Basic knowledge of power capacitors and common operating problems ⑤

7 Common abnormal problems and causes in capacitor test 7.1 Insulation resistance drop (1) Moisture; (2) External pollution; (3) Dielectric material deterioration. 7.2 Reduction in capacitance (1) Oil seepage or oil leakage; (because the dielectric coefficient of oil is larger than that of air) (2) The capacitor layer is disconnected or some components are open. 7.3 Increase in capacitance (1) Partial component breakdown in series components; (2) Moisture ingress (the dielectric coefficient of water is very large, εru003d81). 7.4 Increase in tgδ (1) Low insulation resistance due to moisture and external pollution; (2) Some components are not completely broken down and form a resistance; (3) The connecting wires of the components form contact resistance due to desoldering or virtual welding; (4) There is partial discharge; (5) Dielectric aging. 7.5 tgδ decreases under high voltage (1) There are impurities in the medium; (2) The components that have not been completely broken down are completely broken down under high voltage; (3) The connecting wires of open or virtual welding are completely connected under high voltage. through. 8 Operation and Maintenance of Power Capacitors 8.1 The temperature abnormality can be detected by an infrared thermal imager under operating conditions. In order to prevent the influence of sunlight, it should be checked at night or in a cloudy day. It can effectively detect defects such as internal moisture, short circuit, increased dielectric loss, lack of oil, and poor lead contact. The operating temperature of the capacitor should not exceed the product's regulations (usually should not exceed 55°C) Figure 8.1 Internal Defects of 10kV Shunt Capacitor Figure 8.2 Internal Defects of 220 kV Coupling Capacitor (Tgδ in the upper section is 0.2%, and the lower section is 0.64%) Figure 8.3 110kV Coupling capacitor capacity to reduce heat generation 8.2 There is noise during operation If there is noise inside the capacitor during operation, it means that there has been a serious discharge inside, and it should be withdrawn from operation in time. If it is external noise, it may be that the wire connection is loose. 8.3 Poor sealing If oil leakage or oil leakage is found, it may be caused by the following reasons, and it should be checked and dealt with in time. (1) The fastening bolts are loose; (2) The sealant is aging; (3) The outer casing is broken. 8.4 Shell swelling If the iron shell capacitor is found to be swollen, it may be caused by the following reasons: (1) Internal breakdown; (2) There is open welding or severe discharge inside; (3) The capacitor with harmonic resonance over-current shell bulging should be withdrawn in time Run and can no longer be used. 8.5 The capacitor should be taken out of operation immediately in the following emergency situations (1) The capacitor explodes; (2) The bushing has a serious flashover; (3) The capacitor is sprayed with oil; (4) The capacitor catches fire; (5) The joint is overheated and melted. 9 Technical problems in the capacitor test (1) For the problems of open welding and virtual welding, the measurement of insulation resistance cannot reflect the problem, because the crack will break down and discharge under high voltage, and the capacitor will still show normal charging and discharging phenomena, so use the output Capacitance meters with very low voltages can better find changes in capacitance; (2) For capacitors with multiple elements connected in series, even if the insulation resistance of most elements has dropped to zero, as long as the insulation resistance of one capacitance element is qualified, measure it. The insulation resistance is still qualified, but the capacitance and dielectric loss may increase significantly; (3) After the breakdown of some capacitive elements, there is still a certain insulation resistance between the plates. At this time, if the capacitance is measured with a low voltage, because The breakdown point remains insulated, and the measured capacitance is still normal. Only by increasing the test voltage can the fault point break down again. (4) For multi-element capacitors, it is difficult to find the defects of individual elements in the overall tgδ; (5) Incomplete breakdown of individual elements, decreased insulation resistance, false soldering of leads, or poor contact will all cause increased dielectric loss. The main differences are: a. Individual Generally, the overall insulation resistance does not decrease when the component is incompletely broken down and the lead is soldered; b. When the individual component is not completely broken down, a complete breakdown will gradually form with the increase of the test voltage, and the dielectric loss value may gradually drop to the normal range. And the overall capacitance increases; c. When the component leads are soldered, the dielectric loss will decrease with the increase in voltage, but the capacitance will not increase. (6) The influence of insulation resistance on dielectric loss is related to the capacitance of the capacitor. The smaller the capacitance, the greater the influence. As explained above, dielectric loss, insulation resistance, and capacitance are all important judgment data in the capacitor test. When there are doubts about the test results, comprehensive analysis and judgment should be carried out. 10 Maintenance of power capacitors 10.1 Minor repair items of shunt capacitors (1) Check porcelain bottles and remove oil stains; (2) Check and deal with insulating oil leakage defects; (3) Check grounding and discharge devices; (4) Check and replace fuses; (5) Replace the capacitor with the expansion and deformation of the shell; (6) Tighten the connecting bolts of various parts; (7) Carry out preventive tests.
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