resonance Withstand Voltage Test Device Operation
During routine factory testing, there are times when the test background suddenly increases. This is mainly manifested as the elliptical baseline of the partial discharge oscilloscope suddenly becoming wider and blurry before the test voltage is applied, after the test system is connected to the cable under test. At the same time, large external interference pulse spikes appear at fixed phases of the elliptical baseline. When the interference spikes 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 shipment. How to eliminate and troubleshoot various background noise interferences during testing is one of the headaches and tricky problems faced by every on-duty testing personnel.
Through long-term testing practice and summarization, we have found and discovered some practical methods to eliminate and reduce background noise in 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 be loose.
The length of the high-voltage lead is generally about 10 meters, and the outside is covered with a metal hose to uniformly 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 testing and the pulling force from the test terminal (mainly because the 35kV cable test terminal is heavy after oil filling, causing significant pulling force on the lead), over time, the connection between the quick connector and the metal hose may become loose and have poor contact. This phenomenon can cause the elliptical baseline of the partial discharge oscilloscope to widen and blur, and the background noise to increase exponentially. On the other hand, it may cause frequent false breakdowns during testing 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 to below 5PC, thus meeting the test requirements.
2. There may be repeated grounding during cable testing, causing the background noise displayed by the partial discharge instrument to increase exponentially.
This phenomenon is particularly 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 cart, and rails, resulting in 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 cart for isolation, 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 adopting insulation isolation measures, thereby 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 sealing heads at the ends of the signal leads between the partial discharge instrument and the detection unit may have poor contact due to long-term use, causing an increase in background noise of the partial discharge detector. In severe cases, when marking, the elliptical oscilloscope may not receive the marking return pulse signal. However, after pressing the "HV Close" button or during the high-voltage boosting process, the elliptical baseline suddenly widens and blurs, and the background also changes abruptly. At this time, when marking under high voltage, the marking return pulse signal appears, 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 point through an arc. By re-plugging and checking the sealing heads at both ends of each coaxial cable signal transmission line, or by conducting continuity checks, this phenomenon can be quickly eliminated, ensuring the normal progress of the test.
