Basic knowledge of power capacitors and common operation problems ④

4 Common operation problems of power capacitors 4.1 Common problems (shunt capacitors) (1) Causes of inrush current during operation: LC series resonance, the inrush frequency is several hundred to several thousand Hz, which can reach tens of times the normal current, and its maintenance time Generally in tens to hundreds of ms; main hazards: CT breakdown and electrical wear of switch contacts. (2) Causes of over-voltage during exit: The switch reignites, and the over-voltage multiplier can be up to 5 times or more. Main hazards: Causing overvoltage breakdown of capacitors and related equipment. Figure 4.1 Causes of switch reignition (3) Causes of overcurrent and overvoltage during operation: The high-order harmonics in the power supply resonate with the L and C parameters of the circuit. Main hazards: long-term overcurrent and overvoltage. 4.2 Protection measures a. Series reactor current limit; b. Use non-reignition switch (such as SF 6 switch), when the unsophisticated vacuum switch is just put into use, the reignition probability is 2~6%, and the capacitor is disconnected during operation After 30 times of current, it basically does not reignite; c. Add auxiliary contacts and parallel resistance in the switch; d. Single element fuse protection; e. Install lightning arrester protection; f. Three-phase capacitor bank adopts double star connection Method, when one of the capacitors is damaged, the neutral point unbalanced current is used to start the protection circuit. Figure 4.2 Capacitor protection measures Figure 4.3 Capacitor bank double star connection 5. Capacitor test items 5.1 Acceptance test after delivery (1) Visual inspection; (2) Tightness inspection; (3) Capacitance measurement; (4) Power frequency withstand voltage test (usually 75% of the factory test); (5) tanδ measurement; (not for shunt capacitors and collective capacitors) (6) Insulating oil test (collective capacitors). Users can negotiate with the manufacturer to add some items in the type test or factory test (such as impact test, partial discharge measurement, etc.) according to their needs. 5.2 Acceptance (handover) test after installation (1) Measure insulation resistance; (2) Measure tanδ and capacitance value of coupling capacitor and circuit breaker capacitor; (3) Partial discharge test of 500kV coupling capacitor (when there is doubt about insulation); (4) Parallel capacitor AC withstand voltage test; (5) Impulse closing test 5.3 Preventive test (1) Pole-to-shell insulation resistance measurement (collecting capacitors increase the phase-to-phase); (2) Capacitance measurement; (3) Appearance and seepage Oil leakage inspection (4) Infrared temperature measurement; (5) tanδ measurement (not for shunt capacitors and collective capacitors); (6) Low-voltage end-to-ground insulation resistance (coupling capacitor); (7) AC withstand voltage and partial discharge test ( Coupling capacitor, if necessary); (8) Insulating oil test (collecting capacitor). 6. Capacitor test method 6.1 Appearance inspection The appearance inspection is mainly to observe whether the capacitor has any problems such as deformation, rust, oil leakage, overheating and discoloration, and bulging. 6.2 Tightness check The user generally can only use the heating method. Heat the sample to the highest allowable temperature plus 20°C without powering on, and maintain it for a period of time (more than 2 hours). Check carefully for oil leakage. Location. 6.3 Insulation resistance measurement 6.3.1 Basic concepts After applying a DC voltage to the sandwich insulator, three currents will be generated, as shown in Figure 6.1. Figure 6.1 Equivalent circuit of sandwich insulator 1) Electrical conduction current i R, related to insulation resistance; 2) Capacitive current i C, related to capacitance; 3) Absorption current i 1, caused by the polarization process of the insulating medium. It is generally believed that the capacitive current decays quickly, and the decay time of the absorption current is longer, which has a greater impact on the measurement of insulation resistance. This analysis is only valid when the capacitance C is relatively small. When the capacitance is large and the megohmmeter cannot provide a large charging current, the capacitance current will instead become the main factor that affects the measurement results. The greater the electrical capacity of the test product, the higher the short-circuit output current requirements of the megohmmeter. Table 6. 1 Requirements for the short-circuit current of the megohmmeter (reference value) 6.3.2 Measurement method 1) Measurement location: Parallel capacitors only measure the insulation resistance between the two poles of the shell; voltage divider capacitors and voltage equalization capacitors measure the insulation resistance between poles; The coupling capacitor measures the insulation resistance between the poles and the low-voltage electrode to the ground; 2) Measurement wiring: The L terminal of the megohmmeter is connected to the high-voltage end of the device under test, and the E terminal is connected to the low-voltage end of the device or the ground. When equipment, the shielding end G of the megohmmeter is connected with other non-tested equipment; 3) Measurement steps: a. Before the measurement, the two poles of the capacitor should be shorted to the ground and fully discharged for more than 5 minutes; (discharge) b. After the megohmmeter has established the voltage, short-circuit the L and E terminals and separate the L and E terminals. The megohmmeter should display zero or infinity; (validation) c. The connection or separation of the high-voltage terminal L of the megohmmeter and the tested product should be carried out when the voltage of the megohmmeter is established; (accurate, reverse charging) d. Record the insulation resistance at 15 seconds and 60 seconds when measuring the absorption ratio; record the insulation resistance at 1 minute and 10 minutes when measuring the polarization index; e. After the measurement, the two poles of the capacitor should be short-circuited to the ground and discharged for more than 5 minutes.
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