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 may appear at fixed phases of the elliptical baseline. When these 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 most troublesome and challenging issues faced by every on-duty testing personnel.
Through long-term testing practice and summarization, we have discovered and found 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. There may be loose connections in the high-voltage lead quick connectors of the test system.
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. 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 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. This 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 also cause frequent false breakdowns during testing due to poor contact. By disassembling and re-tightening the quick connectors, the background noise can generally be significantly reduced, quickly lowering the test background to below 5PC, thus meeting the testing 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 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 cart, and rails, resulting in repeated grounding of the semi-finished cable under test, which introduces 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 to isolate them, the interference can be greatly reduced. 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 significantly reduced by taking insulation isolation measures, thus meeting the requirements for factory partial discharge testing.
3. There may be poor contact at the connectors of the coaxial cable signal connection lines in the system.
The sealed ends of the signal lead wires between the partial discharge instrument and the detection unit may develop 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 system "HV Close" button or during the high-voltage boosting process, the elliptical baseline may suddenly widen and blur, and the background may also change 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 loop after high voltage is applied, which connects the poorly contacted parts through arcing. By re-plugging and checking the sealed ends of each coaxial cable signal transmission line, or by conducting continuity checks and treatments, this phenomenon can be quickly eliminated, ensuring the normal progress of the test.
