In the routine factory test process of the series resonance partial discharge test system, sometimes the phenomenon that the test background increases suddenly occurs. The main manifestation is that after the test system is connected to the coiled cable, before the test voltage is applied, the elliptical baseline of the PD oscilloscope will suddenly widen and become blurred. At the same time, there will also be large external interference pulse burrs at the fixed phase of the ellipse baseline. When the interference burrs are large and numerous, and exceed the background requirements of the cable partial discharge test, the test will not be carried out normally. Seriously affects the normal delivery of the cable, and the delivery is on time. How to eliminate and troubleshoot all kinds of background noise interference in the test process is one of the headaches and difficult problems faced by every inspector on duty.
Through long-term experimental practice summary and exploration, we have found and found some practical solutions to eliminate and reduce the background noise of the experimental equipment. The increase of background interference noise after the series resonance partial discharge test system with cables may be caused by the following reasons.
1. There may be looseness in the quick connector part of the high voltage lead wire 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 uniform the surface electric field of the lead. At both ends of the metal hose, there are test equipment manufacturers to handle with wire noses and quick connectors for reliable connection. However, because the lead is often moved frequently and is affected by the tensile force of the test terminal during the test process (mainly, the 35 cable test terminal is heavier after refueling, which has a large tensile force on the lead), a long time will cause quick connector. Looseness and poor contact occur at the connection with the metal hose. On the one hand, the ellipse baseline of the PD oscilloscope will be widened and blurred, and the background noise will be multiplied. On the other hand, false breakdown may occur frequently due to poor contact during the test. By disassembling and re-tightening the quick connector, the background noise can generally be greatly reduced, and the test background of the cable can be quickly reduced to below 5PC, thus meeting the needs of the test.
2. During the cable test, there may be repeated grounding phenomenon, which makes the PD meter display background noise as a result.
This phenomenon is particularly evident when partial discharge tests are performed on shielded semi-finished products mounted on iron tooling. This is because the exposed copper tape shielding layer is connected to the system ground wire through the iron tooling, cable moving trolley and track, resulting in repeated grounding of the semi-finished product of the tested cable, thus introducing a large number of external interference signals. Increase the background noise of the test system. By inserting insulation (plastic sheath or thick epoxy insulation material) between the iron tool and the mobile trolley, the interference can be greatly eliminated. Sometimes the copper tape at the end of the finished test cable If it is left too long and overlaps with the iron and wood tooling angle iron, it will also cause repeated grounding and introduce large interference. The above phenomenon can greatly reduce the interference by taking insulation isolation measures, so as to meet the needs of the factory partial discharge test.
3. There may be poor contact at the connector of the coaxial cable signal cable of the system.
Due to the long-term use of the signal lead ends of the PD detector and the detection unit, there may be poor contact, which will increase the background noise of the PD detector. In severe cases, the elliptical oscilloscope may not receive the marking return pulse signal during marking, and after the system "high pressure close" button is pressed or during the high pressure boosting process, the ellipse baseline will suddenly widen and become blurred, and the background will be blurred. It also followed a sudden change. At this time, when marking under high pressure, the pulse signal returned from the oscilloscope marking appeared, and everything seemed to return to normal under high pressure. This phenomenon is due to the fact that after the system applies high voltage, the current flows through the signal loop, and the signal of the bad contact part is connected through the arc. By re-plugging and checking the encapsulation heads at both ends of each coaxial cable signal transmission line, or conducting a continuity check on them, this phenomenon will be quickly eliminated and the normal operation of the test will be ensured.
The above are some small experiences we have accumulated in the investigation of background noise interference during the partial discharge test of medium-voltage cross-linked cables. Among them, the looseness between the high-voltage lead wire shielded metal hose and the quick connector is more common, and we should pay enough attention during the test. In addition, during the test process, if batches of cables fail to pass the partial discharge test and the equipment displays a large background noise, it is not easy to draw a conclusion that the partial discharge test of the cables is unqualified. We should thoroughly investigate and deal with the causes of the background noise of the equipment, and take the results of the second re-inspection after the equipment returns to normal as the basis for judging whether the partial discharge of the cable is qualified or not. Thus, the phenomenon of misjudgment and missed judgment in the process of partial discharge detection is prevented.
There is another phenomenon worth mentioning. In our daily inspection process, although the partial discharge detection equipment has taken a variety of anti-interference measures, some unforeseen external interference during the test will still appear on the oscilloscope within a certain period of time. popped up. According to our work experience, we know that it may be the momentary interference caused by the temporary opening of a certain equipment in the workshop or the signal transmitted by the external radio station. It usually disappears after a few minutes, or appears at a fixed phase of the ellipse. This situation has no effect on our power frequency AC voltage test, but has a great impact on the partial discharge and positioning work. We can only wait for the interference to disappear or use the window method to conduct the test.