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Cause of accident of coupling capacitor
Addtime:2012-07-20 15:00:20

In recent years, the burning and explosion accidents of the coupling capacitors have occurred, and the preventive tests made before the accidents are all qualified, and the accidents occur shortly after the operation. For example, a OY-220/3 type coupling capacitor, after passing the preventive test, commits a comminution explosion in less than 20 days. Therefore, the cause analysis and preventive measures of coupling capacitors have attracted wide attention.


First, the cause of the accident


Field accident analysis shows that the main cause of the accident is manufacturing quality problems.


(1) the capacitive core is dampened


Some manufacturers do not dry the capacitor core well, remain more water, and some manufacturers do not have time to press the pressure loading in time, causing the components to stay in the air too long, so that the capacitance core is dampened, forming a hidden danger.


(two) bad seal


The main reason is that the rubber sealing pad is of poor quality, the swelling rate of its bubble can not reach the requirement; secondly, the sealing property is not strictly checked; in addition, the bolt is not properly tightened or loosed through long distance transportation, so that the seal is invalid, the leakage oil is caused, and the insulation property is affected. For example, the burning loss of a OY-110/3 type coupling capacitor is caused by leakage oil. Again, a OY-110 /3 coupling capacitor exploded in operation, the main reason is that the sealing bolt on the cover plate is not tight, the oil resistant rubber pad is not sealed, and the rainwater enters the coupling capacitor along the sealing bolt to reduce the insulation and cause the breakdown and explosion.


(three) unreasonable structure design


Some of the finished products can not guarantee the constant pressure at the operating temperature, some do not install or install the dilator, and some are injected under normal pressure, so there will be negative pressure and easy to be dampened. Of the 6 coupling capacitor accidents in Zhejiang, 5 were caused by non pressure oil injection, and 4 in Shandong in 5 accidents.


(four) the splints are defective in manufacturing and processing


In the field, it is found that when the epoxy glass plate or the phenolic board is used as the hot pressing of the bottom village, the impregnability is poor and the bonding force is poor. It is easy to form the air gap or be seriously dampened in the cutting process. These reasons may cause the splint to be partial discharge under the operating voltage, thus reducing the insulating property of the splint. Splint defect is a very important reason for the coupling capacitor accident.


(five) the amount of aromatic hydrocarbons in the used capacitor oil is too low.


During the partial discharge of the capacitor oil under the action of high electric field, the gas (mainly hydrogen) is precipitated (mainly hydrogen) due to the impact of ion impact, and the solid wax (X wax) is generated. Aromatic hydrocarbons are unsaturated hydrocarbons with ring structure, which can be combined with hydrogen precipitated in capacitor oil to prevent gas from precipitation. However, due to the less aromatic hydrocarbon in the oil, the gas can not be absorbed, which aggravates partial discharge and gradually causes the medium to be aged and destroyed.


(six) element open welding


The coupling capacitor is composed of about 100 components. There are many welding heads, if there is a phenomenon of welding or dewelding, it will fire under the operating voltage, which causes the deterioration of the oil quality and the corrosion of the medium, causing the accident. A OY-110/3 type coupling capacitor of a substation in Chu County of Anhui province is an explosion caused by welding.


In addition, during the transportation process, if the equipment is laid down, it is often prone to malposition of components, which may cause defects similar to open welding.


(seven) there is a discharge phenomenon in the lead line of the equipment


The early product leads are not insulated and may be discharged with the potential dilator. A OY-110/3 type coupling capacitor in Yunfeng Power Plant in Northeast China is exploded for the above reasons.


It should be pointed out that the test items specified in the "electrical equipment preventive test regulation" (1985 Edition) are not ideal for detecting the defects of the coupling capacitor. This is because:


(1) normal measurement of insulation resistance is not good for detecting insulation defects or opening welding. For the open welding or non welding of capacitive components, it is generally believed that the insulation resistance table can be judged whether there is charging process or discharge sound during the test process, but because the coupling capacitor has 100 components in series, the connection gap between the components is very small and the voltage of the insulation resistance meter is high. As a result, stable spark discharge is difficult to reflect in the charge and discharge process.


For damp or partial defects of capacitance elements, it is difficult to find only some intact components in the series circuit. If a coupling capacitor has been seriously affected by moisture, its insulation resistance is 750M.


(2) measuring capacitance value is not likely to detect dampness and lack of oil. It is reported that the capacitance changes of the coupling capacitors of the accident are in the qualified range, and the proportion of the total number of the two individual components is very small. Therefore, in practice, it is not likely to detect dampness and defects by using the standard deviation of capacitance value not exceeding (ten 10%-5%) standard. In addition, the accuracy of the measurement results is also influenced by many factors, such as: (1) the standard capacitor is dampened; (2) the external strong electric field interference; (3) the wiring mode of the bridge; (4) the precision of the bridge. These all affect the detection effect.


(3) dielectric loss factor is also difficult to detect insulation defects. Because the coupling capacitor is made up of about 100 capacitance components, the overall dielectric loss factor can not reflect the change of the number of loss mass of the dielectric loss of the individual element. For example, a OY-110/3 type coupling capacitor, the capacitance of each component is about 0.7 F, several components are poorly insulated and the dielectric loss factor is very large, but the total dielectric loss factor changes little. Table 5-8 lists the 6 capacitor capacitors and the overall dielectric loss factor. As shown in table 5-8, even if the dielectric loss factor of a single capacitor element is large, the dielectric loss factor of the capacitor is still within the acceptable range.


Table 5-8 dielectric loss factor of the coupling capacitor element and the whole

Two. Preventive measures


(1) improve the quality of products and eliminate congenital defects.


(2) the leakage oil should be checked according to the prescribed cycle and stop when the oil leakage is found.


(3) the capacitance value, tg&delta, polarity insulation resistance and insulation resistance of the low voltage end should be measured according to the prescribed period. The results of measurement shall be in line with the requirements for electrical equipment preventive test procedures.


(4) actively carry out new test items, such as live measurement of capacitance current, partial discharge, AC voltage withstand test and chromatographic analysis. Chromatographic data analysis should be based on the analysis of characteristic gas content, and the attention value can be used to refer to the attention value of transformers and bushings.


(5) the new capacitor should be selected with "positive pressure at the operating temperature".


(6) it is suggested that the manufacturer should install an oil level indicator and a pressure release device on the capacitor, and equipotential connection for the expander and the pin. The factory test adds the data of "partial discharge measurement".